In this photo provided by NASA, astronaut Scott Kelly sits inside a Soyuz simulator at the Gagarin Cosmonaut Training Center (GCTC), Wednesday, March 4, 2015 in Star City, Russia. On Saturday, March 28, 2015, Kelly and cosmonaut Mikhail Kornienko will travel to the International Space Station to begin a year-long mission living in orbit. (AP Photo/NASA, Bill Ingalls)

An American astronaut and a Russian cosmonaut are moving into the International Space Station this week and staying for an entire year.

After more than two years of training, Scott Kelly and Mikhail Kornienko are eager to get going. It will be the longest space mission ever for NASA, and the longest in almost two decades for the Russian Space Agency, which holds the record at 14 months.

Their Soyuz rocket is scheduled to blast off from Kazakhstan on Friday afternoon in the U.S. (early Saturday morning in Kazakhstan.)

The world's space agencies want to know how the body adapts to an entire year of weightlessness before committing to even longer Mars expeditions. The typical stint at the space station is six months.

  • In this Sept. 5, 2014 photo provided by NASA, cosmonaut Mikhail Kornienko, left, of the Russian Federal Space Agency (Roscosmos), and astronaut Scott Kelly stand together for a picture at the Gagarin Cosmonaut Training Center in Star City, Russia. In late March 2015, Kelly and Kornienko are scheduled to travel to the International Space Station to begin a year-long mission living in orbit. (AP Photo/NASA, Stephanie Stoll)

  • In this Thursday, March 19, 2015 photo provided by NASA, astronaut Scott Kelly, left, plays pool with Russian cosmonaut Mikhail Kornienko, of the Russian Federal Space Agency (Roscosmos), at the Cosmonaut Hotel in Baikonur, Kazakhstan. On Saturday, March 28, 2015, Kelly and Kornienko will travel to the International Space Station to begin a year-long mission living in orbit. (AP Photo/NASA, Bill Ingalls)

  • This August 2010 photo provided by the Gagarin Cosmonaut Training Center shows NASA astronaut Scott Kelly in a Russian Sokol launch and entry suit in Star City, Russia. On Saturday, March 28, 2015, Kelly and cosmonaut Mikhail Kornienko will travel to the International Space Station to begin a year-long mission living in orbit. (AP Photo/GCTC via NASA)

  • This Feb. 6, 2015 photo provided by the Russian Federal Space Agency (Roscosmos) and the Gagarin Cosmonaut Training Center (GCTC) shows cosmonaut Mikhail Kornienko in Star City, Russia. On Saturday, March 28, 2015, Kornienko and astronaut Scott Kelly will travel to the International Space Station to begin a year-long mission living in orbit. (AP Photo/Roscosmos and GCTC)

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