The year is 3000 and humans have conquered space travel. Weve colonized other planets and built self-sufficient societies on interplanetary spaceships. Most of humanity watches the new millennium dawn by the light of suns in other solar systems.

While many science fiction writers imagine a space-faring civilization as almost an inevitability and have envisioned mind-bending food-printing and advanced propulsion technologies to take us there, theres one major problem thats rarely considered: We dont know if humans can procreate in space or on any other planet. And without offspring, the survival of any space colony would be limited to a single generationfar short of the estimated 73,000 years it would take to reach even the closest star, Proxima Centauri, with currently available technology.

For some animals, making babies in space seems to be no problem. Former guests of past space missions like fruit flies, newts, and Japanese rice fish, have all produced offspring in space. Unfortunately, mammalsand especially humansare a lot more complicated, and studies involving astronauts, animals, and lonely cell cultures in Petri dishes floating in space suggest that reproduction outside of Earth could be very difficult for us.

Even if you were an adult in peak physical fitness living in the protective confines of a space station, your environment would be an almost unimaginably inhospitable one where you float in a tub, deprived of gravity, while bathing in a surplus of exotic cosmic radiation.

That punishing environment would not be a particularly safe place for the delicate tissues of your reproductive system. Studies have shown that even short stints in space shrivel testes, shrink ovaries, and plummet sperm counts. (The majority of these studies involve rodents, since astronauts arent particularly keen to have their testes or ovaries removed for dissection.)

There are some glimmers of hope. For trips where astronauts stay relatively close to Earth, say, to the International Space Station (ISS), spaceflight doesnt seem to induce permanent infertility. There are plenty of examples of both male and female astronauts having had kids after a stint in space. Anecdotally, some female astronauts do seem to have difficulty having children upon returning to Earth, but researchers generally attribute this to age rather than spaceflight.

For longer tripslike future missions to Mars, which if past missions are any indication, we should be able to reach within nine monthsthe radiation of deep space might have longer-term consequences, but with different risks for men and women. While sperm production can be impaired in space, it may recover afterward. The testes in rodents and in humans have so-called germline stem cells called spermatogonia, says Ulrike Luderer, a developmental biologist at the University of California, Irvine. Those spermatogonia are relatively resistant to radiation. So, after the radiation exposure has ceased, those spermatogonia can start repopulating the testes and making new sperm.

This isnt necessarily the case in the female reproductive system. In the ovary, we do not have any germline stem cells, says Luderer. Although the existence of stem cells in the ovaries has been the subject of recent controversy among developmental biologists, with some suggesting they do exist, others hold that women are born with all the eggs theyre ever going to have. If this is the case, then once theyre goneeither through the aging process or by exposure to radiation in spacetheyre not coming back.

Without improvements in protective technology, deep-space radiation could cause permanent infertility in female spacefarers.

Luderer wanted to know what effect the radiation exposure during a Mars mission would have on female astronauts ovaries. To find out, her team exposed female mice to different degrees of simulated deep space radiation and then assessed the effect on the ovarian folliclessmall bundles of tissue that contain the egg cells. Eight weeks after the high dose of radiation, there were essentially no follicles of any kind remaining in the ovaries, Luderer says. The high dose was 50 centigray. The total dose for a mission to Mars is estimated to be about 40 centigray. So, our high dose was about the total dose that an astronaut would receive during a Mars mission.

Though the experiment was limited by the fact that the radiation was administered over a relatively short time period rather than the multi-year timeframe of astronaut exposure, it nevertheless suggests that without improvements in protective technology, deep-space radiation could cause permanent infertility in female spacefarers.

The easiest way to solve the problems of fertility for deep space travel may be to harvest the sperm and fertilize the eggs in advance, nurture them, keep them safe from cosmic radiation, and implant the embryos in the womb. But this raises another question: Would the embryo (and later fetus) be able to develop normally? At present, we can only guess based on what we know about how spaceflight affects adult humans, animals, and cells in Petri dishes.

One of the biggest dangers of spaceflight for adult humans is bone loss. Any astronaut who serves a long tour in space will practice routine, rigorous exercises designed to prevent bone loss, and even then, astronauts can lose as much as 1 percent of their bone density every month they spend in space. Without the stress of gravity on the skeleton, the production of bone-building cells called osteoblasts slows down, which eventually leads to bone loss.

How would this affect a fetus, which needs to grow an entire skeleton from scratch and also, for obvious reasons, cannot practice astronaut bone-protecting exercise programs? Would a child whose skeleton developed in space be strong enough to ever live somewhere with Earth-like gravity?

The cardiovascular system is also negatively impacted by space travel. The heart muscle atrophies, and anecdotal evidence suggests an increased risk of arrythmias. In nine-day-old rats, spaceflight altered the development of the aorta, thinning the wall of this major artery. So would a newborn baby whose cardiovascular system developed in space have a heart or blood vessels strong enough to survive to adulthood? We simply dont know.

