Mercury is the planet closest to the Sun. Since it never strays far in the sky from the Sun’s glare, early astronomers had a difficult time viewing it, and considered it a "wandering star" appearing just before sunrise or just after sunset.

	It is fitting that we know Mercury by the name of the swift Roman messenger god, since it travels around the Sun faster than any other planet. During one year on Earth, Mercury makes over four orbits of the Sun. On the other hand, Mercury rotates slowly on its axis – almost 60 times more slowly than does our home planet. The amazing outcome is that a single Mercury day takes two Mercury years. The symbol for Mercury also represents the planet's swift orbit around the sun. Click on Mercury's symbol to the left to learn more about its origin.
	Mercury is the second smallest planet in our Solar System, larger only than Pluto and not much bigger than our own Moon. The surface of Mercury is in fact very Moon-like, covered with large and ancient craters, while its interior is like Earth’s, with a large core of iron. Mercury has a very thin atmosphere, and no moons of its own. It is a world of extreme temperatures in which the surface can heat to over 400°C and cool to almost –200 °C.
	Much of what we know about Mercury was discovered through flybys of the planet during the Mariner 10 space mission in the 1970’s. But Mariner 10 photographed only half the planet’s surface, and many questions remain. In 2004, NASA will launch a spacecraft to make more detailed observations. This planned mission is called MESSENGER, for “MErcury Surface, Space ENvironment, GEochemistry and Ranging”, and will reach orbit around Mercury in 2011.

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