In our mind’s eye, the gray, cratered landscape of the moon is untouched. Up there still are the iconic first human footprints, the American flag, and a plaque that reads, “Here men from the planet Earth first set foot upon the moon, July 1969, A.D. We came in peace for all mankind.” Bleached by harsh UV rays from the sun, the Stars and Stripes have disappeared and the nylon has faded to white. But the Americans didn’t just plant one flag on the moon; they planted six. And space travelers have left a much heavier footprint than simple human tread marks. Littering the lunar surface are almost 200 tons of forgotten trash. According to NASA, along with 96 bags of urine and vomit, there are old boots, towels, backpacks, and wet wipes. With no garbage cans at hand, the astronauts also littered the landing site with magazines, cameras, blankets, and shovels. And after several international missions, there are now 70 spacecraft on the surface, including crashed orbiters and rovers. Compared to Earth, the moon has a very thin atmosphere,1 so it will take some time for the evidence of our visits to erode and disappear. Arizona State University scientist Mark Robinson suggests that with ...