I lost the reference but it was from some journal on Aerospace Engineering.
Basically the aeronautical engineer explained that if a space ship from outer space as many people believe they have seen were to have mass, it would sound like a nuclear bomb going off. Presumably the 'aliens' are subject to the same laws of physics as we are.
For an alien ship of the size and scope as reported by eye witnesses to hover under our gravitational field it would have to apply a reverse thruster(Newton's third law) that will create so much noise it would shatter glass windows in radius of kilometers. Thus whatever the people have seen: the flying objects didn't have mass.
A F-15 fighter jet weighs a few tons and creates a noise audible for kilometers. An alien ship would have to be larger by a magnitude of factors and thus create noise by orders of magnitude more.
What is noted in all these "alien" space ship observations is one common factor: no noise heard by anyone, which implies no mass and thus no "space aliens".