What's it like to spend 6 months in space? Scott Kelly just misses home.

NASA astronaut Scott Kelly has spent 173 days in space, halfway through his nearly year-long mission aboard the International Space Station.

Astronauts Scott Kelly (l.) and Kjell Lindgren, aboard the International Space Station, speak to actors from the movie 'The Martian,' on Tuesday. On Thursday, Kelly and Russian cosmonaut Mikhail Kornienko clocked in for their 173rd day aboard the International Space Station since arriving on March 27. The two are scheduled to spend 342 days in space.

Astronaut Scott Kelly reached the halfway point in his nearly year-long mission aboard the International Space Station, orbiting 250 miles above Earth.

Since blasting off 173 days ago, the retired US Navy captain has travelled 72 million miles around Earth with Russian cosmonaut Mikhail Kornienko, who is sharing this voyage.

"It's a big milestone," Mr. Kelly told the AP, "but I'm not going to start counting days down yet. I think I'm going to wait until March, then I'm going to allow myself to count how many days I have remaining. I'm counting up for the time being."

As part of his objective in allowing scientists to better understand the effects of prolonged space travel, Kelly’s experiments also involve comparisons with his twin Mark, a former astronaut, who remains on Earth. Because they’re identical twins, researchers can monitor the changes to Scott’s body using Mark as the control.

“As Scott reaches the six-month mark of his mission, we are headed into unknown but exciting territory – because we have never had an American in space for such a long period of time,” Mark writes in an essay for NBC. “Scott's mission is effectively doubling the amount of data we have.”

The brothers keep near-constant correspondence. They email several times a day and talk on the phone every other day. Still, they’re looking forward to Scott’s homecoming. He recently told two actors from the upcoming movie, "The Martian,” that if he could do or have anything, it’d be the ability to come home.

"Something people don't recognize is that being on the space station is probably a lot like being in some kind of confinement — like isolation," he says. "Not having the ability to leave is ... an all-present feeling. Although I'm not claustrophobic at all, it would be nice to get outside. And I'm not talking about outside like on a spacewalk, but 'put my feet in the grass and feel the wind on my face' kind of outside."

This report contains material from the Associated Press.

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