Indeed, some scientists have hypothesized that normal fetal development would be impossible in space, predicting that alterations to normal muscle and bone development would cause space-born children to reach developmental milestones like sitting, standing, and walking much later than their Earth-bound counterparts.

Far more fickle may be the brains development. Although we know very little about how spaceflight might alter bone, muscle, and cardiovascular development, we understand even less about how it might affect brain development. Studies on Earth, however, give cause for concern: prenatal radiation exposure is associated with an increased risk of intellectual disability and seizure disorders.

More research needs to be done before we can ethically allow a human fetus to develop in space, but the most dangerous part of space reproduction may not come until the very end. The process of birthwhich can be dangerous enough on Earthis likely to be even more risky in space, an environment which weakens both bone and muscle. Pregnant rats that experienced spaceflight for just 11 days before returning to Earth and giving birth had twice as many labor contractions as rats that stayed on Earth, potentially due to weakening of the muscles needed for giving birth.

Scott Solomon, an evolutionary biologist at Rice University, says that the involuntary muscle contractions during labor are going to be the same whether a woman is giving birth on Earth or in space, but the effect of those forces on a womans body could be very different in space. Those forces could be really dangerous if the woman has a weakened skeleton, he says.

No mammals have ever given birth in space.

Although pregnant rats have been taken to space, none have actually given birth there. In fact, Luderer says, at this time, no mammals have ever given birth in space. Since humans are rarely the first to undergo unknown medical dangers (in the present century, at least), it is perhaps unsurprising that the Dutch startup SpaceLife Origin abruptly aborted its Mission Cradle plan to launch a pregnant woman into space and have her deliver her baby there, which was originally scheduled for 2024.

An earlier Mission Lotus, originally slated for sometime this year, would have sent human sperm and eggs into space to attempt in vitro fertilization, returning the resulting embryos to Earth for transfer into a mothers womb. But this mission was also cancelled. In a statement, SpaceLife Origin CEO Kees Mulder cited serious ethical, safety and medical concerns as the reason for these cancellations. A 2019 Atlantic report noted that the companys top three employees had no background in either medicine or space travel.

Medical experts say its a good thing this mission was cancelled. I couldnt wrap my head around it, says Virginia Wotring, a space medicine researcher at Baylor College of Medicine. Even if you were to just go to the ISS, youd be 250 miles away from medical help in the event of an emergency. And I dont know many pregnant women who would volunteer for that. She says it was too much, too soon to even think about sending women into space to give birth. Mission Lotus was also problematic: Id be really uncomfortable sending something as sensitive as an embryo into space, Wotring says.

Radiation is also a danger, especially in deep space. The type of radiation to which you would be exposed in deep space is unlike anything you will ever experience on Earthit includes high-energy atom fragments from solar flares and ions from beyond our solar system traveling at nearly the speed of light. Even in low-Earth orbit on the International Space Station, our planets magnetic field protects us from this dangerous deep-space radiation that can damage DNA.

But deep space radiation is just the beginning, Wotring says. Spaceflight stresses the human body in a myriad of ways. Your circadian rhythms are disrupted. You are exposed to higher levels of carbon dioxide, prolonged psychological stress, and social isolation. You lack fresh food, use only recycled air and water, and subject your microbiome to unknown stressall of which may affect your fertility and the development of a fetus.

This makes coming up with solutions tricky. For many of the physical effects of spaceflight, Wotring says, we dont know which one of the possible causes is really responsible. And without knowing the etiology of something, its really hard to figure out an appropriate countermeasure or treatment.

Nevertheless, scientists are trying to come up with new technologies to improve radiation shielding and alter the microgravity environment of space vessels, but these are thorny challenges. On Earth, we can use metals such as lead to protect ourselves from radiation in hospitals or industrial settings. But the types of radiation present in space are a completely different beast.

Metal solutions are not very useful when it comes to space radiation, Wotring says. Because with some of the space radiation types, if they encounter a metal ion, they break up and result in actually higher, more dangerous radiation. So, its exactly the wrong thing to do. And then theres the profound payload problem of lifting heavy lead shielding into space. Its just not feasible, Wotring says.

Polyethylene plastics are relatively light and effective for blocking space radiation. Unfortunately, they are not very strong. Astronautical engineers in Italy are experimenting with adding carbon or graphene nanomaterials to polyethylene to see if they can make it stronger while preserving its radiation-blocking properties. Research teams in Europe and Japan are investigating different types of lithium-containing materials, with promising early results. Scientists at NASA and space agencies around the globe are also investigating active shieldingusing magnetic or electric fields to protect spaceships from radiation in the same way that the Earths magnetic field protects terrestrial life. So far, however, these designs have been deemed too heavy to scale up for space travel.

Shielding on other planets, such as Mars, may be slightly easier. Materials that are less efficient at radiation shielding can still be usefulthey just need to be a lot thicker to achieve the proper protection. If we use materials that are already available on Mars, we could use as much as we wanted without worrying about how much fuel it would take to transport them from Earth. People living on Mars might use the Martian regolith (the Martian soil) to block the radiation, Solomon says. They might live underground in lava tubes. Waterwhich is found on Mars and which colonists will need to survive anywayis also effective at blocking radiation.

