ON the afternoon of Nov. 7, 2006, pilots and airport employees at O’Hare International Airport in Chicago saw a disc-like object hovering over the tarmac for several minutes. Because nothing was tracked on radar, the Federal Aviation Administration did not investigate. Yet radar is not a reliable detector of all aircraft. Stealth planes are designed to be invisible to radar, and many radar systems filter out signals not matching the normal characteristics of aircraft. Did it really make sense to entirely ignore the observations of several witnesses? < > On Dec. 26, 1980, for instance, several witnesses at two American...