spase://CNES/Observatory/CDPP-AMDA/PioneerVenusOrbiter

The Pioneer Venus Orbiter was
the first of a two-spacecraft orbiter-probe
combination designed to conduct a comprehensive investigation
of the atmosphere of Venus. The spacecraft was a solar-powered
cylinder about 250 cm in diameter with its spin axis
spin-stabilized perpendicular to the ecliptic plane.
A high-gain antenna was mechanically despun to remain focused
on the earth. The instruments were mounted on a shelf within
the spacecraft except for a magnetometer mounted at the end
of a boom to ensure against magnetic interference from the
spacecraft. Pioneer Venus Orbiter measured the detailed
structure of the upper atmosphere and ionosphere of Venus,
investigated the interaction of the solar wind with the
ionosphere and the magnetic field in the vicinity of Venus,
determined the characteristics of the atmosphere and surface
of Venus on a planetary scale, determined the planet's
gravitational field harmonics from perturbations of the
spacecraft orbit, and detected gamma-ray bursts.
UV observations of comets have also been made. From Venus
orbit insertion on December 4, 1978 to July 1980 periapsis was
held between 142 and 253 km to facilitate radar and
ionospheric measurements. Thereafter, the periapsis was
allowed to rise (to 2290 km at maximum) and then fall, to
conserve fuel. In 1991 the Radar Mapper was reactivated to
investigate previously inaccessible southern portions of the
planet. In May 1992 Pioneer Venus began the final phase of its
mission, in which the periapsis was held between 150 and 250
km until the fuel ran out and atmospheric entry destroyed the
spacecraft the following August. The orbiter cost $125 million
to build and operate for the first 10 years. For further
details see Colin, L. and Hunten, D. M., Space Science
Reviews 20, 451, 1977.