A piece of a SpaceX rocket is about to hit the moon.

A discarded Falcon 9 rocket will collide with the moon at 5,400 miles per hour. A roughly five-ton piece of a SpaceX rocket that sent two lunar landers to the moon is about to join them, but not on purpose. On Aug. 5 at around 2:34 a.m. ET, the upper stage of a discarded Falcon 9 rocket will collide with the moon at 5,400 miles per hour near the crater Einstein on the western lunar limb, according to Bill Gray, who runs Project Pluto and develops software for tracking astronomical objects. Gray was the first to calculate that the object...