Jim Kell was sent to the High-Born rulers' Throne World, with orders only to observe - until he cast away his orders from Earth and proved himself a Wolfling indeed. Gremlins Go Home by Gordon R. Dickson and Ben Bova: Suppose that elves, gremlins, and leprechauns are really tiny aliens marooned on Earth for hundreds of years.
They want to go home, and human technology finally can make it possible - it they can get aboard NASA's Mars rocket and hijack it! Pity the poor human who has to help them with the big heist