Ask most people to rank a list of close approaches and they will sort by miss distance. It is the intuitive number. It is also the one most likely to point attention at the wrong event.
Two passes, same distance, different problems
Take two conjunctions, both with a predicted miss of two kilometres. In the first, both objects are actively tracked spacecraft with element sets a few hours old. In the second, the secondary is a fragment from a decades-old breakup, last observed three days ago, tumbling and poorly characterised.
The distances are identical. The uncertainty is not — and it is the uncertainty that determines whether two kilometres is comfortable or coincidental. The second event can carry a probability an order of magnitude higher than the first while looking identical on a distance-sorted list.
Size matters too. Probability scales with the square of the combined hard-body radius, so a large spacecraft with deployed arrays and a small cubesat present very different targets at the same range.
What ranking by probability changes
Operationally, the useful question is not what is closest but what deserves an hour of an engineer’s attention today. Those are different lists. Sorting by probability, with the hard-body radius and element age folded in, tends to surface a handful of events that a distance sort buries several screens down.
It also removes a category of wasted work. Very close passes between two well-tracked, recently-updated objects are frequently routine, and a distance-first workflow spends real time re-confirming them.
Distance still earns its place
None of this makes miss distance useless. It is the honest, model-free measurement in the whole calculation — the one number that does not depend on an assumed covariance. That is exactly why OrbitIQ still lets you set a distance threshold that raises an alert regardless of what the probability says.
Probability decides what to look at first. Distance is the backstop for when the probability model is wrong. You want both, and you want to know which one is talking to you.
Miss Distance Is the Wrong Number to Panic About