Mercury, the closest planet to the Sun is one of the least well known planets in the solar system. Only one side has been photographed by spacecraft, and the light streak on the image is an area of 'no data'. This should change however, when the Messenger probe does fly-by in January 2008. (However, the image may not be published immediately.) Messenger will perform three fly-bys in total, entering orbit in March 2011. It has also has made two fly-bys of Venus and a fly-by of Earth. This method may seem odd, but saves fuel, making the mission much less expensive.

Mercury has an unusually large iron core which makes it very dense. It also has strange ridges on one side of the planet and a huge crater on the other. It has been theorised that an asteroid hit Mercury, possibly early on in its history when the crust was still soft. The shock waves rippled through the planet and crumpled the crust.

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