In less than a week, Scott J. Kelly will once again feel weight.
On Tuesday, he is to turn over command of the International Space Station to his fellow NASA astronaut Timothy L. Kopra and climb into a Russian Soyuz capsule. A few hours later, he will land in Kazakhstan, ending 340 consecutive days in space — a record for a NASA astronaut.
The previous record was 215 days, reached by Michael Lopez-Alegria during a trip to the space station in 2006 and 2007. Counting his three previous trips to space, Kelly will have spent a total of 520 days in orbit.
"We'll learn a lot about longer-duration spaceflight," Kelly said during a news conference Thursday broadcast on NASA Television. "I'd like to think this is another of many steppingstones to landing on Mars sometime in our future."
Some of the memorable sights from space included auroras and Hurricane Patricia in October, Kelly said.
"The more I look at Earth, at certain parts of Earth, the more I feel more of an environmentalist," Kelly said. "There are definitely areas where the Earth is covered with pollution almost all the time, and it's not good for any of us."
Until now, astronauts have spent six months, give or take, on the International Space Station. By studying Kelly and Mikhail Kornienko, a Russian astronaut who is also part of this nearly one-year mission, scientists hope to gain a better understanding of what health issues astronauts might encounter during the six to eight months a trip to Mars would take.
In the absence of gravity, bones become more brittle, fluids in the body move upward, and vision sometimes shifts toward farsightedness.
Scientists are also interested in how quickly the astronauts readjust to gravity. After landing, Kelly will have a series of tests on his balance, including navigating an obstacle course.
Kelly said he was particularly intrigued by research on genetic changes that occur in space, which scientists will try to glean from comparisons with his twin brother, Mark E. Kelly, a retired NASA astronaut.
But with just two test subjects who spent 340 days in orbit, there will not be definitive answers.
"We simply don't have that many people flying in space," Charles W. Lloyd, a NASA manager for communications about the agency's human research program, said in an interview. "But we still need to understand how space potentially affects you."
Kelly is far short of the overall record for longest human space mission: nearly 438 days, by Valeri Polyakov on the old Russian Mir space station in 1994 and 1995.
"Yeah, I could go another 100 days," Kelly said when asked to reflect about Polyakov's record and whether he would be up for a longer stay. "I could go another year if I had to."
Still, he conceded that the space station never fully became like home. "Even after I've been here nearly a year, you don't feel perfectly normal," he said. "There's always a lingering something you feel. It's not necessarily uncomfortable, but it is a harsh environment. For instance, having no running water. It's kind of like I've been in the woods camping for a year."
Once NASA is done performing tests on him, Kelly said he was looking forward to jumping in his pool.
He said the hardest part of the mission was being physically cut off from friends and family. He said that for a Mars mission, in which crew members would be housed in a space much smaller than the space station, the privacy of their personal space would be important.
"Making that private area as perfect as possible, I think, will go a long way towards reducing fatigue, reducing stress and helping for a successful mission," Kelly said.