Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusm.u23a..02z&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #U23A-02
Physics
6235 Mercury
Scientific paper
The first flyby of Mercury by the MESSENGER spacecraft occurred on January 14, 2008, and both altimeter observations and radio Doppler tracking data were acquired as the spacecraft passed within 201 km of the planetary surface. These were the first new observations of Mercury by a spacecraft since the third encounter by Mariner 10 in-1975 and the first spacecraft laser range measurements at Mercury. A 3300-km-long profile was obtained by the Mercury Laser Altimeter (MLA) along a profile approximately 5° S of the equator and from longitudes 14° to 93° E. The altimetry revealed a rough, cratered surface with a full dynamic range of more than 5 km and a root-mean-square roughness of ~900 m. The apparent mean radius of Mercury decreased by about 1 km over the length of the track, which may be indicative of a fundamental feature of the equatorial shape of the planet. Doppler tracking data showed a change in the direction of the velocity relative to the Sun of 20.1° and a reduction in its magnitude. The spacecraft was occulted from Earth for approximately 48 minutes until approximately 4 minutes prior to closest approach; coherent Doppler tracking data were re-acquired approximately 2 minutes prior to closest approach. From an a priori estimation of the trajectory of MESSENGER, a variation in residual velocity of 1.65 to -2.16 cm/s was observed near closest approach, which occurred at 37.8° E and 4.2° S. The adjustment of the low-degree coefficients of the Mercury gravity field reduced the residual velocity, but the reduction was significantly improved by solving simultaneously for a gravity anomaly centered near the track of the spacecraft. We interpret this result as indicating that the planetary gravity field has short-wavelength power that cannot be explained in the context of the long-wavelength shape of Mercury.
Barmouin-Jha O.
Harmon John K.
Hauck Steven A.
Head James W.
Johnson Clifton L.
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