Scientists have developed a way to triple the shelf life of ready-to-eat macaroni and cheese which, they say, could benefit everything from space travel to military use.

Currently, plastic packaging can keep food safe at room temperature for up to twelve months, but the Washington State University (WSU) researchers demonstrated in a recent paper they could keep ready-to-eat macaroni and cheese safe and edible with selected nutrients for up to three years.

We need a better barrier to keep oxygen away from the food and provide longer shelf-life similar to aluminium foil and plastic laminate pouches, said Shyam Sablani, who is leading the team working to create a better protective film.

Weve always been thinking of developing a product that can go to Mars, but with technology that can also benefit consumers here on Earth.

In addition to having space travel in mind, the researchers are working closely with the US Army, who want to improve their Meals Ready to Eat (MREs) to stay tasty and healthy for three years.

In taste panels conducted by the Army, the mac and cheese, recently tested after three years of storage, was deemed just as good as the previous version that was stored for nine months.

The science behind longer shelf life

The food itself is sterilised using a process called the microwave-assisted thermal sterilisation (MATS) system, developed by WSUs Juming Tang.

The food must be sterilised in plastic, since metal, like tin cans, cant be microwaved and glass is fragile and not a preferred choice of packaging for MREs. Glass is also too heavy for military or space uses.

Adding a metal oxide coating to a layer of the plastic film significantly increases the amount of time it takes for oxygen and other gases to break through.

The metal oxide coating technology has been around for almost 10 years, but it develops cracks when subjected to sterilization processes. That eventually compromises the food shelf-life, Sablani said.

WSU researchers have been working with packaging companies to develop new films that keep oxygen and vapor out longer.

The packaging films are made up of multiple layers of different plastics. These few-micron thin layers have different purposes, like being a good barrier, good for sealing, good mechanical strength, or good for printing, Sablani said.

We are excited that an over-layer of organic coating on metal oxide helped protect against microscopic cracks, he said.

Multiple layers of metal oxide coating have also increased the barrier performance. Our research guided development of newer high barrier packaging.

To ensure the process works fully, the Army plans to do testing under field conditions. So these new MREs will be stored longer, then sent to deployed soldiers to eat in the field.

If they like the taste of the packaged food there, then thats the ultimate test of new films, Sablani said.

The team doesnt wait the three years to test the results of each new film. Keeping the packaged food in a 100-degree Fahrenheit incubator rapidly speeds up the food quality changes at a consistent rate. Six months in the incubator is equivalent to three years at room temperature, while nine months is the equivalent to nearly five years, Sablani said.

The final frontier

For space travel, its not really possible to field-test for a trip to Mars. But Sablani plans to reach out to NASA to talk about how to test the WSU films to make sure that packaged food stays edible on a space mission where failure isnt an option.

NASA knows about our work, but were just now getting to the point where we can talk to them with a proven product, Sablani said.

We hope to work out a way to test these products on the International Space Station in the future to show that the food is safe after long-term storage.

NASA will require storage of up to five years for food, so thats what the team is working on now. They are currently aging other recipes that will be taste tested once they reach the five-year mark.

Several types of mission plans have been proposed for a trip to Mars. The five-year food storage includes some built-in safety requirements, Sablani said.

It may involve an approximately nine-month travel time from Earth to Mars, about five hundred days on or orbiting Mars, and a travel time of about nine months to return to Earth.

The extra storage time is necessary in case the mission is delayed and explorers must stay longer.

See the article here:

Scientists extend shelf life to benefit army and space travel - Food & Drink International

Faster Than Light (FTL) technology has been constricted to the realms of science fiction, but it is theoretically possible, according to one researcher. The research, carried out by Joseph Agnew, an undergraduate engineer and research assistant from the University of Alabama in Huntsvilles Propulsion Research Center (PRC), builds on the expertise of Mexican physicist Miguel Alcubierre. Mr Alcubierre established a concept for an FTL system back in 1994 which was built on Einsteins field equations.

Essentially, the equations dictate that space, time and energy all interact and Mr Alcubierre believed they could be manipulated to travel faster than the speed of light a staggering 299,792,458 metres per second.

Mr Alcubierres warp drive technology would involve stretching the fabric of space-time to form a wave.

Theoretically, the space ahead of the ship would contract while behind it would expand. This would mean that the ship is not moving, but moving space-time itself.

It has now become known as the Alcubierre Metric which involves riding the wave of space-time to achieve FTL travel.

The theory seemed improbable at the time, but the discovery of gravitational wave proved that space-time can warp, as per special relativity.

Mr Agnew told Universe Today: The historically theoretical nature of the idea is also itself a likely deterrent, as its much more difficult to see substantial progress when you are looking at equations instead of quantitative results.

In the past 5-10 years or so, there has been a lot of excellent progress along the lines of predicting the anticipated effects of the drive, determining how one might bring it into existence, reinforcing fundamental assumptions and concepts, and, my personal favourite, ways to test the theory in a laboratory.

The LIGO discovery a few years back was, in my opinion, a huge leap forward in science, since it proved, experimentally, that spacetime can warp and bend in the presence of enormous gravitational fields, and this is propagated out across the Universe in a way that we can measure.

READ MORE:How NASA captured supermassive black hole dance of death tornado'

Before, there was an understanding that this was likely the case, thanks to Einstein, but we know for certain now.

In essence, what is needed for a warp drive is a way to expand and contract spacetime at will, and in a local manner, such as around a small object or ship.

I believe there is a chance that once the effect can be duplicated on a lab scale, it will lead to a much deeper understanding of how gravity works, and may open the door to some as-yet-undiscovered theories or loopholes.

I suppose to summarise, the biggest hurdle is the energy, and with that comes technological hurdles, needing bigger EM fields, more sensitive equipment, etc.

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An EM field is an electromagnetic field produced by electrically charged objects. An EM Drive works by bouncing microwaves around inside a closed engine. The microwaves subsequently push against the side of the container, acting as a propellor.

Even if travelling at light speed can be achieved for reference the fastest man-made machine is NASAs Solar Probe Plus which, when it orbits the Sun, will achieve a speed of 690,000 km/h (430,000 mph), or 0.064 percent the speed of light getting across the Universe would still be problematic.

It may make travelling across the solar system a doddle, but to reach the nearest star system, Proxima Centauri which is 4.2 light-years away, it would take, well, 4.2 years.

To exit the Milky Way and reach the next nearest galaxy, the Canis Major Dwarf Galaxy, it would take a whopping 25,000 years.

View post:

Space travel across the universe could be faster than speed of light with Warp Drive - Express.co.uk

By Lucas NaberUpdated September 25, 2019 1:12 a.m. CTPublished September 25, 2019 10:10 a.m. CT

Writer and director James Grays Ad Astra is equal parts character study and sci-fi epic, exploring both the physical and emotional isolation of its astronaut protagonist.

Set in a bleak take on the near-future, the film stars Brad Pitt (Fight Club, The Big Short) as astronaut Roy McBride, son of legendary U.S. Space Command astronaut and leader of the fictional Lima Project H. Clifford McBride (Tommy Lee Jones).

Its been 26 years since the Lima Project was formed to scour the solar system for signs of intelligent life, and 16 since the projects ship and entire crew went missing somewhere in Neptunes orbit.

Clifford and his crew have long been presumed dead, but Space Command officials reconsider when they link a worldwide series of deadly electric surges back to the Lima Projects experiments. They enlist Roy to try and contact his father, who may be alive and purposely avoiding detection.

Roy agrees to Space Commands terms and finds himself suiting up to leave Earth the way hes done his whole career.

Ad Astra, released Sept. 20, might not actually be a realistic depiction of space travel, but its rule-defined and unflinchingly logical approach is so plausible it might as well be a documentary on the subject.

Roys complicated journey through this detail-oriented world drives the plot, but the film finds its true merit in simplicity.

Roy is numb to the bureaucracy and safeguarding of Space Commands operations. Hes a machine, powering through psychological exams, ignoring repetitive safety videos and sporting a heart rate that has never risen above 80 beats per minute.

Roys character is defined by his ability to robotically excel in the structure around him. His interstellar expertise and introspective narration make the films complex trappings seem commonplace, reducing the need for expository dialogue.

Grays earnest screenplay paints Roy with plenty of emotional depth, but Pitt communicates more with his eyes than any screenplay could. Bearing a hollow fake smile and perfect posture, Pitt carries the weight of loneliness and labor spanning years inside his pupils and along his brow, expressing more with his mannerisms than his words.

Gray understands the talent hes been blessed with in Pitt, and the film reflects this. Other characters linger on the margins, but the film laser-focuses on Pitt. Roys separated wife Eve (Liv Tyler) is his only human tie to Earth, and shes reduced to a hazy memory by the void Roy faces.

As Roy navigates the stars, he reflects on his current position in life and his relationship with his father.

When the Lima Project left Earth, Roy was a teenager. By the time he found out his father wouldnt return, he was a grown man. Now in his 40s, Roy must confront the possibility that his father wasnt taken from him but instead chose not to come back.

After Clifford disappeared, he shifted from a real figure in Roys life to a security blanket. His heroism motivated Roys career path and his tragic presumed death was easier for Roy to cope with than the possibility of abandonment.

For decades, Roy has used these justifications to ignore the painfully obvious. He always wanted more from his father, even before his mission lifted off.

At two hours and four minutes long, Grays film is a masterpiece of pacing. Ad Astra handles the material of a much longer film without rushing and employs a contemplative pace without drawing things out.

The film takes a densely classical approach to its genre with great success, utilizing hard scientific logic to tell a cosmic adventure story spanning years, but this isnt where its main appeal lies.

Ad Astra understands the appeal of futuristic space travel and knowing the unknowable, but lots of films do. Its the films ability to connect its fictional concept to such innate human concerns that makes Ad Astra so special.

Ad Astra, rated PG-13, is playing in theaters nationwide.

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Space-Travel Odyssey 'Ad Astra' Reflects on the Human Condition - Loyola Phoenix

Mac and cheese in the new plastic packaging from WSU. (Image courtesy of CAHNRS News.)

Researchers from Washington State University (WSU) have developed a process that could potentially increase the shelflife of a mealtime favorite in space. Current plastic packaging products can keep food safe at room temperature for only up to 12 months. WSU researchers have figured out how to triple the shelflife of ready-to-eat macaroni and cheese, a development that can largely benefit space travel and military use.

To survive the long travel between Earth and Mars, astronauts will need food that wont spoil during the journey and while theyre on the planets surface.

We need a better barrier to keep oxygen away from the food and provide longer shelflife similar to aluminum foil and plastic laminate pouches, said Shaym Sablani, a professor in WSUs Department of Biological Systems Engineering who lead the research.

The study took form when the team began working closely with the U.S. Army in efforts to improve the Armys Meals Ready to Eat (MREs) to have a shelflife of three years. The Army recently put together a taste panel to test mac and cheese stored for the equivalent of three years and concluded that it was just as good as the current version, which can be stored only for nine months.

The researchers worked with packaging companies to develop new films that prevent oxygen and vapor from escaping for a longer period.

The food is sterilized using a process developed by WSUs Juming Tang called the microwave assisted thermal sterilization (MATS) system. Instead of using metal, like tin cans, the food is sterilized in plastic. Since metal cannot be microwaved, it is the least preferred packaging for MREs. Similarly, glass is too fragile as well as too heavy for either military or space use.

Additionally, the researchers discovered that adding a metal oxide coating to the plastic film significantly speeds up the time it takes for oxygen and other gases to escape. Sablani notes that this compromises the foods shelflife. While metal oxide coating technology has existed for almost 10 years, it can actually be detrimental to the preservation processes, creating cracks when subjected to sterilization.

The packaging films the WSU researchers developed along with packaging companies are composed of multiple layers of different plastics. According to Sablani, each micron thin layer serves a different purpose, such as acting as a barrier or a seal, and can be used for mechanical strength or for printing.

We are excited that an over-layer of organic coating on metal oxide helped protect against microscopic cracks, Sablani said. Multiple layers of metal oxide coating have also increased the barrier performance. Our research guided development of newer high barrier packaging.

The team did not actually wait three years to test the results of each new film. The packaged food was instead kept in a 100F incubator, which rapidly speeds up the change in food quality at a consistent rate. According to Sablani, six months in the incubator is equivalent to three years at room temperature, while nine months is equivalent to five years.

WSU graduate student Juhi Patel, an author on the mac and cheese paper, puts packages of purple potatoes into an incubator, which speeds up the food quality changes at a consistent rate. (Image courtesy of CAHNRS News.)

The Army plans to conduct more testing under field conditions. If they like the taste of the packaged food there, then thats the ultimate test of new films, said Sablani.

The team has already expressed plans to put the technology to use in space, specifically for Mars. While its still not possible to field-test the films through a trip to Mars, Sablani intends to reach out to NASA to discuss how his team can test the WSU films for space missions.

NASA knows about our work, but were just now getting to the point where we can talk to them with a proven product, explained Sablani. We hope to work out a way to test these products on the International Space Station in the future to show that the food is safe after long-term storage.

For food, NASA requires storage allocation of up to five years. The WSU team is currently working on meeting this stipulation. The researchers are also exploring other recipes that will be taste tested when the foods reach the five-year mark. With several types of mission plans proposed for a trip to Mars, Sablani adds that five-year food storage will need to include some built-in safety requirements.

A trip from Earth to Mars may involve approximately nine months of travel, plus five hundred days on or orbiting Mars, then another nine months of travel to return to Earth. Having food that can withstand extra storage time is also crucial in case of unexpected delays or the prolonging of a mission.

The study can be found in the Food and Bioprocess Technology journal. The research was supported by the USDA National Institute of Food and Agriculture Research, AFRI Foundational Grant Program.

For more on the latest developments in space travel, check out how China is building a gigawatt power station in spacehere.

Excerpt from:

Mealtime Favorite Mac and Cheese May Be the Next to Visit Mars - ENGINEERING.com

Forward-looking: At the UK Space Conference 2019, the country's space agency announced that it would be closely working with the Australian Space Agency on an agreement called the "world's first Space Bridge" that includes the prospect of hypersonic space travel between Australia and the UK. A possibility, thanks to the Sabre engine currently in development at the Oxfordshire-based Reaction Engines Limited.

By 2030, hypersonic flights could potentially let people travel from the UK to New York in an hour or reach Australia in four hours. That's the ambition set by the UK and Australia's space agencies as they recently signed up on a 'space bridge' agreement to collaborate and advance in the space industry.

The development took place at this year's UK Space Conference held in Wales. "A space bridge agreement will bring significant benefits to both our thriving space industries, facilitating new trade and investment opportunities and the exchange of knowledge and ideas," commented Dr Graham Turnock, CEO of the UK Space Agency. "It was a pleasure to welcome the Australian Space Agency to the UK Space Conference 2019 and to set out our intent to increase collaboration," he added.

Part of this collaboration involves working on a new hypersonic aircraft powered by UK's Reaction Engines Ltd. The Synergetic Air-Breathing Rocket Engine (SABRE) currently undergoing development at the company is said to have the fuel efficiency of a jet engine combined with the power and high-speed ability of a rocket.

"When we have brought the SABRE rocket engine to fruition, that may enable us to get to Australia in perhaps as little as four hours," said Dr Graham, adding that "This is technology that could definitely deliver that. We're talking the 2030s for operational service, and the work is already very advanced."

Reaction Engines also ran successful tests of a precooler in April this year, in which it simulated conditions at Mach 3.3 (more than three times the speed of sound). These simulations were conducted at a testing facility in the Colorado Air and Space Port in the US.

The precooler was tested to ensure that extremely hot temperatures caused by high-speed air-flow through the engine wouldn't damage any components. The company said that the precooler was able to cool gases over 1,000 C to ambient temperature in less than 1/20th of a second. "This is a hugely significant milestone which has seen Reaction Engines' proprietary precooler technology achieve unparalleled heat transfer performance," said Mark Thomas, CEO of Reaction Engines.

The company's program director Shaun Driscoll said that the Sabre engine was like a hybrid of a rocket engine and an aero engine as it allowed a rocket to breathe air. "Rockets really haven't progressed in 70 years, whereas aero engines have become very efficient, so if you can combine an aero engine and a rocket you can have a very lightweight efficient propulsion system and basically create a space plane," he said.

With over 100 million ($130 million) in funding over the past four years, Reaction Engines has garnered interest of many big names in the industry including BAE Systems, Rolls-Royce and Boeing.

A testing facility at Buckinghamshire, UK, is being finalized for construction that will serve as the location for the first ground-based demonstration of a SABRE engine air-breathing core.

Image(s) Credit: Reaction Engines

Originally posted here:

UK and Australia space agencies are developing a hypersonic 'space plane' - TechSpot

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Letter of the weekSaudi is a great destination for a special-interest holiday. I would start in Jeddah, with a visit to the old towns historic merchant houses and fabulous gold and spice souks, then stroll along the Corniche to see the sunset. North of Medina, you can see the remains of the Hejaz railway, the tracks, stations and rusting rolling stock lying abandoned in the desert, like a scene from Lawrence of Arabia. Continuing north through Al Ula, you come to Madain Saleh, the second largest Nabatean settlement after Petra. This necropolis has more than 131 tombs with intricate inscriptions and carvings of eagles and sphinxes. This is just a small part of the Nabatean kingdom there is much more to discover.Susan Hannis, Dorset

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Letters: Saudi's controversial tourism bid; space travel; and why trains aren't always better than planes - The Times

NASA

The mini robots can come together to form one big robot

Nasa are designing a shapeshifting robot that'll travel to Saturn.

They're hoping to send the new robot to one of Saturn's biggest moons, called Titan.

The robot is called Shapeshifter, and it's formed of lots of mini robots that can roll, fly, float and swim, then morph into a single machine.

This is what the robot looks like at the moment

Nasa hope they'll find out more about Titan. It's the only object in the Solar System other than Earth that has liquid on the surface.

