Scott and Mark Kelly are both NASA astronauts and are participating in a yearlong science experiment in which one will reside approximately 230 miles in the atmosphere.

According to the Associated Press, Scott will attempt an American record one-year stay on the International Space Station (ISS) while Mark will remain on Earth. The experiment will determine the effect a year of weightlessness has on a person compared to his identical twin on the ground.

While on the ground, Mark will undergo medical testing alongside his brother. Mark has also agreed to keep up with Scott's hour-and-a-half to two hours of physical exercise per day, but will not eat "crappy space station food."

Mikhail Kornienko, a 54-year-old former paratrooper turned Russian cosmonaut, will also be spending a year aboard the ISS alongside Scott Kelly. Unlike NASA astronauts, Russians have accomplished long stays on the ISS and one completed a 14-and-a-half month tour from 1994 to 1995.

From 2010 to 2011, Scott spent five months aboard the ISS but that tour is not even half as long as his upcoming trip.

The astronaut told the AP he has heard reactions ranging from "'Oh, that would be really cool to be in space for a year' to 'What, are you out of your mind?'"

"No second thoughts - I'm actually getting kind of excited about the whole idea as we get closer," he previously told the AP.

The ISS normally carries six people, but an emergency doctor will be accompanying Scott, a retired Navy captain, since no NASA astronaut has attempted a stay that long.

"Not only are they the same genetically, but one is an astronaut, one's a retired astronaut. So they've followed very similar career paths. After Scott's mission is done, he'll have 540 days of spaceflight (in four missions). Mark will have 54. So exactly a 10-fold difference," Craig Kundrot, deputy chief scientist for the human research program at NASA's Johnson Space Center in Houston, told the AP. "That's just an uncanny opportunity that we're taking advantage of."