On Friday afternoon, a Russian rocket will carry NASA astronaut Scott Kelly to the International Space Station for a very special stay. Kelly isn't just going to have one of the longest jaunts in orbit ever -- and the longest ever (by several months) on ISS -- he's also part of an exciting new part of NASA's scientific research.

NASA's Human Research Program is examining the effects of spaceflight on the human body by studying identical twin astronauts, Scott and Mark Kelly. (NASA)

While Kelly orbits the Earth at nearly 5 miles per second, his twin brother, Mark, will undergo a battery of tests on the ground. By comparing how the 51-year-olds fare, NASA will start to crack the surface of an important question: What happens to bodies in space, and how long can humans stand to travel through the cosmos?

For now, the only reason to send humans into space for longer and longer stretches is to try to answer that question. But one day, space enthusiasts hope, we'll have missions long enough in their own right to demand answers. It would take at least seven months to get to Mars, the closest planet worth visiting — Venus is closer, but it's an impossibly hot planet with an inhospitable atmosphere — and it would take years to arrive on the icy (but perhaps lively) surface of Jupiter's moon Europa for a scientific scuba dive. So if we want to venture out to new cosmic shores, we absolutely must know how well astronauts can take longer space trips, both physically and psychologically.


A study of one set of twins won't even come close to answering this question definitively. But it's a good start. The NASA program will test everything from the twins' gut microbiomes to their cognitive abilities.

Here's Kelly talking to Destin Sandlin of the YouTube series Smarter Every Day:

At five minutes in, you can enjoy a really thorough explanation of how the speeding Soyuz spacecraft manages to meet up with the International Space Station. Hint: The rocket doesn't just shoot straight up to the space station. It also doesn't just spiral higher and higher into orbit until they meet. It's way cooler than that.

But for Kelly and his ISS-extended-stay partner, Russian cosmonaut Mikhail Kornienko, the launching is old hat. It's the time between launching and landing that might seem a bit daunting. In an interview with the Associated Press, Kelly and Kornienko said they'd miss nature the most. Kornienko expects to miss flowing water — stuff he can swim in, not the floating globs he'll deal with in space — and Kelly will miss the outdoors of his Houston home.

“[The weather] never changes on the space station,” he said. “Even though it’s a pretty nice environment, I guess it’s like living in Southern California, people get sick of it . . . after a while.”

Indeed, it seems that everyone is more concerned about psychological effects than anything physical the men might endure.

“Imagine if you went to work where your office was and then you had to stay in that place for a year and not go outside, right? Kind of a challenge,” Mark Kelly told the AP when asked about his twin's departure. And both Scott Kelly and Kornienko have expressed concerns about leaving their families for so long. If something goes amiss on Earth, they won't be able to come home early.

But Kelly and Kornienko at least have a rotating cast of companions to look forward to: Several normal-length ISS missions will start and end during their tenure on board, so fresh faces will never be far off.