Leonape, PASeptember 1955

On September 23, 1955, at around 12:30 p.m., a housewife in Leonape, Pennsylvania, heard a sputtering sound like a light airplane engine and stepped outside to look. Instead of seeing an aircraft, she spotted an unusual oval-shaped object that was charcoal gray in color, moving across the sky in a westerly direction. She watched it rotate counter-clockwise as it traveled. The sighting lasted between two and five minutes.

The witness retrieved binoculars and noticed additional details: a small blister or bump on the underside of the object, and a circular object nearby that seemed to move independently. She described the main object's appearance as resembling the cover of an alligator skin purse. An unidentified airliner flying low passed beneath and to the right of the object during the sighting. The weather at the time was overcast with a ceiling of about 3,500 feet. No photographs were taken.

Air Force investigators checked with numerous weather stations and airports across Pennsylvania, Delaware, Maryland, and Virginia to determine the object's identity. They discovered that a radiosonde weather balloon, which carries instruments to measure atmospheric conditions, had been released from a station near Swarthmore, Pennsylvania, at 10:16 a.m. that same day. The balloon's tracking data and wind patterns made it theoretically possible, though not certain, that it could have drifted into the area near West Chester around the time of the sighting.

The preparing officer concluded the sighting was probably caused by the radiosonde balloon, noting that the object's appearance resembled such a balloon and that no sound was heard, suggesting it was not self-propelled. The approving officer concurred with this assessment. The Air Force evaluated the case as "probably balloon." The complete case file is reproduced below as held by the National Archives, consisting of 16 pages.