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Mars close approach 2007, Hubble Space Telescope (HST) image. Mars is a cold desert world, with an atmosphere of mostly carbon dioxide. It is smaller than the Earth, and orbits the Sun at an average distance of 230 million kilometres. Here, at its closest approach that year, in December 2007, it was 88 million kilometres from Earth. Image data obtained by Hubble's Wide Field and Planetary Camera 2 (WFPC2). (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA/ESA/STSCI/AURA/J. BELL (CORNELL UNIVERSITY)/M. WOLFF (SPACE SCIENCE INSTITUTE, BOULDER)/ SCIENCE PHOTO LIBRARY)