When astronaut Scott Kelly returned to Earth after a year floating about the International Space Station, he was noticeably different from his identical twin, Mark Kelly. For one, Scott temporarily grew two inches taller, but NASA scientists expected this space-induced growth spurt. What they really cared about was how Scott changed on the inside.

The brothers participated in NASA’s Twins Study, which compared Mark’s grounded lifestyle to how long-term spaceflight influenced Scott’s bodily functions. In the study’s latest findings, researchers discovered an increase in Scott’s methylation rate, a process that turns gene activity on and off. The team documented thousands of genes switch on and off as soon as Scott entered space, and the state continued for a short time after his return.

“There are over 50,000 genes in the human genome, and when floating in zero gravity, the body is trying to manage that situation in new ways,” Chris Mason, one of the principal investigators of the Twins Study and a geneticist at Weill Cornell Medicine, told PBS NewsHour. “Both DNA and RNA were found to express genes in order to compensate for a lifestyle in space.”

Why does gene expression matter for space travel?

The goal of the Twins Study is to see if humans can withstand a future mission to Mars, and knowing how the human body reacts to extraterrestrial environments starts at the molecular level.

Gene expression is the multistep process whereby our genes produce protein, which in turn build our cells and their activities. This is key to the body’s ability to deal with stress and the development of diseases, such as cancer or metabolic disorders. For example, low oxygen levels can switch on certain genes so the body can survive, while exposure to excessive radiation can mutate genes so they’re always off or on, which can lead to cancer.

DNA methylation modifies a gene, dictating whether or not expression can happen in the first place. So the new result might mean spending more time in space can lead to unprecedented changes in cell function, and the Twins Study is planning to publish their full results and their implications in 2018.

“And sometimes gene expression might produce cases where some risk factors and positive responses appear together,” Mason said. “The most notorious of them being telomeres.”

Earlier this year, the Twins Study reported that Scott’s telomeres — tiny caps at the ends of DNA strands — grew longer from his time in space. The upside of having longer telomeres means signs of aging are stable or decreasing, Mason explained. The downside is if longer telomeres are in a place with higher levels of radiation, for instance space, cells are more likely to mutate and become an extremely aggressive cancer.

Can we control gene on/off switches?

After astronauts return from space, some effects linger a bit, such as that temporary height boost and blurrier vision. Mason said these effects, and those at the molecular level, need to be regulated to keep astronauts safe and healthy on lengthy missions.

Researchers plan to use tools such as CRISPR-Cas9 and other genetic engineering methods to prevent methylation from causing detrimental genetic effects. They’re also looking at ways to use these same tools after the fact to repair damaged or disrupted cells, Mason added.

“The cool thing about this study is that we’ll soon have exact coordinates for the human genome that are at risk due to space travel,” Mason said. “With our current molecular engineering technologies, we can counteract and even repair genes at these points in a matter of days after they’re discovered.”