The Pioneer Venus Orbiter, also known as Pioneer Venus 1 or Pioneer 12, was a mission to Venus conducted by the United States as part of the Pioneer Venus project. Launched in May 1978 atop an Atlas-Centaur rocket, the spacecraft was inserted into an elliptical orbit around Venus on December 4, 1978. It returned data on Venus until October 1992.
Manufactured by Hughes Aircraft Company, the Pioneer Venus Orbiter was based on the HS-507 bus. The spacecraft was a flat cylinder, in diameter and long. All instruments and spacecraft subsystems were mounted on the forward end of the cylinder, except the magnetometer, which was at the end of a boom. A solar array extended around the circumference of the cylinder. A despun dish antenna provided S and X band communication with Earth. A Star-24solid rocket motor was integrated into the spacecraft to provide the thrust to enter orbit around Venus. From Venus orbit insertion to July 1980, periapsis was held between to facilitate radar and ionospheric measurements. The spacecraft was in a 24-hour orbit with an apoapsis of. Thereafter, the periapsis was allowed to rise to a maximum of and then fall, to conserve fuel. In 1991, the Radar Mapper was reactivated to investigate previously inaccessible southern portions of the planet, in conjunction with the recently arrived Magellan spacecraft. In May 1992, Pioneer Venus began the final phase of its mission, in which the periapsis was held between, until the spacecraft's propellant was exhausted, after which the orbit decayed naturally. The spacecraft continued to return data until 8 October 1992, with the last signals being received at 19:22 UTC. The Pioneer Venus Orbiter disintegrated upon entering the atmosphere of Venus on October 22, 1992.
Experiments
The Pioneer Venus Orbiter carried 17 experiments with a total mass of :
a cloud photo-polarimeter to measure the vertical distribution of the clouds, similar to Pioneer 10 and Pioneer 11 imaging photo-polarimeter
a surface radar mapper to determine topography and surface characteristics. Observations could only be conducted when the probe was closer than 4700 km over the planet. A 20 watt S-band signal was sent to the surface that reflected it, with the probe analyzing the echo. Resolution at periapsis was 23 x 7 km.
From its orbit of Venus, the Pioneer Venus Orbiter was able to observe Halley's Comet when it was unobservable from Earth due to its proximity to the sun during February 1986. UV spectrometer observations monitored the loss of water from the comet's nucleus at perihelion on February 9.
