After a trip to the moon, there's nowhere to go but down.
Launched in September 2011, NASA's twin moon probes, Ebb and Flow of the Gravity Recovery and Interior Laboratory, have been orbiting the moon. Flying in tandem with the trailing Flow measuring the moon's gravitational tugs on the leading Ebb, the pair have resulted in the most accurate gravitational measurements in the solar system.
Alas, the two probes will end their mission Monday by auguring into the very ground they've been studying. Low on fuel and destined to be more man-made moon junk, the two crafts will be sent to the far side of the moon, where they will be intentionally crashed into a mountain in an act of self-disposal.
The choice of their final resting place was made carefully to avoid any chance of the twin crash landings wiping out historic lunar sites, like a moon buggy or an astronaut's famous footsteps.