Astronaut Scott Kelly will return from the International Space Station later today, having spent 340 consecutive days in orbit. He set the new American record for consecutive days spent in space, and broke the U.S. record for total time in space with 520 days over several missions.

Kelly's year-long mission is about twice as long as the average stay on the ISS. It's also about the amount of time it would take for an astronaut to journey to Mars and return. (However, most manned missions to Mars are expected to take between two and three years, since presumably they will stay once they get there.) One of the greatest challenges of sending humans to Mars is the detrimental effects that microgravity and radiation will have on the body over such a long period of time. Our biology is suited to Earth's gravity, and Earth's atmosphere shields us from harmful radiation. To safely send astronauts to Mars and beyond, we will need fresh solutions to these problems, and Kelly's pioneering missions are crucial in helping researchers create them.

Kelly is an ideal candidate for this biomedical study because he has an identical twin, Mark Kelly, a retired astronaut who remained here on Earth, making comparisons between their bodies possible. Scientists will be able to further understand how zero-g changes astronauts over time.

We already know that astronauts lose 1.5 percent of their bone mass per month. Not only that, but their muscles atrophy, and their vision is compromised. The eyesight problem is just one result of how gravity affects the fluids in the human body. While on the ISS, Kelly used Chibis pants, which pull fluid back down toward the feet.

According to NASA, "The Russian Chibis suit is designed to counteract the tendency for fluids to pool in the upper body by applying lower body negative pressure (LBNP). Chibis works like a household vacuum cleaner to suck astronauts into the pants, load the bottoms of their feet, and expand veins and tissues of the lower body." Technology like Chibis pants will become critical to a manned mission to Mars, as bodies in space face so many challenges.

A video, “So You Want to Go to Mars,” provided by PBS in partnership with Time, explains in depth what a voyage to Mars would do the human body. (It will also air on PBS at 8 p.m. EST.) From microbiome activity in space to losing sense of taste to changes in mental health, there's a lot that will happen to humans in space. While the study of the twin astronauts will reveal a lot about what space does to the human body, it’s limited in scope. Isolating the changes in the men’s bodies that occurred from other factors will be a challenge. However, the Kelly twins' contributions to our understanding of the human body in space will provide an invaluable starting point.

Kelly will rocket back to Earth on March 1 along with cosmonaut Mikhail Korniyenko who also spent a year in space. After landing in Kashakstan, Kelly will undergo initial tests. He will return to Houston 24 hours later for further study. Scientists will continue to study the brothers to determine the long-term effects on Scott Kelly's body.

During his time on the ISS, Kelly treated us to fabulous photographs, lessons in space gardening and a unique insight into his experience. A week before his scheduled return, Kelly took questions from Earth and explained that his favorite activity in orbit was going on space walks: "If I had to pick one thing, I would say going outside for first, second and third time would be for me the most memorable." In a recent video tweeted by his brother, Kelly chases crew members a round the ISS while dressed in a gorilla costume – proving that his sense of humor hasn't been compromised during his time in space. Kelly also said that he's not opposed to returning to space after this mission.

NASA will have live coverage of Kelly’s return. Coverage begins at 4:15 p.m. EST and Kelly is expected to land at 11:27 p.m. You can watch it online or in the space below, which is showing general NASA coverage begins until 4:15 p.m.

Welcome home, Commander Kelly!

​Scott Kelly to return from a year in space
The astronaut spent a record amount of time aboard the International Space Station.