NASA will be studying how Scott Kelly's body reacted to almost a year in space, using his twin brother Mark Kelly, who stayed on Earth, as a control. One big difference to note right away: The brothers will no longer be the same height, as they were before Scott went into space, because he grew 2 inches while aboard the International Space Station, CNN reports. "Astronauts get taller in space as the spine elongates," NASA's Jeff Williams explains.
That's because "without the full strength of gravity pressing down on gel-filled discs between the vertebrae, they expand and lengthen the spine," the Washington Post explains. But Scott won't be able to lord it over Mark for too long: Astronauts "return to preflight height after a short time back on Earth," Williams says. (Read more Scott Kelly stories.)