That is crazy. Like, I always knew that was technically possible, but we quickly shot the thought experiment down as silly in high school/college. I mean, feeling the changes in gravity to monitor the surface of a planet? That's patently absurd.

I'm having a lot of fun being wrong today!

EDIT: Aaaand that's from 2004. Which means I really wasn't paying attention while in college. Considering my roommate went to JPL to work on a sister project, I feel a bit of shame just learning about it now.