I've long since concluded that the "UFO" is not a subject that astronomers can avoid. Over the years, at nearly every public lecture I've given, at least one curious questioner has asked: "Do you believe in UFO's?" or "Have you ever seen a UFO"?
Such questions deserve to be respected and not dismissed out of hand. Knee-jerk rejection or debunking is contrary to what the scientific temperament is all about. My own strategy has been to first clarify the definition of UFO, while challenging the assumption that it is a synonym for "spaceship from another world". It means exactly what the acronym implies: Unidentified Flying Object. As such, UFOs are an observed and recorded fact, as much as the Sun rising every morning. The existence of UFOs is a matter independent of "belief".
What remains open to question is the particular interpretation of the UFO. Here is where the astronomer can provide useful insight by introducing his or her audience to the notion of scientific hypotheses. Several are applicable to UFOs, which I will explore in detail later. They include: 1- a misidentified known object (planet, airplane, or balloon); 2- an unknown natural-meteorological phenomenon; 3- a psychological phenomenon and 4- a genuine craft from another planetary civilization. Note that depending on viewpoint, an astronomer may or may not regard (1) as a valid hypotheses. I do, because further research can be used to verify it.
From all the available UFO statistics, hypothesis (1) has most often been demonstrated by sheer attrition of reported sightings. These statistics show that about 95 per cent of initially reported UFOs turn out to be "IFOs" or identified flying objects. I used to have enormous difficulty coming to grips with this statistic, but I don't any more. The fact is that the majority of sporadic skywatchers really don't know what is up there. Many cannot distinguish a star from a planet, or an extremely bright planet or star from a man-made device. I cannot begin to recount here all the times an excited caller has phoned me to breathlessly report...... Venus! Incredibly, when I correct them they retain an attitude of disbelief that any "natural" object could be that bright! Surely it must be an unnatural object! An alien craft!
As long as the night sky remains unknown to the vast majority of people, misidentifications will be the rule for "UFOs". On the other hand, I'm intrigued by the 5 per cent of reports which persist as unidentified after close scrutiny: what I call the "signal" (as distinct from the 95% "noise".) These reports cannot be solved after years of comprehensive investigations. They are what I call "the genuine UFO reports" (I use the term "UFO reports" rather than UFOs, since the existence of the reports is not in question, though their interpretation is open). These reports, in all probability, will fall into one of the other hypotheses: (2), (3) or (4). However, as we shall see, it is also possible to arrive at other hypotheses which are not so neat and tidy.
The difficulty in formulating UFO hypotheses, and interpreting UFO reports, is a first-hand experience for me. Not only have I investigated other people's reports and published the results but I have seen a "UFO"myself. The incident occurred in the summer of 1962 while at the opening of a shopping center in Carol City, Florida. While awaiting the start of festivities I happened to look up at the night sky, being the amateur astronomer that I was. Amazingly, I witnessed a brilliant orange disc, at least the same diameter as a full Moon, moving rapidly from north to south. It hovered for two to three seconds above the crowd at the shopping center and I detected the odd "Oooh" or "Aaah" from random spectators. Thus, I knew I was not having a simple hallucination (at least not by myself!)
The most ironic and notable thing to me was the complete absence of sound. No whirring, like one would expect from a helicopter's propeller blades, or engine noise. The object - if "object" it was - appeared to be a light source rather than just reflecting light from elsewhere. After about three seconds it took off due south at what I estimated to be an incredible speed. As a seasoned sky observer, even at the age of 16, I was able to quickly eliminate all known man-made or natural objects from consideration. The exceptional luminous and dynamical behavior allowed this. Nevertheless, to this day I am not prepared to pinpoint a specific hypothesis in any dogmatic sense, though up until recently I have gravitated toward (4).
Why? From my knowledge of physics, it is the only one I could fit into a conceptual framework. The key is the fact that the UFO, certainly for the brief time I observed it, exhibited a remarkable degree of intelligent control. Weather phenomena, like ball lightning, simply do not behave in this fashion. They tend to be governed by stochastic or random forces. And, on a probability scale of 0 to 1 (with 1 certainty) I would put the existence of a hitherto "unknown" meteorological phenomenon at about 1 in ten million, or 0.0000001.
There may be rare weather phenomena capable of precisely imitating
the dynamics of my UFO, but I wouldn't bet on it! Ball lightning, which is a known phenomenon, comes closest - but what I saw was not ball lightning by any stretch of imagination. (For one thing there were absolutely no clouds visible at the time, and ball lightning does not move at the speed this moved).
What about hypothesis (3), a psychological effect? This cannot be ruled out absolutely, but it is rendered highly improbable by there being a dozen or so other witnesses. If it was psychological, then it was a shared hallucination, which stretches credibility in yet another direction. That leaves hypothesis (4), or does it? At face value it would seem to be the most reasonable thing to accept that what I observed was an intelligently controlled spacecraft from another world. Somehow, however, it seems to me too facile an explanation.
Forget about my observation for the moment, and consider the vast number of UFO sightings each year. That 5 per cent of permanent unknowns translates, on average, into about ten thousand sightings each year. Surely, there cannot be ten thousand different craft visiting us each year.
Even allowing for repeat sightings, the variety of shapes and sizes would suggest a figure of at least one thousand. One thousand craft from another world - or from a thousand different worlds? No way! I simply can't accept that any race of supposedly intelligent beings would regard the human species and its humdrum little world as that important - and consume so much energy in such an extended endeavor!
Energy, even for very intelligent aliens, must be extremely resource-intensive. And though highly advanced, I can't see how these hypothetical beings could bypass fundamental physical laws such as entropy. To imagine that any civilization can afford to squander vast energy on thousands of yearly visitations to a backwater world borders on the ridiculous.
 See: Transient Optical Phenomena of the Atmosphere - A Case Study, in The Journal of the Royal Astronomical Society of Canada, Vol. 74, No. 3, June 1980.
 Jacques Vallee, in his book: Revelations: Alien Contact and Human Deception (Ballantine Books, 1991, Appendix) presents worldwide data and statistics which extrapolate to 14 million sightings over a thirty-year period. This figure,of course, renders the extraterrestrial hypothesis even more improbable. Is Earth really the "grand central station" of the Milky Way? I somehow doubt it.
 The entropy law, or 2nd law of thermodynamics, states that for every useful conversion of energy/fuel there will be a large amount of waste energy (e.g. heat) accompanying it. A corollary is that as the frequency of useful energy conversion grows, energy efficiency must go down and the amount of waste energy must increase.