NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko returned to earth on Wednesday after a 340-day mission aboard the International Space Station. The tour marked the longest trip by a NASA astronaut and was considered twice the length of a normal stay. Their extended trip was part of an effort to examine the effects of long-duration spaceflight on the body. Scientists will now compare Kelly with his earthbound identical twin, Mark, who is now a retired astronaut. A Soyuz capsule carrying Kelly, Kornienko, and Russian crew member Sergey Volkov parachuted down on to the steppe of Kazakhstan at 11:26 p.m. ET. Kelly, who now has completed a total of 520 days in space over four flights, will be flown to NASA headquarters in Houston to begin a series of tests.