It's been a long year in space for astronaut Scott Kelly, but for those who are studying the effects of space on his body, the work is only beginning.

Kelly will plunge back to Earth on Tuesday as his stint aboard the International Space Station comes to an end, setting a new American spaceflight record , according to Gizmodo. The entire time he's been in space, NASA has also been collecting data from someone else– Kelly's twin brother, Mark, a retired astronaut.

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In this image from video made available by NASA, astronaut Scott Kelly speaks to reporters on Earth during a news conference held on the International Space Station on Thursday, Feb. 25, 2016.

(NASA via AP)

Through studying blood, urine, saliva and fecal samples from the two brothers, NASA hopes to get a better idea of what the effects of space , Wired said. Some of the other impacts of spaceflight– loss in bone density, muscles atrophying, shrinking heart due to less gravity– are already well-known, but there is so much to learn before we send humans on a mission to, say, Mars.

There are other important factors we've yet to completely grasp, and even Kelly's mission may not give us the final answers. For example, we know that Earth's atmosphere shields ISS astronauts from much of space's radiation, but it's not fully known how extreme the effects would be on a human body that's in deeper space for months at a time, Wired also said.

In the next few weeks, Kelly will once again acclimate to the gravity of Earth after , and he'll deal with the same challenges as so many returning astronauts before him. Men and women who spend prolonged amounts of time in space often suffer from vision problems due to the pressure put on the back of the eye by extra blood and other fluids that flow to the head , according to STAT News.

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He's also likely to encounter memory problems, weakened muscles and even issues with blood circulation, the report added. For these and other reasons, Kelly will be closely monitored for years after he returns.

Despite all of those occupational hazards and a desire to see loved ones on Earth, Kelly said he would have stayed in space even longer, if necessary.

“Yeah, I could go another 100 days. ,” he said during a recent video news conference. “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.”

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