Just after 4:00 pm Eastern, astronaut Scott Kelly will head back to Earth after a staggering 340 days aboard the International Space Station — the longest continuous space mission by an American and twice as long as astronauts typically stay in orbit at one time.
After Kelly touches down, he and his identical twin brother Mark are going to be able to provide NASA with a ton of information that will help shape the future of human space exploration, including missions to far-flung destinations like Mars.
Mark Kelly also flew on four missions during the space shuttle program, for a total of 54 days in space. By comparing the changes in his body to that of Scott's, who was in space six times longer, NASA can get a better understanding of what astronauts would face on a 500+ day mission to Mars.
Because humans evolved under the constant pull of Earth's gravity, experiencing weightlessness for an extended period of time has some significant effects on the body, including decreased bone density, muscle atrophy, impaired blood flow, and alterations to the shape of the heart.
This information from the Kellys, combined with ongoing efforts to understand changes in physiology through experiments such as burning astronaut pee, will guide the development of medication, diets, and specialized suits that counter the effects of long-term weightlessness.
There are also a number of technological challenges that need to be addressed before humans can go on longer missions, including the need for faster communications and radiation shields.
Despite the amount of work that lies ahead, people are still pretty excited at the opportunity of being the first humans to go to Mars. NASA's astronaut program continues to be flooded with applications and private companies like Mars One has a long line of people who can't wait to boldly go where no one has gone before.