US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko have set off on an enduring 12-month tour of duty on the International Space Station.
The orbiting platform is 400 km high. This will be the longest stay anyone has ever had up there.
Throughout this time, Scientists hope to get a better idea of the effects on the human body of living with no gravity for long periods of time.
The information collected will help scientists discover whether a trip to Mars is a realistic idea.
Mr Kelly and Mr Kornienko launched from Baikonur in Kazakhstan in a Soyuz vehicle at 1.42am Saturday. They should arrive at the space station six hours after lift-off.
Would you want to live in space for a whole year?