If we’re going to continue to explore space, we need to understand how interstellar travel will effect our bodies. That’s why identical-twin brothers Scott and Mark Kelly are being studied to understand what space does to our genes.

They’ve both been in space, but most recently, Scott Kelly spent 340 days in orbit, and the results are fascinating. It turns out that, when compared to his brother, Scott’s blood samples after returning showed “changes in gene expression, DNA methlyation and other biological markers,” according to a report published in Nature. Scientists are not only surprised by the results, but also that they were able to see such significant changes.

Because they’re identical twins, the Kelly brothers make for ideal test subjects. One of the more interesting findings included differences in their telomeres, the caps on the end of chromosomes that typically shrink as someone ages. In Scott’s case, though, they grew longer than Mark’s, which is the opposite of what you’d expect over time. Does this mean space extends one’s lifespan? Probably not, but it does show that space can and will change our genetic makeup.

This is critical information, because a trip to Mars is expected to take seven years, much longer than the 340 days Scott spent in space. At this point, they can begin to make predictions and understand what such a trip would do to a person. We’ll need to wait at least a year for full results from the study, but for now, know this: space is totally different than earth.