Conjunction (satellites)

Two objects predicted to pass close enough that somebody has to decide whether to move.

Tracking networks screen the catalogue continuously and issue warnings when two objects are projected to pass within a few kilometres. Operators then weigh the probability of collision — typically acting somewhere around one in ten thousand — against the fuel a manoeuvre costs. The station has moved dozens of times. The uncomfortable part is that most debris is too small to track and too large to survive: a centimetre fragment at orbital speed carries the energy of a hand grenade. The screening itself is a probability problem rather than a geometry one. Both objects have position uncertainties measured in hundreds of metres, so the answer is never "they will miss" but a collision probability computed from two overlapping error volumes — and it usually falls as the pass approaches and the tracking improves, which is why most warnings are stood down rather than acted on.