The robot explorer is still in the first stages of production but apparently the early tests are looking promising! The prototype can roll around on the ground and split itself in half.

This is an artist's impression of how the robot might operate

One of the researches at Nasa said: "We have very limited information about the composition of the surface [of Titan]"

So the experts felt they needed to design a machine that was capable of exploring all the different parts of it and could deal with anything!

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Space: Nasa 'Shapeshifter' robot could be on its way to Saturn! - CBBC Newsround

Maybe it will take going to outer space for renewable energy naysayers to realise the devastating impact the human race has on planet Earth.

Just this week, the United Kingdom parliament became the first in the world to pass a motion declaring an environment and climate emergency, amid mounting pressure for the region to adopt a net zero emission target before 2050.

The news follows a recent United Nations report that found seven major countries, including the United States, were well behind achieving the carbon neutral pledges they made in Paris three years ago.

The UN noted that countries were not doing enough to curb their emissions.

But perhaps commercial space travel is what we need to push people to tackle climate change more seriously?

NASA astronaut Terry Virts has spoken publicly about the perspective-altering effects of viewing Earth from above, in particular the increased awareness it brings of humankinds environmental mistakes.

Virts has spent 200 days in space at the International Space Station (ISS) and has taken more photos of Earth from space than any other astronaut.

People ask astronauts 'what's your favourite planet?' and it's not Mars or Jupiter, it's Earth, Virts said upon returning from space.

We have everything we need to survive right here. In saying that, when I tried to film Beijing I never could because all you could see was smog. And in the Amazon you can see deforestation. So there's some man-made environmental messes you can see from space. But 99 percent of the planet really does look beautiful, it's not all doom and gloom.

While 99 percent of the planet may look beautiful right now, how long will it take for emissions from fossil fuels to produce enough smog to obscure the majority of Earth from view?

And by the time commercial space travel becomes available for the likes of you and I, will we be able to capture that perfect holiday snap of Earth that Virts fondly speaks of?

Commercial Space Travel: Coming to An Airport Near You

There are few people who can say theyve been to outer space. And even fewer that can claim to have been among the first in the world to fly on a commercial spacecraft.

But in February this year Beth Moses, chief astronaut instructor at Virgin Galactic, became a household name for doing just that. Years of hard work, dedication and self belief finally paid off and Moses became the first woman to travel to space on a commercial spacecraft.

It was the first time that Moses, an aerospace engineer who had previously worked at NASA, had visited space. The purpose of Moses flight was to ensure she could fully prepare Virgin Galactics future customers, of which there are hundreds in queue, for the journey to view earth from above.

Having Beth fly in the cabin today, starting to ensure that our customer journey is as flawless as the spaceship itself, brings a huge sense of anticipation and excitement to all of us here who are looking forward to experiencing space for ourselves, Virgin Galactic founder Sir Richard Branson said at the time.

Beth Moses celebrating after the flight with the two pilots, Dave Mackay (L) and Mike Sooch Masucci (C), as well as Virgin Galactic CEO George Whitesides (R)

Mosess mission for the flight was clear; experience the flight fully to ensure there were no surprises following the three-day briefing each customer receives pre-flight, so these everyday people could focus on the life-altering view around them.

Taking Risks To Shape Perspective

Moses admits the criticism the Virgin Galactic team received for daring to dream big was unexpected.

Its an experience Moses and I share, with Power Ledger facing its own critics for daring to reimagine the financial models surrounding green energy and accelerate the takeup of renewables.

I blindly assumed the new space community would be as respected as NASA. But within my professional community people laugh about human commercial space travel, Moses told me.

Its true doubters have not been shy about sharing their views publicly.

Australian astronaut Andy Thomas called Sir Richard Bransons quest to take passengers into orbit a dead-end and dangerous technology.

And following a fatal crash in 2014, rocket propulsion expert Carolynne Campbell-Knight was quoted stating Virgin Galactic should go away and do something they might be good at like selling mobile phones. They should stay out of the space business.

Despite the mounting pressure to give up, Moses and the team at Virgin Galactic pushed on, paying close attention to their mistakes and turning them into lessons.

And slowly, some space experts started to change their tune.

In December 2018, NASA selected its own technology experiments to fly on Virgin Galactics SpaceShipTwo commercial research flight. At the time, Christopher Baker of NASAs Armstrong Flight Research Center said that regular commercial access to space will change how NASA approaches technology development, by allowing them to invest in early research validation.

This year, in its fifth supersonic-powered test flight, Virgin Galactic reached space for the second time in 10 weeks. Spaceship VSS Unity, the spacecraft Moses was on, also flew higher and faster than ever before.

Moses herself could feel the tides changing when some of the NASA colleagues, who had laughed off her decision to join the human space startup, began asking to work with Virgin Galactic.

All of a sudden they were saying wow, they did it. Now they want to come work here and they want a job, Moses says.

I like our model as it can bring it (space travel) to more people.

Author and National Geographic Society chairman, Jean Case describes the bold risk-taking behind some of the greatest discoveries and innovations humanity has made in her book Be Fearless. Case specifically calls out space travel as being able to alter culture, geography and political systems.

Big bets are the engine for countless other innovations. They can change a culture, a geography, a mindset and a political system, Case writes in chapter six.

Moses agrees; Space travel can redefine geography and political systems, it encourages a mindset that isnt regional and there is a human and emotional magic to that, she says.

We have to graduate to a sense of planetary unity that we dont have as humans.

Using A Planetary View to Fix Our Problems

Yale has already recognised the power of an outer space perspective to address Earths pressing climate issues.

Yale OpenLabs energy academy, which believes that achieving sustainability requires a collective planetary consciousness, has created an open and globally collaborative space dedicated to promoting planetary sustainability.

The program is currently in proof-of-concept phase, but promises to build an unconventional classroom and teach visually stunning journeys where natural resources are studied from the big perspective.

Our climate and energy challenge requires a population that understands the close and delicate connections between macro and micro scales; from the solar system down to cellular and molecular structures, the project explains.

Yales vision? To empower individuals and communities to re-envision the global energy system, to generate planetary consciousness and foster higher awareness in our delicate relationship to Earth.

A Vision Thats Out of This World

Moses is hopeful that the Virgin Galactic team can replicate their successes and operate from any airport, and admits she is an eternal and self-proclaimed biased optimist.

Were asking questions that have never been asked, Moses says.

I think it is time we all asked ourselves questions that have never been asked.

As that old adage goes; its better to have an impossible dream than no dream at all - if not for commercial aspirations, then at least for the sake of planet Earth.

Maybe it will take going to outer space for renewable energy naysayers to realise the devastating impacts of climate change.

But hopefully it wont take a ticket to the moon to appreciate Earth more and realise the importance of preserving its future.

Continue reading here:

Commercial Space Travel: How Leaving Earth Will Make You ...

Human spaceflight (also referred to as crewed spaceflight or manned spaceflight) is space travel with a crew or passengers aboard the spacecraft. Spacecraft carrying people may be operated directly, by human crew, or it may be either remotely operated from ground stations on Earth or be autonomous, able to carry out a specific mission with no human involvement.

The first human spaceflight was launched by the Soviet Union on 12 April 1961 as a part of the Vostok program, with cosmonaut Yuri Gagarin aboard. Humans have been continuously present in space for 18years and 168days on the International Space Station. All early human spaceflight was crewed, where at least some of the passengers acted to carry out tasks of piloting or operating the spacecraft. After 2015, several human-capable spacecraft are being explicitly designed with the ability to operate autonomously.

Russia and China have human spaceflight capability with the Soyuz program and Shenzhou program. In the United States, SpaceShipTwo reached the edge of space in 2018; this was the first crewed spaceflight from the USA since the Space Shuttle retired in 2011. Currently, all expeditions to the International Space Station use Soyuz vehicles, which remain attached to the station to allow quick return if needed. The United States is developing commercial crew transportation to facilitate domestic access to ISS and low Earth orbit, as well as the Orion vehicle for beyond-low Earth orbit applications.

While spaceflight has typically been a government-directed activity, commercial spaceflight has gradually been taking on a greater role. The first private human spaceflight took place on 21 June 2004, when SpaceShipOne conducted a suborbital flight, and a number of non-governmental companies have been working to develop a space tourism industry. NASA has also played a role to stimulate private spaceflight through programs such as Commercial Orbital Transportation Services (COTS) and Commercial Crew Development (CCDev). With its 2011 budget proposals released in 2010,[1] the Obama administration moved towards a model where commercial companies would supply NASA with transportation services of both people and cargo transport to low Earth orbit. The vehicles used for these services could then serve both NASA and potential commercial customers. Commercial resupply of ISS began two years after the retirement of the Shuttle, and commercial crew launches could begin by 2019.[2]

Human spaceflight capability was first developed during the Cold War between the United States and the Soviet Union (USSR), which developed the first intercontinental ballistic missile rockets to deliver nuclear weapons. These rockets were large enough to be adapted to carry the first artificial satellites into low Earth orbit. After the first satellites were launched in 1957 and 1958, the US worked on Project Mercury to launch men singly into orbit, while the USSR secretly pursued the Vostok program to accomplish the same thing. The USSR launched the first human in space, Yuri Gagarin, into a single orbit in Vostok 1 on a Vostok 3KA rocket, on 12 April 1961. The US launched its first astronaut, Alan Shepard, on a suborbital flight aboard Freedom 7 on a Mercury-Redstone rocket, on 5 May 1961. Unlike Gagarin, Shepard manually controlled his spacecraft's attitude, and landed inside it. The first American in orbit was John Glenn aboard Friendship 7, launched 20 February 1962 on a Mercury-Atlas rocket. The USSR launched five more cosmonauts in Vostok capsules, including the first woman in space, Valentina Tereshkova aboard Vostok 6 on 16 June 1963. The US launched a total of two astronauts in suborbital flight and four into orbit through 1963.

US President John F. Kennedy raised the stakes of the Space Race by setting the goal of landing a man on the Moon and returning him safely by the end of the 1960s.[3] The US started the three-man Apollo program in 1961 to accomplish this, launched by the Saturn family of launch vehicles, and the interim two-man Project Gemini in 1962, which flew 10 missions launched by Titan II rockets in 1965 and 1966. Gemini's objective was to support Apollo by developing American orbital spaceflight experience and techniques to be used in the Moon mission.[4]

Meanwhile, the USSR remained silent about their intentions to send humans to the Moon, and proceeded to stretch the limits of their single-pilot Vostok capsule into a two- or three-person Voskhod capsule to compete with Gemini. They were able to launch two orbital flights in 1964 and 1965 and achieved the first spacewalk, made by Alexei Leonov on Voskhod 2 on 8 March 1965. But Voskhod did not have Gemini's capability to maneuver in orbit, and the program was terminated. The US Gemini flights did not accomplish the first spacewalk, but overcame the early Soviet lead by performing several spacewalks and solving the problem of astronaut fatigue caused by overcoming the lack of gravity, demonstrating up to two weeks endurance in a human spaceflight, and the first space rendezvous and dockings of spacecraft.

The US succeeded in developing the Saturn V rocket necessary to send the Apollo spacecraft to the Moon, and sent Frank Borman, James Lovell, and William Anders into 10 orbits around the Moon in Apollo 8 in December 1968. In July 1969, Apollo 11 accomplished Kennedy's goal by landing Neil Armstrong and Buzz Aldrin on the Moon 21 July and returning them safely on 24 July along with Command Module pilot Michael Collins. A total of six Apollo missions landed 12 men to walk on the Moon through 1972, half of which drove electric powered vehicles on the surface. The crew of Apollo 13, Lovell, Jack Swigert, and Fred Haise, survived a catastrophic in-flight spacecraft failure and returned to Earth safely without landing on the Moon.

Meanwhile, the USSR secretly pursued human lunar orbiting and landing programs. They successfully developed the three-person Soyuz spacecraft for use in the lunar programs, but failed to develop the N1 rocket necessary for a human landing, and discontinued the lunar programs in 1974.[5] On losing the Moon race, they concentrated on the development of space stations, using the Soyuz as a ferry to take cosmonauts to and from the stations. They started with a series of Salyut sortie stations from 1971 to 1986.

After the Apollo program, the US launched the Skylab sortie space station in 1973, manning it for 171 days with three crews aboard Apollo spacecraft. President Richard Nixon and Soviet Premier Leonid Brezhnev negotiated an easing of relations known as dtente, an easing of Cold War tensions. As part of this, they negotiated the Apollo-Soyuz Test Project, in which an Apollo spacecraft carrying a special docking adapter module rendezvoused and docked with Soyuz 19 in 1975. The American and Russian crews shook hands in space, but the purpose of the flight was purely diplomatic and symbolic.

Nixon appointed his Vice President Spiro Agnew to head a Space Task Group in 1969 to recommend follow-on human spaceflight programs after Apollo. The group proposed an ambitious Space Transportation System based on a reusable Space Shuttle which consisted of a winged, internally fueled orbiter stage burning liquid hydrogen, launched by a similar, but larger kerosene-fueled booster stage, each equipped with airbreathing jet engines for powered return to a runway at the Kennedy Space Center launch site. Other components of the system included a permanent modular space station, reusable space tug and nuclear interplanetary ferry, leading to a human expedition to Mars as early as 1986, or as late as 2000, depending on the level of funding allocated. However, Nixon knew the American political climate would not support Congressional funding for such an ambition, and killed proposals for all but the Shuttle, possibly to be followed by the space station. Plans for the Shuttle were scaled back to reduce development risk, cost, and time, replacing the piloted flyback booster with two reusable solid rocket boosters, and the smaller orbiter would use an expendable external propellant tank to feed its hydrogen-fueled main engines. The orbiter would have to make unpowered landings.

The two nations continued to compete rather than cooperate in space, as the US turned to developing the Space Shuttle and planning the space station, dubbed Freedom. The USSR launched three Almaz military sortie stations from 1973 to 1977, disguised as Salyuts. They followed Salyut with the development of Mir, the first modular, semi-permanent space station, the construction of which took place from 1986 to 1996. Mir orbited at an altitude of 354 kilometers (191 nautical miles), at a 51.6 inclination. It was occupied for 4,592 days, and made a controlled reentry in 2001.

The Space Shuttle started flying in 1981, but the US Congress failed to approve sufficient funds to make Freedom a reality. A fleet of four shuttles was built: Columbia, Challenger, Discovery, and Atlantis. A fifth shuttle, Endeavour, was built to replace Challenger, which was destroyed in an accident during launch that killed 7 astronauts on 28 January 1986. Twenty-two Shuttle flights carried a European Space Agency sortie space station called Spacelab in the payload bay from 1983 to 1998.[6]

The USSR copied the reusable Space Shuttle orbiter, which it called Buran. It was designed to be launched into orbit by the expendable Energia rocket, and capable of robotic orbital flight and landing. Unlike the US Shuttle, Buran had no main rocket engines, but like the Shuttle used its orbital maneuvering engines to perform its final orbital insertion. A single unmanned orbital test flight was successfully made in November 1988. A second test flight was planned by 1993, but the program was cancelled due to lack of funding and the dissolution of the Soviet Union in 1991. Two more orbiters were never completed, and the first one was destroyed in a hangar roof collapse in May 2002.

The dissolution of the Soviet Union in 1991 brought an end to the Cold War and opened the door to true cooperation between the US and Russia. The Soviet Soyuz and Mir programs were taken over by the Russian Federal Space Agency, now known as the Roscosmos State Corporation. The Shuttle-Mir Program included American Space Shuttles visiting the Mir space station, Russian cosmonauts flying on the Shuttle, and an American astronaut flying aboard a Soyuz spacecraft for long-duration expeditions aboard Mir.

In 1993, President Bill Clinton secured Russia's cooperation in converting the planned Space Station Freedom into the International Space Station (ISS). Construction of the station began in 1998. The station orbits at an altitude of 409 kilometers (221nmi) and an inclination of 51.65.

The Space Shuttle was retired in 2011 after 135 orbital flights, several of which helped assemble, supply, and crew the ISS. Columbia was destroyed in another accident during reentry, which killed 7 astronauts on 1 February 2003.

After Russia's launch of Sputnik 1 in 1957, Chairman Mao Zedong intended to place a Chinese satellite in orbit by 1959 to celebrate the 10th anniversary of the founding of the People's Republic of China (PRC),[7] However, China did not successfully launch its first satellite until 24 April 1970. Mao and Premier Zhou Enlai decided on 14 July 1967, that the PRC should not be left behind, and started China's own human spaceflight program.[8] The first attempt, the Shuguang spacecraft copied from the US Gemini, was cancelled on 13 May 1972.

China later designed the Shenzhou spacecraft resembling the Russian Soyuz, and became the third nation to achieve independent human spaceflight capability by launching Yang Liwei on a 21-hour flight aboard Shenzhou 5 on 15 October 2003. China launched the Tiangong-1 space station on 29 September 2011, and two sortie missions to it: Shenzhou 9 1629 June 2012, with China's first female astronaut Liu Yang; and Shenzhou 10, 1326 June 2013. The station was retired on 21 March 2016 and remains in a 363-kilometer (196-nautical-mile), 42.77 inclination orbit.

The European Space Agency began development in 1987 of the Hermes spaceplane, to be launched on the Ariane 5 expendable launch vehicle. The project was cancelled in 1992, when it became clear that neither cost nor performance goals could be achieved. No Hermes shuttles were ever built.

Japan began development in the 1980s of the HOPE-X experimental spaceplane, to be launched on its H-IIA expendable launch vehicle. A string of failures in 1998 led to funding reduction, and the project's cancellation in 2003.

