Right now, there are six astronauts aboard the International Space Station, floating 408km above our heads. But soon things could be about to get a lot lonelier up there. Delays in building new spacecraft to get astronauts into space mean that the next trio of astronauts set to join the ISS in April 2020 are facing the possibility of being the space stations lone occupants for six months.

It'll be the first time the ISS has had only three semi-permanent occupants since 2009, when it was expanded so it could comfortably fit six occupants at any one time. But for the last 10 years a crew of six has kept up with the ISS endless list of maintenance tasks and research projects. What will happen when this floating workforce is cut in half?

The next three astronauts to be sent up American Chris Cassidy, and Russians Nikolai Tikhonov and Andrei Babkin will travel in a Soyuz capsule. Since the retirement of the US Space Shuttle in 2011, all journeys to the ISS have taken place in these Russian-made spacecraft, which bring three people at a time. One Soyuz capsule is attached to the ISS, like a lifeboat, at all times, and the crew members who have been there the longest will take this capsule home. Three new astronauts arrive a couple of weeks later so apart from the short periods while crews are changed over, there are usually six astronauts and cosmonauts on the ISS at any one time.

This system has worked worked for nearly a decade, but for 2020, Nasa decided that instead of buying seats on Soyuz, it would rely on contracts with commercial companies SpaceX and Boeing for extra crew launches. But development of the commercial capsules has been delayed, with Boeing having trouble with their parachutes and SpaceXs Crew Dragon capsule exploding during tests.

In 2010 when Nasa prepared to shut down their Space Shuttle program, they gave commercial companies a combined $50m (38m) to design their own transport spacecraft. Its uncertain exactly when the commercial capsules will be ready as they need to pass rigorous safety tests first, so rather than paying for an extra Soyuz, Nasa has decided to prepare the next group of astronauts for what might happen if theyre left alone.

Over the years, ISS expedition experiments have made discoveries which will be vital if humanity wants to explore space further. Previous missions have revealed the effect of microgravity on the human body and the source of cosmic rays. But if the crew need to do general maintenance on the station, with fewer people on board there will be less time for other activities.

A large amount of experiments can be done with commanding from the ground so we're trying to to give preference to those, says Ruediger Seine, space training team leader at ESA's European Astronaut Centre. Space agencies managing experiments will have to pick which of their projects theyd like the astronauts to devote their limited time towards, and press pause on experiments that require more human intervention.

But some experiments might fare between without humans getting in the way. Some experiments actually might benefit from less people because of vibration, says Laura Forzcyk, founder of the consulting firm Astralytical who used to work for the ISS US National Laboratory. Even with three there are still a lot of vibrations. Some experiments just want to be left alone.

The limited crew will also mean more sharing of resources. The ISS itself is split into two sides Russian and American. The Russians usually operate on their side, while astronauts from the United States, Europe, Japan, and Canada operate on the other. However with such a small number of people on board, they need to work more cohesively. With only one American, the two Russians have been trained on how to use the US equipment. For example, each side has its own space suit both with different ways of operating but the astronauts and cosmonauts need to be trained in how to use both. Its a case of being prepared for all situations.

It's not easy to get from the Russian airlock to the American segment of the International Space Station. The additional training for the remaining crew members is to make sure that we're covering all contingencies, says Seine.

And while the experiments may appreciate being alone, the people may not. Experiments only take up part of astronaut's time on board the ISS. Like a typical working week on Earth, they have time off, in the evenings and on the weekends. Chris Cassidy, Nikolai Tikhonov and Andrei Babkin may not see another person from spring 2020, to when their mission finishes at the end of the year.

And 2020 could be the last point the ISS gets this quiet for a while. In 2019, Nasa announced that it would let tourists fly to the ISS from 2020 if they were willing to pay the 27,500 for the privilege. This isnt the first time this has happened seven people who arent employed by a space agency have gone to the ISS before. And it wont mean they are dead weight; anyone who gets sent up, professional or not, will be given tasks and help out in any way they can.

But until then, the trio of astronauts will have to endure the mental stress that comes with isolation. Research has found that along with psychological effects caused by adjusting to the novel situation of being in space, astronauts also often struggle with anxiety and depression. ISS astronauts, despite being kept busy with experiments, will still have opportunities to socialise they try to have at least one shared meal a day, and receive plenty of contact from their friends and family.

Michael Lopez-Alegria, who has flown the longest US space station mission to date, went up to the International Space Station in 2006 when the crew was still small. He was there with only two other people at a time, but managed to not feel too lonely during the mission.

I like the small size crew, because we tend to bond more as a unit, he says. Sometimes there can be a divide by culture when there are more people. We had one Russian, me and then the third person was either a German or an American, and so we tended to be more cohesive and spend more time together.

Lopez-Alegria found that looking out at Earth helped him feel closer to home, and in 2010, an observatory module was added to the ISS to create an even bigger window to help with astronauts mental health. I wish I were going back myself, he says.

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Life on the Space Station is about to get really weird and lonely - Wired.co.uk