In fact, the very first mission to successfully fly by the moon was intended to be an impactor. Once humans began arriving on the moon with NASA's Apollo program, scientists found another reason to direct spent rocket and spacecraft stages into a collision course with the surface. These four images, captured by NASA's Lunar Reconnaissance Orbiter, show craters formed by impacts of Apollo SIV-B stages. Unintentional collisions are fewer in number, as are rocket stages, but are not unprecedented. Litter versus liabilityWithout a large human population on the moon, the risk that these planned and unplanned impacts present at current is minimal.