The Junkyard In Orbit

APublic discussion of orbital debris is conducted almost entirely in units of quantity. Catalogues are quoted, fragment counts are set beside the counts of a decade earlier, and the implied remedy is arithmetic: remove enough objects and the problem shrinks in proportion. Operators of spacecraft experience something rather different. What constrains a working satellite from one day to the next is not how many fragments exist but how imprecisely the position of any single one of them is known, because a warning can only be issued against a predicted location, and predictions carry an error that grows with every hour since the last measurement was taken. For a fragment last observed by radar three days earlier, that error may extend several kilometres along the direction of travel. The consequence is a peculiar economy of caution. Every alert must therefore be answered as though it described a solid object on a known path, even though the great majority of them describe nothing more substantial than the width of a statistical guess, and the fuel is gone whichever of the two turns out to have been the case. A control room receiving notice of an approach cannot tell whether it is looking at a genuine threat or at the ordinary spread of an uncertain estimate, and since the cost of moving is measured in fuel while the cost of not moving is measured in the loss of the spacecraft, the incentives are not symmetrical.

BMira Faltings, who leads the Innsbruck Centre for Orbital Metrology, has set out to establish what that asymmetry costs. Between 2019 and 2022 her group obtained the conjunction alerts issued to eleven operators in low orbit, some 12,000 in all, together with the manoeuvres those alerts provoked and the tracking data gathered afterwards. Her central figure is startling and has been repeated widely since. Of the manoeuvres actually carried out, 98 per cent were shown by later measurements to have been unnecessary, in the sense that the two bodies would have passed one another at a separation no smaller than the width of a city. Faltings argues that the binding constraint on activity in low orbit is therefore not the debris itself but the coarseness of the measurements, and she maintains that a modest investment in tracking would return more usable orbit than any conceivable programme of removal. Fuel spent on avoiding a phantom, she notes, shortens a mission just as surely as fuel spent on avoiding a fragment.

CKwabena Sandoval, an orbital analyst at the Accra Observatory of Near-Earth Traffic, dismisses the headline figure as an artefact of its own definition. An encounter is labelled unnecessary, he objects, only after the fact and only by using the same imprecise measurements whose inadequacy is the point at issue; had the two bodies in fact collided, the identical data would have been read as a narrow escape. His own simulations, which assume a network of optical sensors capable of fixing a fragment to within forty metres, suggest that alerts would fall by four fifths while the number of genuine near misses would remain unchanged. Sandoval is nevertheless in a weaker position than he sometimes sounds. The sensor network his model requires does not exist and has never been costed in public, his simulation treats only objects larger than ten centimetres, and he concedes that for the far more numerous fragments below that size no plausible improvement in tracking would help at all, since they cannot be seen from the ground in the first place.

DA quieter difficulty sits beneath both positions and is not a matter of measurement at all. When two working satellites approach one another there is no rule of the road establishing which of them should give way, and each operator, reasoning from its own estimate of the encounter, may decide independently to move. Two well-intentioned manoeuvres can produce a closer approach than no manoeuvre would have done. Automatic coordination exists but functions only between operators who have adopted a common data format, and in 2022 that group covered fewer than half of the active spacecraft in low orbit. The episode most often cited occurred in that same year, when two satellites belonging to different fleets each shifted orbit within twenty minutes of one another after receiving the same warning from the same source. Nothing was struck. The gap between them, however, turned out to be narrower than either operator had planned for, and what the incident demonstrated was not incompetence but the absence of any authority entitled to say which spacecraft moves and which holds its course.

EOn the narrow question this article sides with Faltings, though not on the terms in which her result is usually reported. Better tracking is the cheapest available intervention, and the case for it does not rest on the exact figure of 98 per cent, which Sandoval is right to treat with suspicion; it rests only on the uncontested point that most alerts are generated by uncertainty rather than by danger. The qualification matters more than the endorsement. Everything Faltings proposes applies solely to the tracked population above ten centimetres, and below that threshold the only defences remain shielding, sensible design and the avoidance of deliberate fragmentation. Nor does any amount of measurement settle who yields. A right-of-way convention costs nothing to write and would remove a category of risk that neither better sensors nor debris removal can touch, which is perhaps why it has attracted so little of the attention that the more photogenic remedies command.