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.

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.

The other major change was found in Scott Kelly’s gene expression, or the way DNA directs cells to make components like proteins.

The researchers saw many genes turning off and on again during flight, especially ones related to circulation and the immune system. These changes hint at how the body attempts to adapt to space.

Mason says that in the first half of the mission, they saw almost 1,500 genes change their expression. In the latter half of the mission, six times as many changed. This suggests that the human body undergoes changes throughout its time in space, and not just when it arrives.

Similar to the telomere results, most of the gene expression changes reversed themselves after Scott Kelly came home. However, several hundred genes seem to have remembered their time in space and held onto the shifts.

Whether that is a lot—and what the direct effects on health are—we just don’t know yet. For most of the numbers in the paper, there is no comparison point. “It’s analogous to the very first time that we measured someone’s blood pressure,” says Mason. “We didn’t know what the actual reference numbers were until we started to measure more people.”

Although he has massive amounts more data on his health than most people can ever dream of, Scott Kelly told MIT Technology Review that he’s not planning on taking any extra action on the basis of this information. He felt unwell right after he landed, as most astronauts do, but he doesn’t notice much of a difference in his health now. “I feel different than when I launched a little bit, but that’s probably because, you know, I’m four years older,” he says.

Next steps for Bailey’s team are trying to create better testing methods for observing telomerase activity, either on the International Space Station or back on Earth. Mason is also looking at technology aimed at removing the time pressure on the sample transportation step. His team even performed the first DNA sequencing on the ISS and looks to advance this further. Most other teams are hoping for a wider sample size and, possibly, longer tests. One astronaut isn’t enough to draw definitive conclusions, so researchers like Mason will be studying additional astronauts during two-month, six-month, and year-long tests. Those missions, however, won’t have the benefit of a ground-based twin for comparison.

“[Astronauts] have the choice of being a human research subject,” says Kelly. “You can choose or not choose to do or not to do the science. But so few people travel to space, and the idea is that we’re there for science. I think it’s kind of your obligation to sign up for all of this stuff.”