Mission Objectives Summary      ==========================        NASA established the Pioneer Venus Science Steering Group in        Jan. 1972 in order to enlist widespread science community        participation in designing the mission science requirements.        This group concluded that a successful mission to Venus would        answer the following list of scientific questions (Fimmel et        al, 1995, Table 2-2, p25):        1) Cloud layers: What is their number and where are they           located? Do they vary over the planet?        2) Cloud forms: Are they layered, turbulent, or merely hazes?        3) Cloud physics: Are the clouds opaque? What are the sizes           of the cloud particles? What is the density of the cloud           particles?        4) Cloud composition: What is the chemical composition of the           clouds? Is it different in the different layers?        5) Solar heating: Where is the solar radiation deposited in           the atmosphere?        6) Deep circulation: What is the nature of the wind in the           lower regions of the atmosphere? Is there any measurable           wind near the surface?        7) Deep driving forces: What are the horizontal differences           in temperature in the deep atmosphere?        8) Driving force for the 4-day circulation: What are the           horizontal temperature differences at the top of the cloud           layer that could cause the high winds there?        9) Loss of water: Has water been lost from Venus? If so, how?       10) Carbon dioxide stability: Why is molecular CO2 stable in           the upper atmosphere?       11) Surface composition: What is the composition of the           crustal rocks?       12) Seismic activity: What is its level?       13) Earth tides: Do tidal effects from Earth exist at Venus,           and if so, how strong are they?       14) Gravitational moments: What is the figure of the planet?           What are the higher order gravitational moments?       15) Extent of the 4-day circulation: How does this circulation           vary with latitude and depth in the Venusian atmosphere?       16) Vertical temperature structure: Is there an isothermal           region? Are there other departures from adiabaticity? What           is the structure near the cloud tops?       17) Ionospheric motions: Are these motions sufficient to           transport ionization from the day to night hemisphere?       18) Turbulence: How much turbulence is there deep in the           atmosphere?       19) Ion chemistry: What is the chemistry of the ionosphere?       20) Exospheric temperature: What is the temperature and does           it vary over the planet?       21) Topography: What features exist on the surface of the           planet? How do they relate to the thermal maps?       22) Magnetic moments: Does the planet have any internal           magnetism?       23) Bulk atmospheric composition: What are the major gases in           the Venus atmosphere? How do they vary with altitude?       24) Anemopause: How does the solar wind interact with the           planet?        The wide range of science questions to be answered by the        Pioneer Venus mission could not be answered by an orbiter        mission alone.  An orbiter could address the questions        regarding the upper atmosphere, ionosphere, solar wind, and        surface topography, but a probe would be needed to make in        situ measurements in the lower atmosphere. Likewise, a single        probe would be inadequate to address the all of the questions        posed about the lower atmosphere. Multiple probes targeting        different parts of the planet simultaneously were required to        meet the science objectives. Thus the Pioneer Venus mission        developed into an Orbiter and Multiprobe spacecraft.        A large number of scientific instruments were required on        both the orbiter and the various probes in order to meet the        mission science objectives. The orbiter carried 4 remote        sensing instruments and 8 in situ experiments. In addition,        several radio science experiments were performed using the        S-band telemetry signal plus a special X-band beacon that was        included on the Orbiter. The first letter of the instrument        acronym designates which component of the spacecraft carried        the instrument: O for Orbiter, L for Large Probe, S for the        Small Probes, and B for the Bus. The remote sensing        instruments were: 1) Cloud Photopolarimeter (OCPP), 2) Radar        mapper (ORAD), 3) Ultraviolet Spectrometer (OUVS), and 4)        Infrared Radiometer (OIR). The in situ instruments included 4        plasma and gamma ray detectors: 1) Ion Mass Spectrometer        (OIMS), 2) Electron Temperature Probe (OETP), 3) Retarding        Potential Analyzer (ORPA), 4) Plasma Analyzer (OPA), and 5)        Gamma Burst Detector (OGBD). In addition, the neutral        atmosphere composition was sampled by the Neutral Mass        Spectrometer (ONMS), the magnetic field was measured by MAG        (OMAG) and the wave electric fields were measured by the        Electric Field Detector (OEFD).        The large probe, bus, and small probes each carried slightly        different instrumentation. The probe instruments were:        Neutral Mass Spectrometers (LNMS, SNMS, BNMS), Gas        Chromatograph (LGC), Atmospheric Structure experiment (LAS,        SAS), Nephelometers (LN, SN), Cloud Particle Size (LCPS),        Solar Flux Radiometer (LSFR), Infrared Radiometer (LIR), Net        Flux Radiometer (SNFR),        Finally, the spacecraft and probe telemetry signal        distortions were used to probe the Venus atmosphere and        ionosphere. The radio science experiments included the        Atmospheric Propagation Experiment (OGPE), Atmospheric Drag        (OAD), Differential Long Baseline Interferometry (DBLI),        Doppler tracking of the probes (MWIN), Atmospheric Turbulence        (MTUR/OTUR), Dual Frequency Occultation (ORO), Internal        Density (OIDD), and finally, Celestial Mechanics experiments        (OCM).