Sounds nutty, but you should read more

Apollo Astronaut Says UFOs Came to Prevent Nuclear War

Edgar Mitchell, who has long believed in ESP and the paranormal, is turning up sounding like a nutter for talking about UFOs and nuclear war.

But - before you dismiss him entirely, you should read the surprisingly good Wikipedia entry explaining the very real controversy and concern in the late 1940's that mysterious green fireballs were indeed spying on the American nuclear program.

While I had read a short account of this before in some UFO book or other, the Wiki explanation makes it pretty clear that many people had seen them, including the scientists and technicians in New Mexico, and many genuinely thought they were so odd that were not a mere natural phenomena.

It does appear to be one of the greatest UFO style mysteries still around. 

Update:   Ooh.  This report, which I don't think I read at the time, contains a suggestion from a physicist in Brisbane that sometimes meteors might cause a ball lightning effect close to the ground.   I have a feeling that there was at least one case of what looked like ground following ball lighting in New Mexico at the time of the green fireball panic, so that idea does sound half plausible:

"A transient electrical link between the ionosphere and ground, created by meteors or some other means, could help to solve the mystery of many UFO sightings," Hughes told LiveScience. "Since such balls would be very insubstantial they would be able to move and change direction very fast as has often been observed."

Hughes detailed his findings online Nov. 30 in the Proceedings of the Royal Society A.