It's likely not an anniversary anyone thought we would meet after the first moon landing, but today marks 40 years since Gene Cernan left the last footprint on the moon as Apollo 17 ended its mission. That was the last of six manned missions to the lunar surface (nine including those that didn't land), which saw twelve men actually walk on the moon in all. The years since have of course seen continued exploration of the moon through other means, though, and next week will see another major event when NASA's twin GRAIL spacecraft conduct a planned crash into a mountain near the lunar north pole. Those have been in orbit since January 1st, creating a high-resolution map of the moon's gravitational field and collecting data that promises to provide more detail than ever about its internal structure and composition. You'll be able to follow along on NASA's website as that happens beginning at 5PM Eastern on Monday, December 17th.

[Image credit: NASA / Eugene Cernan]