Mercury is the small planet closest to the sun.  Although it is less than half the diameter of Earth, Mercury is not the smallest planet, that honor going to the other end of the system, tiny Pluto.  Mercury was once thought to be stationary, and thought to keep the same (hot!) face toward the sun, but is now known to rotate slowly, one Mercury day being about 58 Earth days.  Because of this, it becomes very hot on the side facing the sun, and quickly becomes very cold on the other side, due to the slow rotation and the fact that Mercury's atmosphere is not more than a trace and therefore offers no insulation effect.

Temperatures on the whole planet average 179 degrees Celsius, but the sunward side would reach hundreds of degrees while the dark side was down to -180 degrees.  Mariner 10 flew past Mercury in 1974-75 and photographed most of the planet.

For more information on Mercury:

http://pds.jpl.nasa.gov/planets/choices/mercury1.htm

http://www.seds.org/nineplanets/nineplanets/mercury.html
http://pds.jpl.nasa.gov/planets/welcome/mercury.htm
 

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Page by Francis Perry, April 2002