Under the Bush administration, the Constellation Program included plans for retiring the Shuttle program and replacing it with the capability for spaceflight beyond low Earth orbit. In the 2011 United States federal budget, the Obama administration cancelled Constellation for being over budget and behind schedule while not innovating and investing in critical new technologies.[9] For beyond low Earth orbit human spaceflight NASA is developing the Orion spacecraft to be launched by the Space Launch System. Under the Commercial Crew Development plan, NASA will rely on transportation services provided by the private sector to reach low Earth orbit, such as SpaceX's Falcon 9/Dragon V2, Sierra Nevada Corporation's Dream Chaser, or Boeing's CST-100. The period between the retirement of the shuttle in 2011 and the first launch to space of Spaceshiptwo on December 13, 2018 is similar to the gap between the end of Apollo in 1975 and the first Space Shuttle flight in 1981, is referred to by a presidential Blue Ribbon Committee as the U.S. human spaceflight gap.[10]

Since the early 2000s, a variety of private spaceflight ventures have been undertaken. Several of the companies, including Blue Origin, SpaceX, Virgin Galactic, and Sierra Nevada have explicit plans to advance human spaceflight. As of 2016[update], all four of those companies have development programs underway to fly commercial passengers.

A commercial suborbital spacecraft aimed at the space tourism market is being developed by Virgin Galactic called SpaceshipTwo which reached space in December 2018.[11][12]Blue Origin has begun a multi-year test program of their New Shepard vehicle and carried out six successful uncrewed test flights in 20152016. Blue Origin plan to fly "test passengers" in Q2 2017, and initiate commercial flights in 2018.[13][14]

SpaceX and Boeing are both developing passenger-capable orbital space capsules as of 2015, planning to fly NASA astronauts to the International Space Station by 2019. SpaceX will be carrying passengers on Dragon 2 launched on a Falcon 9 launch vehicle. Boeing will be doing it with their CST-100 launched on a United Launch Alliance Atlas V launch vehicle.[15]Development funding for these orbital-capable technologies has been provided by a mix of government and private funds, with SpaceX providing a greater portion of total development funding for this human-carrying capability from private investment.[16][17]There have been no public announcements of commercial offerings for orbital flights from either company, although both companies are planning some flights with their own private, not NASA, astronauts on board.

12 April 1961

Yuri Gagarin became the first Russian as well as the first human to reach space on Vostok 1 on April 12, 1961.

Sally Ride became the first American woman in space in 1983. Eileen Collins was the first female Shuttle pilot, and with Shuttle mission STS-93 in 1999 she became the first woman to command a U.S. spacecraft.

For many years, only the USSR (later Russia) and the United States had their own astronauts. Citizens of other nations flew in space, beginning with the flight of Vladimir Remek, a Czech, on a Soviet spacecraft on 2 March 1978, in the Interkosmos programme. As of 2010[update], citizens from 38 nations (including space tourists) have flown in space aboard Soviet, American, Russian, and Chinese spacecraft.

Human spaceflight programs have been conducted by the former Soviet Union and current Russian Federation, the United States, the People's Republic of China and by private spaceflight company Scaled Composites.

Currently have human spaceflight programs.

Confirmed and dated plans for human spaceflight programs.

Plans for human spaceflight on the simplest form (suborbital spaceflight, etc.).

Plans for human spaceflight on the extreme form (space stations, etc.).

Once had official plans for human spaceflight programs, but have since been abandoned.

Space vehicles are spacecraft used for transportation between the Earth's surface and outer space, or between locations in outer space. The following space vehicles and spaceports are currently used for launching human spaceflights:

The following space stations are currently maintained in Earth orbit for human occupation:

Numerous private companies attempted human spaceflight programs in an effort to win the $10 million Ansari X Prize. The first private human spaceflight took place on 21 June 2004, when SpaceShipOne conducted a suborbital flight. SpaceShipOne captured the prize on 4 October 2004, when it accomplished two consecutive flights within one week. SpaceShipTwo, launching from the carrier aircraft White Knight Two, is planned to conduct regular suborbital space tourism.[18]

Most of the time, the only humans in space are those aboard the ISS, whose crew of six spends up to six months at a time in low Earth orbit.

NASA and ESA use the term "human spaceflight" to refer to their programs of launching people into space. These endeavors have also been referred to as "manned space missions," though because of gender specificity this is no longer official parlance according to NASA style guides.[19]

On 15th August, 2018 Prime Minister of India Narendra Modi, from rampant of the Red Fort Formally announced Indian Human Spaceflight Programme. Through this Programme, India is planning to send humans into the space on its orbital vehicle Gaganyaan by the end of 2021. The Indian Space Research Organisation (ISRO) began work on this project in 2006.[20] The objective is to carry a crew of three to low Earth orbit (LEO) and return them safely for a water-landing at a predefined landing zone. The program is proposed to be implemented in defined phases. Currently, the activities are progressing with a focus on the development of critical technologies for subsystems such as the Crew Module (CM), Environmental Control and Life Support System (ECLSS), Crew Escape System, etc. The department has initiated activities to study technical and managerial issues related to crewed missions. The program envisages the development of a fully autonomous orbital vehicle carrying 2 or 3 crew members to about 300km low Earth orbit and their safe return.

NASA is developing a plan to land humans on Mars by the 2030s. The first step in this mission begins sometime during 2020, when NASA plans to send an uncrewed craft into deep space to retrieve an asteroid.[21] The asteroid will be pushed into the moons orbit, and studied by astronauts aboard Orion, NASAs first human spacecraft in a generation.[22] Orions crew will return to Earth with samples of the asteroid and their collected data. In addition to broadening Americas space capabilities, this mission will test newly developed technology, such as solar electric propulsion, which uses solar arrays for energy and requires ten times less propellant than the conventional chemical counterpart used for powering space shuttles to orbit.[23]

Several other countries and space agencies have announced and begun human spaceflight programs by their own technology, Japan (JAXA), Iran (ISA) and Malaysia (MNSA).

A number of spacecraft have been proposed over the decades that might facilitate spaceliner passenger travel. Somewhat analogous to travel by airliner after the middle of the 20th century, these vehicles are proposed to transport a large number of passengers to destinations in space, or to destinations on Earth which travel through space. To date, none of these concepts have been built, although a few vehicles that carry fewer than 10 persons are currently in the flight testing phase of their development process.

One large spaceliner concept currently in early development is the SpaceX BFR which, in addition to replacing the Falcon 9 and Falcon Heavy launch vehicles in the legacy Earth-orbit market after 2020, has been proposed by SpaceX for long-distance commercial travel on Earth. This is to transport people on point-to-point suborbital flights between two points on Earth in under one hour, also known as "Earth-to-Earth," and carrying 100+ passengers.[24][25][26]

Small spaceplane or small capsule suborbital spacecraft have been under development for the past decade or so and, as of 2017[update], at least one of each type are under development. Both Virgin Galactic and Blue Origin are in active development, with the SpaceShipTwo spaceplane and the New Shepard capsule, respectively. Both would carry approximately a half-dozen passengers up to space for a brief time of zero gravity before returning to the same location from where the trip began. XCOR Aerospace had been developing the Lynx single-passenger spaceplane since the 2000s[27][28][29] but development was halted in 2017.[30]

There are two main sources of hazard in space flight: those due to the environment of space which make it hostile to the human body, and the potential for mechanical malfunctions of the equipment required to accomplish space flight.

Planners of human spaceflight missions face a number of safety concerns.

The immediate needs for breathable air and drinkable water are addressed by the life support system of the spacecraft.

Medical consequences such as possible blindness and bone loss have been associated with human space flight.[38][39]

On 31 December 2012, a NASA-supported study reported that spaceflight may harm the brain of astronauts and accelerate the onset of Alzheimer's disease.[40][41][42]

In October 2015, the NASA Office of Inspector General issued a health hazards report related to space exploration, including a human mission to Mars.[43][44]

On 2 November 2017, scientists reported that significant changes in the position and structure of the brain have been found in astronauts who have taken trips in space, based on MRI studies. Astronauts who took longer space trips were associated with greater brain changes.[45][46]

Researchers in 2018 reported, after detecting the presence on the International Space Station (ISS) of five Enterobacter bugandensis bacterial strains, none pathogenic to humans, that microorganisms on ISS should be carefully monitored to continue assuring a medically healthy environment for astronauts.[47][48]

In March 2019, NASA reported that latent viruses in humans may be activated during space missions, adding possibly more risk to astronauts in future deep-space missions.[49]

Medical data from astronauts in low Earth orbits for long periods, dating back to the 1970s, show several adverse effects of a microgravity environment: loss of bone density, decreased muscle strength and endurance, postural instability, and reductions in aerobic capacity. Over time these deconditioning effects can impair astronauts performance or increase their risk of injury.[50]

In a weightless environment, astronauts put almost no weight on the back muscles or leg muscles used for standing up, which causes them to weaken and get smaller. Astronauts can lose up to twenty per cent of their muscle mass on spaceflights lasting five to eleven days. The consequent loss of strength could be a serious problem in case of a landing emergency.[51] Upon return to Earth from long-duration flights, astronauts are considerably weakened, and are not allowed to drive a car for twenty-one days.[52]

Astronauts experiencing weightlessness will often lose their orientation, get motion sickness, and lose their sense of direction as their bodies try to get used to a weightless environment. When they get back to Earth, or any other mass with gravity, they have to readjust to the gravity and may have problems standing up, focusing their gaze, walking and turning. Importantly, those body motor disturbances after changing from different gravities only get worse the longer the exposure to little gravity.[53] These changes will affect operational activities including approach and landing, docking, remote manipulation, and emergencies that may happen while landing. This can be a major roadblock to mission success.[citation needed]

In addition, after long space flight missions, male astronauts may experience severe eyesight problems.[54][55][56][57][58] Such eyesight problems may be a major concern for future deep space flight missions, including a crewed mission to the planet Mars.[54][55][56][57][59]

Without proper shielding, the crews of missions beyond low Earth orbit (LEO) might be at risk from high-energy protons emitted by solar flares and associated solar particle events (SPEs). Lawrence Townsend of the University of Tennessee and others have studied the overall most powerful solar storm ever recorded. The flare was seen by the British astronomer Richard Carrington in September 1859. Radiation doses astronauts would receive from a Carrington-type storm could cause acute radiation sickness and possibly even death.[61] Another storm that could have incurred a lethal radiation dose if astronauts were outside the Earth's protective magnetosphere occurred during the Space Age, in fact, shortly after Apollo 16 landed and before Apollo 17 launched.[62] This solar storm of August 1972 would likely at least have caused acute illness.[63]

Another type of radiation, galactic cosmic rays, presents further challenges to human spaceflight beyond low Earth orbit.[64]

There is also some scientific concern that extended spaceflight might slow down the bodys ability to protect itself against diseases.[65] Some of the problems are a weakened immune system and the activation of dormant viruses in the body. Radiation can cause both short and long term consequences to the bone marrow stem cells which create the blood and immune systems. Because the interior of a spacecraft is so small, a weakened immune system and more active viruses in the body can lead to a fast spread of infection.[citation needed]

During long missions, astronauts are isolated and confined into small spaces. Depression, cabin fever and other psychological problems may impact the crew's safety and mission success.[66]

Astronauts may not be able to quickly return to Earth or receive medical supplies, equipment or personnel if a medical emergency occurs. The astronauts may have to rely for long periods on their limited existing resources and medical advice from the ground.

During astronauts' stay in space, they may experience mental disorders (such as post-trauma, depression, anxiety, etc.), more than for an average person.NASA spends millions of dollars on psychological treatments for astronauts and former astronauts.[67] To date, there is no way to prevent or reduce mental problems caused by extended periods of stay in space.

Due to these mental disorders, the efficiency of their work is impaired and sometimes they are forced to send the astronauts back to Earth, which is very expensive. [68] A Russian expedition to space in 1976 was returned to Earth after the cosmonauts reported a strong odor that caused a fear of fluid leakage, but after a thorough investigation it became clear that there was no leakage or technical malfunction. It was concluded by NASA that the cosmonauts most likely had hallucinations of the smell, which brought many unnecessary wasted expenses.

It is possible that the mental health of astronauts can be affected by the changes in the sensory systems while in prolonged space travel.

During astronauts' spaceflight, they are in a very extreme state where there is no gravity. This given state and the fact that no change is taking place in the environment will result in the weakening of sensory input to the astronauts in all seven senses.

Space flight requires much higher velocities than ground or air transportation, which in turn requires the use of high energy density propellants for launch, and the dissipation of large amounts of energy, usually as heat, for safe reentry through the Earth's atmosphere.

Since rockets carry the potential for fire or explosive destruction, space capsules generally employ some sort of launch escape system, consisting either of a tower-mounted solid fuel rocket to quickly carry the capsule away from the launch vehicle (employed on Mercury, Apollo, and Soyuz), or else ejection seats (employed on Vostok and Gemini) to carry astronauts out of the capsule and away for individual parachute landing. The escape tower is discarded at some point before the launch is complete, at a point where an abort can be performed using the spacecraft's engines.

Such a system is not always practical for multiple crew member vehicles (particularly spaceplanes), depending on location of egress hatch(es). When the single-hatch Vostok capsule was modified to become the 2 or 3-person Voskhod, the single-cosmonaut ejection seat could not be used, and no escape tower system was added. The two Voskhod flights in 1964 and 1965 avoided launch mishaps. The Space Shuttle carried ejection seats and escape hatches for its pilot and copilot in early flights, but these could not be used for passengers who sat below the flight deck on later flights, and so were discontinued.

There have only been two in-flight launch aborts of a crewed flight. The first occurred on Soyuz 18a on 5 April 1975. The abort occurred after the launch escape system had been jettisoned, when the launch vehicle's spent second stage failed to separate before the third stage ignited. The vehicle strayed off course, and the crew separated the spacecraft and fired its engines to pull it away from the errant rocket. Both cosmonauts landed safely. The second occurred on 11 October 2018 with the launch of Soyuz MS-10. Again, both crew members survived.

In the only use of a launch escape system on a crewed flight, the planned Soyuz T-10a launch on 26 September 1983 was aborted by a launch vehicle fire 90 seconds before liftoff. Both cosmonauts aboard landed safely.

The only crew fatality during launch occurred on 28 January 1986, when the Space Shuttle Challenger broke apart 73 seconds after liftoff, due to failure of a solid rocket booster seal which caused separation of the booster and failure of the external fuel tank, resulting in explosion of the fuel. All seven crew members were killed.

The single pilot of Soyuz 1, Vladimir Komarov was killed when his capsule's parachutes failed during an emergency landing on 24 April 1967, causing the capsule to crash.

The crew of seven aboard the Space Shuttle Columbia were killed on reentry after completing a successful mission in space on 1 February 2003. A wing leading edge reinforced carbon-carbon heat shield had been damaged by a piece of frozen external tank foam insulation which broke off and struck the wing during launch. Hot reentry gasses entered and destroyed the wing structure, leading to breakup of the orbiter vehicle.

There are two basic choices for an artificial atmosphere: either an Earth-like mixture of oxygen in an inert gas such as nitrogen or helium, or pure oxygen, which can be used at lower than standard atmospheric pressure. A nitrogen-oxygen mixture is used in the International Space Station and Soyuz spacecraft, while low-pressure pure oxygen is commonly used in space suits for extravehicular activity.

Use of a gas mixture carries risk of decompression sickness (commonly known as "the bends") when transitioning to or from the pure oxygen space suit environment. There have also been instances of injury and fatalities caused by suffocation in the presence of too much nitrogen and not enough oxygen.

A pure oxygen atmosphere carries risk of fire. The original design of the Apollo spacecraft used pure oxygen at greater than atmospheric pressure prior to launch. An electrical fire started in the cabin of Apollo 1 during a ground test at Cape Kennedy Air Force Station Launch Complex 34 on 27 January 1967, and spread rapidly. The high pressure (increased even higher by the fire) prevented removal of the plug door hatch cover in time to rescue the crew. All three, Gus Grissom, Ed White, and Roger Chaffee, were killed.[72] This led NASA to use a nitrogen/oxygen atmosphere before launch, and low pressure pure oxygen only in space.

The March 1966 Gemini 8 mission was aborted in orbit when an attitude control system thruster stuck in the on position, sending the craft into a dangerous spin which threatened the lives of Neil Armstrong and David Scott. Armstrong had to shut the control system off and use the reentry control system to stop the spin. The craft made an emergency reentry and the astronauts landed safely. The most probable cause was determined to be an electrical short due to a static electricity discharge, which caused the thruster to remain powered even when switched off. The control system was modified to put each thruster on its own isolated circuit.

The third lunar landing expedition Apollo 13 in April 1970, was aborted and the lives of the crew, James Lovell, Jack Swigert and Fred Haise, were threatened by failure of a cryogenic liquid oxygen tank en route to the Moon. The tank burst when electrical power was applied to internal stirring fans in the tank, causing the immediate loss of all of its contents, and also damaging the second tank, causing the loss of its remaining oxygen in a span of 130 minutes. This in turn caused loss of electrical power provided by fuel cells to the command spacecraft. The crew managed to return to Earth safely by using the lunar landing craft as a "life boat". The tank failure was determined to be caused by two mistakes. The tank's drain fitting had been damaged when it was dropped during factory testing. This necessitated use of its internal heaters to boil out the oxygen after a pre-launch test, which in turn damaged the fan wiring's electrical insulation, because the thermostats on the heaters did not meet the required voltage rating due to a vendor miscommunication.

The crew of Soyuz 11 were killed on June 30, 1971 by a combination of mechanical malfunctions: they were asphyxiated due to cabin decompression following separation of their descent capsule from the service module. A cabin ventilation valve had been jolted open at an altitude of 168 kilometres (551,000ft) by the stronger than expected shock of explosive separation bolts which were designed to fire sequentially, but in fact had fired simultaneously. The loss of pressure became fatal within about 30 seconds.[73]

As of December2015[update], 22 crew members have died in accidents aboard spacecraft. Over 100 others have died in accidents during activity directly related to spaceflight or testing.

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Human spaceflight - Wikipedia

Title 10 USC 2641b: Space-Available Travel on Department of Defense Aircraft

Space-available travel on Department of Defense aircraft:

Program Authorized and Eligible recipients:

(a) AUTHORITY TO ESTABLISH PROGRAM.

