In those same decades, most UFO sightings were made in the daytime and frequently at close range, when shapes and surface features could be distinguished, thus making positive identification of normal sights easier and the descriptions of unusual sights more detailed. When all normal explanations had been eliminated, the witnesses could concentrate on those aspects of the experience which were most abnormal.

Hundreds of thousands of UFO sightings have been made by persons in all walks of life, in all parts of the world. Tens of thousands of UFO reports have been made to governmental and private agencies in the past 55 years. Thousands of these reports have withstood careful scrutiny and appear to represent real objects having a novel nature.

If every UFO report could be convincingly credited to some conventional astronomical or atmospheric phenomenon, there would be no UFO mystery. It is precisely because so many UFO reports cannot logically be blamed on stars, planets, satellites, airplanes, balloons, etc., that a UFO mystery has existed since at least the mid-1940s.

These abnormal aspects included the shapes of UFOs and their behavior. Most of the UFOs seen in the daytime were said to have had simple geometric shapes-discs, ovals, spheres, cylinders-and surfaces that looked like metal. Such shapes are not only nonexistent among known aircraft, but contrary to all known theories of flight, in most cases offering control and performance disadvantages rather than advantages.

Patterns of these UFOs appearance and behavior suggest a limited range of sizes and shapes of unidentified craft, despite the often-desperate efforts on the part of the American and other governments to discount them as nothing more substantial than mistakes made by naïve individuals. Their performance, observed repeatedly by expert witnesses, remains as far off the scale today as it was in the 1940s.

When authoritative reports of radical-design craft having spectacular performance are viewed in the light of a stream of astrobiological discoveries, the possibility that some UFOs are alien does not seem quite so farfetched. Serious-minded scientists in astronomy and other disciplines estimate there could be billions of planets in the universe, and millions that could harbor life. If even a few of those planets were occupied by technological civilizations, their ability (if not desire) to explore other worlds, such as ours, must be a possibility.

For more than 50 years, men and women around the world have reported sights in the sky that are strange to their experience and understanding. Inasmuch as many of these sights appeared to be solid objects having impressive performance, they have become known as Unidentified Flying Objects (UFOs) or, colloquially, flying saucers. Their positive identification is one of the major public scientific challenges of the era, with a significant number of these sights so far defying all reasonable efforts at classification.