Scientists are also working on ways to provide artificial gravity using rotational forces. The Japan Aerospace Exploration Agency has developed an artificial gravity system for mice, which seems to ameliorate at least some of the negative effects of spaceflight. But scaling this system up from a mouse-sized enclosure to an entire space station presents several engineering challenges, from the complicated problem of how to dock a ship with a rapidly rotating space station to the issue of motion sickness. However, recent research from scientists at the University of Colorado Boulder suggests that starting with slow rotations and letting people acclimate over several weeks can reduce motion sickness. But would this device be safe for a developing fetus? We have no idea: To date, there are no data regarding the effects of these artificial gravity generators for embryonic or fetal development.

Others are working on pharmacological options to help protect astronauts (and perhaps one day their developing offspring) from the dangers of space travel. Animal studies from Luderers lab found that an antioxidant called alpha lipoic acid provided at least partial protection for the ovaries of mice exposed to some (but not all) types of simulated space radiation. We know that a lot of the tissue damage from ionizing radiation is by the generation of reactive oxygen species when the radiation interacts with water in the cell, Luderer says. Reactive oxygen species are a type of unstable molecule that can damage our DNA. And so thats why we thought an antioxidant might be beneficial.

Could something as simple as an antioxidant be the key to preserving a womans fertility during space travel? That is something that I think is certainly worth researching further, Luderer says.

A recent study by researchers at the Jackson Laboratory for Genomic Medicine found that using pharmaceuticals to block certain signaling proteins in mice not only prevented the loss of bone and muscle mass that usually takes place in microgravity but actually increased their density. Perhaps some in-utero treatment based on a cocktail of such proteins could help a space fetus grow bones and muscles in microgravity, although much more research is needed before this could be determined.

But maybe the answer isnt taking drugs or changing the environmentmaybe we need to change ourselves. Scientists are studying durable organisms like tardigrades and certain forms of yeast, looking for genes that protect their cells from radiation or help repair damage to their DNA after its occurred. Scientists at the University of Wisconsin Madison are blasting bacteria with high doses of ionizing radiation to watch them evolve radiation resistance in real time and study which genes are involved. Geneticists like George Church, cofounder of Harvard Medical Schools Consortium for Space Genetics, have suggested dozens of genes that might benefit space-faring humans.

But Solomon says that knowing which genes confer protection is only the first step. We need to know that any changes we make arent going to have unintended consequences, he says, and to make sure that a gene that helps protect an adult from radiation doesnt have harmful effects on a fetus.

Maybe we need to get even more creative and think outside the womb, so to speak. Perhaps the future of human reproduction in space doesnt occur inside a human at all. Scientists are already working on creating bioengineered ovaries as well as artificial sperm and eggs from other cells in our bodies. Perhaps these artificial gametes could then be implanted in an artificial womb with an artificial placenta. It might be easier to protect this smaller structure from radiation, and an artificial womb isnt affected by stress or circadian rhythm disruption in the same way a womans womb is. This could also protect female astronauts from the dangers of pregnancymorning sickness, gestational diabetes, preeclampsia, and moreas well as reduce the risks associated with childbirth in space.

Long-term human habitation in space or on other planets might eventually turn us into multiple human species.

But none of this is possible quite yet. Wotring, Solomon, and Luderer agree that we have barely scratched the surface of understanding how living in space or on other planets would affect human reproduction and development. Its early days for these topics, says Wotring. Its far too early to really think about attempting reproduction in space.

However, if we do manage to figure out extraterrestrial reproduction, this will have major implications for the future of our species. Genetic and physiological changesprecipitated by genetic engineering, adaptation to the conditions of space, or to the measures that we took to survive off Earthwill likely occur.

For example, one way to address the difficulty of delivering a baby in space would be to deliver them via Cesarean section, but if all extraterrestrial births had to take place by Cesarean section for the safety of the mother, this changes the evolutionary pressures on human head size, says Solomon. For all of human history, the size of our heads (and therefore the size of our brains) at birth has been limited by the need for the head to be able to squeeze through the birth canal. But without this constraint, Solomon says, the human head would be freed to become larger without the consequence of it being dangerous to the birth of the child and to the mothers life. So, you could then imagine on Mars, if people chose [to have Cesarean sections], that heads could become larger and larger in future generations.

If we ever leave Earth and start to colonize other places in our solar system or beyond, that could trigger evolutionary changes that would be perhaps similar to the types of changes that happen to species here on Earth when they colonize new habitats like islands, says Solomon. Organisms living in drastically different environments dont necessarily stay the same species foreverlong-term human habitation in space or on other planets might eventually turn us into multiple human species. How long this speciation could take is anyones guess: In special circumstances, new species may arise in just a few generations, in other cases, scientists think lasting evolutionary change may occur over a million years.

But to get there, we have to figure out extraterrestrial reproduction first.

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