Welcome Home Astronaut Kelly and Cosmonaut Kornienko!

Cosmonauts Mikhail Kornienko, left, and Gennady Padalka of the Russian Federal Space Agency (Roscosmos), and NASA Astronaut Scott Kelly, right, pose for a photograph on the stairs leading into the Soyuz TMA-16M spacecraft during the final check of the … NASA/Bill Ingalls

On March 27, 2015, U.S. astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko left Earth to join the crew at the International Space Station. Their mission was a bit different from most of the scientific studies aboard the ISS — they were to remain on board the station almost a year (340 days), with their return trip home to take place today, March 1, 2016, courtesy of the Soyuz spacecraft.

Humans have been studying the effects of microgravity for decades, but only a few missions have subjected people to such a long exposure. And even though Kelly and Kornienko will have spent nearly a year up in orbit, they don’t come close to matching cosmonaut Valeri Polyakov’s 437 days in space aboard Mir. But this does mark the longest anyone has called the ISS home in one go.

The mission is important. If we want to send people to distant locations like Mars, we need to have a thorough understanding of what microgravity does to the human body and how we can best take care of the men and women who elect to take those first steps on the red planet and beyond.

This mission also created another amazing opportunity for NASA. Normally, the space agency would have to rely only upon the data gathered by studying the returning astronauts. But with Scott Kelly, that’s not quite the case. That’s because Kelly is a twin — his brother, Mark, remained on Earth while Scott soared above. Oh, and Mark is also a former astronaut. You have to imagine their family reunions get a little competitive.

The Kelly brothers pose for a photo on March 26, 2015, a day before Mark (right) would leave for a 340-day stint on the ISS.

Bill Ingalls/NASA/Getty Images

NASA took advantage of the opportunity by using the Kelly brothers to study everything from responses to spaceflight to developing strategies for individualized medicine. These studies have the potential to help not just the folks who board spacecraft but also everyone here on Earth. Imagine a future in which doctors can really treat patients on the individual level, prescribing personalized treatments that will help heal patients faster and more effectively than ever before.

A year in space is an extraordinary achievement, but it’s still just a step toward our goal of getting to Mars. A trip to Mars and back would last about 30 months (assuming nine months for each leg and about a year in orbit). It’s not just that Mars is far away — it’s that explorers will have to wait for the orbits of Earth and Mars to line up to make best use of fuel and resources. To them, a year in space might seem like a brief layover.

Welcome home, Kelly and Kornienko! And a happy belated birthday to the Kelly twins (their birthday was on Feb. 21).