Mercury's Moon, 1974

Two days before the March 29, 1974, Mariner 10 flyby of Mercury, one instument began registering bright, extreme ultraviolent (UV) emissions that had "no right to be there." The next day, the emissions were gone. Three days later, they reappeared, apparently emanating from an "object" that seemingly detached itself from Mercury. The astronomers first thought they had seen a star. But, they had seen the emissions in two quite different directions, and every astronomer knew that these extreme UV wavelengths couldn't penetrate very far through the interstellar medium. This suggested that the object must be relatively close. Did Mercury have a moon?
After a hectic Friday, during which the "object" had been computed to move at 4 kilometers (2.4 miles) per second, a speed consistent with that of a moon, Jet Propulsion Laboratory (JPL) managers were called in. They turned the then-dying spacecraft over full time to the UV team, and everyone started worrying about a press conference scheduled for later that Saturday. Should the suspected moon be announced? But the press already knew. Some newspapers -- the bigger, more respectable ones -- played it straight; many others ran excited stories about Mercury's new moon.
And the "moon" itself? It headed straight on out from Mercury, and was eventually identified as a hot star, 31 Crateris. The origins of the original emissions remain a mystery. So ended the story of Mercury's moon. At the same time, a new chapter in astronomy began: extreme UV turned out not to be so completely absorbed by the interstellar medium as formerly believed. The Gum nebula has turned out to be a emitter in the extreme UV, and spreads across 140° of the night sky at 540 angstroms. Astronomers had discovered a new window through which to observe the heavens.