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Mercury, the closest planet to the sun, has been known since ancient times. Because its
orbit is inside of the Earth's orbit, it never gets very far from the sun and therefore many
times is difficult to see from Earth. The orbit of Mercury is very peculiar, in that it is extremely
elliptical. Its perihelion is 23.8 million kilometers closer to the sun than its aphelion.
Mercury's average distance to the sun is .387 AU, and takes 88 Earth days to orbit the sun. The planet
helped confirm Albert Einstein's "General Theory of Relativity" when it explained a strange
precession in Mercury's orbit.
Mercury rotates very slowly, taking 58.6 Earth days to make one rotation. The diameter of Mercury
is 4,879 kilometers, making it the second smallest planet in the solar system to Pluto. Mercury's iron core
makes it very dense with a density of 5,427kg/m3 but the mass of Mercury is only .055 Earth masses.
Temperatures on Mercury range from
-183 degrees Celsius to 430 degrees Celsius on the day side.
Most of what we know about Mercury has come from observations from Earth or from Mariner 10,
which is the only probe to fly by the planet. Mariner 10 gathered its information in 1974-1975 and mapped about
50% of the surface of Mercury. A NASA mission called Messenger was launched in August 2004 and is on its way to Mercury
where it will map the rest of the surface of Mercury and investigate peculiarities of Mercury including a magnetic field
that cannot be explained at this time. Messenger will reach Mercury in 2009. Another planned mission to Mercury,
BepiColombo, is a joint project of Japan and the ESA. This mission planned for 2011, will send two orbiters to Mercury.
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