Mercury is the closest planet to the Sun and the eighth largest. Mercury is smaller in diameter than Ganymede and Titan but more massive.

In Roman mythology Mercury is 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 given two names by the Greeks: Apollo for its apparition as a morning star and Hermes 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.

Mercury has been visited by only one spacecraft, Mariner 10. It flew by three times in 1973 and 1974. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by the Hubble Space Telescope (HST).

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