An American astronaut on a historic mission radioed back to nasa late tonight from the international space station after leaving Earth just hours ago. But could you imagine spending a year with... See More
An American astronaut on a historic mission radioed back to nasa late tonight from the international space station after leaving Earth just hours ago. But could you imagine spending a year with limited human contact in very tight quarters, all while floating in space? Hires ABC's David Kerley. Engines at maximum thrust. Reporter: A liftoff making history today. Astronaut Scott Kelly taking off for a year-long mission in space. It's the longest expedition nasa astronaut has ever undertaken. Hours ago, arriving safely at the international space station. Now below 50 meters. You're putting yourself at risk. We've never done this before with an American a year in space. It can mess up your eyes, it can mess up your heart. It can mess you up. There are risks. I'm willing to accept that for what we're going to learn from it. Reporter: Scott, part of a nasa study on whether the human body can survive a trek to Mars. But he's not alone on this mission. We need to figure out how people are going to live in space for really long periods of time. Especially if we want to send somebody to Mars or maybe want to send people to -- maybe one day build a base on the moon. Reporter: That is mark Kelly, former shuttle commander, husband to gabby giffords, twin brother of Scott, and an important part of this mission by staying on Earth. You get the easy part of this. The really easy part. Reporter: His matching genetic code making him the perfect subject to measure against Scott. For nasa scientists this mission could provide a gold mine of data. Looking at the effects of how physical strain, radiation exposure, muscle deterioration, and psychological pressure, will affect Scott. You once told me that when you came back we might be the same age. Reporter: A real-life version of "Interstellar." Leaving Earth is far from easy. What do you expect to miss the most? The human connection. The connection you have with people that you care about on Earth. You still miss the variety that you get by, you know, being on Earth and the people that are important to you. Reporter: But for the Kelly brothers, the rewards outweigh the risks. You both have the chance of actually paving the way to Mars. That's one of the things that makes it exciting and something I'm really happy to be a part of. Reporter: Tonight, the first night for Scott Kelly on the international space station. Contact and capture confirmed. The one-year crew has arrived. Reporter: The Kelly brothers' space expedition just getting started. For "Nightline," I'm David Kerley in Washington. Another giant leap for mankind. What must haves would you pack for a year in space? Head to our "Nightline" Facebook page and let us know what you'd
This transcript has been automatically generated and may not be 100% accurate.