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Craters on Mercury. False-colour MESSENGER image of the cratered surface of the planet Mercury, showing the Caloris impact crater (orange, centre). Mercury is the closest planet to the Sun and the smallest in the solar system. Its diameter (4880 kilometres) is less than half that of Earth. The MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) spacecraft was launched in August 2004. After flybys of Earth, Venus and Mercury, it will enter orbit around Mercury in 2011 to study the surface. Image obtained during the January 2008 Mercury flyby, the first for over 30 years. (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA/JHU-APL/ASU/CARNEGIE INSTITUTION OF WASHINGTON/SCIENCE PHOTO LIBRARY)