[EdTsft]

Why is half an orbit late? Admittedly, my knowledge of orbital dynamics mostly comes from kerbal space program but isn't half an orbit exactly when you'll have the most maneuverability relative to the collision point? 1/2+n orbits and you're just spending more time in the alternate orbit without increasing your ability to avoid the collision.

[TeMPOraL]

It's late because it's the last moment you can do this cheaply in terms of Δv. I expected them to do it n + 1/2 orbit before potential collision, with n = being at least 1, and more like 10 or 20.