Between 1952 and 1969, the U.S. Air Force conducted a series of studies on UFO sightings called Project Blue Book. Though the program ended more than 50 years ago, tales of its findings and inner workings have continued to intrigue people—thanks in part to a recent History Channel series. Get to know the secretive program better.

1. Project Blue Book wasn’t the government’s first UFO study.

In 1947, a private pilot named Kenneth Arnold reportedly spotted nine glowing UFOs zooming over Washington's Mount Rainier. The public went wild for the so-called “flying saucers.” Shortly after, the U.S. government launched Project SIGN to determine if such objects were a threat. In 1948, Project SIGN purportedly published a document called the “Estimate of the Situation,” which suggested that extraterrestrials were a possible explanation for UFO sightings. As the story goes, Air Force officials destroyed the document and launched a more skeptical investigation in the late 1940s called Project GRUDGE. Blue Book came a few years later.

2. The “Estimate of the Situation” was inspired by a mind-boggling event.

In the 1960s, Air Force officials denied that the “Estimate of the Situation” document ever existed. Those who vouch for its authenticity, however, say the report was inspired by a 1948 UFO sighting in Alabama, after two experienced pilots saw a torpedo-shaped “glowing object” zip past their aircraft and rocket into the clouds. The report shocked and baffled many of Project SIGN’s researchers, though scientists would later claim the sighting was consistent with a bolide, or bright meteor.

3. “Blue Book” was named after a college testing staple.

Whether UFOs are extraterrestrial in origin is debatable. What’s undeniable is that, during the 1950s, people routinely spotted (or thought they spotted) objects flying over the United States—and it was the onus of the U.S. military to figure out what they were and whether they posed any danger. Blue Book would earn its name because, at the time, Air Force officials equated studying the phenomenon with preparing for a collegiate “blue book” final exam.

4. Officials developed a special protocol for handling UFO sightings during Project Blue Book.

A central part of Project Blue Book was the creation of a standardized questionnaire for UFO sightings. Some sample prompts: “Draw a picture that will show the shape of the object or objects … What was the condition of the sky? ... Did the object: Suddenly speed up and rush away at any time? Change shape? Flicker, throb, or pulsate?” Eventually, every U.S. Air Force base ended up designating a special officer to collect these UFO reports.

5. Thousands of reports were collected—and some haven’t been explained.

By the time Project Blue Book was closed, officials had gathered 12,618 UFO reports. Of those, 701 were never explained. Nearly half of those unidentified UFOs appeared in 1952 when a whopping 1501 UFOs were sighted. (Interestingly, that following year, it became a crime for military personnel to discuss classified UFO reports with the public; the risk of breaking the law could mean up to two years imprisonment.)

6. Project Blue Book saw five leadership changes.

Each person in command saw the purpose of Project Blue Book differently. Captain Edward J. Ruppelt, for example, treated the job as a serious scientific quest and is often lauded as the project’s most impartial leader. (Notably, he is responsible for coining the term UFO.) Major Hector Quintanilla, who took over the project in 1963, was more interested in turning Blue Book into a PR front and focused on quelling the public’s interest in UFOs—a desire that would eventually lead to charges of a government cover-up.

7. Project Blue Book made such bad scientific mistakes that Congress had to get involved.

In 1965, Oklahoma Police, the Tinker Air Force Base, and a local meteorologist using weather radar independently tracked four unexplained flying objects. Under Quintanilla’s advisement, Project Blue Book would claim that these witnesses had simply observed the planet Jupiter. The problem with this explanation? Jupiter wasn’t even visible in the night’s sky. “The Air Force must have had its star finder upside-down during August,” Robert Riser, an Oklahoma planetarium director, said at the time. A series of more badly botched scientific explanations eventually led to a congressional hearing.

8. Project Blue Book’s desire to dismiss unidentified phenomena bothered its sole scientist.

Project Blue Book had one consistent scientific consultant, astronomer Dr. J. Allen Hynek. In 1968, Hynek wrote: “The staff of Blue Book, both in numbers and in scientific training, is grossly inadequate … there is virtually no scientific dialogue between Blue Book and the outside scientific world … The statistical methods employed by Blue Book are nothing less than a travesty.” Hynek held Quintanilla in particularly low regard, saying, “Quintanilla’s method was simple: disregard any evidence that was counter to his hypothesis.”

9. new government Inquiries into UFOs have been launched.

Between 2007 and 2012, the U.S. government spent $22 million on a new UFO study called the “Advanced Aviation Threat Identification Program.” (Nowadays, UFOs are called UAPs, or “Unidentified Aerial Phenomena.”) In January 2019, more than three dozen of the program’s studies became publicly available, revealing the government’s interest in everything from warp drives to invisibility cloaks. In spring 2020, the Pentagon officially released a small batch of UFO videos; in August that same year, the U.S. Department of Defense approved the establishment of the Unidentified Aerial Phenomena Task Force.

A version of this story originally ran in 2019; it has been updated for 2021.