Three years ago, American astronaut Scott Kelly came back to Earth. His return from the International Space Station on March 1, 2016, ended his US-record-setting run of 340 days in space under a medical microscope. His twin brother, Mark Kelly (who had been an astronaut), was under similar scrutiny here on Earth. The pair offered a unique opportunity to explore how the human body responds to long periods in space—giving us a glimpse at what could happen on trips to, say, Mars.

Now, more than three years later, we are finally getting a clear picture of what microgravity, radiation, and the space environment did to Scott’s body. The first results, published in Science today by dozens of researchers from around the globe, show promise for humankind’s space-based future. “It is predominantly very good news for spaceflight and those interested in joining the ranks of astronauts,” says Cornell professor Chris Mason, principal investigator for the NASA Twins Study. “While the body has an extraordinary number of changes, it also exhibits extraordinary plasticity in reverting back to a normal terrestrial state.”

The study looked at a number of biological markers, from the immune system (it functioned similarly to the way it does on Earth) to the shape of the eyeballs (Scott’s retinal nerve thickened). But two of the standout results came from a closer look at DNA and gene expression.

NASA

Susan Bailey and her colleagues from Colorado State University focused on observing the length of Scott Kelly’s telomeres and the associated enzyme, telomerase. Telomeres are located on the ends of DNA, and their length generally signifies age and health. Things like aging, stress, and radiation can cause them to shrink.

Since spaceflight exposes people to both stress and radiation, the researchers were expecting to see his telomeres shorten. “It was exactly the opposite,” says Bailey. “As soon as our earliest samples [were taken] in flight, which was within about two weeks after him being up there, we saw significantly longer telomeres in Scott than he had before he went.”

And the trend persisted over Kelly’s entire time on the space station. Overall, his telomeres lengthened by about 14.5%.

So what does that mean? Don’t think we’ve suddenly found the fountain of youth in space. Within a week of his return, his telomeres shortened substantially. “They’re very, very spaceflight-specific and very rapid kinds of changes, which really left us scratching our head as to what in the world could be causing such a thing,” says Bailey. Scott Kelly’s average telomere length returned to normal within six months, but an abnormally high number of short telomeres that formed on his return to Earth remained in his body.

A key piece of missing data is creating some of the mystery around why this happened. The data on telomerase, the enzyme related to the length of telomeres, didn’t make it back to the lab. While the samples from Kelly’s body in space got to the researchers in under 48 hours, the environment on the trek to the lab wasn’t controlled well enough to prevent the telomerase activity from being lost. Heading back to Earth in a Soyuz capsule doesn’t equate to perfect laboratory conditions.