Astronauts saw them on the way to the moon. Television screens across the world carried dramatic pictures of them on New Year's Eve 1978. Unidentified flying objects - UFOs - undoubtedly exist. The question is: what are they?

In most cases those who see UFOs are not mad or drunk, but responsible adults who are sincerely convinced that they have witnessed something very strange. And the sightings are not just visual - UFOs have been photographed, filmed and tracked on radar.

So are these sightings evidence of spacecraft from other worlds-or of something more down to earth? Tonight's programme examines some of the more classic UFO cases and comes up with at least one rather startling possibility.