INSTITUTE OF PHYSICS PUBLISHING REPORTS ON PROGRESS IN PHYSICS Rep. Prog. Phys. 65 (2002) 529–560 PII: S0034-4885(02)12697-2 Mercury: the planet and its orbit Andre´ Balogh and Giacomo Giampieri Space and Atmospheric Physics, Imperial College, London SW7 2BW, UK E-mail:
[email protected] Received 7 August 2001, in final form 26 November 2001 Published 20 March 2002 Online at stacks.iop.org/RoPP/65/529 Abstract The planet closest to the Sun, Mercury, is the subject of renewed attention among planetary scientists, as two major space missions will visit it within the next decade. These will be the first to return to Mercury, after the flybys by NASA’s Mariner 10 spacecraft in 1974–5. The difficulties of observing this planet from the Earth make such missions necessary for further progress in understanding its origin, evolution and present state. This review provides an overview of what is known about Mercury and what are the major outstanding issues. Mercury’s orbital and rotation periods are in a unique 2:3 resonance; an analysis of the orbital dynamics of Mercury is presented here, as well as Mercury’s special role in testing theories of gravitation. These derivations provide a good insight into the complexities of planetary motion in general, and how, in the case of Mercury, its proximity to the Sun can be described and exploited in terms of general relativity. Mercury’s surface, superficially similar to that of the Moon, presents intriguing differences, representing a different, and more complex history in which the role of early volcanism remains to be clarified and understood.