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The Giant Impact Hypothesis describes that Earth's moon was formed from collision between a proto-Earth and another Mars-sized object. This collision ejected mass from the Earth into orbit, whereupon the material formed an accretion disk, and eventually, our Moon.

The way I understand it, in order to place an object into an orbital insertion around Earth (from the Earth), two impulses are required. The initial impulse is used to lift the object out of Earth's atmosphere, on a parabolic/elliptical return-path. A second impulse must be applied in order to circularize the object's path and create an orbit.

I also understand how Gravity Capture works. However, since the aggregate matter used to form the moon was ejected from the Earth, I don't believe that this applies.

So how does the Giant Impact Hypothesis reconcile these two seemingly conflicting arguments? Is it really possible to blast an object into orbit without any other influences?

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  • $\begingroup$ If the impactor had missed the Earth, it would have kept going. One might suggest that it had to hit Earth to lose enough momentum to end up with bits in orbit. $\endgroup$ – Jon Custer Jul 15 '16 at 15:59
  • $\begingroup$ This is not necessarily the case. The jury still seems to be out as to the impactor's origin. While it may have been an object with a unique orbit about the Sun, others propose that it was captured within either the L4 or L5 points for the Earth-Sun system (indicating that it wouldn't have kept going). My question is more of the "what goes up must come down" variety. If the material was simply blasted from the surface of the earth it would either have enough velocity to escape the influence of the Earth, or it would eventually fall back to the Earth as meteoric material. $\endgroup$ – SpaceNewbie Jul 15 '16 at 16:07
  • $\begingroup$ I won't risk a full answer since this isn't my area, but my understanding is that viscosity and dynamical friction in the ejected matter circularised its orbit to form the accretion disk. $\endgroup$ – John Rennie Jul 16 '16 at 9:36

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