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?

Reaching orbit from NASA’s Kennedy Space Center, the newest crew includes NASA’s Anne McClain and Nichole Ayers, both military pilots; and Japan’s Takuya Onishi and Russia’s Kirill Peskov, both former airline pilots. They will spend the next six months at the space station, considered the normal stint, after springing Wilmore and Williams free.

SpaceX launch scheduled for Wednesday will carry replacement crew for astronauts who have been stuck on the International Space Station since their troubled Starliner mission.