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Two Russians and an American floated into the International Space Station on Saturday, beginning what is to be a year away from Earth for two of them.

Mikhail Kornienko and Scott Kelly are to spend 342 days aboard the orbiting laboratory, about twice as long as a standard mission on the station. Russia's Gennady Padalka is beginning a six-month stay.

The three astronauts entered the station about eight hours after launching from Russia's manned space facility in Kazakhstan. They were embraced by American Terry Virts and Russia's Anton Shkaplerov who along with Italian Samantha Cristoforetti have been aboard since late November.

Kirill Kudryavtsev/AP Photo

PHOTO: U.S. astronaut Scott Kelly, left, and Russian cosmonaut Gennady Padalka wave prior to the launch of Soyuz-FG rocket at the Russian-leased Baikonur cosmodrome, Kazakhstan, March 27, 2015.

The trip is NASA's first attempt at a one-year spaceflight; four Russians have spent a year or more in space, all on the Soviet-built Mir space station.

The stay is aimed at measuring the effects of a prolonged period of weightlessness on the human body, a step toward possible missions to Mars or beyond.

Kelly's identical twin Mark, a retired astronaut, agreed to take part in many of the same medical experiments as his orbiting sibling to help scientists see how a body in space compares with its genetic double on Earth. They are 51.

Kelly and Kornienko, 54, will remain on board until next March. During that time, they will undergo extensive medical experiments, and prepare the station for the anticipated 2017 arrival of new U.S. commercial crew capsules. That means a series of spacewalks for Kelly, which will be his first.

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Astronauts Board Space Station for 1-Year Mission