[Grig]

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

[boris]

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

Wrong question.

Need assumptions. From what direction? How fast? Etc.

Essentially 'impossible'.

Fuggedaboudit. A big rock hitting Earth is much more likely.

[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.