In Roman mythology Mercury was the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky.
Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was sometimes given separate names for its apparitions as a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit the Sun, not the Earth.
Since it is closer to the Sun than the Earth, the illumination of Mercury’s disk varies when viewed with a telescope from our perspective. Galileo’s telescope was too small to see Mercury’s phases but he did see the phases of Venus.
Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 km. Mercury consists of approximately 70% metallic and 30% silicate material.
Mercury has been now been visited by two spacecraft, Mariner-10 and MESSENGER. Marriner-10 flew by three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by HST). MESSENGER was launched by NASA in 2004 and orbited Mercury starting in 2011 till after several flybys. Its first flyby in Jan 2008 provided new high quality images of some of the terrain not seen by Marriner 10.
The mission is scheduled to end sometime in 2012, when there will no longer be enough fuel to maintain the probe’s orbit.
Amazingly, radar observations of Mercury’s north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters. Mercury has a small magnetic field whose strength is about 1% of Earth’s. Mercury has no known satellites.