The term"flying saucer" refers to strange objects travelling through the earth's atmosphere at very high speeds. Thousands of people all over the world claim to have seen them. Some believe them to be spaceships from other planets. Others insist that they are a secret type of aircraft being built on earth. Many people maintain that they are only natural phenomena happening under special temperature Jight, or weather conditions. All reports, however, agree that the strange objects move at extremely high speeds and fail to follow the laws of motion by reversing (逆转)direction instantly without slowing down.
In the United States,the first flying saucers were reported on June 4,1947,by a private plane pilot. He saw nine objects travelling in the sky. Each was disk-shaped and very bright. Because their movements were compared to those of a"saucer skimming across the water". newspaper reporters called them "flying saucers".
Some weeks after the first reported sighting,the United States Air Force was given the job of investigating all the reports of flying saucers. Because of the confusing variely of shapes,sizes,and colors reported, the Air Force adopted the term Unidentified Flying Objects(UFO) to describe them.
Since the Air Force officials began their investigation of flying saucers, they have identified most of the objects as stars, planes, man-made satellites,experimental missiles(导弹)，or weather phenomena. Some of the UFO reports, including photographs, were deliberate (蓄意的)tricks made to cheat people.
All the research so far has failed to prove that UFOs are spaceships coming from outside the earth. Most investigators agree• however, that more study is needed before the question of whether or not flying saucers really exist can be answered.
The Air Force preferred the term UFO because ______.