Riverside, CaliforniaNovember 1951

In the early morning of November 28, 1951, Leo Swenson, an illustrator at Northrop Aircraft, was driving alone on Highway 91 between Colton and Riverside, California, when he spotted an unusual object in the sky. Swenson had been studying navigation and was reviewing a course when he noticed the object at 3:00 a.m. He stopped his car to observe it for about thirty minutes.

The object appeared greenish with an oval halo around a bright core, similar in character to the nearby star Vega. Swenson estimated it hung in the air at about 4,000 to 5,000 feet altitude. As he watched, the object dilated rapidly, swelling to about five times its original size, then shrank back down until it appeared to be cut in half at its center. It then returned to something close to its original brightness and size. The object moved like a rubber ball bouncing in slow motion, rising and falling vertically without swerving side to side. When it finally departed, it moved eastward at a slow speed. Swenson followed it in his car for about five miles before the object turned south and the chase ended. Eventually the object climbed, gained speed, and disappeared.

The Air Force's Office of Special Investigations became interested in the case after Swenson's account appeared in a May 1952 article in the Northrop News. An agent interviewed Swenson on March 21, 1952, and Swenson confirmed the story. However, the case was complicated by a separate issue: a photograph allegedly showing a "flying saucer" had also been published in newspapers around the same time. The Air Force investigated that photograph and found it to be a hoax, a 1937 Ford tire cover that had been buried at a location and then dug up and photographed. The investigation concluded that the photograph was fake, but Swenson's visual sighting itself remained as a separate report.

The Air Force evaluated Swenson's observation against astronomical explanations. The record notes that at 3:00 a.m. on November 22 (the case file shows some date confusion, as other documents reference November 28), the star Vega was not visible above the horizon, and Venus was not visible either. Jupiter was near the horizon to the west. The Air Force noted that the object had some characteristics consistent with an astronomical body, but concluded that no major planet was visible at the reported time of sighting. The discrepancy between the reported visibility of Vega and the astronomical facts led the Air Force to mark the case with "conflicting data" rather than a definitive explanation.

The full case file is reproduced below as held by the National Archives, 22 pages.