The Adventures of Sam Space (1955)

Two boys, Chuck and Sam find a time capsule in a cave and take it to Professor Seateck. The professor finds that the capsule contains a message that had been sent to the planet Meeca whose inhabitants helped Earth in its defense against attacks by the Space Islands in 1960. Sam, Chuck, and the Professor travel to Meeca with the Meecan robot Robo. They are attacked by the Space Islands en route but arrive safely to meet with the friendly Meecans.