Fireball XL5 - Season 1 Episode 2 -

The Fireball XL5 narrowly misses a planet that has broken out of it's orbit and they realize the planet is on a collision course for the planet, Membrono. After concluding the planets are uninhabited, Steve realizes he saw a flying saucer. After the ship lands at Venus' house, the XL5 is lured back to save the planet Membrono from destruction.