In March 2016, astronaut Scott Kelly completed his yearlong mission on board the International Space Station. As he orbited the Earth at an altitude of 250 miles, he conducted experiments, repaired systems, tweeted photos of our planet, and watched Breaking Bad. But one of Kelly’s most important contributions to science and humanity may well be both invisible and unavoidable: He is aging.

Kelly has racked up nearly 540 days in space over the course of four missions and endured all of the associated stressors of space travel: rocket flight, microgravity, radiation exposure, isolation, and a space station diet.

During Scott’s yearlong mission, his identical twin brother, retired astronaut Mark Kelly, has been in Arizona with his wife, former Sen. Gabrielle Giffords, living a fairly typical Earth life, eating what he wants. (He declined NASA’s offer to provide him with the same diet as his brother.) Aside from Scott’s time in space, the brothers have led remarkably similar lives and are nearly identical genetically, so Mark, who has spent 54 days in space, is the perfect control for NASA’s out-of-this-world experiment.

In a research lab at Colorado State University, Dr. Susan Bailey, professor of environmental and radiological health sciences, and postdoctoral student Miles McKenna examine Scott Kelly’s cells under a microscope, just 36 hours after Kelly’s blood was drawn on the International Space Station – an amazing feat in and of itself. The team, including senior research associate Lynn Taylor, is evaluating both of the Kelly twins’ telomeres and levels of telomerase, the enzyme that regulates telomere length.

Telomeres are repetitive DNA sequences at the ends of chromosomes that keep them from fraying or tangling. However, with each cell division, telomeres incrementally shorten. How much telomeres shorten, and how long cells can divide before they can’t do so any longer, depends on both genetics and lifestyle factors. In essence, telomeres are cellular clocks – clocks that speed up or slow down depending on an individual’s lifestyle and environment. Go for a long walk or enjoy a relaxing weekend, and your clocks might slow down. Smoke a cigarette or argue with a difficult coworker regularly, and your clocks might speed up.

Each time a cell in your body divides, that cell loses 50 to 250 pairs of telomeres. This is material that the cell can afford to lose. It does not contain crucial genetic information, but it prevents the chromosome from fusing together and harming the genetic code. The cell will divide 50 to 70 times, until the telomeres cease to protect the cell and it becomes pathological or dies. Due to age, environment, stress, exercise, and other factors, as you age your cells also age and begin life with shorter and shorter telomeres. If you live to retirement, your cells will typically have 1,500 pairs of telomeres, and so they will age and die – or become dangerous – much more quickly than those of your grandchildren.’

KellyTelomere length is inherited at birth, but the rate at which they erode as we age can be influenced by an ever-growing list of lifestyle factors,” said Bailey. “Telomeres can provide an informative biomarker of the stressors astronauts experience during space flight, especially during extended time in space. How does life in space impact telomere length, and therefore aging and risk of disease?”

Bailey hypothesizes that Scott’s telomeres will be shorter than Mark’s at the end of the yearlong mission. If that is indeed the case, Scott will return to Earth biologically older than his twin, and therefore potentially at an increased risk of diseases associated with aging, such as cancer and cardiovascular disease. This is the first time that NASA has embarked on an aging study, an important aspect of spending longer periods of time in space.

As one of 10 primary investigators working on NASA’s Twin Study, Bailey is responsible for analyzing the cellular changes caused by space radiation, which is far more damaging than the radiation we typically encounter on Earth. Bailey and her co-investigators are all contributing to NASA’s understanding of the health risks of long-duration space travel, with the goal of preparing astronauts for the mission to Mars. NASA plans to send humans to Mars in the 2030s, but before they can launch an astronaut on such a mission, they have to understand the health risks – both acute and chronic – of life in space. Will exposure to space radiation increase an astronaut’s risk of leukemia or heart disease? If astronauts develop ill health effects during a mission, how will we treat them? Can we identify which astronauts are more or less susceptible to age-related diseases prior to space flight? If we can, should that influence mission decisions?

Originally Appeared In: Impact 2016, Sarah Ryan, 03-04-2016