Identical twin blasts off into space
28 March 2015 Last updated at 11:46 GMT
An identical twin has arrived at the International Space Station where he'll stay for a year to find out how space affects his body compared to his brother back on earth.
US astronaut Scott Kelly is joined by Russian cosmonaut Mikhail Kornienko on what will be the longest stop anyone has had aboard the ISS.
Scott's brother, Mark, will stay on earth and the pair will both be monitored regularly for changes to their bodies.
Scientists reckon living 250 miles from our planet without gravity will have lots of different effects on Scott.