Lorenz and Avelar/Workbook Stock/Getty Images You glimpse a light in the night sky -- not a star, not an airplane, but something radically different. It moves with baffling speed, pulsates with radiance beyond anything you've witnessed. Three letters immediately enter your mind: U-F-O. Technically an unidentified flying object can be anything when you get right down to it, but the term has become synonymous with extraterrestrial spacecraft. Alleged sightings began popping up in the 1950s and continue to this day throughout the world. Exact descriptions of alien spacecraft vary with each telling, but witnesses often describe a lighted object capable of hovering silently and zigzagging in midair.