Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

is the closest planet to the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

and lacks any

natural satellites A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...

. 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 31 Crateris is a binary star system in the constellation Corvus (constellation), Corvus. Varying between apparent magnitudes 5.19 and 5.23 over 1.48 days, it has the variable star designation of TY Corvi. It is actually a remote system wi ...

. Observation of a moon of Mercury from Earth would be difficult because Mercury is relatively close to the Sun.Malcolm W. Browne - An Airborne Telescope Risks a Look At Mercury (1995) - The New York Times
/ref> For example, Mercury was not observed in the infrared spectrum until 1995.

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

's ''

MESSENGER ''MESSENGER'' was a NASA robotic space probe that orbited the planet Mercury between 2011 and 2015, studying Mercury's chemical composition, geology, and magnetic field. The name is a backronym for "Mercury Surface, Space Environment, Geoche ...

'' spacecraft, which orbited Mercury from 2011 to 2015, did not detect any moon. Mercury's small

Hill sphere The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sp ...

limits the potential for a natural satellite to exist.

''Mariner 10'' mission

A moon of Mercury was for a short time thought to exist. On March 27, 1974, two days before ''

Mariner 10 ''Mariner 10'' was an American Robotic spacecraft, robotic space probe launched by NASA on 3 November 1973, to fly by the planets Mercury (planet), Mercury and Venus. It was the first spacecraft to perform flybys of multiple planets. ''Ma ...

'' made its flyby of Mercury, instruments began registering large amounts of

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

radiation in the vicinity of Mercury that, according to one astronomer, "had no right to be there".Mercury's Moon
"Hypothetical Planets". Paul Schlyter. 2009. By the next day, the radiation had disappeared; it reappeared three days later, appearing to originate from an object that was, seemingly, detached from Mercury. Some astronomers speculated that they had detected a

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

, but others argued that the object must be a moon, citing not only the two different directions from which the radiation had emanated but also the belief that such high-energy

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

could not penetrate very far through the

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

. Arguments in favor of a moon were strengthened when the object's speed was calculated to be , which matched the expected speed of a moon.

31 Crateris

A "moon" was detected moving away from Mercury in 1974, and was eventually identified as a background star,

. 31 Crateris is a

spectroscopic binary A binary star is a system of two star, stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separa ...

with a period of 2.9 days, and this may have been the source of the ultraviolet radiation detected in 1974. (HD 104337 near 11 58 17.515 -19 22 50.18) The 'moon of Mercury' misidentification in 1974 sparked an important discovery in astronomy: ultraviolet radiation is not as completely absorbed by the

as was formerly thought.

''MESSENGER'' mission

In an

April Fools' Day April Fools' Day or All Fools' Day is an annual custom on 1 April consisting of practical jokes and hoaxes. Jokesters often expose their actions by shouting "April Fools!" at the recipient. Mass media can be involved in these pranks, which may ...

joke in 2012,

announced that the ''

'' spacecraft supposedly discovered a moon in orbit around Mercury, which they named jokingly ''Caduceus'', in reference to the

caduceus The caduceus (☤; ; la, cādūceus, from grc-gre, κηρύκειον "herald's wand, or staff") is the staff carried by Hermes in Greek mythology and consequently by Hermes Trismegistus in Greco-Egyptian mythology. The same staff was also ...

, the staff carried by the Roman god Mercury.NASA - Featured Image
(Note inset seems to show

243 Ida Ida, minor planet designation 243 Ida, is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after a nymph from Greek mythology. ...

) The ''MESSENGER'' mission used the spacecraft to search for moons of Mercury in 2011 and 2013, and confirmed that Mercury had none.MESSENGER to Capture Images of Earth and Moon During Search for Satellites of Mercury (2013)

References

{{Mercury (planet) Mercury (planet) Hypothetical bodies of the Solar System

Planetary satellite systems Mariner program

''Mariner 10'' mission

31 Crateris

''MESSENGER'' mission

See also

References