When the Air Force started high-altitude testing (60,000 feet) of the U-2 spy plane there was increased sightings of “UFOs."
In the mid-1950s, most commercial airliners flew between 10,000 and 20,000 feet, while military aircraft such as B-47s and B57s flew below 40,000 feet.
When the U-2 started flying at over 60,000 feet, the number of reported UFO sightings exploded.
“[UFO] reports were most prevalent in the early evening hours from pilots of airliners flying from east to west. When the sun dropped below the horizon of an airliner flying at 20,000 feet, the plane was in darkness,” the authors explain.
“But, if a U-2 was airborne in the vicinity of the airliner at the same time, its horizon from an altitude of 60,000 feet was considerably more distant, and, being so high in the sky, its silver wings would catch and reflect the rays of the sun and appear to the airliner pilot, 40,000 feet below, to be fiery objects.
“Consequently, once U-2s started flying at altitudes above 60,000 feet, air-traffic controllers began receiving increasing numbers of UFO reports.” The report adds that at the time, no one believed manned flight was possible above 60,000 feet, and so didn’t expect to see objects so high in the sky.
The original 1998 report at CIA (PDF).