In the 1980s, Jim Hollopeter helped design rockets that shot into orbit. Today, some of those launchers are still cluttering up space, and he wants to wash them away with a rocket-powered water gun.

Like many aerospace engineers, Mr. Hollopeter is worried about thousands of pieces of useless equipment circling Earth. Bits of spent rocket boosters, old exploded satellites and tools dropped by space-walking astronauts are just some of the trash racing along in the near-vacuum of space.

The...