[Dead Dog]

How big of an asteroid would it take to knock the moon into a decaying orbit?

I would guess one about the size it took to break the moon from Earth. That would be on the order of real big.

Orbital decay is a relative term, even geo stationary will eventually decay...if the solar systems exists long enough.

[Grig]

It wouldn't have to be that big, it took a lot to break off the moon and toss it up there, you only have to make a small change in the momentum of the moon to cause an orbital change.

Currently the moon's orbit is not stable, it is slowly getting farther from the earth, something like a quarter inch farther away per century. It wouldn't have to hit earth to cause a lot of damage either, if it made a near pass so that earth slings it out into the cosmos (Space 1999 anyone) it would cause massive tidal waves and earthquakes, might even pull some of the atmosphere away or mess with earth's orbit around the sun.