It's been a year since Nasa astronaut Scott Kelly returned to Earth from his 340-day visit to the International Space Station.
Scott has an identical twin brother, Mark, and they're helping scientists plan for future space travel.
Researchers are studying them both to find out how being in space affected Scott's body - and compare it to Mark's on Earth.
Nasa wants to use the twins' information to help work out how to keep astronauts healthy if they ever send them to Mars in the future.
Here's Jenny with what they've found out so far...