ALF's spaceship crashes into the Tanner Family's garage after Melmac explodes. ALF narrowly misses planet Alvin on his approach to Earth. ALF's spaceship contains gold plumbing, which he later sells to purchase a Ferarri.
ALF's spaceship presumably runs on nuclear power since ALF suggests that nuclear waste from his spaceship is stored in a canister. Perhaps unwisely, ALF also stores his crayons in such a canister.
His spaceship was borrowed for a science-fiction movie called Jupiter Guys in The Gambler, and the Tanners were paid $6,000 for it.