Kelly and Kornienko performed several experiments while onboard the ISS to find out how the human body is able to react and adapt to the environment in space.

The impressive jaunt is a record for the United States, and for the International Space Station.

In all, Kelly spent 340 days in space - the longest for any American astronaut.

But this is the first attempt to really study the effects of such a long stay in space.

NASA aims to put astronauts on the red planet in the 2030s, but first wants to know how the body - and mind - will fare during the 2½-year expedition.

"It's great to be back", Kelly said, after hugging his family and friends.

Kelly acknowledged it was bittersweet leaving the space station - his home since last March, now staffed by three men until the arrival of three more in two weeks. Kelly returned to Houston just after midnight, but it wasn't fast enough. Kelly and his identical brother, Mark Kelly - who is a retired fellow astronaut and husband to former U.S. Rep. Gabrielle Giffords - will be examined by NASA doctors.

Specifically, researchers are looking at the effects of space radiation, as well as the effect of visual impairment associated with long-duration space flight, according to Graham Scott, chief scientist at the National Space Biomedical Research Institute and deputy project scientist for the NASA and NSBRI Twin Study.

Mark Kelly said his brother was anxious to get home and joked that he still hadn't showered since returning. The extra height will go away in time, but other effects - less pleasant ones, such as brittle bones, vision problems and muscle weakness - may prove harder to shake.

"I'm used to going 17,500 miles per hour, but this airplane doesn't do quite that", Kelly joked at a brief welcoming ceremony attended by Jill Biden, wife of Vice President Joe Biden.

"I brought you some beer and apple pie - nothing's more American than that", Mrs Biden said.

When Kelly returned he addressed the crowd saying, "It's great to be back in Texas in the US, on USA soil".

The 52-year-old is due to leave the International Space Station after 340 days together with his colleague Mikhail Kornienko and catch a ride back to Earth on a Russian Soyuz spacecraft.

And although Kelly has become something of a celebrity for his record-breaking flight, he remains humble.

"This mission is the latest achievement in our country's space program", Kelly added.

This was America's mission, Scott Kelly said.

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