The United States Air Force Project Blue Book

Previous Investigative Projects

Public USAF UFO studies were first initiated under Project Sign at the end of 1947, following many widely publicized UFO reports (see Kenneth Arnold). Project Sign was initiated specifically at the request of General Nathan Twining, chief of the Air Force Materiel Command at Wright-Patterson Air Force Base. Wright-Patterson was also to be the home of Project Sign and all subsequent official USAF public investigations.

Sign was officially inconclusive regarding the cause of the sightings. However, according to U.S. Air Force Captain Edward J. Ruppelt (the first director of Project Blue Book), Sign's initial intelligence estimate (the so-called Estimate of the Situation) written in the late summer of 1948, concluded that the flying saucers were real craft, were not made by either the Russians or U.S., and were likely extraterrestrial in origin. (See also extraterrestrial hypothesis.) This estimate was forwarded to the Pentagon, but subsequently ordered destroyed by Gen. Hoyt Vandenberg, USAF Chief of Staff, citing a lack of physical proof. Vandenberg subsequently dismantled Project Sign.

Project Sign was succeeded at the end of 1948 by Project Grudge, which had a debunking mandate. Ruppelt referred to the era of Project Grudge as the "dark ages" of early USAF UFO investigation. Grudge concluded that all UFOs were natural phenomena or other misinterpretations, although it also stated that 23 percent of the reports could not be explained.

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