Project Blue Book was one of a series of systematic studies of unidentified flying objects (UFOs) conducted by the United States Air Force. Started in 1952, it was the second revival of such a study (the first two of its kind being Projects Sign and Grudge). A termination order was given for the study in December 1969, and all activity under its auspices ceased in January 1970.
Project Blue Book had two goals:
to determine if UFOs were a threat to national security, and
to scientifically analyze UFO-related data.
From Wikipedia