The Boy Who Cried Comet

The Boy Who Cried Comet is an Arthur episode from Season 6

Premise
Buster thinks he's seen a UFO, but no one believes him because he's said he's seen a UFO too many times. He borrows Muffy's telescope and finds a comet headed for earth. No one believes him, but this time he has proof. They discover that the telescope is malfunctioning.