NASA Astronaut Scott Kelly is traveling to the International Space Station for the second time. Mr. Kelly says that this time in space would present a greater challenge than his first visit.Most visits to the space station last just four to six months. Scott Kelly will be there a year. Researchers hope this longer stay will teach them more about how the human body reacts to long periods of spaceflight.Astronaut Kelly's mental and physical health will be closely watched during his ISS visit and after his return. He also presents NASA with a unique study situation. Researchers will be able to compare medical information from Astronaut Kelly with that from his identical twin brother, former astronaut Mark Kelly.Craig Kundrot is deputy chief scientist of NASA's Human Research Program at Johnson Space Center in Houston, Texas. He says Mark Kelly is volunteering as a private citizen.One main focus will be on the bacteria that live in the human digestive system. Mr. Kundrot says comparisons could be interesting because the twins will be eating very different food in the year ahead. He says Scott Kelly's diet in space will be very limited and sterile.John Charles is also a NASA scientist with the Human Research Program. He says it has been almost twenty years since someone spent so long in space. He says technology has developed a lot since that team of Russians spent a year in space."The Russians themselves are very interested in what we are going to learn from this mission, because it is a chance to revisit and further the studies they did earlier."John Charles says the collection of tests and the ability to watch Scott Kelly's condition every day could produce unexpected results."We are not going to be surprised by surprises. We were surprised several years ago by finding that certain astronauts have changes in their vision during long-duration space flight about halfway through a six-month mission."Another area of interest is the effect of radiation on the human body. This would be even more intense for astronauts on a three-year mission to Mars. NASA scientists think research collected from Scott Kelly's one-year space trip will be important to pave the way for a future piloted flight to Mars.更多听力请访问51voa.com