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Mercury is the closest planet to the Sun with an average distance of 0.387 AU or 5.79 x 107 km. It takes 88 Earth days to orbit the Sun, and rotates very slowly at a rate of 1 rotation every 58.7 Earth days. Its mass is 3.3 x 1023 kg or 0.055 Earth masses. It has no moons, little to no atmosphere and generally a very hot surface. The surface temperature ranges from 90 to 700 K (−183 °C to 427 °C, −297 °F to 801 °F) with the hottest temperatures where the Sun's rays hit perpendicular to the surface, and the coolest temperatures in craters at the poles.
Mercury can be seen with the naked eye for a few hours before or after sunset, depending on which side of the Sun it is on relative to Earth. It never travels more than 28° from the Sun from our point of view. It has a large iron core and a magnetic field about 1% as strong as Earth's. Its surface is covered with craters where there is even water ice! Mercury's orbit has a 3:2 spin–orbit resonance, and rotates three times for every two revolutions around the Sun. Mercury's orbit is eccentric and this resonance makes it stable.
Mercury was visited from 1974 to 1975 by Mariner 10, and again in 2008 and 2009 by MESSENGER. Mariner 10 mapped about 45% of Mercury's surface, and MESSENGER mapped another 30% during its two flybys. It is scheduled to go into orbit around Mercury on March 18, 2011.