Per aspera ad astra. This is the phrase adopted as a statement of intent by space agencies, both real and fictional, that originates in Virgils Aeneid. But exactly what kinds of hardship will the human body have to endure to colonise the cosmos?

When Scott Kelly came back to Earth after 340 days in space, it felt like his skin was on fire. Not on re-entry, but later, when he tried to sit down, get dressed or move. Spending close to a year in microgravity will do that to you; aboard the International Space Station (ISS), Kellys skin got used to feeling no weight and having nothing touching it.

Like other astronauts, he floated around the ISS with little need for furniture. He didnt wear shoes and even his clothes drifted around his body instead of hanging from it. So when he came home, a shirt sleeve bearing down on his arm under the pull of Earths gravity was alien, painful even. As Kelly himself said in a post-flight press conference: Adjusting to space is easier than adjusting to Earth

Since the year-long mission ended in 2016, Kelly has become a guinea pig for scientists studying what happens to the human body when it ventures beyond Earths atmosphere. Even among astronauts hes a rare case. Not only did he spend the best part of a year in orbit, but Kelly has an identical twin brother, Mark. It gave NASA an unprecedented opportunity to study the physiological, molecular and cognitive effects of long-term spaceflight.

Scott went to space. Mark, the perfect control subject, stayed on Earth. The brothers are both retired astronauts now, but their contributions to the landmark Twins Study continue and have produced a wealth of information about how space affects the heart, the microbiome, the immune system and more.

Learning about the challenges of spending prolonged periods in microgravity is vital as space agencies and private companies get serious about sending humans back to the Moon and even to Mars. A mission to the Red Planet is potentially a three-year trip, so we need to understand what might happen to anyone trying to make it.

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One of the scientists prodding and poking the Kelly brothers is Prof Christopher E Mason, the lead geneticist on the Twins Study. Masons lab at Cornell University is nothing if not ambitious. Its work centres on a 500-year plan for the survival of the human species on Earth, in space, and on other planets.

As well as studying what happens to astronauts, it involves laying the genetic groundwork for humans to live among the stars. Mason envisions a future in which the human genome can be bioengineered to adapt to almost any environment, augmented with genes from other species that allow us to explore and settle the farthest corners of the Universe.

Mason is serious. His new book, The Next 500 Years, maps out in detail how well do it. Here, he gives us a sneak peek

So, 500 years from now, well have two key things. I call these the twin engines of discovery. [The first is] a good list of candidate exoplanets to go to. In the past 10 years, thousands have been catalogued, including several hundred Goldilocks planets that are probably not too hot, not too cold and that we could possibly survive on.

In the next 500 years well have, Im sure, thousands or tens of thousands of other candidates. We could use the intervening time to get better spectroscopy and imaging of the atmospheres to figure out whats there and then pick the best ones.

The second thing thatll happen is that well discover a number of genes in the human genome and other genomes, that we could use to regulate our health, design medical treatments or engineer organisms that could survive a long space flight to another planet and survive on it.

And its not just for human cells. Microbial cells, of course, would be engineered to produce products, as they do for us now therapeutically. Well have a genetic tool kit that will let us counteract the ill effects of long-term space travel, and produce the things we need like food and fuel. And, obviously, the more genomes we have in our kit, the more tools we can make.

Astronauts Scott (right) and Mark Kelly are identical twins, but also astronauts. Mark stayed on Earth while Scott spent nearly a year in space Getty Images

At that time, 500 years from now, lets propose that we have generation ships and people could live and die in the same spacecraft as they make their way towards one of the new planets. Then we would have humans in more than one Solar System. It would probably take about 20 generations to get there with current propulsion methods.

Nowhere in the book do I presume anything new is developed that doesnt already exist today. Im hoping thatll be wrong. Maybe well have a new fusion propulsion that makes it much faster or some way to fold space-time. Thatd be fantastic.

Yeah, Id say its even hitting a nice acceleration point. If you look at the number of objects that have been sent into space in the past 60 years, its reached a super exponential pace in the past two years, for sure.

Reduced gravity wreaks havoc with the immune and vascular systems. Thats a challenge and theres not much we can do about it. We could have rotating space platforms or magnetic boots like in [the sci-fi show] The Expanse, but those arent really deployed yet. So, theres no way around it.

The body does what it can to respond in the short term, though. A lot of [astronauts] have spikes in cortisol, or immune stress signatures. They get up [into space] and their bodies are really reacting. Its very uncomfortable. For the first few days, they often have really sensitive skin. But within a few days they adapt.

Maybe the most important thing, in terms of health, is radiation not just in low Earth orbit, but when you get farther out to Mars or on lunar missions. Thats really the biggest risk and its something we can see in the molecular data.

We can see indications of the damaged DNA coming out in the urine and we can see it in the blood. You see telomeres [essentially caps that protect the ends of chromosomes from degradation and unnecessary repair] and DNA getting broken. Its the equivalent of about five or six X-rays per day. You can survive it, obviously, because people have been coming back, but, you know, you probably shouldnt have five X-rays a day.

So thats the biggest hazard, the one that NASA and our staff are most worried about. Thats why were spending so much time thinking about pharmaceutical ways to boost radiation-repair mechanisms or even ways of using genes from other organisms that have already adapted to radiation.

