( 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 would n'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 .
Gennady Padalka aims to set a record for longest stay on the International Space Station . Russian cosmonaut Scott Kelly will add to his all-time human spaceflight record .