CAPE CANAVERAL, Fla.—The moon is about to get walloped by 3 tons of space junk, a punch that will carve out a crater that could fit several semitractor-trailers. The leftover rocket will smash into the far side of the moon at 5,800 mph (9,300 kph) on Friday, away from telescopes’ prying eyes. It may take weeks, even months, to confirm the impact through satellite images. It’s been tumbling haphazardly through space, experts believe, since China launched it nearly a decade ago. But Chinese officials are dubious it’s theirs. No matter whose it is, scientists expect the object to carve out a hole 33 feet to 66 feet (10 to 20 meters) across and send moon dust flying hundreds of miles (kilometers) across the barren, pockmarked surface. Low-orbiting space junk is relatively easy to track. Objects launching deeper into space are unlikely to hit anything and these far-flung pieces are usually …