At 3:42 p.m. on Friday, March 27, 2015, NASA astronaut Scott Kelly embarked on a journey no NASA astronaut has ever before undertaken. Along with Russian cosmonaut Mikhail Kornienko, Kelly headed to the International Space Station to start a — the longest continuous amount of time any American has ever spent in space.Gennady Padalka, also of the Russian Federal Space Agency, launched with Kelly and Kornienko, but will not stay aboard the ISS for a full year.

The two travelers will be “testing the limits of human research, space exploration and the human spirit,” as NASA put it.

Typically, astronauts spend anywhere from four to six months on the ISS, according to NASA. But to take the next leap in space travel — sending people to Mars — it’s crucial to better understand how the human body reacts to prolonged time off Earth. A journey to and from the Red Planet could span 500 or more days. That’s 16 months.

Kelly also has something else distinctive going for him: His identical twin brother, Mike Kelly, a retired NASA astronaut (and spouse of former Arizona Congresswoman Gabby Giffords). “We realized this is a unique opportunity to perform a class of novel studies because we had one twin flying aboard the International Space Station and one twin on the ground,” Craig Kundrot, of NASA’s Human Research Program, said in a release. “We can study two individuals who have the same genetics, but are in different environments for one year.”

Some of what NASA will look at in its includes how extended time in space affects physical factors like heart function and muscle growth and mental factors such as decision-making and reasoning. NASA also said it plans to look at changes to gut function in Scott (in space) as compared to Mike (on the ground).

Just a few hours pre-launch, Kelly was getting his final taste of the comforts of home:“Just awoke from pre-launch nap. Last time in bed for a year,” he tweeted. He also posted this photo, his last walk for awhile, something so simple yet impossible in space.

Kelly leaves Earth younger than his twin, but as Quartz points out, when he returns. In space he’ll age both faster and slower: “Scott will become 25 microseconds a day younger than his already six-minute-older brother Mark. Over 342 days, that’s 8.6 milliseconds — a little less than a hundredth of a second. Even added to the extra milliseconds he’s already gained — he’s spent 180 days in space before, to Mark’s 54 — it’s not enough, probably, to compensate for all the biological effects of living up there.”

At this point, only time will tell, 365 days to be exact.

The Soyuz TMA-16M spacecraft on launch pad at the Baikonur Cosmodrome, Kazakhstan, Wednesday, Mar. 25, 2015. NASA Astronaut Scott Kelly, and Russian Cosmonauts Mikhail Kornienko and Gennady Padalka are scheduled to launch to the International Space Station in the spacecraft on Mar. 27, 2015. (NASA/Bill Ingalls)