Director, Department of Terrestrial Magnetism, Carnegie Institution for Science Mercury, the smallest member of the family of terrestrial planets that includes Earth, is one of the last frontiers of the inner Solar System. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, developed under NASA’s Discovery Program, is the first space probe to visit the planet Mercury in more than 30 years. MESSENGER flew by the innermost planet twice in 2008 and once in 2009. MESSENGER’s flybys confirmed that Mercury’s internal magnetic field is primarily dipolar and documented that the magnetosphere is more responsive to variations in solar wind conditions than that of any other planet. MESSENGER detected magnesium in Mercury’s exosphere, demonstrated that Mercury’s anti-sunward neutral tail contains multiple species, and revealed that the distributions of sodium, calcium, and magnesium in the exosphere and tail vary differently with latitude, time of day, and Mercury’s position in orbit, signatures of multiple source processes. MESSENGER’s laser altimeter showed that the equatorial topographic relief of Mercury exceeds 5 km and documented the relief of numerous impact craters and fault scarps. MESSENGER’s images provided evidence for widespread past volcanism, and candidate sites for volcanic centers have been identified. Also revealed were newly imaged lobate scarps and other tectonic landforms supportive of the hypothesis that Mercury contracted globally in response to interior cooling. Reflectance spectra show no evidence for FeO in surface silicates, and reflectance and color imaging observations support earlier inferences that Mercury’s surface material consists dominantly of iron-poor, calcium-magnesium silicates with an admixture of spectrally neutral opaque minerals, but MESSENGER’s neutron spectrometer showed that the surface abundance of iron plus titanium is comparable to that of several lunar mare regions. MESSENGER is now on course for insertion into orbit about Mercury in March 2011, and a full Earth-year of orbital observations is planned for the remainder of the nominal mission.A planetary scientist, seismologist, and marine geophysicist, Dr. Solomon has worked on a wide range of problems in earthquake seismology, geodynamics, magmatism, and the geological and geophysical evolution of the terrestrial planets. He was a co-investigator on the Magellan mission to Venus and on Mars Global Surveyor, he is currently a co-investigator on GRAIL, and since 1999 he has been the Principal Investigator for the MESSENGER mission. He also served as a member of the MIT faculty for more than 20 years. Dr. Solomon is a member of the National Academy of Sciences, a Fellow of the American Academy of Arts and Sciences, and a past President of the American Geophysical Union. He received the Arthur L. Day Prize from the National Academy of Sciences, the G. K. Gilbert Award from the Geological Society of America, the Harry H. Hess Medal from the American Geophysical Union, the NASA Public Service Medal, the Distinguished Alumni Award from Caltech, and (on behalf of the MESSENGER team) the Nelson P. Jackson Aerospace Award from the National Space Club.