Collision Watch

Pc (probability of collision) is not the literal chance two objects will collide — it's the modeled probability that both objects will pass within a specified combined distance of each other at time of closest approach, given known uncertainty in their tracked positions. It's deliberately conservative: when in doubt, the models overestimate risk rather than underestimate it.

Contrary to common assumption, a smaller miss distance does not automatically produce a higher collision probability. Pc is determined by three interacting variables: physical miss distance, the combined dimensions of both objects, and the positional uncertainty (covariance) of their orbital tracks.

• Precisely tracked objects can have a moderate miss distance but very low Pc, since their true position is tightly known.
• Poorly tracked objects can have a larger miss distance but higher Pc, since their true position could plausibly fall anywhere within a wider uncertainty region — some of which may overlap the collision zone even at a seemingly comfortable distance.
• Counterintuitively, Pc can decrease again at very extreme uncertainty, since the same probability spreads across such a large area that its concentration over the actual collision zone thins out.

This is why sorting by "Highest Risk" (Pc) on this page sometimes surfaces an event with a larger miss distance above one with a smaller miss distance — that's expected behavior, not an error.