Some Opinions on UFOs
I was an amateur astronomy educator for years and one of the most common questions I was asked by the public (apart from "why can't the telescope see through cloud") was "what are UFOs, and are they visitors from another planet?". I don't really blame people for taking that possibility seriously because the media (especially TV) show sensationalised, biased reports on this sort of topic and the relatively uninformed person might easily be persuaded.
After a bit of research I have concluded that there is no good reason to believe UFOs are visitors from other planets, and that all UFO reports can be covered adequately by more mundane explanations. I'm not saying that all UFOs can be explained and I'm not saying its absolutely impossible that some aren't alien visitors - I'm just saying that, given the current evidence, there's no reason to abandon conventional explanations.
There are alternative explanations for many well-known UFO cases. Unfortunately, this sort of news isn't very sensational so it isn't very widely distributed and many people don't know the cases have been "solved". Many UFO books ignore the explanations and continue to offer the alien hypothesis, even when the author knows the conventional explanation is probably true. This bias isn't restricted to UFOs, all paranormal phenomena suffer from it to some extent.
The most famous UFO case of all is probably the "Roswell Incident" and many people are convinced an alien spacecraft was involved. However, there has been almost irrefutable evidence for several years now, that the spacecraft remains were actually the wreckage of a balloon flight launched as part of a top-secret program called Project Mogul. Many of the descriptions of designs and features on the wreckage match the appearance of the balloon equipment closely.
Another source of seemingly reliable sightings are astronauts, however all of the stories have been examined by investigators and quite reasonable explanations, based on well understood visual phenomena associated with space flights, have been provided. In 1976, during a visit to the NASA Johnson Space Center in Houston, Dr. J Allen Hynek (Center for UFO Studies) concluded that none of the authentic cases had anything to do with alien spacecraft or similar phenomena. Also there are many cases atributed to astronauts which are simply fictitious.
I even have a personal experience solving a UFO case. Years ago, when I was an active amateur astronomer, I was called at the observatory about a large bright light in the north-west sky which had been hovering in place for a long time and occasionally flashed bright red and green lights. I couldn't explain all the observations I was given because no conventional source fitted the description. I actually visited the person and was shown the star Arcturus. It wasn't in the north west, it was in the north east; it was an orange point but scintillation made it look bigger and provided occasional flashes of colour; it wasn't hovering, it was moving with all the other stars. My point is that most people are unreliable observers! If I hadn't seen the object I would not have had an explanation, yet when I did see it, the explanation was easy.
The fact is, that in every case, when the evidence is examined closely it is either found to be inaccurate, unreliable, fictitious or capable of being explained better by well known conventional explanations. When a number of explanations of a phenomenon are available, its always best to take the simplest!