In March of 1962, an unknown object going Mach 2 mysteriously “changed appearances” mid flight. It had no visible signs of propulsion. After the Air Force investigated, they concluded it was an unknown.
These are the official documents about it from the Project Blue Book files. It’s officially considered an unknown. The below file was obtained from the National Archives & Records Administration.
Project Blue Book: Ramstein Air Force Base UFO Encounter, March 26, 1962 (Unidentified) [4 Pages, 0.8MB]