When NASA astronaut Scott Kelly returned to Earth after spending 340 days aboard the International Space Station, scientists were ready to examine how nearly a year in space had affected his body. What made the mission especially unusual was that Scott had an identical twin brother, Mark Kelly, who remained on Earth and could serve as a valuable comparison. Both brothers had worked as NASA astronauts, but Scott’s extended mission, which began in March 2015, offered researchers a rare opportunity to study the effects of long-duration spaceflight. Traveling alongside Russian cosmonaut Mikhail Kornienko, Scott and the station’s crew carried out almost 400 experiments. Researchers monitored both twins through extensive medical testing, including blood and saliva samples, vision checks, cognitive assessments, and physical performance evaluations. Their goal was to understand how the human body responds to prolonged weightlessness and whether those changes might create challenges for future missions to Mars.

The findings, published as NASA’s Twin Study, revealed several unexpected results. Scott’s immune system remained relatively stable, and many changes his body experienced during the mission moved back toward normal after he landed. One particularly surprising discovery involved telomeres, protective structures at the ends of chromosomes that generally shorten as people age. During his time in space, Scott’s telomeres lengthened, although the change did not mean that space had reversed his aging process. Many of the telomere changes later returned toward their original state, reminding researchers how complicated the body’s response to space can be. NASA scientist Steven Platts described the results as evidence of the human body’s resilience, while emphasizing their value for future research. The study did not prove that long-term space travel is risk-free, but it offered important clues about how astronauts might remain healthier on extended missions. By comparing Scott with Mark, scientists gained rare insight into changes that could otherwise have been difficult to separate from normal individual differences. Those findings may help NASA develop better safeguards for astronauts preparing to travel farther from Earth than ever before.