During the last four years substantial advances have been made in modeling the gravity field of Venus through improved analyses of the Pioneer Venus Orbiter (PVO) data and the analysis of tracking data from the Magellan spacecraft. Prior to the Magellan mission, the gravity field of Venus had largely been determined through the tracking of PVO. Spherical harmonic gravity models developed in the 1980s from PVO data were complete only to degree 18 or less. A reanalysis of the PVO data in preparation for the Magellan mission resulted in a 21 x 21 model by McNamee et al. [1992], which was later improved through the addition of early Magellan tracking data [ McNamee et al., 1993]. Nerem et al. [1993a] applied the a priori constraint technique discussed earlier to develop a 50 x 50 gravity model for Venus from the PVO data. Reasenberg and Goldberg [1992] also developed an improved high resolution gravity model from a reanalysis of the PVO data.
Konopliv et al. [1993b] combined improved X-band tracking data from the Magellan mission with the PVO data to compute a 60 x 60 gravity model, also using a priori constraints. Although an improvement over previous models, the similar periapse locations of PVO and Magellan did not allow a dramatic improvements. This situation changed considerably in 1993, when aerobraking was used to circularize the Magellan orbit. Tracking data from this phase of the mission have already allowed dramatic improvements in our knowledge of the gravity field of Venus, especially in the polar regions [ Konopliv et al., 1994]. Preliminary analysis of this data by Konopliv et al. [1994] indicate that it might be possible to relax and even eliminate the a priori constraints once more complete coverage of the planet is obtained from this low orbit. The development of improved models of the gravity field of Venus, in concert with improved topography models developed using PVO and Magellan radar altimeter data [ Ford and Pettengill, 1992; Rappaport and Plant, 1994], have resulted in an improved understanding of the geophysics of Venus [e.g. Banerdt et al., 1994; Bindschadler et al., 1992; 1994; Johnson and Sandwell, 1994 Phillips, 1994; Schubert et al., 1994; Smrekar, 1994]. The development of gravity models containing the complete set of Magellan post-aerobraking tracking data will have a significant impact on geophysical investigations of this type during the next several years. Unfortunately, the Magellan mission was terminated in October 1994 before complete post-aerobraking coverage of all planetary longitudes was obtained.