(CNN)NASA astronaut Scott Kelly has started a trip to the record books. He plans to spend 342 days on the International Space Station -- the longest stretch of time any U.S. astronaut has spent in space. Kelly lifted off Friday aboard a Russian Soyuz rocket that launched from Kazakhstan. The rocket docked at the ISS at 9:33 p.m. ET, according to a tweet from NASA, with hatches between the Soyuz and the space station scheduled to open about 11:15 p.m. ET. Kelly will stay twice as long as any U.S. astronaut has ever stayed on the space station, giving scientists a chance to study how the human body responds to long-duration space flights. On Earth, scientists will perform parallel studies on Scott Kelly's identical twin brother, retired astronaut Mark Kelly. "It's a lot of fun," Kelly said at a media briefing in January. "Space station is a magical place." Two cosmonauts are riding up with Kelly: Mikhail Kornienko and Gennady Padalka. Kornienko also will stay up for a year. Padalka will only stay up for six months. The trio will join three crew members already on the space station: U.S. astronaut Terry Virts, Russian cosmonaut Anton Shkaplerov and Italian astronaut Samantha Cristoforetti. Other crew members also will rotate in and out during their year in orbit so it will never be just Kelly and Kornienko on the station. Kelly, 51, hopes to break the record for the longest mission on the space station set by NASA astronaut Michael Lopez-Alegria and Russian cosmonaut Mikhail Tyurin. They spent over seven months on the ISS -- from September 18, 2006, to April 21, 2007. He is expected to spend 342 days off the planet on this mission. When added to his previous space missions, that would give him a total of 522 days in space, breaking the record of U.S. astronaut Mike Fincke. Fincke spent a combined 381 days, 15 hours and 11 minutes in space, according to his official NASA bio. While Kelly shoots to set space station and other U.S. records, Padalka is on schedule to set a record for the most total time in space for a human. He's already spent more than 710 days in space, including stays on Russia's Mir space station and three previous stints on the International Space Station. Why do it? Why stay up on the station so long? NASA wants to know more about the impact of long-duration spaceflights on the human body to help plan missions to Mars and deeper into space. And the space station, in orbit 250 miles above Earth, is the best place available to study that. Spending that much time in a weightless environment will take a toll. NASA says space station astronauts have vision changes, bone loss, muscle atrophy and other problems. NASA says the experiments should have benefits on Earth, such as helping patients recover from long periods of bed rest to improved monitoring of people whose bodies are unable to fight infections. The space station has about the same living space as a six-bedroom, two bathroom house. And it has a 360-degree bay window with a great view. But astronauts and cosmonauts still report feeling isolated and confined at times. Kelly will document his feelings about being away from his two children and the rest of his family. "I'm going to keep a personal journal of the experience," Kelly said. He also will share some of his journal with researchers studying the psychological impacts of long-term space flight. Will he tell all? "I plan to be completely honest about it," he said, but ... "who knows, maybe there are some crazy thoughts I'll have at the end that I wouldn't want to share." Mark Kelly has volunteered for NASA's "Twins Study" to see how the identical twins change over the year in two very different environments. He is the husband of former U.S. Rep. Gabrielle Giffords of Arizona, who left office after being wounded in a January 2011 assassination attempt. "He thinks it's great that he can still be a participant in this," Scott Kelly said. So Mark will be on Earth getting poked and prodded by researchers -- while Scott makes history in orbit. Final candidates announced for one-way trip to Mars .
Scott Kelly aims to set a record for longest stay on the International Space Station . Russian cosmonaut Gennady Padalka will add to his all-time human spaceflight record .