The Two Moons

When they found the corpse in a grave on the Moon, wearing a spacesuit of unfamiliar design, his identity was a complete mystery. Analysis showed that the deceased was 50,000 years old - meaning that he had somehow died on the Moon before the human race even existed. On another moon, Jupiter's Ganymede, another mystery was found: a wrecked spaceship, which had been there for millennia, and was obviously designed for beings larger than the humans of Earth. The mystery seemed insoluble until another ship, manned by the same humanoid giants arrived, and were very surprised to find humans inhabiting their Solar System.