Kirill Kudryavtsev / AFP / Getty Images

Two astronauts lift off historic year at International Space Station

Scott Kelly and Mikhail Kornienko blast off on marathon mission, the longest-ever for an American astronaut

An American astronaut and Russian cosmonaut left Earth on Friday en route to the International Space Station for an entire year.

American Scott Kelly and Russian Mikhail Kornienko began their marathon mission with a Soyuz rocket launch from Kazakhstan at 1:42 a.m. Saturday morning, or 3:42 Friday afternoon on the American east coast. They should arrive at the orbiting outpost six hours later.

It is NASA's first stab at a one-year spaceflight, a predecessor for Mars expeditions that could 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.

Both Kelly and Kornienko have lived on the space station before. Both former military men, they were selected as an astronaut and cosmonaut, respectively, in the 1990s. Kelly, 51, is a retired Navy captain and former space shuttle commander. Kornienko, 54, is a former paratrooper. The pair blasted off with Russian Gennady Padalka, who will spend six months at the orbiting lab.

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.

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.

NASA's scientists will also have an unusual opportunity to study how a body in space body compares with its genetic double on the ground. Kelly's identical twin brother, Mark, a retired astronaut, agreed to take part in many of the same medical experiments as his orbiting sibling. They won't follow the same diet or exercise regime, however. Mark, who is married to former congresswoman Gabrielle Giffords, said he has no intentions of consuming bland space-type food or working out and running two hours a day on a treadmill, as his brother will be doing.

NASA and the Russian Space Agency announced Kelly and Kornienko as the one-year crew in late 2012. This will be new territory for NASA, which has never flown anyone longer than seven consecutive months. The Russians hold the world record of 14 months, set by a physician-cosmonaut aboard the former Mir station in 1994-1995. Several other Russians spent between eight and 12 months at Mir. All but one of those long-timers are still alive.

The International Space Station (ISS) is one of the rare areas of U.S.-Russian cooperation that has not been hit by the Ukraine crisis.

"We do our work that we love, and we respect each other," Russian cosmonaut Alexander Samokutyaev said of life aboard the ISS after returning to Earth this month. 

"Whatever the politicians want to get up to, that is their business," he told journalists at a press conference just after landing.   

First launched as an international project back in 1998, the station was heralded as a symbol of the cooperation that emerged from the Cold War rivalry of the space race between the Soviet Union and United states. 

And while the research outpost may technically be divided into Russian and American sections, analysts say that neither country can run the space station on their own. 

"The U.S. and Russia need each other," American expert John Logsdon, a member of NASA's Advisory Council, told AFP. "It is like a marriage where divorce is almost impossible."

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