Since December 1978, the Pioneer Venus Orbiter (PVO) has been orbiting Venus once every 24 hours in a near polar, highly eccentric orbit with apoapsis of about 12 Venus radii and a periapsis as low as 140 km. The spacecraft carries instruments for remote measurements of the surface, the atmosphere, and the clouds of Venus, as well as instrumentation for in situ measurements of its thermosphere, ionosphere, and magnetic field environment and the incident solar wind parameters. These instruments were described in a special issue of the IEEE Transactions on Geoscience and Remote Sensing . A dedicated issue of the Journal of Geophysical Research  described the mission, its goals, and many of its early scientific results. The Venus book [Hunten et al., 1983] presented many review articles that summarized the more complete results and analysis available at that time. Two of the several major goals of the mission have been to perform exploratory measurements of the upper atmosphere and ionosphere of Venus and to investigate the interaction of the solar wind with the atmosphere of such an unmagnetized planet. The purpose of this paper is to call attention to the way in which secular changes in the PVO orbit are permitting these goals to be achieved. Note that we have addressed only the effect of orbital evolution only upon the in situ measurements. Clearly, the remote measurements of cloud patterns and ultraviolet emissions from the atmosphere will benefit from both the long baseline of observing time and the changing observing altitude and angles.
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