The commander of the International Space Station will break the record for the longest single flight by a US astronaut on Thursday by clocking up 216 days in orbit.

Scott Kelly, who performed his debut spacewalk on Wednesday, will surpass the previous record of 215 days set by the Spanish-American astronaut Michael López-Alegría in 2007.



The former US navy captain has already amassed more cumulative time in space than any other US astronaut, having spent a combined total of 396 days falling around the planet.



Scott Kelly’s debut spacewalk from the International Space Station. Guardian

Kelly’s latest stint on the station is scheduled to last 342 days. During that time, he and the Russian cosmonaut Mikhail Kornienko will endure a battery of medical investigations as part of the One-Year Mission. The results will help space agencies understand the varied effects that long-duration space travel can have on humans.



But Kelly is taking part in another study made possible by the unique genetic relationship he has with a former US astronaut: his identical twin, Mark, who retired to look after his wife, Gabrielle Giffords, the Democratic congresswoman who was shot in the head in January 2011. Scott and Mark have signed up for the Nasa Twins Study, which will examine how Scott’s brain and body change in space, compared with Mark’s on the ground.



Before Scott blasted off for the space station, he and Mark provided saliva, urine, blood and faecal samples to give doctors baseline readings on their biological makeup. The twins, who share the same genome, will give plenty more samples over the course of the mission, and for at least six months after Scott has returned to Earth.



“We are looking at just about every level in the biological spectrum from the molecular level to the whole body integrated,” said John Charles, a doctor on Nasa’s human research programme. “This is possibly the most complex biomedical investigation ever done on the space station.”



The blood and saliva samples will be pored over for changes in the twins’ genomes, including any differences in the way genes are switched on and off over the duration of the mission, and how those changes drive physiological effects. The urine contains substances that reveal a person’s metabolism, while stool samples give an insight into the complex ecosystem of microbes that sets up home in the gut and play a crucial role in health.



Astronauts in orbit experience immediate changes to their physiology. Without gravity to push against, their bones and muscles waste away, though resistance training in orbit slows down the effects. Body fluids move up to their chests and heads. The shift is thought to raise the pressure in astronauts’ brains, squeezing their eyeballs from behind and making them long-sighted.

Nasa has signed up 10 scientific teams to study data gathered from the twins. One group, led by Mike Snyder at Stanford University, will perform a complete analysis of all the biomedical and molecular data to produce the single most comprehensive portrait of the human biological response to spaceflight.



“We hope to understand in incredible detail what happens as people are launched into space and live there,” Snyder said. “What does it do to the human body? Humans in space experience dramatic differences, from high force during launch to loss of gravity and radiation exposure. We want to see how all these things affect the human body using modern tools.”



Though Kelly will hold the record for the longest a US astronaut has spent in space, a Russian will still have the lead in endurance missions. In the 1995, Valeri Polyakov returned to Earth after spending 438 days aboard the Mir space station.



“That was 20 years ago, and much has changed in that time. Our technology has improved,” said Charles. “This is not the first time people have done this, but it’s the first time using 21st-century technology.”



Susan Bailey at Colorado State University will look at blood samples from the twins for evidence that space travel causes damage to the protective caps on chromosomes called telomeres. Telomeres tend to shorten with a person’s age and their length can reflect a person’s risk of developing age-related diseases.

The idea Bailey will test is whether the stresses of spaceflight, ranging from diet and radiation exposure to physical and psychological demands, speed up the loss of telomeres, and so the rate of ageing and risk of related diseases. “The twins make it possible to evaluate spaceflight-specific factors, because the genetic factors are controlled,” she said, since the twins are identical. “This is the first ageing study Nasa has done on the astronauts, so it will be the first time a biological marker of ageing has been is assessed in the context of spaceflight.”



Another part of the twin study will look for differences in how the brothers’ immune systems react to the seasonal flu vaccine, a jab that both had before Scott blasted off to the space station.



The haul of body fluids gathered from the twins will be analysed only once Scott returns to Earth in March 2016. Charles admits that looking at just two twins has its scientific shortcomings, but argues that the opportunity to study identical twins was too good for Nasa to pass up. “There is not likely to be identical twin astronauts again, at least in my career, and probably in my lifetime. So this is really a once in a career, and perhaps a once in a lifetime opportunity,” he said.