An eight-day mission has turned into a nine-month space stint for two NASA astronauts who are finally set to come home. But why have they been away for so long - and what are the effects of such a lengthy stay in space?

NASA and SpaceX on Wednesday night scrubbed the Falcon 9 launch of a flight intended to retrieve from the International Space Station astronauts Suni Williams and Butch Wilmore, who have spent nine months in orbit.

Wednesday's launch attempt was called off after concerns over a critical hydraulic system popped up less than four hours before planned liftoff.