I have the feeling most UFO sightings have their basis in people accidentally catching a glimpse of secret government aircraft that are entirely terrestrial in origin but to people who have never seen them before look like something out of science fiction. That's because they are cutting-edge and unusually futuristic looking.
Let's say it's 1983, and you somehow see something like this around Area 51:
Pretty futuristic looking, huh? That's an F-117 Nighthawk stealth bomber, in use as early as 1981. But the government never acknowledged its existence until 1988. Just a few years later, it was basically retired, because the government had been developing another stealth bomber to replace it, in secret.
Basically, the planes we know about are probably already on their way out or retired. The ones actually being used for missions are top secret.
Take for example the following. This is going to look about as much like a UFO as you can get:
Pretty...alien looking, right? If I had this in a sci-fi movie or something you wouldn't even blink. So, what is it? Is it just a fake prop made up by a sci-fi studio or something? No, it's a U.S. stealth drone called the U.S. RQ -170 that crashed in Iran.
http://en.wikipedia.org/wiki/Iran%E2%80%93U.S._RQ-170_incident
No one outside top secret officials and the people flying and test piloting them knew of its existence until this one accidentally crashed in Iran. We asked them nicely for it back, but they refused.
Let's say you're hanging around out in the southwest in the '50s. Roswell, New Mexico, that kind of thing. We know now that there are dozens of secret military installations in that area and that they were doing testing on all kinds of weird looking military aircraft, aircraft which, to a random person, would look alien, beyond anything they'd ever seen before. But in fact they were simply test flights of advanced top secret aircraft by the U.S. government and companies like Lockheed Martin.
http://en.wikipedia.org/wiki/Skunk_Works
For fun, here is a top secret stealth sea ship built by Lockheed:
Pretty crazy looking, huh?
Who knows how many funky prototypes have cruised the skies in the past 60 years? With enough radar shielding and weird angles, anything can look like a futuristic alien space craft. And I have the feeling ones in the future are going to look even weirder.
You want UFOs? Here they are. You don't need to look beyond planet Earth.
Here's a fun article about weird shaped planes of the Cold War:
http://www.environmentalgraffiti.co...ost-incredible-experimental-aircraft-cold-war
Look familiar? I see several flying saucers.
Here is an article on 6 modern day stealth planes routinely mistaken for extraterrestrial UFOs:
http://www.popularmechanics.com/technology/aviation/ufo/4304207