Whenever there’s news of an asteroid expected to pass closely by Earth (like this one did on Halloween 2015) at least one person will typically ask “what if it hit the Moon?” (as if that’s a scenario that somehow all of the astronomers around the world who specialize in near-Earth asteroids failed to take into consideration.) I assume the expected answer would be that such an impact would offset our Moon’s oh-so-delicate position in Earth orbit and send it tumbling inwards toward an inevitable and catastrophic collision with our planet, or possibly shatter it apart completely.
As it turns out the Moon is a lot tougher than many people think. (Maybe they’d just watched too many Saturday morning cartoons.)
Any asteroid strike on the Moon would basically just leave a bright new crater. In fact even if the largest object in the asteroid belt — the dwarf planet Ceres — were to for some reason whack into the Moon it wouldn’t be destroyed nor knocked out of orbit. Not only is Ceres considerably less dense than the Moon but at 587 miles across it’s three and a half times smaller. Even a direct hit would be “the equivalent of a four-year-old trying to knock over an NFL lineman,” according to University of Hawaii astronomer Gareth Wynn-Williams in a 2013 Popular Science article.
That would be quite a show from Earth though. And Ceres itself probably wouldn’t fare so well.
So what would it take to knock the Moon out of orbit? According to the same article it’d take an object about the same size and density of the Moon itself impacting at a similar velocity — 0.635 miles per second — from the opposite direction to stop the Moon in its orbit, at which point it (or whatever was left of it) would begin to fall inwards.
Although most travel much faster than that, since none of the known NEOs or comets fall anywhere near this weight class I’d say that scenario is a quite unlikely one.
Besides, the Moon has been impacted by countless objects of all sizes over the course of its 4.53-billion-year life (and has the scars to prove it) and it hasn’t gotten knocked from orbit. If anything it’s still steadily moving farther away from Earth at a rate of 1.5 inches a year, despite still getting struck on a regular basis today.
Read more on PopSci here, and watch the video below to see an observed lunar impact event from March 17, 2013.