When NASA astronauts Butch Wilmore and Suni Williams inadvertently spent nearly nine months in space, their bodies likely underwent a series of physiological changes due to the pressures of living in space.The duo left Earth on June 5, 2024, in what was supposed to be a roughly weeklong test mission to the International Space Station (ISS). Instead, they spent 286 days waiting to hitch a ride back to their home planet after their spacecraft experienced multiple issues.Upon their arrival on Earth, the two astronauts were assisted out of the SpaceX Crew Dragon “Freedom” as their bodies acclimate to gravity on March 18, 2025.Astronauts, Genetics, and Space EffectsSusan Baily, professor of radiation cancer biology at Colorado State University, says that the longer an astronaut spends in space, the more complex their transition to Earth will be.“It’s all very disorienting when you first get up into space, and that disorientation can last anywhere from days to sometimes weeks,” adds Chris Mason, a professor of genomics at Cornell University.Because each body has a different genetic makeup and unique lifestyle, how a person responds to space can vary between individuals.Bailey and Mason, among others, authored the NASA Twins Study, which followed identical twin astronauts, one of whom spent a year on the ISS and the other on Earth. The research examined “molecular and physiological traits that may be affected by time in space” to establish a basis for assessing the hazards of long-term space travel.From genetic changes and an increased cancer risk to bone and muscle loss, astronauts are affected mentally and physically both in the long and short term, depending on their experience in space.Short-term EffectsComing back to Earth’s gravity serves as the “universal cellular surprise or, in a nutshell, the ‘holy crap’ moment,” says Mason.On Earth, gravity’s force is constant, forcing the body to bear weight and maintain muscle and bone strength through consistent use. ISS, however, is a microgravity environment, meaning that it experiences about 90 percent of Earth’s gravity, causing objects and fluids to float through space.“In microgravity, there is no resistance or weight bearing, so muscles and bones don’t get the exercise and use that they require,” says Bailey. “Even with exercise, there is still a period of adjustment that astronauts have to go through to get the body and the brain used to supporting itself again after returning to Earth.“Astronauts often gain one or two inches of height because of a lack of compression on the spinal column. There is also increased pressure in the brain because gravity is no longer pulling the body’s fluids down to the feet. This can impact the eyes and heart and, in some cases, lead to cognitive decline. However, most of these eventually return to normal once they are back on Earth.“The good news is that the body does adjust again; it adjusts to space and then it adjusts back to the Earth environment and gravity. It just takes a little while,” says Bailey.Astronauts may also experience skin irritation upon their return, as clothing hasn’t been rubbing on their skin as usual. It’s also possible that these stressors can result in autoimmune issues.“In many astronauts, the immune system is very activated; we like to say that it’s perturbed,” says Mason. “More than half of astronauts are observed to have oral herpes reactivations of cold sores, indicating the immune system is under stress.” Long-term effectsAccording to the twin study, long-term space travel has been shown to “prompt mitochondrial dysfunction, immunological stress, vascular changes and fluid shifts, and cognitive performance decline, as well as alterations in telomere length, gene regulation, and genome integrity.”Radiation exposure during space flight can also affect health, aging, and disease trajectories as it damages the DNA in cells, leading to short-term effects like skin changes or fatigue and long-term consequences like an increased cancer risk.“We’re very protected on Earth from a lot of the more damaging types of radiations from the Sun and our atmosphere, as well as the magnetosphere around the Earth,” says Bailey. “As a person goes further and further away from the Earth, and even in low Earth orbit like the space station – about 250 miles or so above the surface of Earth – they get much higher doses of more damaging radiation.”Cancer develops over long periods of time, so it’s not immediately clear how space radiation may influence astronauts’ risk of cancer. However, Bailey notes some radiation-induced forms of leukemia can appear within three to five years.Space as a Living Laboratory to Model Accelerated AgingRadiation can also accelerate aging and certain age-related health conditions, like cardiovascular disease and dementia. Telomeres at the ends of chromosomes, threadlike structures of DNA found in each cell’s nucleus, serve as a biomarker of aging.“Telomeres are very important because they protect the end of the chromosome from degradation and help prevent DNA damage,” says Bailey, adding that telomeres help maintain genomic stability.Telomeres shorten as a person ages through cell division and psychological and physical stress. In other words, the length of telomeres indicates how quickly someone may be aging.Stressors associated with microgravity, radiation, diet, exercise, and even loneliness have all been shown to induce aging in astronauts. Many of these factors cannot be altered while people are in space.“On Earth, for example, a person can manage their lifestyle better or make healthier choices; all the things that we associate with a healthy lifestyle can help maintain telomere length,” says Bailey.In a very controlled environment like space, astronauts can’t make these changes to alter the level of radiation or oxidative, psychological, or emotional stress they’re exposed to. Even diet and exercise are limited in space.To better understand how aging may be accelerated in space, NASA launched CIPHER, a project to “pinpoint how the human body reacts to long-duration missions in space” to prepare astronauts for future missions to the Moon and Mars. The CIPHER database includes many investigations to better understand the health effects of long-duration space flight.An Astronaut’s Work Isn’t Finished when Back on EarthMuch of coming back to Earth is coming back to the human experience and relishing in small things, like seeing family or taking a bath (there are no showers in space).“Just the simple pleasure of taking a hot shower is almost universally deeply loved upon return,” says Mason.Because the long-term effects of humans in space aren’t completely known, astronauts should be monitored for “the rest of their lives” because their work doesn’t end when they get back to Earth or when they retire from NASA.“The body really is amazingly adaptive; it adapts to the space flight environment fairly rapidly, and then on the return, even though it may take a little longer, the body adapts back to gravity as well,” says Bailey.But humanity’s experience with space is “fairly limited,” and scientists don’t fully understand what the long-term implications of space travel mean for human health.As commercial spaceflight becomes more common, Bailey adds that people who haven’t been trained their whole lives to fly in space will also require new considerations: How do they perform in space, and how are they monitored upon their return to Earth?The varying effects recorded in astronauts are logged in the Space Omics and Medical Atlas, or SOMA, a collection of known research on the effects of space on the human body. Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:NASA. CIPHER Nature Portfolio. Space Omics and Medical Atlas (SOMA) across orbitsMadison’s reporting focuses on marine and environmental issues, climate change, and novel scientific discoveries related to health and technology. Raised on an island in southeast Alaska, Madison is now based in western Montana. Her writing has been featured in Time, Snopes, Business Insider, Mountain Journal, EcoWatch, and Alaska Magazine, among others. When not writing, Madison teaches yoga, raises chickens, and fosters adoptable dogs and cats.