That is what is so unsettling about T-symmetry. What you describe is merely improbable, not impossible. With appropriate starting conditions, it is certain. Bas

That is what is so unsettling about T-symmetry. What you describe is merely improbable, not impossible. With appropriate starting conditions, it is certain. Basically we'd require all the particles in the comet to bounce into each other in such a way that their heat motion cancels and organizes into momentum in one direction, away from the planet. This is physically permitted in either time-direction, but we only see it in one.