The Earth’s atmosphere is large and complicated. It is therefore not surprising that occasionally phenomena are observed in it that are not immediately understood. However, just because something unusual is seen in the sky does not imply that it is an extraterrestrial spaceship.

“It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.” – Sherlock Holmes.

For some of my personal history with the subject see UFOs? OK. Alien Spacecraft? No so fast.

Some references:

• Martin Gardner’s Fads and Fallacies in the Name of Science, chapter 5. Second editon 1957, and still relevant. I first read the book in the mid 1960s and it has permanently affected my thinking about UFOs and the other phenomena discussed in it.
• Philip J. Klass, UFO’s Explained (New York: Vintage Books), 1976. Klass was a senior editor of Aviation Week and Space Technology. An on-line archive of his Skeptics UFO Newsletter is available at The Klass Files.
• James Oberg, UFO’s & Outer Space Mysteries: A sympathetic skeptic’s report (Norfolk: Donning, 1982). Oberg has written extensively on space travel. He is particularly well known as an expert on the Soviet and Russian space programs.
• 99 Frequently Asked Questions {FAQs} about astronauts and UFOs
• Proof of the Prosaic Nature of the STS-48 Zig-Zag Video
• Area 51 and Gordon Cooper’s ‘Confiscated Camera’
• Robert Sheaffer, The UFO Verdict: Examining the Evidence (Buffalo: Prometheus Books, 1981).
• Ian Ridpath’s UFO skeptic pages
• President Jimmy Carter’s Sighting of a UFO
• A belief that SETI (the Search for ExtraTerrestrial Intelligence) is worthwhile does not imply acceptance of UFO’s as extraterrestrials, see UFOs.
• What about the U.S. Military and UFO stories? Again with the Aliens and Balloons points out that you need to consider their motives. They prefer that you think something is an alien spacecraft if that draws attention away from their own secret projects.