TCA — time of closest approach — is the predicted instant two space objects reach minimum separation during a conjunction. Every close-approach warning is anchored to it: miss distance, relative velocity and collision probability are all quoted at TCA, and every avoidance decision counts down to it.
Understanding TCA
The encounter plane picture
Because the dangerous window is so short, analysts collapse the 3-D fly-by into a 2-D picture: a plane through one object, perpendicular to the relative velocity, crossed by the other object at TCA. Miss distance becomes a point on that plane, and both objects' covariances project onto it as ellipses. The probability of collision is then the chance that the crossing point lands within the combined hard-body radius — an integral over a disc on the encounter plane. The geometry also explains why the radial component of miss distance is the most trusted: orbital uncertainty stretches along-track, so a healthy radial gap survives data errors that would swallow an along-track one.
Why manoeuvring early beats manoeuvring hard
Orbital mechanics rewards patience. A tiny along-track burn changes the orbital period slightly, and the displacement it buys compounds every revolution — a few centimetres per second applied a day before TCA can shift the encounter by kilometres, while the same burn an hour before achieves metres. This is why the decision timeline in conjunction operations is so disciplined: wait as long as possible for tracking to firm up (late CDMs carry the truth), but never so long that the remaining time cannot convert a feasible burn into a safe distance. Screening the post-manoeuvre trajectory closes the loop, confirming the dodge does not create a new conjunction elsewhere.