When it comes to seasoned astronaut Scott Kelly - who last year broke the US record for most time spent in space - NASA sure got lucky, because he happens to have an identical twin brother who is his exact genetic copy.
That means the space agency had the perfect opportunity to figure out how extended periods in microgravity can affect the human body, and now that the first results from NASA's Twin Study are in, it's becoming clear that in some cases, the effects are the total opposite of what scientists were expecting.
Figuring out how humans are affected by long-term exposure to the conditions of outer space is something researchers have been grappling with for decades.
For one thing, the sample size is incredibly small - only 10 people can be living on the International Space Station at any one time, and we've only been sending humans into space over the past 50 years.
And then there's the control group problem - each human is different, so if a particular person experiences biological or genetic changes out in space, there's no way of knowing if they would have experienced the same thing on Earth had their situation been different.
That's why Mark Kelly is so important - being Scott's identical twin brother, he possesses the exact same genetics, which gives scientists the opportunity to compare how his body changes here on Earth to how Scott's body changes in space over a specific period of time.
That's the premise of NASA's landmark Twin Study, which is being carried out in 10 individual investigations across 12 universities, looking at various aspects of Scott and Mark Kelly's biology.
To give you an idea of the brother's history, Scott Kelly spent 340 days in space between 2015 and 2016, and has spent a total of 520 days in space during his lifetime.
Mark Kelly, who is also an astronaut, has spent 54 days in space across multiple missions between 2001 and 2011.
The results are only just coming in now, and while the researchers are a long way off making any conclusions just yet, what we're seeing are significant differences between the brothers, and not necessarily in the way you might expect.
"Almost everyone is reporting that we see differences," one of the team, geneticist Christopher Mason from Weill Cornell Medicine in New York City, told Nature.
"The data are so fresh that some of them are still coming off the sequencing machines."
Based on previous animal studies, which have shown worrying signs of liver damage in space-faring mice, and brain damage that could lead to anything from memory loss and anxiety to full-on dementia, it's easy to assume that any changes to Scott Kelly would be far worse than what Mark experienced in the comfort of Earth's protective atmosphere.
But one of the most striking results from the Twin Study so far is that during his year in space, Scott's telomeres - DNA-protecting structures on the ends of chromosomes - actually grew to be longer in his white blood cells than his brother's.
"That is exactly the opposite of what we thought," says one of the team, Susan Bailey, a radiation biologist at Colorado State University.
A second lab confirmed this unexpected increase in telomere length.
That's pretty nuts, because telomere length is thought to be one of the most important indications of health and longevity.
Each time a cell divides, its telomeres get shorter - to the point where the cell can't replicate anymore. A cell that can no longer replicate either dies or becomes senescent, which means it can no longer grow or function as it should.
This telomere shortening process has been associated with a higher risk of cancer and premature death, so scientists think that the longer your telomeres are, the more likely you are to avoid the most damaging effects of ageing.
Why would life in space, with all its provably harmful cosmic radiation, gravity levels humans have not evolved to cope with, and strange and limited diet choices, result in the apparent benefit of longer telomeres?
It's not clear yet, but one early hypothesis is that Scott was exercising more and eating more lean foods in space than Mark was down on Earth, and these benefits not only counteracted the damaging effects of space - they overtook them.
While it might seem like good news that Scott appears to have gained an apparent health benefit from being in space, what's perhaps more striking is the fact that it's so unexpected.
The last thing that anyone wants if we're considering six-month journeys to Mars is any surprises when it comes to astronauts - and potential colonists' - health and wellbeing, and this could be just the tip of the iceberg.
Other initial results from NASA's Twin Study have revealed significant changes in gut bacteria between the brothers, and shifts in gene expression - as in, space appears to alter the way our cells process basic genetic information.
"Such changes happen in Earthbound people all the time, associated with environmental shifts such as changes in diet and sleep habits," Alexandra Witze reports for Nature.
"But the changes in Scott seemed to be larger than normal - perhaps due to the stress of eating freeze-dried food and trying to sleep while floating in space.
Peer-reviewed papers detailing the findings are not expected for at least a few months while analyses are carried out and the results confirmed, so we'll have to wait and see what the researchers turn up.
But if this is anything to go on, we should expect a whole lot of surprises, because it looks like when it comes to space, we've barely even scratched the surface of what can happen out there.
And there's nothing quite so unnerving as the unknown.