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1947–1969 · THE FRAME, STATED ONCE
Every case in this section is measured against one file system, and it is stated here once, from the National Archives' published record. On June 24, 1947, the private pilot Kenneth Arnold saw nine objects over Mount Rainier, Washington, moving at an estimated 1,700 miles per hour, and told reporters they flew "like saucers skipping on water"; the newspapers shortened it to "flying saucers," and hundreds of reports followed within weeks. The Air Force answered with a succession of three small offices at Wright-Patterson Air Force Base, Dayton, Ohio. The Chief of Staff ordered an investigation on December 30, 1947, and Project Sign ran from December 1947 to February 1949, examining 243 sightings and reporting in February 1949 that there was "no definite and conclusive evidence" either way. Project Grudge, its scaled-down successor, handled 244 reports to August 1949 and concluded the reports were misinterpretations of natural phenomena, man-made aircraft, fabrications, or hoaxes. Project Blue Book, ordered in March 1952 by Major General Charles P. Cabell and directed first by Captain Edward J. Ruppelt, ran until Secretary of the Air Force Robert C. Seamans Jr. terminated it on December 17, 1969; operations ended in January 1970.
The system's totals are the Air Force's own: 12,618 reports investigated across the three projects, of which 701 were left "unidentified" — about one in eighteen. The peak year was 1952, with 1,501 reports and 303 unidentified. Each case file covers one sighting or one group of closely related sightings, and carries a control sheet stating the Air Force's explanation or conclusion: an identified cause (aircraft, balloon, astronomical, and the like), "insufficient data," or, rarely, "unidentified." The files passed to the National Archives in 1975 and were opened to the public after a name-redaction pass concluded in 1976; they survive as Record Group 341, microfilm publication T-1206, and in the Archives' sanitized case-file series of 10,622 files. That series — the index, the control sheets, and the official list of cases left Unknown — is the record every local claim in this section is checked against. A sighting that never entered it is a press report, however often it is retold; a sighting that did enter it stands at exactly the disposition its control sheet gives it, no higher.
Sources: National Archives, "Public Interest in UFOs Persists 50 Years After Project Blue Book Termination" (archives.gov, Dec. 5, 2019), read whole; U.S. Air Force fact sheet figures (12,618 reports, 701 unidentified; 1952: 1,501 reports, 303 unidentified) as carried in the Archives' record and its parsed mirrors (RearAccess, National Archives case-file index, 10,276 files parsed with place and date).