Astronauts on the space station have no washing machine, because water is far too precious to waste on laundry, so they wear the same clothes for days and then pack the dirty ones into a cargo capsule that burns up on reentry, which means a fireball crossing the sky could, in theory, be a bundle of an astronaut's old underwear.

The International Space Station has been in continuous crewed operation since November 2000, which means somebody has been living roughly four hundred kilometres above the surface of the Earth every single day for the past twenty-five years without a break. During that time the station has been visited by more than two hundred and eighty astronauts and cosmonauts from more than twenty countries, and each of them has faced the same practical problem that any long-term resident of anywhere eventually has to solve.

Their clothes get dirty. And the space station, for reasons that come down to the physics of orbital resupply, does not have and has never had a washing machine on board. What NASA and the Russian space agency have therefore developed over the intervening decades is one of the more counterintuitive laundry solutions in any modern human institution. There is no laundry. The clothes go into a cargo capsule when the astronauts are done with them. The cargo capsule burns up on the way home.

Why nobody bothered to send a washing machine up

The reason is water. Aboard the space station every drop of water is a precious resource that either has to be lifted up from Earth at roughly $10,000 per kilogram in launch costs, or has to be extracted and recycled from the crew’s own urine, sweat and exhaled breath through a purification system that operates continuously in the background of station life. The station recycles roughly 90 per cent of the water it produces internally, which is considerably better than any comparable industrial recycling system on Earth. But even at that recycling rate, adding a washing machine to the water demand of the station would consume enough additional water per cycle to fundamentally change the economics of resupply. According to a 2017 VICE interview with Robert Trevino, a senior engineer with NASA’s Crew and Thermal Systems Division who is responsible for astronaut clothing on the station, the engineering team has been examining this question for years. NASA’s own internal trade studies compared the water and power requirements of a hypothetical station laundry system against the cost of simply replacing dirty clothes with fresh ones sent up on cargo runs. Replacing the clothes turned out to be cheaper. So that is what NASA does.

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What this means in practical terms is that astronauts on the station wear each piece of clothing for as long as it can reasonably be worn before it becomes unusable. Exercise clothes get roughly two weeks of use because the mandatory hour of daily exercise required to prevent bone and muscle loss in microgravity produces enough sweat to defeat the antibacterial coating on the fabric within a fortnight. Regular daily-wear clothes, meaning the cotton polos and cargo shorts that most station astronauts prefer, get anywhere between one and three months of wear per garment. Underwear rotation is up to the individual crew member. Trevino told VICE that former astronaut Don Pettit was known to have worn the same pair of shorts for months at a stretch, which was apparently a source of considerable amusement among his crewmates. The clothing is sourced by NASA’s contractor Lockheed Martin from ordinary commercial suppliers, including Lands’ End, Cabela’s, and Blackhawk. There is nothing technologically special about the fabric.

Where the clothes actually end up

Once a garment has reached the end of its useful wearing life, it goes into the station’s waste system alongside the packaging, food waste, expired equipment, and general trash that the crew generates every day. The waste is bagged, taped, and stored in whichever visiting cargo vehicle happens to be docked at the time. There are currently three different cargo vehicles that regularly deliver supplies to the station. Russia’s Progress capsule and Northrop Grumman’s Cygnus vehicle are both single-use, meaning they are launched from Earth full of supplies, unloaded at the station, refilled with trash, undocked, and then deliberately deorbited over the Pacific Ocean where they burn up in the upper atmosphere on the way down. SpaceX’s Dragon capsule is the exception. It is reusable, and it returns to Earth intact via parachute for a splashdown in the Pacific and eventual reuse on a subsequent mission. So while Dragon comes home in one piece, the trash from every Progress and Cygnus run ends up as a spectacular fireball crossing the sky somewhere in the upper atmosphere above the South Pacific.

According to Space.com’s November 2023 reporting by managing editor Brett Tingley on the reentry of the Russian Progress MS-23 cargo spacecraft, the reentry of a fully loaded trash-carrying capsule is a visually striking event. The Progress MS-23 undocked from the station on 29 November 2023 carrying, in the words of the Roscosmos cosmonaut Oleg Kononenko who saw it off, “old equipment and household waste, or everything the experts have decided to toss from the station.” Four hours later NASA astronaut Jasmin Moghbeli, who had been alerted by mission control to look out of a specific window at a specific time, was able to photograph the reentry from the ISS itself. Her photographs show what looks like a spectacular meteor. Bright trails of fire streak across the darkness of the upper atmosphere. The main capsule breaks apart into several distinct fragments as the atmospheric heating destroys its structural integrity. Then everything disperses into smaller and smaller sparks until nothing recognisable is left. Moghbeli wrote on X afterwards that the whole event lasted only two or three minutes and reminded her of fireworks.

What was inside the capsule during those two or three minutes was, among many other things, the accumulated clothing that seven astronauts and cosmonauts on the station had been wearing over the preceding several months. Somewhere in the fireball was Kononenko’s old exercise gear, along with the daily-wear cotton polos of the American, Japanese, and Danish crew members who had been aboard during the same period. Whether any of the specific garments in question had been observed by anyone on the ground as bright flashes in the night sky is impossible to say with certainty. But the fireball that briefly crossed the upper atmosphere above the South Pacific on the evening of 29 November 2023 did in fact contain, among its many other contents, the discarded underwear of at least seven people who had been living in low Earth orbit at the time.

Which is to say that the surprising claim in the title turns out to be more or less exactly what actually happens. The station has no washing machine. The clothes get worn until they can’t be worn any longer. They get packed into whichever cargo vehicle is next scheduled to depart. And the cargo vehicle, on its final trajectory back through the upper atmosphere, becomes a briefly visible streak of light in the night sky before the whole thing burns away to nothing over the Pacific Ocean. If you happen to be looking up at the right moment from the right latitude, and if you know the timing of the current Progress or Cygnus deorbit schedule, you might catch one. And what you would be watching, in the strict literal sense, is somebody’s old socks and shirts on their final journey home.