It is just a matter of using the process of elimination. For an example, do we have flying vehicles capable of flying at hypersonic speeds within the atmosphere and not create a sonic boom? Some UFOs have been tracked at over thousands of miles per hour and in 1952, one UFO was tracked in the Washington D.C. area at 7200 mph and no sonic boom was heard. In another case, multiple UFOs flew near a B-29 in 1952 near Galveston, Texas at over 5000 mph.
Based on my own experience and what I know from declassified government intelligence and military documents --among other many other things--ET is here and has been for centuries. Here is one small example of thousands of references where flying saucers were tracked by Project Mogul and other balloon teams.
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UFO encounters by military and civilian pilots are nothing knew and the advanced technology nature of UFOs encountered underlines the fact the objects are not ours. In the America West encounter, the object was hundreds of feet long, and I might add the UFO was confirmed by a F-117 stealth fighter pilot flying in the general area.
Audio tape recordings between civilian and military radar personnel made during the UFO encounter have been available to the public where the military confirms on tape the tracking of that huge UFO, but I find the Japan Airlines 1628 UFO encounter over Alaska even more interesting because of the huge size of the UFO that was described as larger than two aircraft carriers, which was also confirmed by FAA and Air Force ground-based radars. FAA documents on the Japan Airlines UFO encounter are now available to the public. The radar data was flown to the East Coast and examined by hardware and software specialist who later confirmed the huge size of the UFO that maneuvered around the B-747, which was accompanied by two smaller objects.
A bit more than two months later, an Air Force KC-135 encountered a similar object of size and shape in the general region and communication tapes of that UFO encounter with the KC-135 are now available to the public along with letters from the FAA controllers who were involved in the JAL 1628 incident. Not long after the KC-135 encounter, an Alaska Airlines jet encountered a UFO in that general region.
I have also been aware that a detachment from Wright-Patterson AFB was sent to Alaska and they also encountered a UFO. Photos taken of the UFO during that encounter were then classified and sent via courier back to Wright-Patterson AFB, and I might add the photos are still classified to this very day, but I can also add that gun video footage taken by a climbing Air Force jet of a UFO that was hovering above 30,000 feet over Wright-Patterson AFB has been released to the public.
I find the Navy pilot UFO encounters interesting but expect more such revelations to be revealed by the government in the future.