Editing Project Blue Book (section)

==Previous projects==
{{See also|Investigation of UFO reports by the United States government}}
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 F. 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.

Project 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&nbsp;— the so-called [[Project Sign|Estimate of the Situation]]&nbsp;— written in the late summer of 1948, concluded that the flying saucers were real craft, were not made by either the [[Soviet Union]] or [[United States]], and were likely [[extraterrestrial life|extraterrestrial]] in origin. (See also [[Extraterrestrial UFO hypothesis]].) This was subsequently rejected by Gen. [[Hoyt Vandenberg]], U.S. Air Force Chief of Staff, citing a lack of physical proof. Vandenberg subsequently dismantled Project Sign.<ref name="Ruppelt">{{cite book|last=Ruppelt|first=Edward J|url=http://www.gutenberg.org/cache/epub/17346/pg17346-images.html|title=The Report on Unidentified Flying Objects|publisher=Doubleday & Company|year=1960|edition=2nd}}</ref>

Project Sign was succeeded at the end of 1948 by [[Project Grudge]], which was criticized as having a [[debunker|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.