NASA astronaut Scott Kelly is back home. A historic 340 day-trip made Kelly the all-time American record holder for most cumulative days in space at 520 days. That's a total of four space missions.
"When you launch in March and you think about coming back the next March, it is not something you can really comprehend," Kelly said.
While the nearly in space went quickly for Scott's twin brother Mark, and the rest of us, it felt a lot slower for Kelly in orbit.
"When you get to two and a half to three months you think: 'I've really been here for a long time' and know that you have nine months to go, that's kind of hard to get your head around," Kelly said.
There were 450 investigations on Kelly's mission and only 18 were human research program missions, said John Charles, the human research program associate manager for international science.
One of those human missions was one that involved former astronaut Mark Kelly and determining the similarities and differences between one person in space and a person with the same DNA on Earth for 340 days. Because of medical privacy law, it was up to the Kellys to come to NASA to volunteer for the twin study.
"Looking at your genes and understanding how they're going to react to whatever experience or whatever medical problem you have this is an important advance for NASA that they'd made possible by coming to us,"said Julie Robinson, International Space Station program chief scientist.
"By looking at Mark's results collected over the course of the year we can see what the normal variations might be and then by looking at Scott's over the course of the year, we can see where his variations are greater than Mark's have been," Charles said.
"I've made four trips or so here to the Johnson Space Center where I spent some time in a laboratory hooked up to all kinds of devices," Mark Kelly said. "It might be an ultrasound machine or a lower body negative pressure garment, giving a lot of blood and urine and other samples."
Mark said he had done more tests on his body while his brother was in space for a year, than he did any of the four times he'd been in space himself.
All of these tests NASA scientists and astronauts did this past year will give them a better idea of how they can eventually send people to Mars.
Scott Kelly landed on Earth at the beginning of this week and he said it was most difficult to adjust to gravity again coming back home versus zero gravity in space.
"I tried to shoot some basketballs yesterday and I didn't get any of them in the net, not that I'm a good basketball player anyway," Kelly said jokingly.
NASA said it plans to collect data from Kelly's mission for another nine months before it analyzes it and releases it to the public.
As for Kelly, he plans to undergo a year of medical testing.
On March 18, NASA astronaut Jeff Williams will launch into space and attempt to break Kelly's record for most cumulative days in space for an American.
"Records are made to be broken," Kelly said.
Williams has already racked up 362 days in space on three separate flights. Later this month he plans to shoot for six months in orbit and break Kelly's record on Aug. 24.
Right now, Kelly ranks 17th on the all-time list for most cumulative days in space.