What Is Space Junk? Forty Thousand Tracked Objects and Millions Too Small to See
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A fleck of paint travelling at ten kilometres a second carries the kinetic energy of a rifle bullet, and a bolt that size has cracked the window of a space station. There are somewhere around forty thousand objects large enough to be tracked in orbit, well over a million between one and ten centimetres, and hundreds of millions smaller, and the problem is not the mass but the speed, since the energy of an impact scales with the square of the closing velocity, which in low orbit routinely exceeds ten kilometres a second.
Where it comes from
The debris population has several identifiable sources and the largest contributors are not what people assume:
- •Defunct satellites, of which thousands remain in orbit with no means of manoeuvre, along with spent rocket upper stages, which are individually massive and are the most dangerous single class
- •Explosions of leftover propellant and pressurised tanks in abandoned stages, which have historically produced more fragments than any other cause
- •Deliberate destruction, with anti-satellite weapon tests by China in 2007, the United States in 2008, India in 2019 and Russia in 2021 each producing large debris clouds, the Chinese test alone creating over three thousand trackable fragments
- •Collisions, most significantly the 2009 impact between a defunct Russian satellite and an operational Iridium communications satellite, which produced around two thousand trackable pieces
- •Mission-related objects including lens caps, separation bolts, tools and insulation blankets, and degradation products such as paint flakes and material shed by thermal cycling
- •The rapid growth of large commercial constellations, which now account for a majority of active satellites and which are operated in low orbits that decay naturally, which is the mitigating factor
Kessler syndrome
In 1978 Donald Kessler and Burton Cour-Palais published an analysis proposing that once the density of objects in a given orbital band passes a threshold, collisions generate fragments faster than the fragments decay, each collision raising the probability of the next, so the debris population grows without further launches. The process is slow rather than cinematic, playing out over decades rather than hours, and the concern is that particular orbital shells could become unusable for generations. Current assessments suggest the most crowded regions of low Earth orbit may already be past the point where the population would grow slowly even with no new launches, which is why mitigation has moved from a recommendation to a licensing requirement in several jurisdictions. The effect on astronomy is a separate complaint, since large constellations leave bright trails across long-exposure images and interfere with radio observation, which professional bodies have raised repeatedly.
What is done about it
Mitigation is the main effort and it is mostly about design and disposal. International guidelines, now written into national licensing in several countries, require that a satellite in low orbit be removed within twenty-five years of the end of its mission, a period the American regulator reduced to five years in 2022. Removal means either lowering the orbit so that atmospheric drag brings it down, which requires reserved fuel, or raising it to a graveyard orbit above the geostationary belt, where there is no atmosphere to help. Upper stages are now routinely passivated, venting residual propellant and discharging batteries so they cannot explode. Operators receive conjunction warnings from tracking networks and manoeuvre to avoid close approaches, which the space station does several times a year and which commercial operators now handle by the thousand. Shielding protects crewed vehicles against small particles using layered designs that vaporise an impactor rather than stopping it.
Cleaning up
Active removal is technically demonstrated and economically unresolved. Proposed methods include capture with a net or a harpoon, both tested in orbit by the RemoveDEBRIS mission in 2018, robotic arms, magnetic capture of prepared targets, and drag augmentation devices that deploy a sail to increase atmospheric drag. A European mission has been contracted to remove a specific piece of launcher hardware. The obstacles are cost, since a dedicated mission to retrieve one object is expensive relative to the value of removing it, and law, since under the Outer Space Treaty an object remains the property of its launching state indefinitely, so removing someone else's debris without permission is not permitted and any removal capability is also, by construction, a capability to interfere with an active satellite, which is why proposals attract suspicion. There is also a straightforward free-rider problem, since the benefit of removal is shared by every operator and the cost falls on whoever pays.
The takeaway
The danger comes from speed rather than mass, since closing velocities above ten kilometres a second give a small fragment the energy of a bullet, and the tracked population is around forty thousand objects with over a million too small to track. Abandoned upper stages exploding, deliberate anti-satellite tests and one major collision in 2009 have produced most of the fragments. Kessler's 1978 analysis described a threshold past which collisions generate debris faster than it decays, mitigation now requires deorbiting within five to twenty-five years, and active removal is technically demonstrated and legally awkward.