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When Scott J. Kelly goes up to the International Space Station — he and two Russian astronauts are scheduled to launch on Friday — he will not come back down again for a year.

That year is to be the longest space mission a NASA astronaut has ever undertaken. This trip — Mr. Kelly’s fourth to space — will also push him to the top in cumulative time in space among NASA’s astronauts. When he lands in March 2016, health he will have spent more than 500 days of his life floating in orbit, viagra sale including a 159-day trip to the space station that ended in 2011.

With his trip this time twice as long, pills “my expectation is that it’ll be more challenging and, as a result, more rewarding,” Mr. Kelly, 51, said in a recent interview.

Like all travelers to the space station since the retirement of NASA’s space shuttles in 2011, Mr. Kelly and the two Russians, Mikhail Kornienko and Gennady Padalka, will be launching on a Russian Soyuz rocket from the Baikonur Cosmodrome in Kazakhstan. Liftoff is scheduled for 3:42 p.m. Friday Eastern time, which is 1:42 a.m. Saturday in Baikonur. The Soyuz capsule will dock at the space station about six hours later.

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Until now, stays at the International Space Station have lasted about six months. In the 1980s and 1990s, Russian astronauts made longer trips to the Mir space station.

NASA scientists hope to learn more about the physical and psychological toll of long space missions, including the effects of factors like weightlessness and radiation on bones, the circulatory system and muscles.

“All of those things really affect the bodies of astronauts,” Julie A. Robinson, NASA’s chief scientist for the space station, said during a news conference in January. “They push them to something not at all unlike aging on Earth, where their balance is disrupted, their hearts are weaker, their immune system isn’t functioning as well, their muscles are weaker and their bones are being lost.”

That is part of the groundwork NASA wants to complete before it would be ready, in a couple of decades, to send astronauts to Mars.

“We’re doing this so that we can mitigate those effects, so we can eventually go beyond low-Earth orbit one day and explore further than we’ve gone before,” Mr. Kelly said.

NASA scientists will see whether Mr. Kelly’s experience differs from his earlier trip, especially during the second six months. They will also conduct experiments comparing his health to that of his twin brother, Mark Kelly, a retired NASA astronaut, on Earth.

“Mark will participate as a sort of a ground control to really help us understand this nature-versus-nurture question,” Dr. Robinson said.

Mr. Kornienko will also remain in the space station for a year, but he will not set any Russian space records by doing so. In 1995, Valery Polyakov set the world’s record for longest single stay in space at nearly 438 days when he returned from Mir.

However, Mr. Padalka, who will be spending the typical six months at the space station, will set a record, for the most total time by any astronaut in space, about 878 days. A retired Russian astronaut, Sergei Krikalev, is the current record-holder, with 803 days.

Mr. Kelly said that during his year off the planet he thought he would miss the same things he missed last time. “The human connection with your friends and family,” Mr. Kelly said. “Getting away from work. Going outside.”

He described the space station as a “magical place,” but added, “You never get to leave.”