Possible explanations for UFOs
(in order of probability)

Common Phenomena

Military

Psychological Phenomena

Hoaxes

Extra-natural Phenomena

Space Aliens

Common Phenomena

A good percentage of UFOs can be explained by common phenomena. These include mirages caused by atmospheric thermal inversions, meteorites, moon reflections off of clouds, space junk reentering the atmosphere, flares from distant oil rigs, distant search lights, headlights reflected off low clouds or fog banks, ball lighting, Auroras, Earthquake lights, major solar flares, Venus, weather balloons, artificial satellites, St. Elmo's fire, strange cloud formations, swamp gas, light houses, etc. The list is long but can be summed up as natural and man made light sources of which the most common are listed above.

Military Craft

Military aircraft are often mistaken for UFOs since they are very fast, fly very high and maneuver unlike commercial aircraft. It is fairly certain that many of the flights of experimental and secret aircraft are the objects of UFO sightings. Everything from the U2 spy plane to the stealth fighters and bombers were the stuff of science fiction until they were revealed to be part of our arsenal. Add to that nearly silent helicopters with powerful searchlights, exotic drones that are as small as a dragon fly to as large as some of the largest aircraft and in odd shapes like saucers and triangles.

There is also speculation that the so called large triangular shaped craft are lighter than air military craft. Three apparently fixed lights are assumed to indicate that a large triangular craft is between the lights. Carbon fiber construction would make very large lighter than air crafts like this possible so its a good bet that it's military if it exists since carbon fibre construction is still very expensive.

Much is made out of UFOs that seem to maneuver and accelerate beyond the limits of human endurance. In the case of drones those limits are no longer valid. For drones the limits are set by power available and the strength of materials.

Psychological Phenomena

This is the most intriguing area. There are at least four areas that can cause people to believe things that aren't necessarily there.

The first is hypnosis which has been known for centuries. It was only in the 1970s that it was found that hypnosis could induce false memories that are as real as actual memories. It doesn't necessarily take another person, the hypnotist, for this to happen. A well known phenomena called road hypnosis can put a person in this altered state as well. A day dream, in this condition, can become a false reality. This state can be induced by repetitive sounds and vibrations like the hum of a power station, some music or even highly repetitive assembly line work.

The second is the hypergogic state that precedes sleep. In this state the body goes into a form of paralysis and a dream like state can commence. These have only recently been studied and are barely understood. The paralysis is thought to be a pre-stage to sleep, a mind-body disconnect, so that you don't sleepwalk. This is thought to be behind most of the experiences where people claim they have been abducted by aliens.

A third phenomena is the magnetic field induced brain waves in the temporal lobe. Very small magnetic pulses (about half the intensity of the Earth's magnetic field) can induce rather wild sensations in people if they're exposed to them for intervals as short as five minutes. These fields can be from small earthquakes (in granite rock since it contains quartz), from unshielded house wiring and from some appliances and devices. The kinds of experiences that people have under these conditions varies from nothing to full blown hallucinations.

The forth area is the desire to believe. All you have to do is look around at the religions around you to see that people believe rather bizarre things because it gives them hope or solace. In spite of all the evidence to the contrary 35% of the population of the US doesn't believe in Evolution. That seems harmless enough until you realize that many if not most of these people come under the spell of powerful preachers that can convert them into action on a whim like the Jonestown or more recently the Hale-Bopp mass murder and suicide. What if Ted Haggard (the head the Evangelicals before he got dethroned for having homosexual relationships) said he has God's word that all non believers should be destroyed and it should be done in the next 24 hours? Tens of thousands of Evangelicals would immediately go out and become murders. All it takes is one nut, look at Hitler who mesmerized a populace. It's best to believe nothing, question everything and take control and responsibility for one's own actions.

Hoaxes

I've pulled off a few hoaxes in my youth and probably damaged the psyches of a couple of people. I wouldn't repeat these now even though they seemed fun to do at the time. However, fun may not be the only motivator to perpetrate a hoax. The military may want to create disinformation. The US government in general and the Military in particular can have their own reasons to create disinformation and have a history of doing so. What better way to cover up advanced aircraft development activity than to create an imaginary alien presence. This then fired the Soviet imagination and they began seeing things and become even more paranoid than they already were. If this was our motivation it probably backfired.

Several sighting involved a bright light and motor vehicles which stall. The military has had silent helicopters with powerful searchlights for sometime. Add to that an EM pulse device and you have all the ingredients necessary.

If you've ever seen air shows where they show model remote control helicopters do some incredible maneuvers then you can see some of what is possible for drones.

Extra-natural Phenomena

There's probably more than a few things about nature that we haven't fully figured out yet. Science relies on repeatability so events that are rare tend not to be believed or studied. Ball lightning is a case in point. No theoretical model existed for it and it's only recently that miniature versions of it have been recreated in the lab. Unfortunately it was not created in such a way as to be easily studied. There may be a number of natural phenomena that fall into the rare but real category. They may explain some incidents of UFO and even ghost sightings. Most likely go unreported since people don't want their credibility questioned.

Space Aliens

To quote Sherlock Holmes, "If all logical solutions are exhausted the illogical, however impossible, must be the solution." The real problem is, it is impossible to eliminate all possible logical conclusions. And the reality is the alien conclusion, though unlikely, is not impossible. We have no idea of where we will be a million years from now let alone ten years into the future. Today's magic is tomorrows science.

A hundred pedaflop (one followed by seventeen zeros floating-point operations) computer may be the size of today's cell phones. Emulated humans in android bodies hundreds even thousands of years old may be the rule. Spending hundreds of years in space mostly turned off wouldn't be a problem since a copy of the same emulated human could be here on Earth carrying on business. Their memories could be merged on the return of the explorer copy. Actually the explorer wouldn't even need to return, the memory information could be communicated back at light speed. It's hard to imagine such a future but space travel, at least to nearby stars, shouldn't be a problem. So called warp drives that modify space takes incredible energies. This takes star size matter-antimatter annihilation to produce which puts this into the realm of fantasy, for us at least, for a very long time. Sorry Star Trek fans.