Mercury

Merucry is the innermost and smallest planet in the solar system (since Pluto was re-labelled as a dwarf planet), orbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°: It can only be seen in morning and evening twilight. Comparatively little is known about it; the first of two spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only about 45% of the planet’s surface.[7] The second was the MESSENGER spacecraft, which mapped another 30% of the planet during its flyby of January 14, 2008.[7] MESSENGER will make two more passes by Mercury, followed by orbital insertion in 2011, and will survey and map the entire planet.

Physically, Mercury is similar in appearance to the Moon. It is heavily cratered, has no natural satellites and no substantial atmosphere. It has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.[8] It is an exceptionally dense planet due to the large size of its core. The surface temperatures on Mercury range from about 90 to 700 K,[9] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes.[10] The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.[11]

Moon of Mercury

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 on Earth began registering large amounts of ultraviolet radiation in the vicinity of Mercury which, according to one astronomer, "had no right to be there".[citation needed] By the next day, the radiation had disappeared; it reappeared three days later, appearing to originate with an object which was, seemingly, detached from Mercury.[citation needed] 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.[citation needed] 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.[citation needed] The object was even given a name: in an interview, the discoverer said if it was confirmed, he'd like to name it 'Charley'.[citation needed] When asked why, he said "I once had a dog named that, and I've always wanted to name something after the dog".[citation needed]

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.[citation needed]

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.