How did our moon achieve orbit?

[Oldfart]

I've read on PF that you can't shoot something, like from a big gun, into Earth orbit; it will either fall back to Earth or attain escape velocity and be gone forever. Instead, you need a separate rocket burn to insert the object into orbit following launch. Makes good sense to me.

So we are told that our moon was the end result of a Mars-size body crashing into the Earth. The artist's rendition of this event always shows a big SPLAT, with collision debris showered up into space, and it's explained that a fair amount of this debris eventually collected to form our moon.

OK, it visually looks like this debris was effectively "shot" into space by the force of the impact, but where is the orbital burn? How did it get inserted into Earth orbit? What am I missing here?

Confused in Virginia, OF

 

[tony873004]

I'll take a guess. Earth was far from spherical immediately after the big splat, so the rules of the game change a bit. Any material blasted off the lobes of an elongated Earth would not need an apogee kick as Earth returns to its spherical shape.

 

[Chronos]

That is a pretty fair guess, Tony. The giant impactor hypothesis has problems. It is merely the best among current models. I lean towards the capture thing, although it invokes certain improbabilities to work. The most compelling evidence, IMO, is iron oxide abundance. The iron oxide abundance of the moon fits nicely between those of Mars and earth. That, along with other chemical signatures, suggests our moon has a unique history.