Mercury's moon would be an undiscovered natural satellite orbiting the planet Mercury. One was briefly thought to exist in the early 1970s, but it turned out to be misinterpreted data from a star. Observation of a Mercurian moon from Earth would be difficult because Mercury is relatively close to the Sun. For example, Mercury was not observed in the infrared spectrum until 1995. NASA's MESSENGER spacecraft, currently orbiting Mercury, had not conclusively detected any moon as of 2012, although data from separate 2011 and 2013 searches by MESSENGER are still being analyzed.
A moon of Mercury was thought to exist for a short time. 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". 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. 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. 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.
Actually a star
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. 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.
The moon of Mercury, 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.
NASA jokingly proposed the name Caduceus, after the staff carried by the Roman god Mercury, during an April Fools' Day joke in which MESSENGER supposedly discovered a moon from Mercurial orbit. MESSENGER mission used the spacecraft to search for moons of Mercury in 2011 and 2013. As of 2012, MESSENGER mission had not conclusively observed any moon during multiple passes of Mercury, nor in its terminal orbit around the planet, although data collected in 2011 is still being examined. The Mariner 10 mission also conducted a search.
- Mercury's Moon
- Malcolm W. Browne - An Airborne Telescope Risks a Look At Mercury (1995) - The New York Times
- "MESSENGER Provides New Look at Mercury's Landscape, Metallic Core, and Polar Shadows" (Press release). Johns Hopkins University. March 21, 2012. Retrieved 2012-03-22.
- MESSENGER to Capture Images of Earth and Moon During Search for Satellites of Mercury (2013)
- Stratford, R.L. (1980). "31 Crateris reexamined". The Observatory 100: 168. Bibcode:1980Obs...100..168S. (HD 104337 near 11 58 17.515 -19 22 50.18)
- NASA - Featured Image (Note inset seems to show 243 Ida)