Description:

An expedition sets out from Earth to discover whether the asteroid belt was once a planet, and whether the ancient Greek myth of Phaeton is connected with this.

The theory that there could once have been a planet between Mars and Jupiter was first conceived in the early 19th century - see the Wikipedia article Phaeton (hypothetical planet). Since the latter half of the 20th century (around when this film was made), most astronomers have considered it more likely that the gravity of Jupiter would have prevented a planet from ever having formed there.