The Mantell UFO incident

Yesterday was the anniversary of the event; here are excerpts from the Wikipedia entry:

The Mantell UFO incident was among the most publicized early UFO reports. The incident resulted in the crash and death of 25-year-old Kentucky Air National Guard pilot, Captain Thomas F. Mantell, on 7 January 1948, while in pursuit of a UFO... Mantell was an experienced pilot; his flight history consisted of 2,167 hours in the air, and he had been honored for his part in the Battle of Normandy during World War II. 

On 7 January 1948, Godman Army Airfield at Fort Knox, Kentucky received a report from the Kentucky Highway Patrol of an unusual aerial object near Maysville, Kentucky. Reports of a westbound circular object, 250 feet (76 m) to 300 feet (91 m) in diameter, were received from Owensboro, Kentucky, and Irvington, Kentucky. At about 1:45 p.m., Sgt Quinton Blackwell saw an object from his position in the control tower at Fort Knox. Two other witnesses in the tower also reported a white object in the distance. Base commander Colonel Guy Hix reported an object he described as "very white," and "about one fourth the size of the full moon... It remained stationary, seemingly, for one and a half hours." Observers at Clinton County Army Air Field in Ohio described the object "as having the appearance of a flaming red cone trailing a gaseous green mist" and observed the object for around 35 minutes...

Four P-51 Mustangs of C Flight, 165th Fighter Squadron Kentucky Air National Guard already in the air—one piloted by Mantell—were told to approach the object... Only one of Mantell's companions, Lt. Albert Clemmons, had an oxygen mask, and his oxygen was in low supply. Clemmons and a Lt. Hammond called off their pursuit at 22,500 feet (6,900 m). Mantell continued to climb, however. According to the Air Force, once Mantell passed 25,000 feet (7,600 m) he supposedly blacked out from the lack of oxygen (hypoxia), and his plane began spiraling back towards the ground...

..although Venus was roughly in the same position as the UFO, astronomers working for Project Sign ruled that Venus would have been nearly invisible to observers at that time of day. The cause of Mantell's crash remains officially listed as undetermined by the Air Force...

Dr. J. Allen Hynek, a professor of astronomy and a scientific consultant to Project Sign, suggested Mantell had misidentified a US Navy Skyhook weather balloon. Others disputed this idea, noting that no particular Skyhook balloon could be conclusively identified as being in the area in question during Mantell's pursuit. Despite its shortcomings, others thought the Skyhook solution was plausible: the balloons were a secret Navy project at the time of Mantell's crash, were made of reflective aluminum, and were about 100 feet (30 m) in diameter, perhaps consistent with Mantell's description of a large metallic object, and may furthermore be consistent with the motions reported by the other UFO witnesses. Since the Skyhook balloons were secret at the time, neither Mantell nor the other observers in the air control tower would have been able to identify the UFO as a Skyhook.

Photo from imgur, via Dan Duran.