U.S. astronaut Scott Kelly and Russian Cosmonaut Mikhail Kornienko left on March 28 for one year aboard the International Space Station. Kelly’s identical twin brother, Mark Kelly, was an astronaut, too. He is now retired, but he’ll have a very special role to play on this mission from his home on Earth. Since Scott and Mark are twins, their bodies are more similar than most two people. NASA scientists will be able to study the brothers as they live thousands of miles apart. This could reveal valuable information about how spaceflight affects one’s body. The information could be particularly useful if the United States ever sends a manned mission to Mars.