How did the Moon get into orbit?

Question

It's the accepted theory that Moon was formed when an object collided with Earth, throwing off a lump of matter which became the Moon.

But also, an object cannot be launched into orbit from a planet without extra force being applied once it's in space, because otherwise, either it leaves at faster than escape velocity and never returns, or if it does return, its orbital path - being an ellipse - ends up where it began and so the object would just collide back into the planet.

So how did the Moon manage to achieve orbit?

Answer 1

You're right in that, absent post-collision interactions, material ejected during the impact would either leave on a hyperbolic orbit or be placed in an elliptical orbit that would have it falling back to the proto-Earth within one orbit.

However, the hypothesized collision is an extremely messy environment, and you do have significant interactions between all components of the system, both mechanically and gravitationally, which are able to re-distribute both energy and angular momentum within the ejected material (and to exchange both quantities with the proto-Earth core). This is what enables enough material to be placed on an accretion disk with (i) enough energy to be in orbit, but not enough energy to reach escape velocity, and also (ii) enough angular momentum that the perigee is high enough to avoid falling back to the surface. This accretion disk will then re-coalesce into the Moon on relatively short timescales.