Mercury's gravity field from the first six months of MESSENGER data

The Mercury Surface, Space Environment, GEochemestry, and Ranging (MESSENGER) spacecraft, launched on August 3, 2004, was inserted in a highly elliptical polar orbit around the planet on March 18, 2011. One of the main mission goals is the determination of the interior structure of the planet, enabled by a suite of instruments that includes the radio system and a laser altimeter. Thanks to altimetric and radio observables, the topography and the gravity field of the planet have been retrieved with good accuracy, especially in the north polar region, where the spacecraft altitude is lower. In September, 2011, the radio tracking data of the first 6 months of operations were published with the ancillary information necessary for the MESSENGER orbit determination. This data set offers an excellent opportunity to test the orbit determination procedures developed in view of a similar, but more accurate, experiment hosted onboard BepiColombo, the ESA mission to Mercury. We present here the results of our analysis, which provide the spacecraft orbit, a 20×20 gravity field and a linear update of Mercury's ephemeris. The estimated gravity field is fully compatible with the one published by Smith et al. (2012).