Hypothetical moon of Mercury

An undiscovered moon orbiting the planet was hypothesized to exist in the early 1970s, but it turned out to be misinterpreted data from the star 31 Crateris.

[3][4] Mercury's small Hill sphere limits the potential for a natural satellite to exist.

[1] A "moon" was detected moving away from Mercury in 1974, and was eventually identified as a background star, 31 Crateris.

[6] The 'moon of Mercury' misidentification in 1974 sparked an important discovery in astronomy: ultraviolet radiation is not as completely absorbed by the interstellar medium as was formerly thought.

In an April Fools' Day joke in 2012, NASA announced that the MESSENGER spacecraft supposedly discovered a moon in orbit around Mercury, which they named jokingly Caduceus, in reference to the caduceus, the staff carried by the Roman god Mercury.

31 Crateris may be an eclipsing binary ^{[

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