Here's what we'll learn from Scott Kelly's year in space

A year in space

At around 4:40pm ET today, American astronaut and devoted Instagrammer Scott Kelly will hop in a Soyuz capsule and, several hours later, plummet through Earth's atmosphere until he and Russian cosmonaut Mikhail Kornienko land in the barrens of Kazakhstan (Update: They landed safely at 11:26pm ET).

It'll be their first time on Earth in 340 days. That's not the longest time anyone's ever spent in space, but it's a little less than double the time most astronauts spend on shifts to the International Space Station (ISS).

More to the point, both Kelly and Kornienko have traveled around 143,846,525 miles while hanging around in the ISS, which is decently close to the distance astronauts will end up traveling on a trip to Mars. With Kelly's return, earthbound scientists can start the long examination of how well he's held up, which will ultimately prove if we're truly, physically ready to go to the Red Planet.

Here's why Kelly's examiners will be so happy to see him on the ground.

What we can learn from Scott's identical twin

Not only is Scott Kelly a model astronaut, he's also blessed with his twin brother Mark Kelly, himself (incredibly enough) a retired astronaut. Mark, who's been on Earth this whole time, thus serves as a perfect control to gauge how space has affected Scott's body.

Both of the Kelly brothers have been taking blood, urine and fecal samples so researchers can discover any significant changes to Scott's blood cells, fluid distribution and immune system.

The comparison won't be perfect: after all, Mark hasn't spent all of his time eating astronaut food like Scott has. But considering the brothers essentially share the same DNA, the circumstances for accurate data are virtually miraculous.

The long-term effects of weightlessness

Mark Kelly will be particularly useful for painting a more or less accurate picture of how weightlessness affects the body after so long.

Some of the dangers are fairly intuitive, such as the way Scott Kelly's muscles and bones are expected to have weakened significantly despite his daily exercise sessions on the station (which generally used bungee cords to mimic gravity).

Yet plenty of other side effects aren't immediately obvious. Kelly's vision, for example, has started to deteriorate a bit on account of the extra pressure inside his skull after prolonged time spent in microgravity (body fluid moves upward - facial puffiness is another side effect). The heart shrinks, too, as it no longer has to transport blood with the same force it has to on the ground.

Astronauts who've spent a long time in space also have a troubling tendency to develop kidney stones, and it's thought the reason stems from the fact that there's 10 to 20 times more carbon dioxide on the ISS than we normally find here on earth.

Can we walk on Mars right away?

Kelly will have little time to savor the sight of things like grass and wild animals when he gets off the craft tonight, as researchers will be closely watching how he handles himself in a Field Test and a Functional Task Test.

Among other things, he'll be expected to try to walk in a straight line, heel-to-toe, and then try to complete some form of small obstacle course. In addition, he'll have to try to sit up from a lying position and stand still for three minutes.

These are easy feats for most of us here on the ground, and they'll need to be easy for anyone who plans on leaving that first footprint on Mars soon after arriving.

All of the data gathered on Kelly and how he performs in these tests will be used to see whether so-called countermeasures - like in-flight exercises and special body-fluid suits - are effective or need to be updated in order to help humans better cope with the effects of space travel and can land on a planet with some degree of functionality intact. These countermeasures will be useful when we eventually send humans to Mars.

The effects of prolonged exposure to radiation

Here's one of the darker ones. There's nothing like an ozone layer up in space to deflect the brunt of the sun's radiation, and, indeed, you're bombarded with it every time you fly in a plane.

Kelly and Kornienko have been up where the sun's radiation pummels them (even inside the space station) every second of the day, to the point that NASA says one of us groundlings would have to fly from Los Angeles to New York 5,250 times to get the same amount of radiation exposure. That puts Kelly at high risk for cancer, cardiovascular disease, and immunity complications.

Yet even up there you'll find some scant protection from the Earth's own magnetic field. In the vast distances between Mars and Earth, though, there's practically nothing. But with Kelly's data from living through what basically amounts to the same time needed for a trip to Mars, researchers will have a much better starting point than they had previously.

The psychological toll of long-term space flight

Even though NASA is actively funding related research, we're still not anywhere near the point where we can put astronauts in suspended animation for long trips to places like Mars. That's still largely the domain of science fiction. This means the astronauts who go to Mars will have to put up with each other, and only with each other, during their long trip.

For 340 days now, Kelly and Kornienko have only seen one another and the other astronauts who come and go on the station. Kelly himself has held up well, saying in a recent CNN interview that he "could go for another 100 days or 100 years." But the comparative isolation has certainly affected him.

"The hardest part is being isolated from people on the ground who are important to you," he said.

But Kelly at least had the shuffle of ISS visitors and his daily Instagram interactions to keep him company, and he could always look out the window and see us living our lives below him. The astronauts who go to Mars won't have that luxury. They'll be stuck in a smaller craft alone for the better part of three years, with e-mail likely being their only communication with the blue marble. Will that break them?