Lowry AFB, ColoradoApril 1958

On April 3, 1958, shortly after sunset near Lowry Air Force Base in Colorado, two experienced Air Force officers spotted an unusual object moving smoothly across the northwestern sky. Captain W.H. Gibson, a navigation instructor and amateur astronomer with roughly 1,500 hours of flying experience, and Flight Lieutenant D.J. Connolly, a Canadian exchange officer and navigation instructor with approximately 3,900 flying hours, were using a six-inch amateur telescope for celestial observations when one of them noticed the object's motion. Both men watched it with their naked eyes and through the telescope for three to four minutes as it traced an orbit-like path across the sky, passing close to the bright star Spica and the planet Jupiter before disappearing into a cloud deck toward the southeast.

The object appeared in two distinct sections with different brightness levels. The leading section resembled a third or fourth magnitude star (moderately bright), while the trailing part shone much brighter, comparable to a second magnitude star (among the brighter stars visible). Neither section showed any tumbling motion. The observers could not estimate the object's actual size because of its apparent great distance, but based on its reflectivity they judged it must be as large as or larger than Sputnik I, the first artificial satellite launched by the Soviet Union in October 1957. The object had a faint greenish tint and left no visible trail or exhaust. Its smooth, steady flight path suggested it was a satellite-type vehicle following a predictable orbital course.

Within five minutes of the sighting, the observers contacted the head of the Denver Moon Watch Team, a civilian group that tracked artificial satellites using telescopes. That contact person in turn reached the Smithsonian Institution in Washington, which coordinated satellite tracking across North America. However, within approximately 18 hours of the sighting, the Smithsonian reported no confirmation of any known vehicle matching the description, although they did establish a "Moon Watch Alert" in response to the report. The only other air activity noted near the time of the sighting was a B-52 or B-47 bomber conducting a practice bombing run over Denver about 2 to 3 minutes before the UFO appeared. Weather conditions were favorable for observation: clear skies with scattered clouds toward the horizon, unlimited visibility, temperature around 35 degrees Fahrenheit, and light variable winds. The two observers' background in astronomy and flying, along with their familiarity with how satellites actually behave in the sky, indicated a relatively high degree of reliability.

Officials at the U.S. Air Force Academy who reviewed the officers' report noted that the sighting did not match any known American satellites, and the excessive brightness made it unlikely to be a Soviet Sputnik, since such a bright object would normally have been detected by the extensive Moonwatch tracking network. A letter from Dr. J. Allen Hynek at the Smithsonian's Astrophysical Observatory suggested the case was puzzling enough to warrant direct conversation with the witnesses, though no such follow-up appears in this file. The Air Force concluded that this case was unidentified. The full case file, comprising 21 scanned pages, is reproduced below as held by the National Archives.