The prime minister met Canadian astronaut Jeremy Hansen and other crew members in Ottawa.
The BBC's Science editor tries the health checks Nasa carries out on its astronauts.
Susan Charlesworth trains astronauts for space and says Nasa's mission will impact millions.
BBC science correspondent Pallab Ghosh explains what happens if an astronaut gets ill in space.
Four astronauts who left the International Space Station (ISS) a month early due to one of the crew members falling ill have returned to Earth on Thursday.
A medical issue prompted the first-ever emergency return from the International Space Station, which was put into Earth's orbit in 1998.
Four crew members are scheduled to depart earlier than scheduled later this week after a medical issue involving an unidentified crew member.
It is the first time in Nasa's over-65 year history that a mission will return early due to a health issue.
Nasa's Jonny Kim and cosmonauts Sergey Ryzhikov and Alexey Zubritsky have touched down in Earth after 245 days on the ISS.
