His mission to space is now complete, but another one is still underway: Scott is half of a novel NASA study designed to reveal “the subtle effects and changes that may occur in spaceflight as compared to Earth.”

The study’s other half? His identical twin brother Mark, himself a retired NASA astronaut. While Scott was in space following a strict schedule, diet and exercise routine, Mark was leading a normal life in Arizona.

Now, using Mark as a control, NASA researchers are performing a series of 10 studies to determine how Scott’s biology was altered from a year spent circling the Earth at 17,000 miles per hour. Samples from the twins’ were taken before and during the mission, and are being collected now that Scott has returned home.

Related: The Best Reddit AMAs of 2016 (So Far)

While the results won’t be in for at least a year, some of the researchers’ involved hosted a Reddit AMA to answer users’ questions about the study and its possible implications.

Here’s what they know so far:

Scott and Mark are ideal test subjects. They’re about “as similar in nature and nurture as you can get – identical twins, both astronauts,” said Susan Bailey, a professor of radiation cancer biology and oncology at Colorado State University, and one of the study’s investigators. Using a stockpile of baseline data, the researchers should be able to establish “molecular changes that are most impacted by space travel, so we can plan for ways to mitigate and protect against them,” added Christopher Mason, an associate professor of physiology and biophysics and another investigator on the project.

Sustained time in space leaves the body vulnerable. “Spaceflight is altogether different from living on Earth. In addition to weightlessness, the atmospheric composition (CO2 levels), ambient noise levels, stress levels, etc. There are A LOT of variables,” Kjell Lindgren, a NASA astronaut, said in the AMA.

These variables, particularly microgravity, have been shown to put added stress on the body, leading to bone loss, muscle loss, cardiovascular deconditioning and vestibular changes. Because Scott spent an extended period of time in space, he may exhibit more serious, permanent effects. In the AMA, Lindgren said he’d spoken to Scott about his recovery, “and he would say that [it’s] taking longer.”

On a genetic level, the researchers will “examine whether and how Scott’s DNA and his DNA packaging material gets modified during space travel,” said Michael Snyder, a professor of genetics at Stanford and one of the study’s investigators. In addition, they will take a close look at how Scott’s telomeres — the end portion of a chromosome that protects it against deterioration — reacted to his year aboard the ISS. “We will see if telomeres get shorter in Scott relative to Mark,” a sign his chromosomes have aged in comparison to his brother’s.

Radiation is primed to be the largest obstacle in future missions to Mars. In part, the “Twin Study” is intended to help NASA strategize for a future voyage to the Red Planet. Scientists already know that space travel exposes astronauts to cosmic ray radiation, which previous research suggests could increase the likelihood of developing aggressive tumors.

By studying Scott and Mark, NASA should be able to paint a more detailed picture of the specific health risks associated with this type of radiation exposure. And, potentially, strategies to reduce its impact.

Radiation levels in Scott and Mark have already been tested. Unsurprisingly, they’re “much higher in Scott,” said Mason.

Bailey, of Colorado State University, put it bluntly: “Exposure to space radiation has the potential to be a showstopper as we make our way to Mars – longer periods of time deeper and deeper in space.”