A moon orbiting Mercury was, for a short time, believed to exist. On March 27, 1974, two days before Mariner 10 made its flyby of Mercury, instruments began registering large amounts of ultraviolet radiation in the vicinity of Mercury which, according to one astronomer, "had no right to be there".[1] By the next day, the radiation had disappeared; it reappeared three days later, appearing to originate from an object which was, seemingly, detached from Mercury.[1] Some astronomers speculated that they had detected a star, but others argued that the object must be a moon, citing the two different directions the radiation had emanated from and the belief that such high-energy radiation could not penetrate very far through the interstellar medium.[1] Adding to their arguments, the object's speed was calculated to be 4 kilometers per second (2.4 miles per second), which matched the expected speed of a moon.[1]
Soon, however, the "moon" was detected moving away from Mercury, and was, eventually, identified as a star, 31 Crateris. The origin of the radiation detected on March 27 is still unknown.[1] 31 Crateris happens to be a spectroscopic binary with a period of 2.9 days, and this may be the source of the ultraviolet radiation.[2]
Mercury's moon, although non-existent, did spark an important discovery in astronomy: ultraviolet radiation, it was found, was not as completely absorbed by the interstellar medium as was formerly thought.
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