Project Blue Book investigated unidentified flying objects (UFOs). This UFO was photographed over Trindade Island, Brazil. Although never proven to be a hoax, the photographer was famous for publishing an article entitled "A Flying Saucer Hunted Me at Home," that used trick photography.
Photograph by Alan Barauna, courtesy National Geographic

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    On December 17, 1969, the United States Air Force closed Project Blue Book, its last program to investigate reports of unidentified flying objects (UFOs). 
     
    Since its founding in 1947, Project Blue Book recorded more than 12,000 UFO sightings. Military leaders were concerned that some UFO sightings may have been experimental technology developed by the Soviet Union, the U.S.’s rival in the Cold War. Many civilians, on the other hand, associated UFOs with outer space and extraterrestrial intelligence.
     
    In the end, Project Blue Book found no evidence of technology “beyond the range of modern scientific knowledge” or threats to national security. Almost all UFO sightings investigated by the program were explained by weather phenomena, existing technology, or experimental technology developed by the U.S. itself. In fact, some of the unusually shaped UFOs spotted over Air Force bases were top-secret prototypes of U.S. aircraft such as the U-2 and A-12 spy planes.
  • Term Part of Speech Definition Encyclopedic Entry
    aircraft Noun

    vehicle able to travel and operate above the ground.

    Air Force Noun

    nation's military aviation forces.

    associate Verb

    to connect.

    civilian Noun

    person who is not in the military.

    cloud Noun

    visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

    Encyclopedic Entry: cloud
    Cold War Noun

    (1947-1991) conflict between the Soviet Union (and its allies) and the United States (and its allies). The two sides never confronted each other directly.

    concern Verb

    to worry or take an interest in.

    discover Verb

    to learn or understand something for the first time.

    evening star Noun

    planet seen in the western sky soon after sunset, usually Venus.

    evidence Noun

    data that can be measured, observed, examined, and analyzed to support a conclusion.

    experimental Adjective

    having to do with a test or trial version.

    extraterrestrial Adjective

    located or formed outside Earth's atmosphere.

    intelligence Noun

    ability to learn and understand.

    investigate Verb

    to study or examine in order to learn a series of facts.

    military Noun

    armed forces.

    national security Noun

    programs to protect a nation through economic, social, and political policy.

    outer space Noun

    space beyond Earth's atmosphere.

    phenomena Plural Noun

    (singular: phenomenon) any observable occurrence or feature.

    planet Noun

    large, spherical celestial body that regularly rotates around a star.

    Encyclopedic Entry: planet
    prototype Noun

    early version or model.

    puzzle Verb

    to confuse or baffle.

    record Verb

    to preserve observations with notes, drawings, photographs, audio or video recordings.

    rival Noun

    opponent.

    Soviet Union Noun

    (1922-1991) large northern Eurasian nation that had a communist government. Also called the Union of Soviet Socialist Republics, or the USSR.

    spacecraft Noun

    vehicle designed for travel outside Earth's atmosphere.

    technology Noun

    the science of using tools and complex machines to make human life easier or more profitable.

    UFO Noun

    (unidentified flying object) any unexplained moving object observed in the sky.

    weather Noun

    state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

    Encyclopedic Entry: weather
    weather balloon Noun

    hydrogen-filled balloon equipped with tools to measure temperature, humidity, pressure, and other aspects of the atmosphere.

    weather pattern Noun

    repeating or predictable changes in the Earth's atmosphere, such as winds, precipitation, and temperatures.