That video creeped me out! I mean, we'd be screwed. Luckily there are no known astroids, let alone moon-sized ones, heading towards Earth that we know of.
But if we discovered one, even with ample lead time, what could we do?
Depends on the lead time. We have mapped out most possible asteroids for 100 years which presents something of a reasonable upper limit on lead time. Deflecting a 500 mile wide asteroid ~10,000 miles on those timescales might be vaguely possible. Trying to abandon earth or build some kind of ultra deep bunkers given 100 years seems doomed to fail, but it’s not like a lot of other options exist.
But if we discovered one, even with ample lead time, what could we do?