An American astronaut and Russian cosmonaut will leave Earth this week and move into the International Space Station for an entire year, all in the name of science.

Scott Kelly and Mikhail Kornienko begin their marathon mission with a Soyuz rocket launch from Kazakhstan early Saturday Friday in the U.S. They should arrive at the orbiting outpost six hours later.

It will be NASA's first stab at a one-year spaceflight, a predecessor for Mars expeditions that would last two to three times as long. The Russians are old hands at this, but it's been nearly two decades since a cosmonaut has spent close to a year in orbit.

Five things to know about the duo's extraordinary endeavor:

THE CREW

Both Scott Kelly and Mikhail Kornienko have lived on the space station before. No-nonsense former military men, they were selected as an astronaut and cosmonaut in the 1990s. Kelly, 51, is a retired Navy captain and former space shuttle commander. Kornienko, 54, is a former paratrooper. The pair will blast off with Russian Gennady Padalka, a veteran spaceman who will spend six months at the orbiting lab.

THE MISSION

Kelly and Kornienko 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. They also will oversee the comings and goings of numerous cargo ships, as well as other Russian-launched crews. Soprano superstar Sarah Brightman will stop by as a space tourist in September.

THE SCIENCE

Doctors are eager to learn what happens to Kelly and Kornienko once they surpass the usual six-month stay for space station residents. Bones and muscles weaken in weightlessness, as does the immune system. Body fluids also shift into the head when gravity is absent, and that puts pressure on the brain and the eyes, impairing vision for some astronauts in space. Might these afflictions peter out after six months, hold steady or ramp up? That's what researchers want to find out so they can protect Mars-bound crews in the decades ahead.

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American, Russian Leaving Earth for Year at Space Station