(1) The Secretary of Defense may establish a program (in this section referred to as the "travel program") to provide transportation on Department of Defense aircraft on a space-available basis to the categories of individuals eligible under subsection (c)

(2) If the Secretary makes a determination to establish the travel program, the Secretary shall prescribe regulations for the operation of the travel program not later than one year after the date on which the determination was made. The regulations shall take effect on that date or such earlier date as the Secretary shall specify in the regulations.

(3) Not later than 30 days after making the determination to establish the travel program, the Secretary shall submit to the congressional defense committees an initial implementation report describing

(A) the basis for the determination;

(B) any additional categories of individuals to be eligible for the travel program under subsection (c)(S);

(C) how the Secretary will ensure that the travel program is established and operated in compliancewith the conditions specified in subsection (b); and

(D) the metrics by which the Secretary will monitor the travel program to determine the efficient and effective execution of the travel program.

(b) CONDITIONS ON ESTABLISHMENT AND OPERATION.

(1) The Secretary of Defense shall operate the travel program in a budget-neutral manner.

(2) No additional funds may be used, or flight hours performed, for the purpose of providing transportation under the travel program.

(c) ELIGIBLE INDIVIDUALS. Subject to subsection (d), the Secretary of Defense shall provide transportation under the travel program (if established) to the following categories of individuals:

(1) Members of the armed forces on active duty.

(2) Members of the Selected Reserve who hold a valid Uniformed Services Identification and Privilege Card

(3) Retired members of a regular or reserve component of the armed forces, including retired members of reserve components who, but for being under the eligibility age applicable under section 12731 of this title, would be eligible for retired pay under chapter 1223 of this title.

(4) Such categories of dependents of individuals described in paragraphs (1) through (3) as the Secretary shall specify in the regulations under subsection (a), under such conditions and circumstances as the Secretary shall specify in such regulations.

(5) Such other categories of individuals as the Secretary, in the discretion of the Secretary, considers appropriate.

(d) PRIORITIES AND RESTRICTIONSIn operating the travel program, the Secretary of Defense shall-

(1) in the sole discretion of the Secretary, establish an order of priority for transportation under the travel program for categories of eligible individuals that is based on considerations of military necessity, humanitarian concerns, and enhancement of morale;

(2) give priority in consideration of transportation under the travel program to the demands of members of the armed forces in the regular components and in the reserve components on active duty and to the need to provide such members, and their dependents, a means of respite from such demands; and

(3) implement policies aimed at ensuring cost control (as required by subsection (b)) and the safety, security, and efficient processing of travelers, including limiting the benefit under the travel program to one or more categories of otherwise eligible individuals if considered necessary by the Secretary.

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Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically using a hypothetical device known as a time machine. Time travel is a widely-recognized concept in philosophy and fiction. The idea of a time machine was popularized by H. G. Wells' 1895 novel The Time Machine.

It is uncertain if time travel to the past is physically possible. Forward time travel, outside the usual sense of the perception of time, is an extensively-observed phenomenon and well-understood within the framework of special relativity and general relativity. However, making one body advance or delay more than a few milliseconds compared to another body is not feasible with current technology.[1] As for backwards time travel, it is possible to find solutions in general relativity that allow for it, but the solutions require conditions that may not be physically possible. Traveling to an arbitrary point in spacetime has a very limited support in theoretical physics, and usually only connected with quantum mechanics or wormholes, also known as Einstein-Rosen bridges.

Some ancient myths depict a character skipping forward in time. In Hindu mythology, the Mahabharata mentions the story of King Raivata Kakudmi, who travels to heaven to meet the creator Brahma and is surprised to learn when he returns to Earth that many ages have passed.[2] The Buddhist Pli Canon mentions the relativity of time. The Payasi Sutta tells of one of the Buddha's chief disciples, Kumara Kassapa, who explains to the skeptic Payasi that time in the Heavens passes differently than on Earth.[3] The Japanese tale of "Urashima Tar",[4] first described in the Nihongi (720) tells of a young fisherman named Urashima Taro who visits an undersea palace. After three days, he returns home to his village and finds himself 300 years in the future, where he has been forgotten, his house is in ruins, and his family has died.[5] In Jewish tradition, the 1st-century BC scholar Honi ha-M'agel is said to have fallen asleep and slept for seventy years. When waking up he returned home but found none of the people he knew, and no one believed his claims of who he was.[6]

Early science fiction stories feature characters who sleep for years and awaken in a changed society, or are transported to the past through supernatural means. Among them L'An 2440, rve s'il en ft jamais (1770) by Louis-Sbastien Mercier, Rip Van Winkle (1819) by Washington Irving, Looking Backward (1888) by Edward Bellamy, and When the Sleeper Awakes (1899) by H.G. Wells. Prolonged sleep, like the more familiar time machine, is used as a means of time travel in these stories.[7]

The earliest work about backwards time travel is uncertain. Samuel Madden's Memoirs of the Twentieth Century (1733) is a series of letters from British ambassadors in 1997 and 1998 to diplomats in the past, conveying the political and religious conditions of the future.[8]:9596 Because the narrator receives these letters from his guardian angel, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel."[8]:85 Madden does not explain how the angel obtains these documents, but Alkon asserts that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backward from the future to be discovered in the present."[8]:9596 In the science fiction anthology Far Boundaries (1951), editor August Derleth claims that an early short story about time travel is Missing One's Coach: An Anachronism, written for the Dublin Literary Magazine[9] by an anonymous author in 1838.[10]:3 While the narrator waits under a tree for a coach to take him out of Newcastle, he is transported back in time over a thousand years. He encounters the Venerable Bede in a monastery and explains to him the developments of the coming centuries. However, the story never makes it clear whether these events are real or a dream.[10]:1138 Another early work about time travel is The Forebears of Kalimeros: Alexander, son of Philip of Macedon by Alexander Veltman published in 1836.[11]

Charles Dickens's A Christmas Carol (1843) has early depictions of time travel in both directions, as the protagonist, Ebenezer Scrooge, is transported to Christmases past and future. Other stories employ the same template, where a character naturally goes to sleep, and upon waking up find themselves in a different time.[12] A clearer example of backward time travel is found in the popular 1861 book Paris avant les hommes (Paris before Men) by the French botanist and geologist Pierre Boitard, published posthumously. In this story, the protagonist is transported to the prehistoric past by the magic of a "lame demon" (a French pun on Boitard's name), where he encounters a Plesiosaur and an apelike ancestor and is able to interact with ancient creatures.[13] Edward Everett Hale's "Hands Off" (1881) tells the story of an unnamed being, possibly the soul of a person who has recently died, who interferes with ancient Egyptian history by preventing Joseph's enslavement. This may have been the first story to feature an alternate history created as a result of time travel.[14]:54

One of the first stories to feature time travel by means of a machine is "The Clock that Went Backward" by Edward Page Mitchell,[15] which appeared in the New York Sun in 1881. However, the mechanism borders on fantasy. An unusual clock, when wound, runs backwards and transports people nearby back in time. The author does not explain the origin or properties of the clock.[14]:55 Enrique Gaspar y Rimbau's El Anacronpete (1887) may have been the first story to feature a vessel engineered to travel through time.[16][17] Andrew Sawyer has commented that the story "does seem to be the first literary description of a time machine noted so far", adding that "Edward Page Mitchell's story 'The Clock That Went Backward' (1881) is usually described as the first time-machine story, but I'm not sure that a clock quite counts."[18] H. G. Wells's The Time Machine (1895) popularized the concept of time travel by mechanical means.[19]

Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime or specific types of motion in space might allow time travel into the past and future if these geometries or motions were possible.[20]:499 In technical papers, physicists discuss the possibility of closed timelike curves, which are world lines that form closed loops in spacetime, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves, such as Gdel spacetime, but the physical plausibility of these solutions is uncertain.

Many in the scientific community believe that backward time travel is highly unlikely. Any theory that would allow time travel would introduce potential problems of causality.[21] The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill one's own grandfather before one's father was conceived? Some physicists, such as Novikov and Deutsch, suggested that these sorts of temporal paradoxes can be avoided through the Novikov self-consistency principle or to a variation of the many-worlds interpretation with interacting worlds.[22]

Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light, such as cosmic strings, transversable wormholes, and Alcubierre drive.[23][24]:33130 The theory of general relativity does suggest a scientific basis for the possibility of backward time travel in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed.[25] These semiclassical arguments led Stephen Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel,[26] but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.[27][28]:150

The theory of general relativity describes the universe under a system of field equations that determine the metric, or distance function, of spacetime. There exist exact solutions to these equations that include closed time-like curves, which are world lines that intersect themselves; some point in the causal future of the world line is also in its causal past, a situation which is akin to time travel. Such a solution was first proposed by Kurt Gdel, a solution known as the Gdel metric, but his (and others') solution requires the universe to have physical characteristics that it does not appear to have,[20]:499 such as rotation and lack of Hubble expansion. Whether general relativity forbids closed time-like curves for all realistic conditions is still being researched.[29]

Wormholes are a hypothetical warped spacetime which are permitted by the Einstein field equations of general relativity.[30]:100 A proposed time-travel machine using a traversable wormhole would hypothetically work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less, or become "younger", than the stationary end as seen by an external observer; however, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around.[20]:502 This means that an observer entering the "younger" end would exit the "older" end at a time when it was the same age as the "younger" end, effectively going back in time as seen by an observer from the outside. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine;[20]:503 in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backward in time.

According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance with negative energy, often referred to as "exotic matter". More technically, the wormhole spacetime requires a distribution of energy that violates various energy conditions, such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition,[30]:101 and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics.[31] Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.[32]

In 1993, Matt Visser argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other.[33] Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is possible.[34]

Another approach involves a dense spinning cylinder usually referred to as a Tipler cylinder, a GR solution discovered by Willem Jacob van Stockum[35] in 1936 and Kornel Lanczos[36] in 1924, but not recognized as allowing closed timelike curves[37]:21 until an analysis by Frank Tipler[38] in 1974. If a cylinder is infinitely long and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral). However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it does not seem to be possible to create a new cosmic string. Physicist Ronald Mallett is attempting to recreate the conditions of a rotating black hole with ring lasers, in order to bend spacetime and allow for time travel.[39]

A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a "time machine with the compactly generated Cauchy horizon") in a region where the weak energy condition is satisfied, meaning that the region contains no matter with negative energy density (exotic matter). Solutions such as Tipler's assume cylinders of infinite length, which are easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough,[37]:169 he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy."[28]:96 This result comes from Hawking's 1992 paper on the chronology protection conjecture, where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "there will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon."[26] This theorem does not rule out the possibility of time travel by means of time machines with the non-compactly generated Cauchy horizons (such as the Deutsch-Politzer time machine) or in regions which contain exotic matter, which would be used for traversable wormholes or the Alcubierre drive.

When a signal is sent from one location and received at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity in the theory of relativity show that all reference frames agree that the transmission-event happened before the reception-event. When the signal travels faster than light, it is received before it is sent, in all reference frames.[40] The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a tachyonic antitelephone.[41]

Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles.[42] This effect was referred to as "spooky action at a distance" by Einstein.

Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern quantum field theories, and therefore modern theories do not allow for time travel or FTL communication. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used.[43] The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals.

A variation of Everett's many-worlds interpretation (MWI) of quantum mechanics provides a resolution to the grandfather paradox that involves the time traveler arriving in a different universe than the one they came from; it's been argued that since the traveler arrives in a different universe's history and not their own history, this is not "genuine" time travel.[44] The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories.[45] However, some variations allow different universes to interact. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that a time traveler should end up in a different history than the one he started from.[46][47] On the other hand, Stephen Hawking has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history, so that time travelers remain within their own world rather than traveling to a different one.[48] The physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI". Everett also argues that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time through a wormhole, with different particles emerging in different worlds.[22]

Certain experiments carried out give the impression of reversed causality, but fail to show it under closer examination.

The delayed choice quantum eraser experiment performed by Marlan Scully involves pairs of entangled photons that are divided into "signal photons" and "idler photons", with the signal photons emerging from one of two locations and their position later measured as in the double-slit experiment. Depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an interference pattern is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can only be observed after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, only by gathering classical information from the entire system; thus causality is preserved.[49]

The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry, but a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see Fourier analysis), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light,[50] so this experiment is understood not to violate causality either.

The physicists Gnter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons traveled "instantaneously" between a pair of prisms that had been moved up to 3ft (0.91m) apart, using a phenomenon known as quantum tunneling. Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of." However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the speed of the center of the train exceeds the speed of any of the individual cars.[51]

Shengwang Du claims in a peer-reviewed journal to have observed single photons' precursors, saying that they travel no faster than c in a vacuum. His experiment involved slow light as well as passing light through a vacuum. He generated two single photons, passing one through rubidium atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at c in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than c and, thus, no possibility of violating causality.[52]

The absence of time travelers from the future is a variation of the Fermi paradox. As the absence of extraterrestrial visitors does not prove they do not exist, so the absence of time travelers fails to prove time travel is physically impossible; it might be that time travel is physically possible but is never developed or is cautiously used. Carl Sagan once suggested the possibility that time travelers could be here but are disguising their existence or are not recognized as time travelers.[27] Some versions of general relativity suggest that time travel might only be possible in a region of spacetime that is warped a certain way, and hence time travelers would not be able to travel back to earlier regions in spacetime, before this region existed. Stephen Hawking stated that this would explain why the world has not already been overrun by "tourists from the future."[48]

Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth's Destination Day or MIT's Time Traveler Convention heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet.[53] In 1982, a group in Baltimore, Maryland, identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future.[54][55] These experiments only stood the possibility of generating a positive result demonstrating the existence of time travel, but have failed so farno time travelers are known to have attended either event. Some versions of the many-worlds interpretation can be used to suggest that future humans have traveled back in time, but have traveled back to the meeting time and place in a parallel universe.[56]

There is a great deal of observable evidence for time dilation in special relativity[57] and gravitational time dilation in general relativity,[58][59][60] for example in the famous and easy-to-replicate observation of atmospheric muon decay.[61][62][63] The theory of relativity states that the speed of light is invariant for all observers in any frame of reference; that is, it is always the same. Time dilation is a direct consequence of the invariance of the speed of light.[63] Time dilation may be regarded in a limited sense as "time travel into the future": a person may use time dilation so that a small amount of proper time passes for them, while a large amount of proper time passes elsewhere. This can be achieved by traveling at relativistic speeds or through the effects of gravity.[64]

For two identical clocks moving relative to each other without accelerating, each clock measures the other to be ticking slower. This is possible due to the relativity of simultaneity. However, the symmetry is broken if one clock accelerates, allowing for less proper time to pass for one clock than the other. The twin paradox describes this: one twin remains on Earth, while the other undergoes acceleration to relativistic speed as they travel into space, turn around, and travel back to Earth; the traveling twin ages less than the twin who stayed on Earth, because of the time dilation experienced during their acceleration. General relativity treats the effects of acceleration and the effects of gravity as equivalent, and shows that time dilation also occurs in gravity wells, with a clock deeper in the well ticking more slowly; this effect is taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a large gravity well such as a black hole.[24]:33130

A time machine that utilizes this principle might be, for instance, a spherical shell with a diameter of 5 meters and the mass of Jupiter. A person at its center will travel forward in time at a rate four times that of distant observers. Squeezing the mass of a large planet into such a small structure is not expected to be within humanity's technological capabilities in the near future.[24]:76140 With current technologies, it is only possible to cause a human traveler to age less than companions on Earth by a very small fraction of a second, the current record being about one-fiftieth of a second for the cosmonaut Sergei Krikalev.[65]

Philosophers have discussed the nature of time since at least the time of ancient Greece; for example, Parmenides presented the view that time is an illusion. Centuries later, Isaac Newton supported the idea of absolute time, while his contemporary Gottfried Wilhelm Leibniz maintained that time is only a relation between events and it cannot be expressed independently. The latter approach eventually gave rise to the spacetime of relativity.[66]

Many philosophers have argued that relativity implies eternalism, the idea that the past and future exist in a real sense, not only as changes that occurred or will occur to the present.[67] Philosopher of science Dean Rickles disagrees with some qualifications, but notes that "the consensus among philosophers seems to be that special and general relativity are incompatible with presentism."[68] Some philosophers view time as a dimension equal to spatial dimensions, that future events are "already there" in the same sense different places exist, and that there is no objective flow of time; however, this view is disputed.[69]

Presentism is a school of philosophy that holds that the future and the past exist only as changes that occurred or will occur to the present, and they have no real existence of their own. In this view, time travel is impossible because there is no future or past to travel to.[67] Keller and Nelson have argued that even if past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to travel back to the present date could explain the time traveler's actual appearance in the present;[70] these views are contested by some authors.[71]

Presentism in classical spacetime deems that only the present exists; this is not reconcilable with special relativity, shown in the following example: Alice and Bob are simultaneous observers of event O. For Alice, some event E is simultaneous with O, but for Bob, event E is in the past or future. Therefore, Alice and Bob disagree about what exists in the present, which contradicts classical presentism. "Here-now presentism" attempts to reconcile this by only acknowledging the time and space of a single point; this is unsatisfactory because objects coming and going from the "here-now" alternate between real and unreal, in addition to the lack of a privileged "here-now" that would be the "real" present. "Relativized presentism" acknowledges that there are infinite frames of reference, each of them has a different set of simultaneous events, which makes it impossible to distinguish a single "real" present, and hence either all events in time are realblurring the difference between presentism and eternalismor each frame of reference exists in its own reality. Options for presentism in special relativity appear to be exhausted, but Gdel and others suspect presentism may be valid for some forms of general relativity.[72] Generally, the idea of absolute time and space is considered incompatible with general relativity; there is no universal truth about the absolute position of events which occur at different times, and thus no way to determine which point in space at one time is at the universal "same position" at another time,[73] and all coordinate systems are on equal footing as given by the principle of diffeomorphism invariance.[74]

A common objection to the idea of traveling back in time is put forth in the grandfather paradox or the argument of auto-infanticide.[75] If one were able to go back in time, inconsistencies and contradictions would ensue if the time traveler were to change anything; there is a contradiction if the past becomes different from the way it is.[76][77] The paradox is commonly described with a person who travels to the past and kills their own grandfather, prevents the existence of their father or mother, and therefore their own existence.[27] Philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backward time travel could be possible but that it would be impossible to actually change the past in any way,[78] an idea similar to the proposed Novikov self-consistency principle in physics.

