Earlier this week, two probes that had spent the past year orbiting the moon for NASA's GRAIL missionslammed into the lunar surface, destroying themselves and their communications connection to Earth.
None of this was an accident: Crash-landings like this are a typical method of bringing unmanned lunar missions -- and unmanned planetary missions in general -- to a close. This means, however, that NASA's typical method of mission conclusion involves, inevitably, leaving debris strewn on planets across our solar system. And it means that the moon, in particular, currently hosts nearly 400,000 pounds of man-made material. In epic terms, the lunar surface bears human footprints that are as figurative as they are literal, objects of earthly origin that have found their final resting place in the most otherworldly mausoleum imaginable. In less epic terms: We regularly leave trash on the moon.