There are roughly 40,000 pieces of debris larger than a softball in Earth orbit, about 1.1 million between a marble and a softball, and an estimated 130 million smaller than a millimetre. All of it is travelling at around 17,000 mph. So what is space debris in practical terms? It is the reason the International Space Station has to fire thrusters to dodge objects several times a year, and the reason a 2021 anti-satellite test put the ISS crew into emergency shelter.
The Short Answer
Space debris — also called orbital debris or space junk — is any human-made object in orbit that no longer serves a purpose. Dead satellites, spent rocket stages, fragments from explosions, bolts, paint flecks, and frozen coolant. Meteoroids do not count; those are natural. Only the human-made material is debris.
The definition sounds pedantic, but what is space debris legally determines who is allowed to move it. Under the 1967 Outer Space Treaty a defunct satellite remains the property of the state that launched it, indefinitely. Nobody can simply remove someone else’s junk without permission.
Where It All Came From
| Source | Share of tracked debris | Notes |
|---|---|---|
| Fragmentation from explosions and collisions | Largest single source | Old rocket bodies still holding fuel tend to burst |
| Defunct satellites | Thousands | Some have been dead for 50+ years |
| Spent rocket stages | Thousands | Often large and massive |
| Mission-related objects | Significant | Lens caps, adapters, tools dropped during spacewalks |
| Anti-satellite tests | A few events, huge yields | 2007 Fengyun-1C: ~3,500 tracked pieces |
The single worst event was China’s 2007 test against its Fengyun-1C weather satellite, which produced more than 3,500 tracked fragments and an estimated 150,000 smaller ones. Russia’s 2021 test against Kosmos-1408 added roughly 1,500 more, in an orbit that crosses the ISS’s path every 90 minutes.
Why Speed Makes Everything Dangerous
At orbital velocity, kinetic energy rather than mass does the damage. A 1-centimetre aluminium sphere hitting at 17,000 mph carries roughly the energy of a small car crash. A paint fleck a fraction of a millimetre across was enough to chip a Space Shuttle window; another flight returned with a crater in its radiator. There is no armour solution at these speeds — only avoidance and shielding.
The ISS uses Whipple shields: a thin outer bumper shatters the incoming particle into a spray, and a thicker inner wall stops the spray. It works for particles up to about a centimetre. Anything bigger has to be dodged.
The Kessler Syndrome
In 1978 NASA scientist Donald Kessler described a scenario where the density of objects in low Earth orbit becomes high enough that collisions generate more debris than atmospheric drag removes. Each collision creates fragments that cause more collisions — a self-sustaining cascade that could make certain altitudes unusable for generations.
Kessler’s own assessments have been measured: he has said the cascade may already be underway in some orbital bands, but that it operates over decades to centuries, not the days-long collapse depicted in films. The honest position is that we do not know the timeline, and the risk is concentrated in the busiest band, roughly 500-620 miles up.
Tracking and Avoidance
The catalogue maintained by the US Space Force tracks more than 30,000 objects, most of them 4 inches or larger — about the size of a softball. Operators run conjunction assessments and manoeuvre when collision probability crosses a threshold, typically around 1 in 10,000. A satellite may manoeuvre a few times a year; the ISS has dodged more than 40 times since 1999.
Objects below about 4 inches are the real problem. Too small to track reliably, too big to shield against, and there are more than a million of them.
Cleanup: What Actually Works
Proposals are cheaper than hardware. Nets, harpoons, robotic arms, drag sails, and lasers have all been tested or flown. The ClearSpace-1 mission planned by ESA aims to capture a discarded rocket adapter with a four-arm grapple. Astroscale in Japan has flown a demonstration that approached a spent upper stage.
The unglamorous answer is that mitigation beats cleanup. Deorbiting within 25 years of end of mission — the guideline most agencies have adopted — costs an operator some propellant, which is far cheaper than removing debris later. Compliance is voluntary and incomplete, which is why the numbers keep rising.
Keep Exploring
- How do satellites work? — the machines that become debris when they stop
- What is a black hole? — the other place extreme speeds show up
- How far is the Moon? — the nearest place debris is not a problem