According to the philosophical theory of compossibility, what can happen, for example in the context of time travel, must be weighed against the context of everything relating to the situation. If the past is a certain way, it's not possible for it to be any other way. What can happen when a time traveler visits the past is limited to what did happen, in order to prevent logical contradictions.[79]

The Novikov self-consistency principle, named after Igor Dmitrievich Novikov, states that any actions taken by a time traveler or by an object that travels back in time were part of history all along, and therefore it is impossible for the time traveler to "change" history in any way. The time traveler's actions may be the cause of events in their own past though, which leads to the potential for circular causation, sometimes called a predestination paradox,[80] ontological paradox,[81] or bootstrap paradox.[81][82] The term bootstrap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps".[83] The Novikov self-consistency principle proposes that the local laws of physics in a region of spacetime containing time travelers cannot be any different from the local laws of physics in any other region of spacetime.[84]

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes"[85] that in a scenario involving a physical object whose world-line or history forms a closed loop in time there can be a violation of the second law of thermodynamics. Ross uses "Somewhere in Time" as an example of such an ontological paradox, where a watch is given to a person, and 60 years later the same watch is brought back in time and given to the same character. Ross states that entropy of the watch will increase, and the watch carried back in time will be more worn with each repetition of its history. The second law of thermodynamics is understood by modern physicists to be a statistical law, so decreasing entropy or non-increasing entropy are not impossible, just improbable. Additionally, entropy statistically increases in systems which are isolated, so non-isolated systems, such as an object, that interact with the outside world, can become less worn and decrease in entropy, and it's possible for an object whose world-line forms a closed loop to be always in the same condition in the same point of its history.[24]:23

Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel where the past must be self-consistent.[86][87]

Time travel themes in science fiction and the media can generally be grouped into three categories: immutable timeline; mutable timeline; and alternate histories, as in the interacting-many-worlds interpretation.[88][89][90] Frequently in fiction, timeline is used to refer to all physical events in history, so that in time travel stories where events can be changed, the time traveler is described as creating a new or altered timeline.[91] This usage is distinct from the use of the term timeline to refer to a type of chart that illustrates a particular series of events, and the concept is also distinct from a world line, a term from Einstein's theory of relativity which refers to the entire history of a single object.

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Time travel - Wikipedia

A wormhole (or EinsteinRosen bridge) is a speculative structure linking separate points in spacetime, and is based on a solution of the Einstein field equations. A wormhole can be visualized as a tunnel with two ends, each at separate points in spacetime (i.e., different locations or different points of time). More precisely it is a transcendental bijection of the spacetime continuum, an asymptotic projection of the CalabiYau manifold manifesting itself in Anti-de Sitter space.

Wormholes are consistent with the general theory of relativity, but whether wormholes actually exist remains to be seen.

A wormhole could connect extremely long distances such as a billion light years or more, short distances such as a few meters, different universes, or different points in time.[1]

For a simplified notion of a wormhole, space can be visualized as a two-dimensional (2D) surface. In this case, a wormhole would appear as a hole in that surface, lead into a 3D tube (the inside surface of a cylinder), then re-emerge at another location on the 2D surface with a hole similar to the entrance. An actual wormhole would be analogous to this, but with the spatial dimensions raised by one. For example, instead of circular holes on a 2D plane, the entry and exit points could be visualized as spheres in 3D space.

Another way to imagine wormholes is to take a sheet of paper and draw two somewhat distant points on one side of the paper. The sheet of paper represents a plane in the spacetime continuum, and the two points represent a distance to be traveled, however theoretically a wormhole could connect these two points by folding that plane so the points are touching. In this way it would be much easier to traverse the distance since the two points are now touching.

In 1928, Hermann Weyl proposed a wormhole hypothesis of matter in connection with mass analysis of electromagnetic field energy;[2][3] however, he did not use the term "wormhole" (he spoke of "one-dimensional tubes" instead).[4]

American theoretical physicist John Archibald Wheeler (inspired by Weyl's work)[4] coined the term "wormhole" in a 1957 paper co-authored by Charles Misner:[5]

This analysis forces one to consider situations... where there is a net flux of lines of force, through what topologists would call "a handle" of the multiply-connected space, and what physicists might perhaps be excused for more vividly terming a "wormhole".

Wormholes have been defined both geometrically and topologically.[further explanation needed] From a topological point of view, an intra-universe wormhole (a wormhole between two points in the same universe) is a compact region of spacetime whose boundary is topologically trivial, but whose interior is not simply connected. Formalizing this idea leads to definitions such as the following, taken from Matt Visser's Lorentzian Wormholes (1996).[6][pageneeded]

If a Minkowski spacetime contains a compact region , and if the topology of is of the form ~ R , where is a three-manifold of the nontrivial topology, whose boundary has topology of the form ~ S2, and if, furthermore, the hypersurfaces are all spacelike, then the region contains a quasipermanent intrauniverse wormhole.

Geometrically, wormholes can be described as regions of spacetime that constrain the incremental deformation of closed surfaces. For example, in Enrico Rodrigo's The Physics of Stargates, a wormhole is defined informally as:

a region of spacetime containing a "world tube" (the time evolution of a closed surface) that cannot be continuously deformed (shrunk) to a world line (the time evolution of a point).

The equations of the theory of general relativity have valid solutions that contain wormholes. The first type of wormhole solution discovered was the Schwarzschild wormhole,[7] which would be present in the Schwarzschild metric describing an eternal black hole, but it was found that it would collapse too quickly for anything to cross from one end to the other. Wormholes that could be crossed in both directions, known as traversable wormholes, would only be possible if exotic matter with negative energy density could be used to stabilize them.[citation needed]

Schwarzschild wormholes, also known as EinsteinRosen bridges[7] (named after Albert Einstein and Nathan Rosen),[8] are connections between areas of space that can be modeled as vacuum solutions to the Einstein field equations, and that are now understood to be intrinsic parts of the maximally extended version of the Schwarzschild metric describing an eternal black hole with no charge and no rotation. Here, "maximally extended" refers to the idea that the spacetime should not have any "edges": it should be possible to continue this path arbitrarily far into the particle's future or past for any possible trajectory of a free-falling particle (following a geodesic in the spacetime).

In order to satisfy this requirement, it turns out that in addition to the black hole interior region that particles enter when they fall through the event horizon from the outside, there must be a separate white hole interior region that allows us to extrapolate the trajectories of particles that an outside observer sees rising up away from the event horizon. And just as there are two separate interior regions of the maximally extended spacetime, there are also two separate exterior regions, sometimes called two different "universes", with the second universe allowing us to extrapolate some possible particle trajectories in the two interior regions. This means that the interior black hole region can contain a mix of particles that fell in from either universe (and thus an observer who fell in from one universe might be able to see light that fell in from the other one), and likewise particles from the interior white hole region can escape into either universe. All four regions can be seen in a spacetime diagram that uses KruskalSzekeres coordinates.

In this spacetime, it is possible to come up with coordinate systems such that if a hypersurface of constant time (a set of points that all have the same time coordinate, such that every point on the surface has a space-like separation, giving what is called a 'space-like surface') is picked and an "embedding diagram" drawn depicting the curvature of space at that time, the embedding diagram will look like a tube connecting the two exterior regions, known as an "EinsteinRosen bridge". Note that the Schwarzschild metric describes an idealized black hole that exists eternally from the perspective of external observers; a more realistic black hole that forms at some particular time from a collapsing star would require a different metric. When the infalling stellar matter is added to a diagram of a black hole's history, it removes the part of the diagram corresponding to the white hole interior region, along with the part of the diagram corresponding to the other universe.[9]

The EinsteinRosen bridge was discovered by Ludwig Flamm in 1916,[10] a few months after Schwarzschild published his solution, and was rediscovered by Albert Einstein and his colleague Nathan Rosen, who published their result in 1935.[8][11] However, in 1962, John Archibald Wheeler and Robert W. Fuller published a paper[12] showing that this type of wormhole is unstable if it connects two parts of the same universe, and that it will pinch off too quickly for light (or any particle moving slower than light) that falls in from one exterior region to make it to the other exterior region.

According to general relativity, the gravitational collapse of a sufficiently compact mass forms a singular Schwarzschild black hole. In the EinsteinCartanSciamaKibble theory of gravity, however, it forms a regular EinsteinRosen bridge. This theory extends general relativity by removing a constraint of the symmetry of the affine connection and regarding its antisymmetric part, the torsion tensor, as a dynamical variable. Torsion naturally accounts for the quantum-mechanical, intrinsic angular momentum (spin) of matter. The minimal coupling between torsion and Dirac spinors generates a repulsive spinspin interaction that is significant in fermionic matter at extremely high densities. Such an interaction prevents the formation of a gravitational singularity.[clarification needed] Instead, the collapsing matter reaches an enormous but finite density and rebounds, forming the other side of the bridge.[13]

Although Schwarzschild wormholes are not traversable in both directions, their existence inspired Kip Thorne to imagine traversable wormholes created by holding the "throat" of a Schwarzschild wormhole open with exotic matter (material that has negative mass/energy).

Other non-traversable wormholes include Lorentzian wormholes (first proposed by John Archibald Wheeler in 1957), wormholes creating a spacetime foam in a general relativistic spacetime manifold depicted by a Lorentzian manifold,[14] and Euclidean wormholes (named after Euclidean manifold, a structure of Riemannian manifold).[15]

This Casimir effect shows that quantum field theory allows the energy density in certain regions of space to be negative relative to the ordinary matter vacuum energy, and it has been shown theoretically that quantum field theory allows states where energy can be arbitrarily negative at a given point.[16] Many physicists, such as Stephen Hawking,[17] Kip Thorne,[18] and others,[19][20][21] therefore argue that such effects might make it possible to stabilize a traversable wormhole.[22][23] Physicists have not found any natural process that would be predicted to form a wormhole naturally in the context of general relativity, although the quantum foam hypothesis is sometimes used to suggest that tiny wormholes might appear and disappear spontaneously at the Planck scale,[24]:494496[25] and stable versions of such wormholes have been suggested as dark matter candidates.[26][27] It has also been proposed that, if a tiny wormhole held open by a negative mass cosmic string had appeared around the time of the Big Bang, it could have been inflated to macroscopic size by cosmic inflation.[28]

Lorentzian traversable wormholes would allow travel in both directions from one part of the universe to another part of that same universe very quickly or would allow travel from one universe to another. The possibility of traversable wormholes in general relativity was first demonstrated in a 1973 paper by Homer Ellis[30]and independently in a 1973 paper by K. A. Bronnikov.[31]Ellis thoroughly analyzed the topology and the geodesics of the Ellis drainhole, showing it to be geodesically complete, horizonless, singularity-free, and fully traversable in both directions. The drainhole is a solution manifold of Einstein's field equations for a vacuum space-time, modified by inclusion of a scalar field minimally coupled to the Ricci tensor with antiorthodox polarity (negative instead of positive). (Ellis specifically rejected referring to the scalar field as 'exotic' because of the antiorthodox coupling, finding arguments for doing so unpersuasive.) The solution depends on two parameters: m {displaystyle m} , which fixes the strength of its gravitational field, and n {displaystyle n} , which determines the curvature of its spatial cross sections. When m {displaystyle m} is set equal to 0, the drainhole's gravitational field vanishes. What is left is the Ellis wormhole, a nongravitating, purely geometric, traversable wormhole.Kip Thorne and his graduate student Mike Morris, unaware of the 1973 papers by Ellis and Bronnikov, manufactured, and in 1988 published, a duplicate of the Ellis wormhole for use as a tool for teaching general relativity. For this reason, the type of traversable wormhole they proposed, held open by a spherical shell of exotic matter, was from 1988 to 2015 exclusively referred to in the literature as a MorrisThorne wormhole. Later, other types of traversable wormholes were discovered as allowable solutions to the equations of general relativity, including a variety analyzed in a 1989 paper by Matt Visser, in which a path through the wormhole can be made where the traversing path does not pass through a region of exotic matter. However, in the pure GaussBonnet gravity (a modification to general relativity involving extra spatial dimensions which is sometimes studied in the context of brane cosmology) exotic matter is not needed in order for wormholes to existthey can exist even with no matter.[32] A type held open by negative mass cosmic strings was put forth by Visser in collaboration with Cramer et al.,[28] in which it was proposed that such wormholes could have been naturally created in the early universe.

Wormholes connect two points in spacetime, which means that they would in principle allow travel in time, as well as in space. In 1988, Morris, Thorne and Yurtsever worked out explicitly how to convert a wormhole traversing space into one traversing time by accelerating one of its two mouths.[18] However, according to general relativity, it would not be possible to use a wormhole to travel back to a time earlier than when the wormhole was first converted into a time 'machine'. Until this time it could not have been noticed or have been used.[24]:504

To see why exotic matter is required, consider an incoming light front traveling along geodesics, which then crosses the wormhole and re-expands on the other side. The expansion goes from negative to positive. As the wormhole neck is of finite size, we would not expect caustics to develop, at least within the vicinity of the neck. According to the optical Raychaudhuri's theorem, this requires a violation of the averaged null energy condition. Quantum effects such as the Casimir effect cannot violate the averaged null energy condition in any neighborhood of space with zero curvature,[33] but calculations in semiclassical gravity suggest that quantum effects may be able to violate this condition in curved spacetime.[34] Although it was hoped recently that quantum effects could not violate an achronal version of the averaged null energy condition,[35] violations have nevertheless been found,[36] so it remains an open possibility that quantum effects might be used to support a wormhole.

In some hypotheses where general relativity is modified, it is possible to have a wormhole that does not collapse without having to resort to exotic matter. For example, this is possible with R^2 gravity, a form of f(R) gravity.[37]

The impossibility of faster-than-light relative speed only applies locally. Wormholes might allow effective superluminal (faster-than-light) travel by ensuring that the speed of light is not exceeded locally at any time. While traveling through a wormhole, subluminal (slower-than-light) speeds are used. If two points are connected by a wormhole whose length is shorter than the distance between them outside the wormhole, the time taken to traverse it could be less than the time it would take a light beam to make the journey if it took a path through the space outside the wormhole. However, a light beam traveling through the same wormhole would -of course- beat the traveler.

If traversable wormholes exist, they could allow time travel.[18] A proposed time-travel machine using a traversable wormhole would hypothetically work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less, or become "younger", than the stationary end as seen by an external observer; however, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around.[24]:502 This means that an observer entering the "younger" end would exit the "older" end at a time when it was the same age as the "younger" end, effectively going back in time as seen by an observer from the outside. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine;[24]:503 It is more of a path through time rather than it is a device that itself moves through time, and it would not allow the technology itself to be moved backward in time.[38][39]

According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance with negative energy, often referred to as "exotic matter". More technically, the wormhole spacetime requires a distribution of energy that violates various energy conditions, such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition,[6]:101 and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics.[40] Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.[41]

In 1993, Matt Visser argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other,[42] or otherwise prevent information from passing through the wormhole.[43] Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is possible.[44]

A possible resolution to the paradoxes resulting from wormhole-enabled time travel rests on the many-worlds interpretation of quantum mechanics.

In 1991 David Deutsch showed that quantum theory is fully consistent (in the sense that the so-called density matrix can be made free of discontinuities) in spacetimes with closed timelike curves.[45] However, later it was shown that such model of closed timelike curve can have internal inconsistencies as it will lead to strange phenomena like distinguishing non-orthogonal quantum states and distinguishing proper and improper mixture.[46][47] Accordingly, the destructive positive feedback loop of virtual particles circulating through a wormhole time machine, a result indicated by semi-classical calculations, is averted. A particle returning from the future does not return to its universe of origination but to a parallel universe. This suggests that a wormhole time machine with an exceedingly short time jump is a theoretical bridge between contemporaneous parallel universes.[48]

Because a wormhole time-machine introduces a type of nonlinearity into quantum theory, this sort of communication between parallel universes is consistent with Joseph Polchinski's proposal of an Everett phone[49] (named after Hugh Everett) in Steven Weinberg's formulation of nonlinear quantum mechanics.[50]

The possibility of communication between parallel universes has been dubbed interuniversal travel.[51]

Theories of wormhole metrics describe the spacetime geometry of a wormhole and serve as theoretical models for time travel. An example of a (traversable) wormhole metric is the following:[52]

first presented by Ellis (see Ellis wormhole) as a special case of the Ellis drainhole.

One type of non-traversable wormhole metric is the Schwarzschild solution (see the first diagram):

The original EinsteinRosen bridge was described in an article published in July 1935.[53][54]

For the Schwarzschild spherically symmetric static solution

( d s {displaystyle ds} = proper time, c {displaystyle c} = 1)

If one replaces r {displaystyle r} with u {displaystyle u} according to u 2 = r 2 m {displaystyle u^{2}=r-2m}

The four-dimensional space is described mathematically by two congruent parts or "sheets", corresponding to u {displaystyle u} > 0 and u {displaystyle u} < 0, which are joined by a hyperplane r = 2 m {displaystyle r=2m} or u {displaystyle u} = 0 in which g {displaystyle g} vanishes. We call such a connection between the two sheets a "bridge".

