A discarded SpaceX rocket stage, left adrift after completing its mission more than a year ago, will collide with the moon Wednesday near Einstein Crater at 5,400 miles per hour — not by design, but by the quiet arithmetic of orbital mechanics. The event, visible to telescopes across the Americas as a plume of lunar dust, is the second known accidental rocket impact on the moon and arrives at a moment when humanity is preparing to send its people there to stay. Scientists welcome the rare chance to study what debris does to a world we are learning to inhabit, even as the crash itself asks a de