Where the name comes from
When an orbit gets crowded enough, one collision makes the debris that causes the next. Fragments then pile up faster than they fall out of the sky, and the orbit fills with wreckage nobody can clear.
Most satellites fly in low orbit, up to about 2,000 km above the ground, where everything moves at about seven or eight kilometres a second. Two objects on crossing orbits can meet at more than ten kilometres a second. Nothing survives a hit like that: both shatter into a cloud of fragments, and each fragment flies on an orbit of its own, a little higher or lower, a little tilted, so the cloud spreads until it crosses other orbits.
Two satellites on orbits that cross reach the crossing at the same moment.
Both shatter. Every fragment flies on an orbit of its own, close to the one it came from.
The fragments spread around the planet and cross other orbits. Every hit makes another cloud.
The only thing that clears an orbit is the thin top of the atmosphere, which drags objects down until they burn up. Below about 600 km that takes several years. At 800 km, where many satellites fly, it can take centuries, and above 1,000 km a thousand years or more. Past a certain crowding, collisions make debris faster than drag removes it, and the count keeps rising even if nobody launches anything again.
Two NASA scientists, Donald Kessler and Burton Cour-Palais, worked this out in 1978. They warned that, as launches went on, a belt of debris could begin to form around the Earth before the end of the century, and that limits on launches could delay it. The effect was later named after Kessler.
China tested an anti-satellite missile on one of its own retired weather satellites, Fengyun-1C, about 860 km up. NASA called the result “the single worst contamination of low Earth orbit (LEO) during the past 50 years”. Within three months more than 1,600 pieces were being tracked, and NASA estimated there were up to 35,000 of a centimetre or more. Most were thrown into orbits that last decades or even centuries.
A working American phone satellite and a retired Russian one crossed paths over Siberia, 790 km up, at more than 11 km/s. It was the first accidental collision between two whole satellites, and both were destroyed. Within seven weeks 823 pieces had been catalogued, with many more too small to track.
Neither set off a runaway on its own. But each added thousands of pieces that can hit something else, and that is how a cascade begins.
About 40,000 objects in orbit are tracked today. ESA estimates there are more than 1.2 million larger than a centimetre: most too small to track, all big enough to wreck a satellite. ESA's summary of its 2025 report says that even with no more launches, the amount of debris would keep growing, because break-ups add fragments faster than the atmosphere takes them away.
Low orbit is a commons: it belongs to no one and is open to all. Whoever launches a satellite gets all the good of it, but the risk of the debris it might one day make is shared by everyone in orbit, including people who haven't launched yet: what Garrett Hardin called the tragedy of the commons (1968).
There are rules, but they reach only part of the sky. In 2022 the US Federal Communications Commission ruled that satellites it licenses in low orbit, if launched after September 2024, must be disposed of within five years of their mission ending; the guideline before it allowed twenty-five. Satellites outside its reach, and debris already up there, aren't covered, and nothing yet removes debris at scale.
This app's sky runs the same physics, simplified so you can watch it happen. What it's for, and what it says about the attention we all share, is on Why.
| In real orbit | In this sky |
|---|---|
| Low orbit is open to every country and company. | One sky, open to everyone who visits. Anyone can launch as often as every minute, and nobody owns it. |
| The most useful altitudes are the most crowded. | The low band comes round past the three ground stations every 3 minutes, the high band every 24 minutes, so the low band is where everyone crowds. |
| A collision makes fragments that hit other things. | Some orbits go the other way round, and two objects going opposite ways at nearly the same height can meet head-on. Each pass is a small chance; when one hits, both break into fragments, and a fragment that hits a satellite destroys it too. The more of you crowd a height, the more collisions there are. (Real collisions come from orbits crossing at an angle, and are far rarer per pass; this sky is flat and tuned so you can watch one happen.) |
| Drag clears low orbits in years and high ones over centuries. | Everything slowly falls, faster the lower it is, and burns up when it meets the atmosphere: 1 to 8 hours from the low band, 31 hours to 3 days from the high. Nothing resets the sky. |
| An operator can bring a satellite down at the end of its life. Debris can't be steered. | You can bring your own satellite down: it leaves orbit in two minutes and burns up. You can also boost one up a band, once, to stay up longer. Nobody can clear debris. |
| Every tracked object has a catalogue entry. | The catalogue keeps every launch for good. |
| Each piece of debris can be traced to the event that made it. | Every fragment traces back to the launches that made it, and every collision names who launched what, on every screen as it happens and in the catalogue. |