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For a long time, they were research subjects. During the mission and immediately after, when we were doing a lot of sampling, it was very hands-off. If youre researching subjects youre not supposed to grab a beer with them, much as you might like to.

But Scott and Mark are both retired now. Theyre no longer NASA employees, theyre just retired dudes walking around the planet. So weve started to do more together, were talking about what weve learnt.

Yeah, very much. And theyre very competitive because theyre brothers and astronauts. When we got some of the first genetic sequencing data back, they were emailing me and saying, Okay, whos got more mutations? Whos got the longest telomeres? Theyre competing down to the molecular level.

Some features changed within a matter of hours and days. For example, his telomeres got longer in space, but got shorter within days [of being back on Earth]. He got 5cm taller in space because of the lack of compression of his spinal column. That was gone within minutes.

Other measures like metabolites pretty small molecules in the blood that can indicate inflammation some of them were spiking extremely high, like thousands of per cent higher than his baseline before the flight. After two or three days, they went back down again, but it was very painful his ankles swelled up to the size of basketballs for a little while. Really painful.

I think we have to use the evolutionary lessons that every creature has learned as part of our own lesson plan, if you will, for humanity. What I mean by that is other creatures have been surviving in harsh environments.

So these different organisms on Earth have found interesting ways to deal with high amounts of radiation, high and low temperatures, salinity. And they serve as, I think, a field guide of adaptations that we can deploy. Say its a protein thats made by an organism thats resistant to radiation. You can use that as a therapeutic, like the way you grow insulin now. Some cancer therapies and antibody therapies are done this way too.

Yes. Human genome editing is something you want to do very slowly and very carefully, ideally over multiple generations. I write in my book that theres never been a clinical trial going across many generations.

There have been observational studies that have done this. But if we ever do anything to the human genome, really, it would require probably a three-generation study to make sure we didnt screw anything up.

Prof Christopher E Mason

I would think that if after two generations there are no obvious signs of changes in longevity or disease, or obvious deformities, I would feel at least reasonably safe that we can deploy it for more individuals. This is not something were going to do in the next five years or even 10 years, although we are engineering cells and infusing them into many patients as we speak.

Were in the middle of screening lots of organisms to see which extremophiles can survive either in space on the ISS, we published the papers of organisms that have adapted there or just in places on Earth.

We have something called the Extreme Microbiome Project. Were working to categorise all these different sites where we see extreme organisms, like nuclear power reactor cooling water, strange places like that. Were still discovering so many organisms. Its a continual process of discovery and the accretion of possible genes.

For some genes weve already shown that its working. We have a paper in review now where weve shown it works well with immune cells, at least with genes from tardigrades. But these alien genes being put into humans has not been tried outside of cell cultures, to my knowledge.

What has been done, though, is weve also looked at engineered T cells [T cells are a type of white blood cell and are important in the immune system]. Youre not modifying all the cells in a person, but youre basically taking out T cells, genetically modifying them and putting them back in for therapeutic purposes. This is actually an extremely common feature of a lot of immunotherapies now.

Yes. I just got the book. I wanted to watch the show first, but I havent read the books yet.

Off-world settlements, as portrayed in The Expanse, will only be feasible if our bodies can withstand the journey to reach them Shutterstock

I love the concept that humans have a capacity and almost this inevitable future in which weve settled the Solar System. The word colony has fallen out of favour because of all the historical baggage. But I love the concept because I really hope and believe that it will come to pass.

The thing I take some issue with is this: it still seems theres been no sociological or intergovernmental advance, were just as petty and tribal and backstabbing as we are, as we always have been. Maybe its more accurate. If you look back thousands of years, theres never been a case where cultures have expanded and had a Kumbaya moment and everythings been fine.

But I would hope that if we reached that stage [wed also have] a more advanced philosophical and sociological framework for people. But I might be too optimistic there.

Yeah, and you get something like Star Trek, where all of humanitys united and were all exploring together. In Star Trek, it wasnt until [humans] encountered aliens that humanity kind of nucleated together and said, Well, were all in this together.

Yeah, something I talk about in the book is this idea of cellular liberty or even planetary liberty. If youre engineering cells you should make it so they can go anywhere you want in the Solar System, or eventually in the Universe. You want to be able to turn on facets of biology that can enable you to live in different places.

Yeah, making it more modular, so youve got that capacity to go places as needed.

Well, you know, theres the Revive and Restore project where were cloning the woolly mammoth and putting it in elephant embryos. Were working with George Church on that. So thats happening.

In that case, we have fully preserved, whole blood cells with a completely intact genome. So if you can get the same thing for a dinosaur, you can give it a shot. In the absence of that, its going to be really hard. Even with that, its really hard. I think Id put it in the unlikely category. But interesting.

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Christopher is a geneticist and computational biologist who has been a principal investigator and co-investigator of seven NASA missions and projects. He is a professor at Weill Cornell Medicine. As well as preparing human beings for space travel, his research interests include novel techniques for next-generation genome sequencing and editing.

His book,The Next 500 Years (MIT Press), is out now.

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Biological space race: NASA doctor reveals the future of genetically edited astronauts - BBC Science Focus Magazine