US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko have returned to Earth after spending almost a year in space on board the International Space Station.
Their 340-day mission aboard the International Space Station was twice the length of a normal stay - but how has time in space affected their bodies?
Space wreaks havoc on almost every part of the human body because there is less gravity to create the conditions we experience living on Earth.
"Astronauts lose a lot of calcium essential to their bones - it's a bit like osteoporosis here on Earth," says space scientist Dr Maggie Aderin-Pocock from University College London.
Osteoporosis is a disease where bones become more brittle, which sometimes affects older people and women. It can mean you're more likely to break your bones.
It's thought that this happens in space because astronauts don't do load-bearing exercise - like walking and running, or lifting things - pulling against the Earth's gravity.
To try to avoid this, while they're in space astronauts have a special diet and have to do up to two hours of exercise per day.
After five months in orbit above the Earth, an astronaut would typically lose as much as 40% of muscle and 12% of bone mass, says Jeremy Curtis from the UK Space Agency.
"The muscle loss is the equivalent of a 20-year-old turning into a 60-year-old over a period of three months," he says.
Astronauts returning to Earth will experience problems standing up and balancing - and some won't be able to drive a car to begin with.
They have to undergo a special rehabilitation programme a year after returning to earth to rebuild muscles and bones - and may never regain their previous bone mass.
The exposure to higher levels of radiation in space also means astronauts may be more likely to suffer from cancer later on in life.
But on the plus side, scientists say studying the effects of space flight on the human body can help with developing new treatments for diseases like osteoporosis and cancer here on Earth.