Project Blue Book: The Air Force and Twenty-Two Years of UFO Investigation
From 1947 to 1969 the U.S. Air Force investigated more than twelve thousand UFO reports under Sign, Grudge, and Blue Book — the largest formal government study until the modern UAP era.
Between 1948 and 1969, the United States Air Force operated a formal program to investigate reports of unidentified flying objects, conducted in three successive phases: Project Sign (1948), Project Grudge (1949–1951), and Project Blue Book (1952–1969). Over those two decades the Air Force collected and analyzed more than 12,600 reports, ending with the University of Colorado's Condon Report and the program's formal closure in December 1969 — concluding that no UFO report represented a threat to national security and none demonstrated technology beyond the era's known science.
Sign and Grudge
Project Sign, established at Wright-Patterson Air Force Base in late 1947, produced what its staff called the "Estimate of the Situation," arguing that the best explanation for the accumulated reports was an interplanetary origin. Chief of Staff Hoyt Vandenberg rejected the estimate; Sign was dissolved into the more dismissive Project Grudge in 1949, which briefly shut down meaningful reporting before the 1952 Washington radar events revived institutional concern.
Blue Book
Reactivated as Project Blue Book in 1952 and led at its fullest by Captain Edward Ruppelt, the program standardized reporting and introduced the term "unidentified flying object" to replace the press's "flying saucer." Each report was categorized as either "identified" — an astronomical object, aircraft, balloon, weather phenomenon — or "unidentified." By the program's end, roughly 701 of the 12,618 cases remained categorized unidentified, a residual rate of about 5.6 percent.
The Condon Report and closure
In 1966, facing a rising caseload, the Air Force contracted the University of Colorado to conduct an independent review under the physicist Edward Condon. The Condon Report, published in 1968, concluded that two decades of study had produced no evidence that unidentified flying objects represented advanced technology or a national-security threat, and that further study was unlikely to be justified. On the basis of that report, the Air Force closed Project Blue Book in December 1969.
The gap that followed
The closure of Blue Book left a gap in formal U.S. government reporting that lasted more than half a century. Reporting didn't stop; it simply reverted to ad hoc channels and private organizations. It was not until the Navy's UAP reporting guidelines in 2019, the 2020 Pentagon video releases, and the eventual establishment of the All-domain Anomaly Resolution Office that the United States returned to organized study of the subject at anything approaching Blue Book's institutional scale.
What the record still offers
Blue Book's full case files — more than 12,600 reports, microfilmed and released to the National Archives — remain the single largest historical dataset of government-recorded sightings in the public record. They are the foundation against which both modern civilian reports and contemporary government studies can be compared.