Astronaut Grew Taller In Space

The homecoming where the taller twin had changed

For their whole lives, Scott Kelly and Mark Kelly stood exactly the same height, which is what you would expect from identical twins. Then, in March 2016, Scott came back from orbit, and for a short while the match was broken. The brother who had spent 340 days aboard the International Space Station was now the taller of the two.

Newspapers reported a gain of about two inches, roughly five centimetres. Scott himself was a little more modest about it. He told reporters that when he measured his own height, the difference was nearer an inch and a half, and he suspected somebody had simply rounded the number up. Either way, the effect was real, and by the time the brothers hugged in Houston, Mark happily reported that Scott had already been, in his word, squished back to normal size.

The height was never the main point of the mission, though. It was one visible clue in a much bigger question NASA had been waiting years to ask properly: what does a long stay in space actually do to a human being?

An experiment only these two brothers could give science

Comparing astronauts with each other is tricky, because every body is different to begin with. If one astronaut’s blood changes in space, was it the spaceflight, or just that person’s natural makeup? The Kelly brothers removed that doubt. They share the same genes, so any clear difference between them during that year pointed a finger at the environment one of them was living in.

NASA built a whole programme around the pair, called the Twins Study. Ten research teams, supported by 84 researchers at 12 locations, followed both men before, during and after the mission, collecting 27 months of data in total. One brother would live in orbit. The other would live an ordinary life on the ground, giving blood, taking tests and keeping the same schedule of measurements, so the two sets of results could be laid side by side.

In space: a spine with nothing pressing on it

Your back is a stack of bones called vertebrae, and between each pair of bones sits a soft disc. Those discs work a lot like sponges. All day long, gravity squeezes them from above, the way a thumb presses a sponge down into a sink. Take the pressure away, and a sponge slowly soaks up water and puffs back to its full size.

That is nearly what happens in orbit. With no gravity load bearing down, the discs between the vertebrae take in fluid and spread, and the whole spine stretches out a little. NASA researcher John Charles explained after the flight that the extra height comes from fluid building up in those discs, which is also why it disappears so fast: once gravity returns, the squeezing starts again, the fluid is pushed back out, and the astronaut settles to their usual height within days.

Scott’s borrowed centimetres were never going to last. They were simply proof of how much of your height on Earth is gravity quietly leaning on you.

On Earth: the brother who kept the measuring honest

Mark’s part in the study sounds easy — stay home — but ground duty had its own discipline. While Scott orbited above him, Mark gave blood samples, sat through scans and cognitive tests, and let researchers track his body through the same seasons. His results became the baseline, the picture of what a Kelly body does during a normal year of gravity, weather, work and stress.

That baseline mattered enormously. When Scott’s numbers moved, scientists could check whether Mark’s had drifted the same way. If both brothers changed, the cause was probably ordinary life. If only Scott changed, space had some explaining to do.

In space: thousands of genes changing their tune

A gene is an instruction written into your DNA, and gene expression is about which of those instructions the body is actively reading at any moment. Think of DNA as a huge library where every book stays on the shelf, but only some books are open on the desk. Space, it turned out, opened and closed a lot of books. Researchers watching Scott saw thousands of genes shift how they were switched on and off, especially in the second half of the mission.

His immune system was among the busiest areas, behaving as if it were on alert in an unfamiliar environment. Reassuringly, it still did its job properly: a flu vaccine given to Scott in orbit worked exactly as it would have done on Earth, according to NASA. Changes in his gut bacteria, meanwhile, were no larger than the kind stress alone can produce in people who never leave the ground.

On Earth: telomeres that stayed boringly stable

One of the strangest findings involved telomeres, the protective caps sitting at the ends of chromosomes. They work a little like the plastic tips on shoelaces, stopping the strand from fraying. Telomeres normally shorten slowly as we age, so scientists expected space, with its radiation and stress, to wear Scott’s down faster.

Mark’s telomeres behaved exactly as predicted during the study period: they stayed stable. Scott’s did the opposite of what everyone expected, which takes us back up to the station.

In space: telomeres that grew longer instead

Against all predictions, the telomeres in Scott’s white blood cells grew longer while he was in orbit. The lead researcher on that part of the study, Susan Bailey of Colorado State University, admitted the team’s first reaction was simple surprise. Nobody is claiming Scott grew younger up there — longer telomeres in space did not turn out to be a fountain of youth, as the next chapter of the story showed.

Back on Earth: almost everything quietly resets

The return to Earth was, by Scott’s own account, harder than the trip up. In the days after landing in Kazakhstan in March 2016, he described deep muscle soreness, heavy fatigue and skin so sensitive it burned where it touched anything, because for nearly a year almost nothing had pressed against it.

Underneath those rough first weeks, the big measurements were settling down:

Height. The extra centimetres vanished quickly once gravity resumed its squeezing.
Telomeres. Scott’s telomeres shortened rapidly after landing, and their average length was back to normal within about six months — although he was left with more unusually short telomeres than he had before the flight, something researchers want to keep studying.
Gene expression. Within six months of landing, 91.3 percent of the gene-expression changes had returned to their normal baseline levels.
The remainder. A small percentage of genes, mostly ones connected to the immune system and to DNA repair, were still behaving differently months later. NASA is careful to stress that this does not mean Scott’s DNA itself was mutated or rewritten; the code stayed the same, only the reading of it shifted.

To me, that 91.3 percent figure is the most comforting number in the whole study — the human body turns out to be less like a machine that space breaks, and more like a spring that space stretches and home mostly releases.

Why NASA put one brother through all this

The point of the experiment reaches far beyond one family’s height chart. A round trip to Mars will keep astronauts away from Earth for years, far longer than Scott’s 340 days. Before sending people on journeys like that, NASA needs to know which changes space causes, which ones heal, and which ones might need medical protection.

The honest limits matter too. NASA itself points out that a study with a single person in space cannot prove what will happen to every astronaut. Scott is one man, not a statistic. But he is the best-matched comparison science has ever had, and the Twins Study results, published in the journal Science in 2019, now guide the health planning for the Moon and Mars missions ahead.

Common questions

So did Scott Kelly really grow two inches? That is the figure most reports used, and it is close — but Scott said his own measurement was about an inch and a half, and joked that someone had rounded it up. The safe answer is that he gained somewhere around four to five centimetres, briefly.

Is the extra height permanent? No. It fades within days of coming home, because gravity compresses the discs in the spine again and pushes the extra fluid out.

Did space change Scott Kelly’s DNA? No. His DNA code did not change. What changed was gene expression — how actively certain genes were being used. Most of that returned to normal within six months, and NASA has corrected stories that claimed otherwise.

Did longer telomeres mean Scott got younger? No. The lengthening surprised researchers, but after he landed his telomeres shortened quickly, and he ended up with more very short telomeres than before the mission, which scientists see as a reason for more research, not celebration.

Was Mark Kelly an astronaut too? Yes. Both brothers flew with NASA before retiring, which is part of what made this comparison possible at all.

Two brothers, one measuring tape

There has only ever been one pair of identical twins who both became astronauts, and there may never be a cleaner experiment than the year one of them spent in orbit while the other stayed home as his mirror. Scott brought back the data, the soreness and a few borrowed centimetres. Mark brought the patience and the baseline that made the data mean something. Somewhere in Houston, two men who started life as exact copies stood back to back one more time — same height again, but no longer quite the same, and both of them part of the reason future crews will travel farther with a little more confidence.