Published October 12, 2026 · Updated October 12, 2026
An increasingly crowded orbit
Working satellites aren't the only things circling Earth. There are also spent rocket stages, dead satellites, loose parts and, above all, fragments from explosions and collisions. Tracking networks have cataloged tens of thousands of objects larger than 10 cm (4 in), and there are estimated to be more than a million fragments between 1 and 10 cm (0.4 and 4 in), too small to track one by one.
The ISS flies right through that zone. At its altitude, about 400 km (250 miles), drag from the thin traces of atmosphere makes fragments fall and burn up within a few years, which clears the orbit somewhat. But new ones keep drifting down from higher up, and the station is a big target: more than 100 m (330 ft) from end to end.
Why a small fragment is so dangerous
The problem is speed. Two objects in low orbit can pass each other at about 10 km (6 miles) per second, more than ten times the speed of a bullet. At that speed, a 1 cm (0.4 in) aluminum sphere, which weighs just over a gram, carries about as much energy as a 1,000 kg (2,200 lb) car traveling at around 40 km/h (25 mph), all concentrated on a single point.
| Size | How many (estimated) | How they're handled |
|---|---|---|
| Over 10 cm (4 in) | Tens of thousands, cataloged | Tracked from the ground; the ISS moves out of the way if one gets close |
| 1 to 10 cm (0.4 to 4 in) | More than a million | Too small to track; the shielding protects against many of them |
| Under 1 cm (0.4 in) | Hundreds of millions | The shielding stops them; they leave small craters and marks |
Keeping watch: a box around the station
The US space surveillance network follows cataloged objects with radar and telescopes and checks several times a day whether any of them will pass close to the ISS. To do that, it draws a virtual box around the station, about 2 km (1.2 miles) tall and 50 km (31 miles) long and wide, informally known as the “pizza box.” If an object is going to enter it, NASA and Roscosmos look into the case.
What matters is the probability of a collision. If it rises above roughly 1 in 10,000 and there's time, a debris avoidance maneuver is ordered: an engine burn of a few minutes that changes the station's speed slightly so the fragment passes in front of it or behind it. The burn is usually done by the engines of a docked Progress spacecraft or by those of the Zvezda module.
The ISS has made several dozen avoidance maneuvers since 1999. On our real-time map, you'll notice the predicted orbit gets recalculated every few minutes: after a maneuver like this, pass times can shift by a few seconds. We explain how this affects predictions in how we calculate ISS passes.
When there's no time: taking shelter in the spacecraft
Sometimes a fragment is spotted too late to maneuver. Then the crew takes shelter in their spacecraft, the Soyuz and the Crew Dragon, with the hatches closed, ready to undock and return to Earth if the station takes a serious hit. This has happened several times. The best-known case was on November 15, 2021, when Russia destroyed one of its old satellites, Kosmos 1408, with a missile in a military test. The impact created more than 1,500 trackable fragments, and the crew had to shelter in their spacecraft while the station passed through the debris cloud.
Shielding: layers with a gap between them
For small fragments, the defense is shielding. The modules have a thin outer wall set a few centimeters away from the pressurized wall. When a particle hits that first layer, it breaks apart and vaporizes into a cloud of debris that spreads over a larger area and no longer has the punch to get through the second wall. This design is called a Whipple shield, after the astronomer who proposed it in the 1940s, and in the most exposed areas it's reinforced with layers of special fabrics such as Kevlar.
With this protection, the station can withstand hits from particles up to about 1 cm (0.4 in). The range between 1 and 10 cm, too small to track and too big for the shielding, is the biggest concern.
Impact scars
- One of the Cupola windows has a small chip about 7 mm (0.3 in) across, photographed by British astronaut Tim Peake in 2016. It was most likely caused by a flake of paint or a bit of metal a few thousandths of a millimeter in size.
- The Canadarm2 robotic arm took a hit in 2021 that punched through its thermal cover. The arm kept working normally.
- The solar arrays and radiators collect small holes, which are studied whenever parts are brought back to Earth to learn more about the debris population.
Kessler syndrome: in 1978, NASA scientist Donald Kessler warned that if there are enough objects in an orbit, collisions between them can create fragments that cause more collisions, in a chain reaction. Keeping that from happening in the most heavily used orbits is now one of the space industry's biggest concerns.
What's being done
Space agencies and regulators are increasingly requiring new satellites to be retired at the end of their working lives, lowered so they burn up in the atmosphere within a few years, and missions to capture debris are being tested. Large constellations such as Starlink fly in low orbits precisely so their satellites will fall back on their own if they fail. If you'd like to know how they share the sky with the ISS, see our guide Starlink vs. the ISS.
Frequently asked questions
Has space debris ever hit the ISS?
Small fragments, yes, many times: there are marks on windows, solar arrays and the robotic arm. Large objects, no, because they're tracked and the station moves out of their way.
How often does the ISS dodge space debris?
There's no fixed schedule. Since 1999, it has made several dozen avoidance maneuvers, sometimes several in a single year and sometimes none for months.
What does the crew do if there's no time to dodge a fragment?
They take shelter in their spacecraft, the Soyuz and the Crew Dragon, with the hatches closed, ready to undock and return to Earth if the station takes a serious hit.
How big a fragment can the ISS withstand?
Its shielding is designed to stop particles up to about 1 cm (0.4 in). Objects larger than 10 cm (4 in) are tracked and avoided. The range in between is the most worrying.
Do avoidance maneuvers change ISS pass times?
Yes, a little. A maneuver changes the orbit, and pass times can shift by a few seconds. That's why it's a good idea to check the prediction on the day itself.
Follow the station in real time
Open the map