What Is Space Debris? The Junk That Could Close Orbit

Tracking networks catalogue objects larger than around ten centimetres in low orbit, and the numbers are only the visible part of the problem:

• •Tens of thousands of tracked objects, of which only a small fraction are working satellites
• •Spent upper stages of rockets, which are large, massive and remain in orbit for decades or centuries
• •Dead satellites that were never deorbited, including several that failed before reaching their disposal orbit
• •Fragments from explosions, mostly caused by leftover fuel or pressurised tanks rupturing years after a mission ended, which was historically the largest single source
• •Fragments from collisions and from deliberate anti-satellite weapon tests, several of which produced thousands of trackable pieces in single events
• •An estimated million objects between one and ten centimetres, which are too small to track reliably and large enough to destroy a spacecraft
• •Hundreds of millions of pieces below a centimetre, which erode surfaces, pit windows and sever exposed cables

Donald Kessler, working at NASA in 1978, described the mechanism that makes debris different from ordinary pollution. Each collision creates fragments, and those fragments raise the probability of further collisions, which create more fragments. Above a certain density the process becomes self-sustaining, so that the debris population continues growing even if nothing further is launched, a condition now called the Kessler syndrome. The important consequence is that the problem has a threshold rather than being linear, and that stopping new launches would not stop it once that threshold is passed. Two events demonstrated the mechanism in practice: a Chinese anti-satellite test in 2007 that destroyed a defunct weather satellite and created the largest debris cloud on record, and the accidental collision in 2009 between a dead Russian satellite and an operational communications satellite, which produced thousands more fragments. Between them those two events substantially increased the tracked population in the most congested altitudes.

Working around debris consumes real resources. Conjunction analysis compares predicted orbits and flags close approaches, and operators perform avoidance manoeuvres when the calculated probability of collision exceeds a threshold, which costs fuel and therefore mission life and which happens routinely for large constellations and for crewed vehicles. Uncertainty is the practical difficulty, since orbit predictions degrade with atmospheric drag variations driven by solar activity, so a warning frequently cannot distinguish a real threat from a near miss and operators manoeuvre anyway. Shielding protects against small particles, using spaced layers that break up an impactor and spread its energy, which works below about a centimetre and is useless against anything larger. Crewed stations have sheltering procedures for high-risk conjunctions. Insurance, launch scheduling and even ground-based astronomy are affected, since the growth of large constellations has added both collision risk and bright streaks across telescope images.

Mitigation guidelines, agreed internationally and implemented in national licensing, now require operators to passivate spacecraft at end of life by venting fuel and discharging batteries so they cannot explode, and to remove them from congested regions within a set period, commonly twenty-five years and being shortened to five in some jurisdictions. Compliance has improved and remains incomplete. Active removal, meaning capturing and deorbiting objects already up there, has been demonstrated in small-scale missions using nets, harpoons and magnetic capture, and the obstacles are as much legal and financial as technical: an object remains the property of the launching state indefinitely under the outer space treaty, so nobody may remove someone else's debris without permission, and no mechanism exists to pay for removing objects that benefit everyone. Proposed economic instruments include orbital use fees and deposits refunded on successful disposal. The physics is unforgiving on one point: the objects most worth removing are the large spent stages, because each one that fragments would add more debris than decades of ordinary operations.

Objects in low orbit meet at closing speeds of tens of thousands of kilometres an hour, so even centimetre-scale fragments destroy spacecraft, and each destruction creates thousands more fragments. Kessler described in 1978 how that becomes self-sustaining above a density threshold, and a 2007 weapon test and a 2009 accidental collision together added thousands of tracked pieces. Mitigation now requires passivation and timely disposal, while active removal is blocked as much by ownership law and funding as by engineering.