Next stop Mars? Maybe. Two astronauts who have spent a year on the International Space Station (ISS) have returned to Earth, and it’s hoped that the research gained from this mission could be invaluable for future missions to the Red Planet.

NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko landed in Kazakhstan at 11:26 p.m. EST last night (4:26 a.m. this morning) following a 340-day stay aboard the ISS. This is the longest single mission ever to the space station. Russian cosmonaut Sergey Volkov also returned, having spent a shorter six months on the station, which is the usual stay for crews on the ISS.

“Scott Kelly’s one-year mission aboard the International Space Station has helped to advance deep space exploration and America’s Journey to Mars,” NASA Administrator Charles Bolden said in a statement after the landing. “Scott has become the first American astronaut to spend a year in space, and in so doing, helped us take one giant leap toward putting boots on Mars.”

^{The Soyuz capsule landed exactly on schedule. NASA/Bill Ingalls}

During their time on the station, Kelly and Kornienko (pictured below) took part in more than 400 experiments, many of which were designed to see how astronauts would cope with the longer stays required with missions to Mars. These included investigations into how the human body copes with weightlessness, isolation, and the additional radiation from spaceflight.

To aid in this research, Scott Kelly’s identical twin brother Mark, a former astronaut, remained on Earth and took part in parallel twin studies to his brother. Scientists hope to examine how the mental and physical attributes of the two twins have differed. Tests will continue for months, or even years, especially as bone loss can take three years to recover.

NASA currently has a tentative goal of sending humans on return trips to Mars in the 2030s. But there are a huge number of unknowns regarding such missions, considering the astronauts will be off Earth for up to three years – which includes launch, transit time to Mars, exploration activities on the surface, and the return to Earth.

For example, astronauts on prolonged stays in weightlessness in space experience significant muscle and bone loss. On their return to Earth, they are barely able to walk. Following a transit of eight months or so to Mars, it’s not going to be ideal if the crew is unable to stand on the Red Planet, which has about two-thirds of Earth's gravity, so NASA wants to develop techniques to cope with this limitation. Kelly, though, joked while in orbit: “I could go another year if I had to.”

^{Image in text: Kelly was joined on his one-year mission by Mikhail Kornienko, pictured. NASA/Bill Ingalls}

^{NASA hopes research from the mission could aid future missions to Mars. NASA}

Kelly will perform a few tasks, such as running through an obstacle course and opening hatches, to see how he has coped with his stay in microgravity. It’s expected to take him 45 days to return to full fitness.

During their time on the station, Kelly and Kornienko saw six resupply spacecraft arrive, performed a few spacewalks between them, and even grew lettuce on the ISS for the first time. Kelly also became widely known for his fantastic imagery of Earth, which you can check out on his Twitter account.

Kelly and Kornienko were joined by eight other crew members at various times on their one-year mission. But their 340-day mission, although impressive, only comes in at fourth on the list of longest single spaceflight missions. In first is Russian cosmonaut Valeri Polyakov, who spent 437 days aboard the Mir space station from January 1994 to March 1995.

^{Scott Kelly tweeted the final farewell before leaving the ISS}

 

My last #sunrise from space! 5 of 5. #YearInSpace pic.twitter.com/p9ZOvS4HcN

— Scott Kelly (@StationCDRKelly) March 1, 2016

 

^{Images via Flickr (CC BY-NC-ND 2.0)}