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You might have thought about what it's like to be an astronaut in space, but have you ever thought about how long you can stay in space?

Two Nasa astronauts are currently staying onboard the International Space Station (ISS), with no set return date, while crews work to repair the Starliner spacecraft that they travelled in.

Butch Wilmore and Suni Williams were initially expected to stay on the ISS for seven days, but their return has been delayed multiple times.

Nasa is carrying out number of tests before deciding when it is safe to bring them home, so it's currently unknown how long they will be there.

But how long can you stay in space and what kind of affect can that have? Read on to find out more.

Space travel affects almost every part of the human body and the main reason for this is gravity

Earth's gravity is what keeps you on the ground and what makes things fall.

In space, astronauts do not walk on the floor like people on Earth do. They float around inside their spacecraft.

It's not because of a lack of gravity like on the Moon, but because the ISS is in free fall - falling towards Earth and moving forward around the Earth at roughly the same speed, meaning the astronauts inside the station float.

It also means astronauts are able to move what we'd normally expect to be very heavy objects, easily.

And that means their muscles have to do very little to move or lift things - the UK Space Agency says that after about five months, astronauts can lose as much as 40% of muscle and 12% of bone mass.

And you don't necessarily need to be in space for that long, research published in the Canadian Medical Association Journal suggests muscle mass can fall by 20% after just two weeks.

Eyesite can be impacted too, astronauts can come back with worse vision than when they left as a result of high pressure with lower gravity.

What's the longest amount of time someone has been in space?

Frank Rubio broke the record for the longest amount of time in space by a US astronaut when he returned last year.

He stayed for a whopping 371 days, but was still a way off the all-time record.

Russian cosmonaut Valeri Polyakov holds the record for the most days spent in space in one go.

He was onboard Russia's Mir space station for 437 days between 1994 and 1995.

When will the Starliner astronauts be back?

Nasa has not yet set a date when it will bring its two astronauts home, this will be decided following a high-level review of the technical problems with the spacecraft.

The space agency has stressed though, that the astronauts are not stranded and that Starliner can return to Earth if there is an emergency on the ISS.

There is also the option that Nasa and Boeing can return their astronauts on SpaceX's Dragon capsule.

But this would be a last resort, with the astronauts not being in any danger.