Scott Kelly came home from 340 days on the International Space Station in March 2016 fractionally younger than the brother he had left behind. Not metaphorically. Measurably. By the calculation NASA ran on his return, the commander was roughly 5 milliseconds younger than Mark Kelly, his identical twin and a former astronaut himself, because the station Scott had been living on was screaming around the planet at about 17,500 miles per hour.
That speed is the whole story. At that velocity, time inside the station ticks slightly slower than time on the ground due to relativistic effects. The effect is tiny. It is also real, and it is happening to the seven astronauts currently aboard the orbital outpost as you read this sentence.
The speed that bends the clock
17,500 mph is hard to picture. It is roughly 7.66 kilometres every second. A rifle bullet leaves the muzzle at about 1 km/s. The station is moving more than seven times faster than that, continuously, every second of every day, and has been doing so for more than 25 years.
At that speed, the ISS completes one full lap of the planet every 90 minutes. The crew sees 16 sunrises and 16 sunsets in every 24-hour period, a fact USA Today notes in its guide to spotting the station from the ground. The station sits about 260 miles up, which sounds high until you realise that is roughly the distance from New York to Washington, just pointed straight at the sky.
The orbital mechanics demand the speed. To stay in a stable low Earth orbit at that altitude, an object has to be falling around the planet fast enough that the curve of its fall matches the curve of the Earth itself. Go slower and you spiral down. Go faster and you climb away. 17,500 mph is the number that keeps the station threading the needle.
Why time slows down up there
A clock moving relative to an observer ticks slower than a clock at rest with that observer. The faster the motion, the bigger the gap. At everyday speeds the effect is so small you would never notice it. At 17,500 mph, with hundreds of days of accumulation, it becomes something a precise atomic clock can read off.
For Scott Kelly, who spent 340 consecutive days on the ISS between March 2015 and March 2016 as part of NASA’s year-long mission, the math worked out to about 5 milliseconds. That is the figure NASA published when comparing his accumulated relativistic time dilation to his twin Mark’s on the ground. Five thousandths of a second. Less time than it takes a hummingbird to flap its wing once.
There is a second relativistic effect working in the opposite direction. Clocks tick faster the further you get from a gravitational mass. The ISS sits 260 miles above the surface, in slightly weaker gravity, so by gravitational time dilation alone the astronauts should age slightly faster than people at sea level. The catch is that at the station’s altitude, the speed effect dominates the gravity effect. Net result: astronauts come home younger, not older.
This is the opposite of what happens to GPS satellites, which orbit much higher and much slower. Those clocks run faster than Earth clocks, and the GPS system has to correct for it every day or your phone’s navigation would drift by kilometres within hours.
The twin who stayed on the ground
The Kelly brothers were a gift to biomedical research. Identical twins, both selected as NASA astronauts, with one willing to spend a year in orbit while the other stayed put in Tucson, Arizona. NASA ran a coordinated effort across ten research teams that compared everything from Scott’s gene expression to his gut bacteria to his cognitive performance before, during, and after the mission.
The headline biological findings were strange. Scott’s telomeres, the protective caps on the ends of chromosomes that normally shorten with age, actually got longer in orbit. They snapped back to roughly baseline within months of his return, with some shorter than before he left. About 7 percent of his gene expression had not returned to preflight patterns six months after landing. His carotid artery thickened. His cognitive speed dipped slightly post-mission. His gut microbiome shifted in orbit and shifted back on the ground.
The 5-millisecond age gap is the cleanest, most quantifiable result of the entire study, because unlike biology it is not subject to interpretation. It is just arithmetic, applied to relativistic principles.
How big is 5 milliseconds, really
A human heartbeat takes about 800 milliseconds. The blink of an eye takes 100 to 400 milliseconds. The fastest a human nervous system can register a sound is roughly 10 milliseconds. Five milliseconds is half of that. It is the time it takes light to travel about 1,500 kilometres, or roughly the distance from London to Warsaw.
