Introduction to the Mission and the Orbiter Neutral Gas Mass Spectrometer (PVONMS)

Venus

The Pioneer Venus mission, launched in 1978, included an orbiter, an entry bus and four entry probes. The Pioneer Venus Orbiting Neutral Mass Spectrometer (ONMS) Experiment was the first planetary mass spectrometer developed by the Atmospheric Experiments Laboratory. The mass spectrometer design was based on experience gained from similar experiments carried out previously in the upper atmosphere of the Earth. The ONMS was designed to measure neutral gas compositions in the upper atmosphere of Venus at altitudes between approximately 150 and 300km above the surface. The mass spectrometer detected and quantitatively measured the abundances of neutral gases in this region. The species measured included carbon dioxide, carbon monoxide, atomic and molecular nitrogen, atomic oxygen, and helium. These data provided a firm basis for theoretical studies of the dynamics, thermal structure, and photochemistry in the upper atmosphere of Venus.

The orbiter sampled the planet with remote sensing experiments and sampled the upper atmosphere with a set of insitu experiments. The Orbiter Neutral Mass Spectrometer obtained fundamental measurements of the compositions of atomic and molecular species in the upper atmosphere. Density variations with altitude allowed scale heights to be determined and thermospheric temperatures.

Rendition of Pioneer over Venus

The configuration of the Pioneer Venus Orbiter is shown in this illustration. The spacecraft was spin stabilized and once every rotation, the mass spectrometer would point in the forward direction.

This mass spectrometer continued to function well many years after the end of the nominal mission. For much of this time the altitude of the spacecraft was above the detectable neutral atmosphere. The unexpected detection by the mass spectrometer of energetic, positively charged ions such as O+ in this region has contributed to understanding mechanisms of atmospheric loss. Reentry of the spacecraft into the Venusian atmosphere in 1992 provided the opportunity for this mass spectrometer to obtain additional information from the atmosphere of Venus. The ONMS instrument operated successfully for almost fourteen years in the Venus environment until the spacecraft entered the atmosphere on orbit 5055.