A. Einstein, N. Rosen, "The Particle Problem in the General Theory of Relativity"

For the combined field, gravity and electricity, Einstein and Rosen derived the following Schwarzschild static spherically symmetric solution

( {displaystyle epsilon } = electrical charge)

The field equations without denominators in the case when m {displaystyle m} = 0 can be written

In order to eliminate singularities, if one replaces r {displaystyle r} by u {displaystyle u} according to the equation:

and with m {displaystyle m} = 0 one obtains[55][56]

The solution is free from singularities for all finite points in the space of the two sheets

A. Einstein, N. Rosen, "The Particle Problem in the General Theory of Relativity"

Wormholes are a common element in science fiction because they allow interstellar, intergalactic, and sometimes even interuniversal travel within human lifetime scales. In fiction, wormholes have also served as a method for time travel.

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Wormhole - Wikipedia

NASA has a mission protocol which says that if a Low Earth Orbit mission increases the lifetime risk of the crew getting cancer by more than 3% they wont go ahead with it but the upcoming mars missions may expose the crews to levels that would be beyond that limit and other hazards, so how dangerous is deep space travel to Mars and beyond.

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With Elon Musk pushing to get men on Mars by the mid 2020s and NASA looking to do the same for the 2030s, just how much have we learned since Apollo and from the space stations.50 years on from the beginning of the Apollo missions and we have yet to send any man back to the moon let alone on the much more arduous journey to our nearest viable planet Mars.Now whilst much of this has been lack of political will in the face of our own manmade problems here on earth, its also down to the increasing sophistication of robotic probes and landers and that they are much cheaper to make, launch, can go where no man could and can continue working for sometimes years at a time, the Voyager probes are still going 40 years after their launch. If we relied on manned discovery only we would know a fraction what we do now.With data from the probes which we have sent around the solar system since then, we have built up a picture which is far from the vision of just whizzing through inter planetary space. That and along with the joint NASA Russian experiment of having men in space for a year onboard the ISS, we now have a much better understanding of what they may experience on the two and half year round trip to Mars.

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How Dangerous is Deep Space Travel to Mars and Beyond ?

According to the theory of relativity, time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently situated relative to a gravitational field. As a result of the nature of spacetime,[2] a clock that is moving relative to an observer will be measured to tick slower than a clock that is at rest in the observer's own frame of reference. A clock that is under the influence of a stronger gravitational field than an observer's will also be measured to tick slower than the observer's own clock.

Such time dilation has been repeatedly demonstrated, for instance by small disparities in a pair of atomic clocks after one of them is sent on a space trip, or by clocks on the Space Shuttle running slightly slower than reference clocks on Earth, or clocks on GPS and Galileo satellites running slightly faster.[1][2][3] Time dilation has also been the subject of science fiction works, as it technically provides the means for forward time travel.[4]

Time dilation by the Lorentz factor was predicted by several authors at the turn of the 20th century.[5][6] Joseph Larmor (1897), at least for electrons orbiting a nucleus, wrote "... individual electrons describe corresponding parts of their orbits in times shorter for the [rest] system in the ratio: 1 v 2 c 2 {displaystyle scriptstyle {sqrt {1-{frac {v^{2}}{c^{2}}}}}} ".[7] Emil Cohn (1904) specifically related this formula to the rate of clocks.[8] In the context of special relativity it was shown by Albert Einstein (1905) that this effect concerns the nature of time itself, and he was also the first to point out its reciprocity or symmetry.[9] Subsequently, Hermann Minkowski (1907) introduced the concept of proper time which further clarified the meaning of time dilation.[10]

Special relativity indicates that, for an observer in an inertial frame of reference, a clock that is moving relative to him will be measured to tick slower than a clock that is at rest in his frame of reference. This case is sometimes called special relativistic time dilation. The faster the relative velocity, the greater the time dilation between one another, with the rate of time reaching zero as one approaches the speed of light (299,792,458m/s). This causes massless particles that travel at the speed of light to be unaffected by the passage of time.

Theoretically, time dilation would make it possible for passengers in a fast-moving vehicle to advance further into the future in a short period of their own time. For sufficiently high speeds, the effect is dramatic.[2] For example, one year of travel might correspond to ten years on Earth. Indeed, a constant 1g acceleration would permit humans to travel through the entire known Universe in one human lifetime.[12]. At a constant 1g traveling up to 0.99999999 c it would take 30 years to reach the edge of the universe 13.5 billions lightyears away. [13] Space travelers could then return to Earth billions of years in the future. A scenario based on this idea was presented in the novel Planet of the Apes by Pierre Boulle, and the Orion Project has been an attempt toward this idea.

With current technology severely limiting the velocity of space travel, however, the differences experienced in practice are minuscule: after 6 months on the International Space Station (ISS) (which orbits Earth at a speed of about 7,700m/s[3]) an astronaut would have aged about 0.005 seconds less than those on Earth. The current human time travel record holder is Russian cosmonaut Sergei Krikalev.[14] He gained 22.68 milliseconds of lifetime on his journeys to space and therefore beat the previous record of about 20 milliseconds by cosmonaut Sergei Avdeyev.[15]

Time dilation can be inferred from the observed constancy of the speed of light in all reference frames dictated by the second postulate of special relativity.[16][17][18][19]

This constancy of the speed of light means that, counter to intuition, speeds of material objects and light are not additive. It is not possible to make the speed of light appear greater by moving towards or away from the light source.

Consider then, a simple clock consisting of two mirrors A and B, between which a light pulse is bouncing. The separation of the mirrors is L and the clock ticks once each time the light pulse hits either of the mirrors.

In the frame in which the clock is at rest (diagram on the left), the light pulse traces out a path of length 2L and the period of the clock is 2L divided by the speed of light:

From the frame of reference of a moving observer traveling at the speed v relative to the resting frame of the clock (diagram at right), the light pulse is seen as tracing out a longer, angled path. Keeping the speed of light constant for all inertial observers, requires a lengthening of the period of this clock from the moving observer's perspective. That is to say, in a frame moving relative to the local clock, this clock will appear to be running more slowly. Straightforward application of the Pythagorean theorem leads to the well-known prediction of special relativity:

The total time for the light pulse to trace its path is given by

The length of the half path can be calculated as a function of known quantities as

Elimination of the variables D and L from these three equations results in

which expresses the fact that the moving observer's period of the clock t {displaystyle Delta t'} is longer than the period t {displaystyle Delta t} in the frame of the clock itself.

Given a certain frame of reference, and the "stationary" observer described earlier, if a second observer accompanied the "moving" clock, each of the observers would perceive the other's clock as ticking at a slower rate than their own local clock, due to them both perceiving the other to be the one that's in motion relative to their own stationary frame of reference.

Common sense would dictate that, if the passage of time has slowed for a moving object, said object would observe the external world's time to be correspondingly sped up. Counterintuitively, special relativity predicts the opposite. When two observers are in motion relative to each other, each will measure the other's clock slowing down, in concordance with them being moving relative to the observer's frame of reference.

While this seems self-contradictory, a similar oddity occurs in everyday life. If two persons A and B observe each other from a distance, B will appear small to A, but at the same time A will appear small to B. Being familiar with the effects of perspective, there is no contradiction or paradox in this situation.[20]

The reciprocity of the phenomenon also leads to the so-called twin paradox where the aging of twins, one staying on Earth and the other embarking on a space travel, is compared, and where the reciprocity suggests that both persons should have the same age when they reunite. On the contrary, at the end of the round-trip, the traveling twin will be younger than his brother on Earth. The dilemma posed by the paradox, however, can be explained by the fact that the traveling twin must markedly accelerate in at least three phases of the trip (beginning, direction change, and end), while the other will only experience negligible acceleration, due to rotation and revolution of Earth. During the acceleration phases of the space travel, time dilation is not symmetric.

Minkowski diagram and twin paradox

Clock C in relative motion between two synchronized clocks A and B. C meets A at d, and B at f.

In the Minkowski diagram from the second image on the right, clock C resting in inertial frame S meets clock A at d and clock B at f (both resting in S). All three clocks simultaneously start to tick in S. The worldline of A is the ct-axis, the worldline of B intersecting f is parallel to the ct-axis, and the worldline of C is the ct-axis. All events simultaneous with d in S are on the x-axis, in S on the x-axis.

The proper time between two events is indicated by a clock present at both events.[27] It is invariant, i.e., in all inertial frames it is agreed that this time is indicated by that clock. Interval df is therefore the proper time of clock C, and is shorter with respect to the coordinate times ef=dg of clocks B and A in S. Conversely, also proper time ef of B is shorter with respect to time if in S, because event e was measured in S already at time i due to relativity of simultaneity, long before C started to tick.

From that it can be seen, that the proper time between two events indicated by an unaccelerated clock present at both events, compared with the synchronized coordinate time measured in all other inertial frames, is always the minimal time interval between those events. However, the interval between two events can also correspond to the proper time of accelerated clocks present at both events. Under all possible proper times between two events, the proper time of the unaccelerated clock is maximal, which is the solution to the twin paradox.[27]

In addition to the light clock used above, the formula for time dilation can be more generally derived from the temporal part of the Lorentz transformation.[28] Let there be two events at which the moving clock indicates t a {displaystyle t_{a}} and t b {displaystyle t_{b}} , thus

Since the clock remains at rest in its inertial frame, it follows x a = x b {displaystyle x_{a}=x_{b}} , thus the interval t = t b t a {displaystyle Delta t^{prime }=t_{b}^{prime }-t_{a}^{prime }} is given by

where t is the time interval between two co-local events (i.e. happening at the same place) for an observer in some inertial frame (e.g. ticks on his clock), known as the proper time, t is the time interval between those same events, as measured by another observer, inertially moving with velocity v with respect to the former observer, v is the relative velocity between the observer and the moving clock, c is the speed of light, and the Lorentz factor (conventionally denoted by the Greek letter gamma or ) is

Thus the duration of the clock cycle of a moving clock is found to be increased: it is measured to be "running slow". The range of such variances in ordinary life, where v c, even considering space travel, are not great enough to produce easily detectable time dilation effects and such vanishingly small effects can be safely ignored for most purposes. It is only when an object approaches speeds on the order of 30,000km/s (1/10 the speed of light) that time dilation becomes important.[29]

In special relativity, time dilation is most simply described in circumstances where relative velocity is unchanging. Nevertheless, the Lorentz equations allow one to calculate proper time and movement in space for the simple case of a spaceship which is applied with a force per unit mass, relative to some reference object in uniform (i.e. constant velocity) motion, equal to g throughout the period of measurement.

Let t be the time in an inertial frame subsequently called the rest frame. Let x be a spatial coordinate, and let the direction of the constant acceleration as well as the spaceship's velocity (relative to the rest frame) be parallel to the x-axis. Assuming the spaceship's position at time t = 0 being x = 0 and the velocity being v0 and defining the following abbreviation

the following formulas hold:[30]

Position:

Velocity:

Proper time as function of coordinate time:

In the case where v(0) = v0 = 0 and (0) = 0 = 0 the integral can be expressed as a logarithmic function or, equivalently, as an inverse hyperbolic function:

As functions of the proper time {displaystyle tau } of the ship, the following formulae hold:[31]

Position:

Velocity:

Coordinate time as function of proper time:

The clock hypothesis is the assumption that the rate at which a clock is affected by time dilation does not depend on its acceleration but only on its instantaneous velocity. This is equivalent to stating that a clock moving along a path P {displaystyle P} measures the proper time, defined by:

The clock hypothesis was implicitly (but not explicitly) included in Einstein's original 1905 formulation of special relativity. Since then, it has become a standard assumption and is usually included in the axioms of special relativity, especially in the light of experimental verification up to very high accelerations in particle accelerators.[32][33]

Gravitational time dilation is experienced by an observer that, being under the influence of a gravitational field, will measure his own clock to slow down, compared to another that is under a weaker gravitational field.

Gravitational time dilation is at play e.g. for ISS astronauts. While the astronauts' relative velocity slows down their time, the reduced gravitational influence at their location speeds it up, although at a lesser degree. Also, a climber's time is theoretically passing slightly faster at the top of a mountain compared to people at sea level. It has also been calculated that due to time dilation, the core of the Earth is 2.5 years younger than the crust.[34] "A clock used to time a full rotation of the earth will measure the day to be approximately an extra 10 ns/day longer for every km of altitude above the reference geoid." [35] Travel to regions of space where extreme gravitational time dilation is taking place, such as near a black hole, could yield time-shifting results analogous to those of near-lightspeed space travel.

Contrarily to velocity time dilation, in which both observers measure the other as aging slower (a reciprocal effect), gravitational time dilation is not reciprocal. This means that with gravitational time dilation both observers agree that the clock nearer the center of the gravitational field is slower in rate, and they agree on the ratio of the difference.

High accuracy timekeeping, low earth orbit satellite tracking, and pulsar timing are applications that require the consideration of the combined effects of mass and motion in producing time dilation. Practical examples include the International Atomic Time standard and its relationship with the Barycentric Coordinate Time standard used for interplanetary objects.

Relativistic time dilation effects for the solar system and the earth can be modeled very precisely by the Schwarzschild solution to the Einstein field equations. In the Schwarzschild metric, the interval d t E {displaystyle dt_{text{E}}} is given by[38][39]

where

The coordinate velocity of the clock is given by

The coordinate time t c {displaystyle t_{c}} is the time that would be read on a hypothetical "coordinate clock" situated infinitely far from all gravitational masses ( U = 0 {displaystyle U=0} ), and stationary in the system of coordinates ( v = 0 {displaystyle v=0} ). The exact relation between the rate of proper time and the rate of coordinate time for a clock with a radial component of velocity is

where

The above equation is exact under the assumptions of the Schwarzschild solution. It reduces to velocity time dilation equation in the presence of motion and absence of gravity, i.e. e = 0 {displaystyle beta _{e}=0} . It reduces to gravitational time dilation equation in the absence of motion and presence of gravity, i.e. = 0 = {displaystyle beta =0=beta _{shortparallel }} .

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Time dilation - Wikipedia

Yusaku Maezawa: Japanese spaceman with a taste for artTokyo (AFP) Sept 18, 2018 Billionaire Yusaku Maezawa, confirmed as SpaceX's first Moon tourist, is a former wannabe rock star now worth $3 billion with a penchant for pricey modern art as well as space travel. The 42-year-old tycoon, chief executive of Japan's largest online fashion mall, is the country's 18th richest person, according to business magazine Forbes. His Instagram feed is peppered with shots of his luxury living - including private jets, yachts and designer watches, but also his beloved art. Maezawa hi ... read moreJapanese billionaire businessman revealed as SpaceX's first Moon traveler Hawthorne, United States (AFP) Sept 18, 2018 A Japanese billionaire and online fashion tycoon, Yusaku Maezawa, will be the first man to fly on a monster SpaceX rocket around the Moon as early as 2023, and he plans to bring six to eight artists along. ... moreJuno image showcases Jupiter's brown barge Washington (UPI) Sep 17, 2018 Jupiter's "brown barge" feature is the subject of a new photograph snapped by Juno's camera. ... moreBaikonur Facilities to Undergo Overhaul Before OneWeb Satellites Launch - Source Baikonur, Kazakhstan (Sputnik) Sep 17, 2018The assembly and testing facility of the Baikonur cosmodrome which will be used for the launch of OneWeb satellites atop Russian rockets will go through a reconstruction ahead of the beginning of th ... moreFly me to the Moon? A look at the space-tourism race Washington (AFP) Sept 14, 2018 SpaceX is among a handful of companies racing to propel tourists into space. Here are the top projects in the works, and what they involve. ... more

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Weve all asked this question at some point in our lives: How long would it take to travel to the stars? Could it be within a persons own lifetime, and could this kind of travel become the norm someday? There are many possible answers to this question some very simple, others in the realms of science fiction. But coming up with a comprehensive answer means taking a lot of things into consideration.

Unfortunately, any realistic assessment is likely to produce answers that would totally discourage futurists and enthusiasts of interstellar travel. Like it or not, space is very large, and our technology is still very limited. But should we ever contemplate leaving the nest, we will have a range of options for getting to the nearest Solar Systems in our galaxy.

The nearest star to Earth is our Sun, which is a fairly average star in the Hertzsprung Russell Diagrams Main Sequence. This means that it is highly stable, providing Earth with just the right type of sunlight for life to evolve on our planet. We know there are planets orbiting other stars near to our Solar System, and many of these stars are similar to our own.

In the future, should mankind wish to leave the Solar System, well have a huge choice of stars we could travel to, and many could have the right conditions for life to thrive. But where would we go and how long would it take for us to get there? Just remember, this is all speculative and there is currently no benchmark for interstellar trips. That being said, here we go!

Over 2000 exoplanets have been identified, many of which are believed to be habitable. Credit: phl.upl.edu

As already noted, the closest star to our Solar System is Proxima Centauri, which is why it makes the most sense to plot an interstellar mission to this system first. As part of a triple star system called Alpha Centauri, Proxima is about 4.24 light years (or 1.3 parsecs) from Earth. Alpha Centauri is actually the brightest star of the three in the system part of a closely orbiting binary 4.37 light years from Earth whereas Proxima Centauri (the dimmest of the three) is an isolated red dwarf about 0.13 light years from the binary.

And while interstellar travel conjures up all kinds of visions of Faster-Than-Light (FTL) travel, ranging from warp speed and wormholes to jump drives, such theories are either highly speculative (such as the Alcubierre Drive) or entirely the province of science fiction. In all likelihood, any deep space mission will likely take generations to get there, rather than a few days or in an instantaneous flash.

So, starting with one of the slowest forms of space travel, how long will it take to get to Proxima Centauri?

The question of how long would it take to get somewhere in space is somewhat easier when dealing with existing technology and bodies within our Solar System. For instance, using the technology that powered the New Horizons mission which consisted of 16 thrusters fueled with hydrazine monopropellant reaching the Moon would take a mere 8 hours and 35 minutes.