For Scott Kelly’s body, the gap is meaningless. He did not gain 5 milliseconds of life. He simply experienced 340 days minus 5 milliseconds while his brother experienced 340 days flat. If both brothers live to be 80, the relativistic gap will still be 5 milliseconds. It does not compound. It does not buy you extra years.
What it does is prove, in human flesh and bone, a prediction made in a Bern patent office at the start of the 20th century. The same equations that govern the ISS govern particle accelerators, GPS satellites, and the muons created by cosmic rays slamming into the upper atmosphere. Time really is relative. Scott Kelly is the proof you can put a name to.
The station that produced the data
The ISS has been continuously inhabited since November 2000. More than 290 people from 26 countries have lived aboard it, including 170 Americans. It is operated jointly by NASA, Roscosmos, the European Space Agency, JAXA, and the Canadian Space Agency, and it weighs roughly 420 tonnes, about the mass of 100 elephants, all of it whipping around the planet 16 times a day.
The station’s future is now a live political question. NASA’s plan is to deorbit the complex in 2031, sending it through a destructive atmospheric reentry with surviving debris falling into a remote stretch of the South Pacific. Ars Technica reported in February that the House Science Committee advanced a bill requiring NASA to reconsider that plan and study whether the station could instead be parked in a higher orbital harbour for possible reuse. The BBC has explored how a controlled fiery reentry of the station would actually work, including the SpaceX-built deorbit vehicle NASA selected for the job.
Either way, the clock-bending continues until the station is gone. Every astronaut currently on board, including the Expedition 74 crew described in USA Today’s January viewing guide, is accumulating their own tiny relativistic offset every second they stay aboard.
Who holds the record for the biggest age gap
Scott Kelly’s 5 milliseconds is famous because of the twin comparison, but it is not the largest accumulated time dilation a human has experienced. That record belongs to Russian cosmonaut Sergei Krikalev, who has logged more than 803 days in orbit across multiple missions. By the standard calculation, Krikalev is roughly 1/48th of a second, about 20 milliseconds, younger than he would have been had he stayed on Earth. He is, in the technical sense, the most time-travelled human alive.
Frank Rubio, the NASA astronaut who set the American single-mission record of 371 days on the ISS in 2023, picked up his own milliseconds. So did Christina Koch, who holds the record for the longest single spaceflight by a woman at 328 days. Every long-duration crew member is essentially a slow-motion time traveller, and the longer the mission, the bigger the offset.
The human consequence is the part that lands. There are people, right now, for whom your watch and theirs no longer agree.
What the body notices and what it doesn’t
The brain has no sensor for relativistic time dilation. Scott Kelly did not feel time slowing. His coffee did not take longer to cool. His pulse, from his perspective on the station, kept ticking at its usual rate, because inside his own reference frame nothing was strange. The slowing is only visible when you compare his clock to Mark’s clock after the fact.
What Scott did feel was everything else. The fluid shift in microgravity that puffs the face and thins the legs. The loss of about 1 percent of bone mineral density per month without aggressive exercise. The constant low hum of station equipment that astronauts report follows them into their sleep cycles. The view, which dozens of returning crew members have described as permanently altering how they think about the planet, a phenomenon researchers call the overview effect.
None of that is relativity. All of it is the price of admission to the only environment where humans can demonstrably experience time at a slightly different rate from everyone else.
The hallway lamp version
If you want a useful comparison, here is one. 5 milliseconds is roughly the time it takes a household LED bulb to flicker imperceptibly between AC cycles in a 60 Hz electrical system. It is the time a sprinter’s foot is in contact with the track during the push-off phase of a 100-metre dash. It is faster than any sound a human ear can fully resolve.
Scott Kelly spent a year of his life accumulating that gap. He came home in March 2016, sat across the table from his identical twin, and was, by the laws that govern the universe, very slightly less old. The brothers no longer share a birthday in the strictest physical sense. They share a birth date.
The station will keep producing tiny age gaps until it falls from the sky. The astronauts inside it will keep, in their own quiet way, time-travelling. And the next time the ISS passes over your house as a bright dot moving west to east, the people inside that dot will be ageing, by a measurable fraction, slower than you.
