The problem with your question is that you do not know what a UFO is. Your question is based on the false premiss that a UFO is a spacecraft from a planet other than Earth. This is completely wrong. The letters UFO, or U.F.O., are an abbreviation for unidentified flying object. It refers to anything a person sees overhead which may be in or beyond the Earth's atmosphere. It may be a natural object or it can be a human-made object. At the time the observer views the object he cannot identify what it is. That does make it unnatural or from an alien civilisation. It simply means the observer does not know what he is seeing. Even if he is unable to ever discover what he saw it will be something that could be readily explained. Possible natural objects are bright stars, planets, meteors, birds. Human-made objects could be artificial satellites, large research balloons, aircraft, etc.