Scientists took care to not crash the probes into historic sites where humans have previously landed probes on the moon. Right now, the likelihood of hitting one of those sites is eight in 1,000,000, said GRAIL project manager David Lehman of NASA's Jet Propulsion Laboratory.

As for exactly what will happen to the successful twins on Monday, Lehman describes it like this:

"It's going to make a crater on the moon, and they're going to be completely blown apart at the time."