“If an asteroid hits the moon, it will just get another crater,” says Gareth Wynn-Williams, an astronomer at the University of Hawaii. It would take a moon-size object to move the moon, says Clark Chapman, a planetary scientist at the Southwest Research Institute, and most likely the moon wouldn’t survive. Hitting it with a much larger, denser object would be like whacking an egg with a golf club.
But let’s say that the moon and the thing hitting it will react like solid billiard balls. None of the known asteroids larger than 60 miles in diameter orbit anywhere near the moon. OK, how about if the largest known asteroid, Ceres—which at 600 miles across is roughly the size of California and Nevada combined—did manage to slip out of its place in the asteroid belt and set out on a collision course for the moon?
Hardly a budge, Wynn-Williams says. It’s the equivalent of a four-year-old trying to knock over an NFL lineman. The moon orbits the Earth at some 0.635 miles per second. This orbital momentum is so great that it would overwhelm the impact force of a collision and just continue zinging around the planet.
By now it should be clear that the moon is staying put, but what could send it toward Earth? At minimum, you’d need an object of the same size and density as the moon to hit it at the same speed, and in the opposite direction of its orbit. This could stop the moon in its tracks, and it would fall onto Earth. Even if the collision only pushed the moon into a lower or less-circular orbit, that doesn’t mean we would escape unscathed, though: If its new orbit halved its current distance from the Earth, ocean tides would get about eight times as big, Wynn-Williams says. “A lot of New Yorkers would get very wet.”