American astronaut Scott Kelly and his roommate, Russian cosmonaut Mikhail Kornienko, are returning to Earth next week after a year aboard the International Space Station – the longest human stay in space since 1999.

Yet if Kelly needs to stay another year in orbit 240 miles above the planet, he would.

"Yeah, I could go another 100 days. I could go another year if I had to. It would just depend on what I was doing and if it made sense, although I do look forward to getting home here next week,” Kelly told reporter in a video news conference from orbit last Thursday.

Kelly said a year of no running water, which made hygiene a tough task, as well as his distance from his loved ones were the main challenges of his space mission.

Kelly will check out Tuesday from what is deemed NASA’s longest single spaceflight, and will then ride a Russian spacecraft going to a Kazakhstan landing.

On the bright side, the 52-year-old former space shuttle commander – who will be met by his girlfriend, two daughters, and his identical twin and former astronaut Mark Kelly in Houston – looks forward to dining at a real table at home and jumping into his pool.

Kelly’s stay in space offers NASA an intimate look at what happens to astronauts’ physical and mental health while staying in such micro-gravity environment. Kelly’s test results will be compared with his twin to serve as research on the extreme effects of weightlessness on the human body based on genetically identical brothers’ data.

The findings will help inform a future human mission to Mars, which is expected to last up to three years round trip and finally happen in about two decades.

“The post-flight data are as important as the inflight data to help us learn how to send humans safely to Mars and return them safely to Earth,” said John Charles, NASA’s Human Research Program chief scientist.

Staying in space is no laughing matter. Kelly, for instance, is bound to undergo rehabilitation in order to recover from the effects of his prolonged stay in zero gravity, such as bone and muscle loss and degraded vision.

Things that are normal on Earth and they lacked on the ISS, including fresh air, will feel different once astronauts from space return to planet life.

“I craved the aroma of leaves and grass and flowers and trees… When you get back to Earth they are literally intoxicating," recalled Doug Wheelock, an astronaut who had his share of 178 days in space from two NASA missions, in an ABC News interview last year.

An upside for Kelly: he will come back to earth a fraction of a second younger than his twin as an effect of time dilation. According to Einstein’s theory of special relativity, time moves more slowly where conditions are of great speed.

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