On the other hand, there is the European Space Agencys (ESA) SMART-1 mission, which took its time traveling to the Moon using the method of ionic propulsion. With this revolutionary technology, a variation of which has since been used by the Dawn spacecraft to reach Vesta, the SMART-1 mission took one year, one month and two weeks to reach the Moon.

So, from the speedy rocket-propelled spacecraft to the economical ion drive, we have a few options for getting around local space plus we could use Jupiter or Saturn for a hefty gravitational slingshot. However, if we were to contemplate missions to somewhere a little more out of the way, we would have to scale up our technology and look at whats really possible.

When we say possible methods, we are talking about those that involve existing technology, or those that do not yet exist, but are technically feasible. Some, as you will see, are time-honored and proven, while others are emerging or still on the board. In just about all cases though, they present a possible, but extremely time-consuming or expensive, scenario for getting to even the closest stars

Ionic Propulsion:Currently, the slowest form of propulsion, and the most fuel-efficient, is the ion engine. A few decades ago, ionic propulsion was considered to be the subject of science fiction. However, in recent years, the technology to support ion engines has moved from theory to practice in a big way. The ESAs SMART-1 mission for example successfully completed its mission to the Moon after taking a 13 month spiral path from the Earth.

SMART-1 used solar powered ion thrusters, where electrical energy was harvested from its solar panels and used to power its Hall-effect thrusters. Only 82 kg of xenon propellant was used to propel SMART-1 to the Moon. 1 kg of xenon propellant provided a delta-v of 45 m/s. This is a highly efficient form of propulsion, but it is by no means fast.

Artists concept of Dawn mission above Ceres. Since its arrival, the spacecraft turned around to point the blue glow of its ion engine in the opposite direction. Image credit: NASA/JPL

One of the first missions to use ion drive technology was the Deep Space 1 mission to Comet Borrelly that took place in 1998. DS1 also used a xenon-powered ion drive, consuming 81.5 kg of propellant. Over 20 months of thrusting, DS1 was managed to reach a velocity of 56,000 km/hr (35,000 miles/hr) during its flyby of the comet.

Ion thrusters are therefore more economical than rocket technology, as the thrust per unit mass of propellant (a.k.a. specific impulse) is far higher. But it takes a long time for ion thrusters to accelerate spacecraft to any great speeds, and the maximum velocity it can achieve is dependent on its fuel supply and how much electrical energy it can generate.

So if ionic propulsion were to be used for a mission to Proxima Centauri, the thrusters would need a huge source of energy production (i.e. nuclear power) and a large quantity of propellant (although still less than conventional rockets). But based on the assumption that a supply of 81.5 kg of xenon propellant translates into a maximum velocity of 56,000 km/hr (and that there are no other forms of propulsion available, such as a gravitational slingshot to accelerate it further), some calculations can be made.

In short, at a maximum velocity of 56,000 km/h, Deep Space 1 would take over 81,000 years to traverse the 4.24 light years between Earth and Proxima Centauri. To put that time-scale into perspective, that would be over 2,700 human generations. So it is safe to say that an interplanetary ion engine mission would be far too slow to be considered for a manned interstellar mission.

Ionic propulsion is currently the slowest, but most fuel-efficient, form of space travel. Credit: NASA/JPL

But, should ion thrusters be made larger and more powerful (i.e. ion exhaust velocity would need to be significantly higher), and enough propellant could be hauled to keep the spacecrafts going for the entire 4.243 light-year trip, that travel time could be greatly reduced. Still not enough to happen in someones lifetime though.

Gravity Assist Method:The fastest existing means of space travel is known the Gravity Assist method, which involves a spacecraft using the relative movement (i.e. orbit) and gravity of a planet to alter is path and speed. Gravitational assists are a very useful spaceflight technique, especially when using the Earth or another massive planet (like a gas giant) for a boost in velocity.

The Mariner 10 spacecraft was the first to use this method, using Venus gravitational pull to slingshot it towards Mercury in February of 1974. In the 1980s, the Voyager 1 probe used Saturn and Jupiter for gravitational slingshots to attain its current velocity of 60,000 km/hr (38,000 miles/hr) and make it into interstellar space.

However, it was the Helios 2 mission which was launched in 1976 to study the interplanetary medium from 0.3 AU to 1 AU to the Sun that holds the record for highest speed achieved with a gravity assist. At the time, Helios 1 (which launched in 1974) and Helios 2 held the record for closest approach to the Sun. Helios 2 was launched by a conventional NASA Titan/Centaur launch vehicle and placed in a highly elliptical orbit.

A Helios probe being encapsulated for launch. Credit: Public Domain

Due to the large eccentricity (0.54) of the 190 day solar orbit, at perihelion Helios 2 was able to reach a maximum velocity of over 240,000 km/hr (150,000 miles/hr). This orbital speed was attained by the gravitational pull of the Sun alone. Technically, the Helios 2 perihelion velocity was not a gravitational slingshot, it was a maximum orbital velocity, but it still holds the record for being the fastest man-made object regardless.

So, if Voyager 1 was traveling in the direction of the red dwarf Proxima Centauri at a constant velocity of 60,000 km/hr, it would take 76,000 years (or over 2,500 generations) to travel that distance. But if it could attain the record-breaking speed of Helios 2s close approach of the Sun a constant speed of 240,000 km/hr it would take 19,000 years (or over 600 generations) to travel 4.243 light years. Significantly better, but still not in the ream of practicality.

Electromagnetic (EM) Drive:Another proposed method of interstellar travel comes in the form of the Radio Frequency (RF) Resonant Cavity Thruster, also known as the EM Drive. Originally proposed in 2001 by Roger K. Shawyer, a UK scientist who started Satellite Propulsion Research Ltd (SPR) to bring it to fruition, this drive is built around the idea that electromagnetic microwave cavities can allow for the direct conversion of electrical energy to thrust.

Whereas conventional electromagnetic thrusters are designed to propel a certain type of mass (such as ionized particles), this particular drive system relies on no reaction mass and emits no directional radiation. Such a proposal has met with a great deal of skepticism, mainly because it violates the law of Conservation of Momentum which states that within a system, the amount of momentum remains constant and is neither created nor destroyed, but only changes through the action of forces.

The EM Drive prototype produced by NASA/Eagleworks. Credit: NASA Spaceflight Forum

However, recent experiments with the technology have apparently yielded positive results. In July of 2014, at the 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference in Cleveland, Ohio, researchers from NASAs advanced propulsion research claimed that they had successfully tested a new design for an electromagnetic propulsion drive.

This was followed up in April of 2015 when researchers at NASA Eagleworks (part of the Johnson Space Center) claimed that they had successfully tested the drive in a vacuum, an indication that it might actually work in space. In July of that same year, a research team from the Dresden University of Technologys Space System department built their own version of the engine and observed a detectable thrust.

And in 2010, Prof. Juan Yang of the Northwestern Polytechnical University in Xian, China, began publishing a series of papers about her research into EM Drive technology. This culminated in her 2012 paper where she reported higher input power (2.5kW) and tested thrust (720mN) levels. In 2014, she further reported extensive tests involving internal temperature measurements with embedded thermocouples, which seemed to confirm that the system worked.

Artists concept of an interstellar craft equipped with an EM Drive. Credit: NASA Spaceflight Center

According to calculations based on the NASA prototype (which yielded a power estimate of 0.4 N/kilowatt), a spacecraft equipped with the EM drive could make the trip to Pluto in less than 18 months. Thats one-sixth the time it took for the New Horizons probe to get there, which was traveling at speeds of close to 58,000 km/h (36,000 mph).

Sounds impressive. But even at that rate, it would take a ship equipped with EM engines over 13,000 years for the vessel to make it to Proxima Centauri. Getting closer, but not quickly enough! and until such time that technology can be definitively proven to work, it doesnt make much sense to put our eggs into this basket.

Nuclear Thermal and Nuclear Electric Propulsion (NTP/NEP):Another possibility for interstellar space flight is to use spacecraft equipped with nuclear engines, a concept which NASA has been exploring for decades. In a Nuclear Thermal Propulsion (NTP) rocket, uranium or deuterium reactions are used to heat liquid hydrogen inside a reactor, turning it into ionized hydrogen gas (plasma), which is then channeled through a rocket nozzle to generate thrust.

A Nuclear Electric Propulsion (NEP) rocket involves the same basic reactor converting its heat and energy into electrical energy, which would then power an electrical engine. In both cases, the rocket would rely on nuclear fission or fusion to generates propulsion rather than chemical propellants, which has been the mainstay of NASA and all other space agencies to date.

Artists impression of a Crew Transfer Vehicle (CTV) using its nuclear-thermal rocket engines to slow down and establish orbit around Mars. Credit: NASA

Compared to chemical propulsion, both NTP and NEC offers a number of advantages. The first and most obvious is the virtually unlimited energy density it offers compared to rocket fuel. In addition, a nuclear-powered engine could also provide superior thrust relative to the amount of propellant used. This would cut the total amount of propellant needed, thus cutting launch weight and the cost of individual missions.

Although no nuclear-thermal engines have ever flown, several design concepts have been built and tested over the past few decades, and numerous concepts have been proposed. These have ranged from the traditional solid-core design such as the Nuclear Engine for Rocket Vehicle Application (NERVA) to more advanced and efficient concepts that rely on either a liquid or a gas core.

However, despite these advantages in fuel-efficiency and specific impulse, the most sophisticated NTP concept has a maximum specific impulse of 5000 seconds (50 kNs/kg). Using nuclear engines driven by fission or fusion, NASA scientists estimate it would could take a spaceship only 90 days to get to Mars when the planet was at opposition i.e. as close as 55,000,000 km from Earth.

But adjusted for a one-way journey to Proxima Centauri, a nuclear rocket would still take centuries to accelerate to the point where it was flying a fraction of the speed of light. It would then require several decades of travel time, followed by many more centuries of deceleration before reaching it destination. All told, were still talking about 1000 years before it reaches its destination. Good for interplanetary missions, not so good for interstellar ones.

Using existing technology, the time it would take to send scientists and astronauts on an interstellar mission would be prohibitively slow. If we want to make that journey within a single lifetime, or even a generation, something a bit more radical (aka. highly theoretical) will be needed. And while wormholes and jump engines may still be pure fiction at this point, there are some rather advanced ideas that have been considered over the years.

Nuclear Pulse Propulsion:Nuclear pulse propulsion is a theoretically possible form of fast space travel. The concept was originally proposed in 1946 by Stanislaw Ulam, a Polish-American mathematician who participated in the Manhattan Project, and preliminary calculations were then made by F. Reines and Ulam in 1947. The actual project known as Project Orion was initiated in 1958 and lasted until 1963.

The Project Orion concept for a nuclear-powered spacecraft. Credit: silodrome.co

Led by Ted Taylor at General Atomics and physicist Freeman Dyson from the Institute for Advanced Study in Princeton, Orion hoped to harness the power of pulsed nuclear explosions to provide a huge thrust with very high specific impulse (i.e. the amount of thrust compared to weight or the amount of seconds the rocket can continually fire).

In a nutshell, the Orion design involves a large spacecraft with a high supply of thermonuclear warheads achieving propulsion by releasing a bomb behind it and then riding the detonation wave with the help of a rear-mounted pad called a pusher. After each blast, the explosive force would be absorbed by this pusher pad, which then translates the thrust into forward momentum.

Though hardly elegant by modern standards, the advantage of the design is that it achieves a high specific impulse meaning it extracts the maximum amount of energy from its fuel source (in this case, nuclear bombs) at minimal cost. In addition, the concept could theoretically achieve very high speeds, with some estimates suggesting a ballpark figure as high as 5% the speed of light (or 5.4107 km/hr).

But of course, there the inevitable downsides to the design. For one, a ship of this size would be incredibly expensive to build. According to estimates produced by Dyson in 1968, an Orion spacecraft that used hydrogen bombs to generate propulsion would weight 400,000 to 4,000,000 metric tons. And at least three quarters of that weight consists of nuclear bombs, where each warhead weights approximately 1 metric ton.

Artists concept of Orion spacecraft leaving Earth. Credit: bisbos.com/Adrian Mann

All told, Dysons most conservative estimates placed the total cost of building an Orion craft at 367 billion dollars. Adjusted for inflation, that works out to roughly $2.5 trillion dollars which accounts for over two thirds of the US governments current annual revenue. Hence, even at its lightest, the craft would be extremely expensive to manufacture.

Theres also the slight problem of all the radiation it generates, not to mention nuclear waste. In fact, it is for this reason that the Project is believed to have been terminated, owing to the passage of the Partial Test Ban Treaty of 1963 which sought to limit nuclear testing and stop the excessive release of nuclear fallout into the planets atmosphere.

Fusion Rockets:Another possibility within the realm of harnessed nuclear power involves rockets that rely on thermonuclear reactions to generate thrust. For this concept, energy is created when pellets of a deuterium/helium-3 mix are ignited in a reaction chamber by inertial confinement using electron beams (similar to what is done at the National Ignition Facility in California). This fusion reactor would detonate 250 pellets per second to create high-energy plasma, which would then be directed by a magnetic nozzle to create thrust.

Like a rocket that relies on a nuclear reactor, this concept offers advantages as far as fuel efficiency and specific impulse are concerned. Exhaust velocities of up to 10,600km/s are estimated, which is far beyond the speed of conventional rockets. Whats more, the technology has been studied extensively over the past few decades, and many proposals have been made.

Artists concept of the Daedalus spacecraft, a two-stage fusion rocket that would achieve up to 12% he speed of light. Credit: Adrian Mann

For example, between 1973 and 1978, the British Interplanetary Society conducted feasibility study known as Project Daedalus. Relying on current knowledge of fusion technology and existing methods, the study called for the creation of a two-stage unmanned scientific probe making a trip to Barnards Star (5.9 light years from Earth) in a single lifetime.

The first stage, the larger of the two, would operate for 2.05 years and accelerate the spacecraft to 7.1% the speed of light (o.071 c). This stage would then be jettisoned, at which point, the second stage would ignite its engine and accelerate the spacecraft up to about 12% of light speed (0.12 c) over the course of 1.8 years. The second-stage engine would then be shut down and the ship would enter into a 46-year cruise period.

According to the Projects estimates, the mission would take 50 years to reach Barnards Star. Adjusted for Proxima Centauri, the same craft could make the trip in 36 years. But of course, the project also identified numerous stumbling blocks that made it unfeasible using then-current technology most of which are still unresolved.

For instance, there is the fact that helium-3 is scare on Earth, which means it would have to be mined elsewhere (most likely on the Moon). Second, the reaction that drives the spacecraft requires that the energy released vastly exceed the energy used to trigger the reaction. And while experiments here on Earth have surpassed the break-even goal, we are still a long way away from the kinds of energy needed to power an interstellar spaceship.

Artists concept of the Project Daedalus spacecraft, with a Saturn V rocket standing next to it for scale. Credit: Adrian Mann

Third, there is the cost factor of constructing such a ship. Even by the modest standard of Project Daedalus unmanned craft, a fully-fueled craft would weight as much as 60,000 Mt. To put that in perspective, the gross weight of NASAs SLS is just over 30 Mt, and a single launch comes with a price tag of $5 billion (based on estimates made in 2013).

In short, a fusion rocket would not only be prohibitively expensive to build, it would require a level of fusion reactor technology that is currently beyond our means. Icarus Interstellar, an international organization of volunteer citizen scientists (some of whom worked for NASA or the ESA) have since attempted to revitalize the concept with Project Icarus. Founded in 2009, the group hopes to make fusion propulsion (among other things) feasible by the near future.

Fusion Ramjet:Also known as the Bussard Ramjet, this theoretical form of propulsion was first proposed by physicist Robert W. Bussard in 1960. Basically, it is an improvement over the standard nuclear fusion rocket, which uses magnetic fields to compress hydrogen fuel to the point that fusion occurs. But in the Ramjets case, an enormous electromagnetic funnel scoops hydrogen from the interstellar medium and dumps it into the reactor as fuel.

Artists concept of the Bussard Ramjet, which would harness hydrogen from the interstellar medium to power its fusion engines. Credit: futurespacetransportation.weebly.com

As the ship picks up speed, the reactive mass is forced into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs. The magnetic field then directs the energy as rocket exhaust through an engine nozzle, thereby accelerating the vessel. Without any fuel tanks to weigh it down, a fusion ramjet could achieve speeds approaching 4% of the speed of light and travel anywhere in the galaxy.

However, the potential drawbacks of this design are numerous. For instance, there is the problem of drag. The ship relies on increased speed to accumulate fuel, but as it collides with more and more interstellar hydrogen, it may also lose speed especially in denser regions of the galaxy. Second, deuterium and tritium (used in fusion reactors here on Earth) are rare in space, whereas fusing regular hydrogen (which is plentiful in space) is beyond our current methods.

This concept has been popularized extensively in science fiction. Perhaps the best known example of this is in the franchise of Star Trek, where Bussard collectors are the glowing nacelles on warp engines. But in reality, our knowledge of fusion reactions need to progress considerably before a ramjet is possible. We would also have to figure out that pesky drag problem before we began to consider building such a ship!

Laser Sail:Solar sails have long been considered to be a cost-effective way of exploring the Solar System. In addition to being relatively easy and cheap to manufacture, theres the added bonus of solar sails requiring no fuel. Rather than using rockets that require propellant, the sail uses the radiation pressure from stars to push large ultra-thin mirrors to high speeds.

IKAROS spaceprobe with solar sail in flight (artists depiction) showing a typical square sail configuration. Credit: Wikimedia Commons/Andrzej Mirecki

However, for the sake of interstellar flight, such a sail would need to be driven by focused energy beams (i.e. lasers or microwaves) to push it to a velocity approaching the speed of light. The concept was originally proposed by Robert Forward in 1984, who was a physicist at the Hughes Aircrafts research laboratories at the time.

