A discarded SpaceX rocket stage, left adrift after delivering its payloads to the moon over a year ago, will complete its unplanned journey Wednesday by striking the lunar surface at 5,400 miles per hour near Einstein Crater. No one intended this collision, yet scientists are receiving it not as a failure but as an uninvited teacher — a chance to observe how human-made debris behaves on a world humanity is preparing to inhabit. The event arrives at a moment when the moon is transitioning from a destination of rare visits to a place of sustained presence, and the questions it raises about stewa