system
Given the following post, respond as if you're commenting on Reddit:
human
I am no astrophysicist.  The consensus is that the moon was formed when the proto-Earth was struck by a body the size of Mars.  The ejecta and (mostly) the Mars-sized body became the moon.  Why should it have a remotely stable orbit?

I understand why the Earth still has a stable orbit (it and the impactor were moving near the same speed and were in the same orbit), but why should the mass left over from the collision have enough kinetic energy remain in orbit and not fall back?  I'm picturing the collision as something of a bounce, as inaccurate as that may be, with massive heat, magma, taking many hours to take place.  But why did the left-overs fall into a stable orbit when in it seems they should have just fallen back to Earth again?
gpt
It is believed the earth and this hypothetic planet (Theia to some) shared an orbit, with Theia occupying the stable Lagrangian points 60^o ahead (L4) or behind (L5) of the earth.

However, these points are only stable if Theia's mass is low relative to earth's.  As it accumulated more matter, the orbit became unstable, and it eventually caught up with the earth, or the earth caught up with it.

Since the two planets would have similar orbital velocities, the collision would have been slower and less energetic that most.  Instead of being like two cars crashing head on, imagine a car going 50 kph being rear ended by one going 51 kph.

Still plenty of energy to mess up the planets themselves, but not enough to change their gravitational interaction with the sun.