The footprints are still there, the striped tread of Neil Armstrong’s boots, caked into dust. There’s no atmosphere on the moon, no wind and no water. Footprints don’t blow away and they don’t wash away and there’s no one up there to trample them. Superfast micrometeorites, miniature particles traveling at 33,000 miles per hour, are bombarding the surface of the moon all the time, but they’re so infinitesimal that they erode things only at the more or less unobservable rate of 0.04 inches every million years. So unless those footprints are hit by a meteor and blasted into a crater, they’ll last for tens of millions of years.