Pioneer Venus Orbiter

The Pioneer Venus Orbiter, also known as Pioneer Venus 1 or Pioneer 12, was a mission to Venus conducted by NASA 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 from Venus until October 1992.

Launch and arrival at Venus
The Pioneer Venus Orbiter was launched by an Atlas SLV-3D Centaur-D1AR rocket, which flew from Launch Complex 36A at the Cape Canaveral Air Force Station. The launch occurred at 13:13:00 (8:13 a.m. local time) on May 20, 1978, and deployed the Orbiter into heliocentric orbit for its coast to Venus. Venus orbit insertion occurred on December 4, 1978.

Spacecraft
Manufactured by Hughes Aircraft Company, the Pioneer Venus Orbiter was based on the HS-507 bus. The spacecraft was a flat cylinder, 2.5 m in diameter and 1.2 m 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 4.7 m boom. A solar array extended around the circumference of the cylinder. A 1.09 m despun dish antenna provided S and X band communication with Earth. A Star-24 solid 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 142 and 253 km (at 17 degrees north latitude) to facilitate radar and ionospheric measurements. The spacecraft was in a 24-hour orbit with an apoapsis of 66900 km. Thereafter, the periapsis was allowed to rise to a maximum of 2290 km 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 150 and 250 km, 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.

Instruments
The Pioneer Venus Orbiter carried 17 experiments with a total mass of 45 kg:


 * a cloud photo-polarimeter (OCPP) to measure the vertical distribution of the clouds, similar to Pioneer 10 and Pioneer 11 imaging photo-polarimeter (IPP)
 * a surface radar mapper (ORAD) 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 (1.757 gigahertz) was sent to the surface that reflected it, with the probe analyzing the echo. Resolution at periapsis was 23 x.
 * an infrared radiometer (OIR) to measure IR emissions from Venus' atmosphere
 * an airglow ultraviolet spectrometer (OUVS) to measure scattered and emitted UV light
 * a neutral mass spectrometer (ONMS) to determine the composition of the upper atmosphere
 * a solar wind plasma analyzer (OPA) to measure properties of the solar wind
 * a magnetometer (OMAG) to characterize the magnetic field at Venus
 * an electric field detector (OEFD) to study the solar wind and its interactions
 * an electron temperature probe (OETP) to study the thermal properties of the ionosphere
 * an ion mass spectrometer (OIMS) to characterize the ionospheric ion population
 * a charged particle retarding potential analyzer (ORPA) to study ionospheric particles
 * two radio science experiments to determine the gravity field of Venus
 * a radio occultation experiment to characterize the atmosphere
 * an atmospheric drag experiment to study the upper atmosphere
 * a radio science atmospheric and solar wind turbulence experiment
 * a gamma ray burst (OGBD) detector to record gamma ray burst events

The spacecraft conducted radar altimetry observations allowing the first global topographic map of the Venusian surface to be constructed.

The instruments can also be classified by what they are meant to measure or analyze:
 * Composition and structure of the atmosphere
 * Large Probe Mass Spectrometer (LNMS)
 * Large Probe Gas Chromatograph (LGC)
 * Bus Neutral Mass Spectrometer (BNMS)
 * Orbiter Neutral Mass Spectrometer (ONMS)
 * Orbiter Ultraviolet Spectrometer (OUVS)
 * Large/Small Probe Atmosphere Structure (LAS/SAS)
 * Atmospheric Propagation Experiments (OGPE)
 * Orbiter Atmospheric Drag Experiment (OAD)
 * Clouds
 * Large/Small Probe Nephelometer (LN/SN)
 * Large Probe Cloud Particle Size Spectrometer (LCPS)
 * Orbiter Cloud Photopolarimenter (OCPP)
 * Thermal balance
 * Large Probe Solar Flux Radiometer (LSFR)
 * Large Probe Infrared Radiometer (LIR)
 * Small Probe Net Flux Radiometer (SNFR)
 * Orbiter Infrared Radiometer (OIR)
 * Dynamics
 * Differential Long Baseline Interferometry (DLBI)
 * Doppler Tracking of Probes (MWIN)
 * Atmospheric Turbulence Experiments (MTUR/OTUR)
 * Solar wind and ionosphere
 * Bus Ion Mass Spectrometer (BIMS)
 * Orbiter Ion Mass Spectrometer (OIMS)
 * Orbiter Electron Temperature Probe (OETP)
 * Orbiter Retarding Potential Analyzer (ORPA)
 * Orbiter Magnetometer (OMAG)
 * Orbiter Plasma Analyzer (OPA)
 * Orbiter Electric Field Detector (OEFD)
 * Orbiter Dual-Frequency Occultation Experiments (ORO)
 * Surface and interior
 * Orbiter Radar Mapper (ORAD)
 * Orbiter Internal Density Distribution Experiments (OIDD)
 * Orbiter Celestial Mechanics Experiments (OCM)
 * High-energy astronomy
 * Orbiter Gamma Burst Detector (OGBD)

Observations of Comets
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.

The extended mission allowed the spacecraft controllers to make several comet observations that were never part of the original mission objectives. The tilt of the spacecraft was altered during these comet observations so that the Ultraviolet Spectrometer (OUVS) could view the comets rather than Venus. Comets Encke (April 13-16, 1984), Giacobini-Zinner (September 8-15, 1985), Halley (December 27, 1985 - March 9, 1986), Wilson (March 13 - May 2, 1987), NTT (April 8, 1987), and McNaught (November 19-24, 1987) were all observed in this way.