The concept retains the benefits of a solar sail, in that it requires no on-board fuel, but also from the fact that laser energy does not dissipate with distance nearly as much as solar radiation. So while a laser-driven sail would take some time to accelerate to near-luminous speeds, it would be limited only to the speed of light itself.

According to a 2000 study produced by Robert Frisbee, a director of advanced propulsion concept studies at NASAs Jet Propulsion Laboratory, a laser sail could be accelerated to half the speed of light in less than a decade. He also calculated that a sail measuring about 320 km (200 miles) in diameter could reach Proxima Centauri in just over 12 years. Meanwhile, a sail measuring about 965 km (600 miles) in diameter would arrive in just under 9 years.

However, such a sail would have to be built from advanced composites to avoid melting. Combined with its size, this would add up to a pretty penny! Even worse is the sheer expense incurred from building a laser large and powerful enough to drive a sail to half the speed of light. According to Frisbees own study, the lasers would require a steady flow of 17,000 terawatts of power close to what the entire world consumes in a single day.

Antimatter Engine:Fans of science fiction are sure to have heard of antimatter. But in case you havent, antimatter is essentially material composed of antiparticles, which have the same mass but opposite charge as regular particles. An antimatter engine, meanwhile, is a form of propulsion that uses interactions between matter and antimatter to generate power, or to create thrust.

Artists concept of an antimatter-powered spacecraft for missions to Mars, as part of the Mars Reference Mission. Credit: NASA

In short, an antimatter engine involves particles of hydrogen and antihydrogen being slammed together. This reaction unleashes as much as energy as a thermonuclear bomb, along with a shower of subatomic particles called pions and muons. These particles, which would travel at one-third the speed of light, are then be channeled by a magnetic nozzle to generate thrust.

The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket. Whats more, controlling this kind of reaction could conceivably push a rocket up to half the speed of light.

Pound for pound, this class of ship would be the fastest and most fuel-efficient ever conceived. Whereas conventional rockets require tons of chemical fuel to propel a spaceship to its destination, an antimatter engine could do the same job with just a few milligrams of fuel. In fact, the mutual annihilation of a half pound of hydrogen and antihydrogen particles would unleash more energy than a 10-megaton hydrogen bomb.

It is for this exact reason that NASAs Institute for Advanced Concepts (NIAC) has investigated the technology as a possible means for future Mars missions. Unfortunately, when contemplating missions to nearby star systems, the amount if fuel needs to make the trip is multiplied exponentially, and the cost involved in producing it would be astronomical (no pun!).

What matter and antimatter might look like annihilating one another. Credit: NASA/CXC/M. Weiss

According to report prepared for the 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit (also by Robert Frisbee), a two-stage antimatter rocket would need over 815,000 metric tons (900,000 US tons) of fuel to make the journey to Proxima Centauri in approximately 40 years. Thats not bad, as far as timelines go. But again, the cost

Whereas a single gram of antimatter would produce an incredible amount of energy, it is estimated that producing just one gram would require approximately 25 million billion kilowatt-hours of energy and cost over a trillion dollars. At present, the total amount of antimatter that has been created by humans is less 20 nanograms.

And even if we could produce antimatter for cheap, you would need a massive ship to hold the amount of fuel needed. According to a report by Dr. Darrel Smith & Jonathan Webby of the Embry-Riddle Aeronautical University in Arizona, an interstellar craft equipped with an antimatter engine could reach 0.5 the speed of light and reach Proxima Centauri in a little over 8 years. However, the ship itself would weigh 400 Mt, and would need 170 MT of antimatter fuel to make the journey.

A possible way around this is to create a vessel that can create antimatter which it could then store as fuel. This concept, known as the Vacuum to Antimatter Rocket Interstellar Explorer System (VARIES), was proposed by Richard Obousy of Icarus Interstellar. Based on the idea of in-situ refueling, a VARIES ship would rely on large lasers (powered by enormous solar arrays) which would create particles of antimatter when fired at empty space.

Artists concept of the Vacuum to Antimatter Rocket Interstellar Explorer System (VARIES), a concept that would use solar arrays to power lasers that create particles of antimatter to be used as fuel. Credit: Adrian Mann

Much like the Ramjet concept, this proposal solves the problem of carrying fuel by harnessing it from space. But once again, the sheer cost of such a ship would be prohibitively expensive using current technology. In addition, the ability to create dark matter in large volumes is not something we currently have the power to do. Theres also the matter of radiation, as matter-antimatter annihilation can produce blasts of high-energy gamma rays.

This not only presents a danger to the crew, requiring significant radiations shielding, but requires the engines be shielded as well to ensure they dont undergo atomic degradation from all the radiation they are exposed to. So bottom line, the antimatter engine is completely impractical with our current technology and in the current budget environment.

Alcubierre Warp Drive:Fans of science fiction are also no doubt familiar with the concept of an Alcubierre (or Warp) Drive. Proposed by Mexican physicist Miguel Alcubierre in 1994, this proposed method was an attempt to make FTL travel possible without violating Einsteins theory of Special Relativity. In short, the concept involves stretching the fabric of space-time in a wave, which would theoretically cause the space ahead of an object to contract and the space behind it to expand.

An object inside this wave (i.e. a spaceship) would then be able to ride this wave, known as a warp bubble, beyond relativistic speeds. Since the ship is not moving within this bubble, but is being carried along as it moves, the rules of space-time and relativity would cease to apply. The reason being, this method does not rely on moving faster than light in the local sense.

Artist Mark Rademakers concept for the IXS Enterprise, a theoretical interstellar warp spacecraft. Credit: Mark Rademaker/flickr.com

It is only faster than light in the sense that the ship could reach its destination faster than a beam of light that was traveling outside the warp bubble. So assuming that a spacecraft could be outfitted with an Alcubierre Drive system, it would be able to make the trip to Proxima Centauri in less than 4 years. So when it comes to theoretical interstellar space travel, this is by far the most promising technology, at least in terms of speed.

Naturally, the concept has been received its share of counter-arguments over the years. Chief amongst them are the fact that it does not take quantum mechanics into account, and could be invalidated by a Theory of Everything (such as loop quantum gravity). Calculations on the amount of energy required have also indicated that a warp drive would require a prohibitive amount of power to work. Other uncertainties include the safety of such a system, the effects on space-time at the destination, and violations of causality.

However, in 2012, NASA scientist Harold Sonny White announced that he and his colleagues had begun researching the possibility of an Alcubierre Drive. In a paper titled Warp Field Mechanics 101, White claimed that they had constructed an interferometer that will detect the spatial distortions produced by the expanding and contracting spacetime of the Alcubierre metric.

In 2013, the Jet Propulsion Laboratory published results of a warp field test which was conducted under vacuum conditions. Unfortunately, the results were reported as inconclusive. Long term, we may find that Alcubierres metric may violate one or more fundamental laws of nature. And even if the physics should prove to be sound, there is no guarantee it can be harnessed for the sake of FTL flight.

In conclusion, if you were hoping to travel to the nearest star within your lifetime, the outlook isnt very good. However, if mankind felt the incentive to build an interstellar ark filled with a self-sustaining community of space-faring humans, it might be possible to travel there in a little under a century if we were willing to invest in the requisite technology.

But all the available methods are still very limited when it comes to transit time. And while taking hundreds or thousands of years to reach the nearest star may matter less to us if our very survival was at stake, it is simply not practical as far as space exploration and travel goes. By the time a mission reached even the closest stars in our galaxy, the technology employed would be obsolete and humanity might not even exist back home anymore.

So unless we make a major breakthrough in the realms of fusion, antimatter, or laser technology, we will either have to be content with exploring our own Solar System, or be forced to accept a very long-term transit strategy

We have written many interesting articles about space travel here at Universe Today. Heres Will We Ever Reach Another Star?, Warp Drives May Come With a Killer Downside, The Alcubierre Warp Drive, How Far Is A Light Year?, When Light Just Isnt Fast Enough, When Will We Become Interstellar?, and Can We Travel Faster Than the Speed of Light?

For more information, be sure to consult NASAs pages on Propulsion Systems of the Future, and Is Warp Drive Real?

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How Long Would It Take To Travel To The Nearest Star ...

This week, Elon Musk dragged space fashion into the 21st century with the newly revealed SpaceX spacesuit . But can he do the same for space tourism?

The allure of space travel is deeply embedded in our psyche. Jules Vernes 1865 novel From Earth to the Moon captured some of this drive. But it was JFKs 1961 Moon Shot speech, and the space programs that followed, that encouraged ordinary people to imagine they might one day be able to travel beyond the Earth.

That possibility came closer in 2004 when Burt Rutans SpaceShip One became the first private vessel to carry its three pilots into suborbital flight. Since then, a handful of companies have been pushing hard to kickstart the future of space tourism.

$250,000 will secure you a seat on Sir Richard Bransons Virgin Galactic, even though the company has yet to make its maiden passenger voyage. And Jeff Bezos is also gearing up to give budding space tourists a similar experience with Blue Origins Space Capsule.

Both Blue Origin and Virgin Galactic are promising a few minutes of weightlessness and stunning views of the Earth from spacealbeit at the cost of a second mortgage. But these are little more than titillating carnival rides compared to true space travel.

For this, aspiring space tourists need to look to SpaceX. In February, Musk announced plans to fly two paying passengers around the moon in 2018. This is still the equivalent of a stroll down the street given the vastness of the solar system. But unlike the toe-dipping experiences promised by Virgin Galactic and Blue Origin, its more likely to capture the full space experience.

And that includes the risks.

If theres one thing weve learned in recent decades, its that space is dangerous. For space tourism to come close to succeeding, companies offering trips beyond the Earths atmosphere are going to have to grapple with a complex and shifting risk landscape.

Space travel encapsulates a remarkable frisson between risk and safety. For many people, the anticipated experience of being in space seems to far outweigh perceived personal risksjust look at the number of people willing to risk their lives on a one-way trip to Mars!

Yet irrespective of what individuals are willing to accept, the possibility of civilian injuries and deaths present a major challenge to the future of space tourism. Expect to see crippling insurance premiums, cold-footed investors, and the specter of regulations that potentially suck the lifeblood out of a fragile industry. But also expect public backlashes against seemingly reckless private ventures that potentially leave deep public scars if they fail.

These and similar risks dont spell the death of space tourism by any stretch of the imagination. But success will depend on weaving a subtle course through new risk territory. Of course, itll mean ensuring that passengers are adequately protected in the event of system failures, and that theyre kept as safe as possible without restricting the experience theyve paid for. But it will also mean granting companies the social and legal license to operate.

And trivial as it may seem, a well-designed spacesuit taps in to all of these. Naturally, you cant succeed in space tourism simply by creating a sexy spacesuit. But you can do a lot with a suit thats functional, desirable, and iconic. And you can excel with one that makes the complete experience worthwhilenot only for the wearer, but for the rest of us who are vicariously experiencing this new adventure from a distance, and everything it promises for the future.

This is a tall order. But maybe Musks sleek new spacesuit will bring us a step closer toward a viable and vibrant future of space tourism.

Andrew Maynard is a professor in the Arizona State University (ASU) School for the Future of Innovation in Society, and director of the ASU Risk Innovation Lab.

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Elon Musk's Sexy Spacesuit Is One Giant Leap for Space Tourism - Fortune

Transforming science fiction to reality, UC Santa Barbara physics professor Philip Lubin is creating a laser-cannon system to propel miniature spaceships with solar sails more than 25 trillion miles to the suns nearest star Proxima Centuari.

Loaded with cameras, other sensors, historical records of humanity, greetings from Earth and possibly human DNA, the smartphone-sized crafts, or interstellar arks, would be thrust on an historic journey that would take about 20 years a blink of an eye in space travel.

People understood roughly 100 years ago that it was possible using then- technology to send a human to the moon and return them, Lubin said, noting that one challenge was scaling down equipment. If you look at the popular literature at that time, the idea was treated as science fiction, like Flash Gordon.

Lubins ambitious vision is showcased in Laser-Sailing Starships, one of eight new books in the Out of this World Series (World Book, 2017). Targeted to middle- school students, the books focus on research fellows involved in the NASA Innovative Advanced Concepts program. NASAs aim is to foster the next generation of scientific talent.

The great part about the whole series is that it doesnt talk down to kids, but addresses the science head-on, said Jason Derleth, the program executive for NASA, which helps fund Lubins research.

In 2009, Lubin began examining how to use directed energy a phased laser array to deflect asteroids bound for Earth. But there was limited outside interest in the UCSB research, he said, because the planet doesnt get hit often.

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Bold Space Travel - Santa Barbara Edhat

A strain of yeast that can recycle urine and carbon dioxide into omega-3 fatty acids and polymers has been developed by US scientists, who say it could help astronauts turn waste products into food on long interplanetary journeys.

Biomolecular engineer Mark Blenner from Clemson University in South Carolina presented the work at the 254th American Chemical Society National Meeting and Exposition in Washington, DC, as part of a broader session on getting people to Mars.

Our yeast not only grow on human urine, they actually prefer it to other nitrogen sources

Mark Blenner, Clemson University

Blenners research focuses on the yeast species Yarrowia lipolytica whose cells naturally produce and accumulate omega-3 fatty acids. He says that these products could be used as nutritional supplements for astronauts, as theyve been implicated in preventing bone loss and maintaining cardiovascular and ocular health, but dont have a long enough shelf life for adequate supplies to be brought from Earth. His group showed that the yeast could grow using human urine as a source of nitrogen, something that there would be a plentiful supply of on manned space missions.

Our yeast not only grow on human urine, they actually prefer it to other nitrogen sources, Brenner says. His group have also used synthetic biology to engineer a strain of the same yeast to produce polyhydroxyalkanoates, which shows they have the potential to manufacture polymer inks that could be used to fabricate objects in a 3D printer. In particular, he said this could be very useful in situations where an astronaut has lost a tool or a piece of equipment that they need.

Blenner admits they dont currently know how the biology would react to being in space. But in the meantime there are several more terrestrial applications they can explore, such as producing omega-3 supplements for fish farms and making other speciality chemicals. He says the next stepis for his team to demonstrate that they can get usable quantities of both the polyestersand the omega-3 fatty acids from these astronaut waste stream. We are going to be doing genetic engineering to the cell to really try and force it to make the products that we want, by knocking out certain pathways that might syphon off intermediates, Blenner explains. The team is also still at the early stages of characterising how the yeast go about taking up a lot of these waste substrates. We havent really done a full analysis yet of whats left over to try and see if there is any way to get the yeast to use some of the leftovers, he says.

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Space travel microbes turn urine into polymers - Chemistry World (subscription)

Imagine you're on your way to Mars, and you lose a crucial tool during a spacewalk. Not to worry, you'll simply re-enter your spacecraft and use some microorganisms to convert your urine and exhaled carbon dioxide (CO2) into chemicals to make a new one. That's one of the ultimate goals of scientists who are developing ways to make long space trips feasible.

The researchers are presenting their results this week at the 254th National Meeting and Exposition of the American Chemical Society (ACS). ACS, the world's largest scientific society, is holding the meeting here through Thursday. It features nearly 9,400 presentations on a wide range of science topics.

Astronauts can't take a lot of spare parts into space because every extra ounce adds to the cost of fuel needed to escape Earth's gravity. "If astronauts are going to make journeys that span several years, we'll need to find a way to reuse and recycle everything they bring with them," Mark A. Blenner, Ph.D., says. "Atom economy will become really important."

The solution lies in part with the astronauts themselves, who will constantly generate waste from breathing, eating and using materials. Unlike their friends on Earth, Blenner says, these spacefarers won't want to throw any waste molecules away. So he and his team are studying how to repurpose these molecules and convert them into products the astronauts need, such as polyesters and nutrients.

Some essential nutrients, such as omega-3 fatty acids, have a shelf life of just a couple of years, says Blenner, who is at Clemson University. They'll need to be made en route, beginning a few years after launch, or at the destination.

"Having a biological system that astronauts can awaken from a dormant state to start producing what they need, when they need it, is the motivation for our project," he says.

Blenner's biological system includes a variety of strains of the yeast Yarrowia lipolytica. These organisms require both nitrogen and carbon to grow. Blenner's team discovered that the yeast can obtain their nitrogen from urea in untreated urine.

Meanwhile, the yeast obtain their carbon from CO2, which could come from astronauts' exhaled breath, or from the Martian atmosphere. But to use CO2, the yeast require a middleman to "fix" the carbon into a form they can ingest. For this purpose, the yeast rely on photosynthetic cyanobacteria or algae provided by the researchers.

One of the yeast strains produces omega-3 fatty acids, which contribute to heart, eye and brain health. Another strain has been engineered to churn out monomers and link them to make polyester polymers.

Those polymers could then be used in a 3-D printer to generate new plastic parts. Blenner's team is continuing to engineer this yeast strain to produce a variety of monomers that can be polymerized into different types of polyesters with a range of properties.

For now, the engineered yeast strains can produce only small amounts of polyesters or nutrients, but the scientists are working on boosting output. They're also looking into applications here on Earth, in fish farming and human nutrition. For example, fish raised via aquaculture need to be given omega-3 fatty acid supplements, which could be produced by Blenner's yeast strains.

Although other research groups are also putting yeast to work, they aren't taking the same approach. For example, a team from DuPont is already using yeast to make omega-3 fatty acids for aquaculture, but its yeast feed on refined sugar instead of waste products, Blenner says. Meanwhile, two other teams are engineering yeast to make polyesters. However, unlike Blenner's group, they aren't engineering the organisms to optimize the type of polyester produced, he says.

Whatever their approach, these researchers are all adding to the body of knowledge about Y. lipolytica, which has been studied much less than, say, the yeast used in beer production.

"We're learning that Y. lipolytica is quite a bit different than other yeast in their genetics and biochemical nature," Blenner says. "Every new organism has some amount of quirkiness that you have to focus on and understand better."

A video on the research is available here

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Turning human waste into plastic, nutrients could aid long-distance space travel - Space Daily