Since the first widely-publicized UFO (unidentified flying object) incident, in Roswell, New Mexico, on 7 July 1947, there have been thousands of UFO sightings per year. Since then, many have speculated that UFOs may be spaceships used by extraterrestrials to visit Earth. A 2006 survey found that 24.6% of Americans agree or strongly agree that at least some UFOs are spaceships from other worlds.
Working against this 24.6%, however, is an absence of physical evidence. Despite decades of eyewitness reports, including those of first-hand contact with extraterrestrials, no physical evidence has turned up. Not a single extraterrestrial machine, scrap of extraterrestrial fabric, fleck of paint, strand of extraterrestrial hair, or anything else that could be definitively put under a microscope and identified as not originating from the Earth.
Low earth orbit is constantly being extensively monitored by powerful radar and long-exposure space-based cameras. Tracking systems are capable of detecting any object larger than a golf ball. This is for the safety of those currently in space, as well as protecting against interpretations of falling space junk as enemy missiles. These space watch systems have never observed any extraterrestrial spacecraft. Astronomers, who watch the sky with high-magnification telescopes for a living, almost never observe unidentified flying objects.
UFO sightings experience a marked jump after events that publicize other UFO sightings, such as the Roswell incident, and movies such as ET. This is similar to the "copycat" phenomenon witnessed when serial killer stories are on the national news. This data might be interpreted as an increase in the number of imagined sightings, while legitimate UFO sightings remain constant, but if so, why are there barely any stories of UFOs being sighted in America prior to World War II?
There are several cases of unusually well-corroborated UFO sightings that could represent genuine craft. Mass sightings of low-flying, silent black triangles in 1989 and 1990 in Belgium were reported by hundreds of credible witnesses, as well as tracked via NATO radar and jet interceptors. This is an intriguing case, and there are a few others. However, the vast majority of UFO reports are very unreliable, and the complete absence of physical evidence is damning. Occam's razor dictates that UFOs should be attributed to causes other than extraterrestrial visitation.