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The launch phase of the Pioneer Venus Multiprobe spacecraft and cruise phases of both the Pioneer Venus Orbiter and the Multiprobe spacecraft are covered. Material pertinent to the Venus encounter is included.
A comprehensive set of television pictures of Venus taken by the Mariner 10 spacecraft is presented. Included is a chronological sequence of television images illustrating the development, variety, and circulation of Venus upper-atmospheric phenomena as viewed in the near-ultraviolet. The higher-resolution images have been assembled into global mosaics to facilitate comparison. Figures and tables describing the imaging sequences have been included to provide a guide to the more complete set of 3400 Venus images on file at the National Space Science Data Center.
The reported study is based on the 12.5 cm wavelength data of Rumsey et al. (1974). The considered low resolution (80 km) radar image covers an area equivalent to 19% of the surface of Venus. The study had the objective to map potential large impact structures and relate their size frequency distribution to those of Mercury and the moon. The Venus radar map was analyzed using a color television film density slicer system for enhancement of subtle changes and gross patterns of contrasting radar reflectivity. Analysis by this technique permitted the recognition of 12 possible basins with diameters exceeding 600 km on about 8% of the total surface area of the planet. The preliminary basin size frequency distribution determined for Venus from these low resolution data suggests that the cloud-covered planet could be more cratered per unit area by basins than either the moon or Mercury.
Using a barotropic model, the upper atmosphere of Venus is examined for stability to small perturbations of a mean zonal wind. Several assumed mean zonal wind profiles, compatible with Mariner 10 UV observations, are examined with the result that some are stable, while others are unstable. At least one of the unstable profiles exhibits a maximum in the growth rate for wavenumber 3, in agreement with some UV observations of the clouds of Venus. The barotropic model should be useful for performing instability analyses using Pioneer Venus data in 1979.
On the basis of a presented theory, it is suggested that Venus' current rotation is a stable balance between atmospheric and solar body tides. The theory is concerned with Venus' atmospheric tides, driven by solar heating, and how these tides could serve as a third torque to balance the effects of solar body torque and to maintain a stable equilibrium resonance with regard to the earth's gravitational effects. In the absence of the atmospheric tidal torque, or some other torque, it would be expected that Venus would be despun until synchronous rotation (one side always facing the sun) is attained, rather than retain the retrograde rotation period of 243 days.
The encounter time line and mission profile are presented for the Pioneer Venus 1 Spacecraft and for the four probes and transponder bus that comprise Pioneer Venus 2. Known facts about Venus are reviewed and the history of discoveries about the planet is related. Operations of the orbiter and multiprobe are described as well as the 30 instruments being carried and provisions for data transmission.
Zonal velocity profiles in the Venus atmosphere above the clouds were estimated from measured asymmetries of HCl and HF infrared absorption lines in high-resolution Fourier interferometer spectra of the planet. These asymmetries are caused by both pressure-induced shifts in the positions of the hydrogen-halide lines perturbed by CO2 and Doppler shifts due to atmospheric motions. Particularly in the case of the HCl 2-0 band, the effects of the two types of line shifts can be easily isolated, making it possible to estimate a profile of average Venus equatorial zonal velocity as a function of pressure in the region roughly 60 to 70 km above the surface of the planet. The mean profiles obtained show strong vertical shear in the Venus zonal winds near the cloud-top level, and both the magnitude and direction of winds at all levels in this region appear to vary greatly with longitude relative to the sub-solar point.
A numerical simulation of the process of formation of the terrestrial planets is carried within the framework of a new theory for the accumulation of planetary and satellite systems. The numerical simulation permitted determining the parameters of the protoplanetary disk from which Mercury, Venus and the Earth were formed as result of the evolution. The acquisition of a slow retrograde rotation for Venus was discovered during the course of the investigation, whereas Mercury and the Earth acquired direct rotation about their axes. Deviations of the semimajor axes of these three planets as well as the masses of the Earth and Venus from the true values are small as a rule (l 10%). It is shown that during the accumulation of the terrestrial planets, there existed a profound relationship between the process of formation of the orbits and masses of the planet and the process of formation of their rotation about their axes. Estimates are presented for the radii of the initial effective bodies and the time of evolution for the terrestrial accumulation zone.
The Pioneer Venus program consists of two spacecraft: an orbiter and a multiprobe. Both will arrive at Venus in early December 1978. The orbiter will collect data on the upper atmosphere and fields and particles and will sense the clouds and surface remotely from a 75 deg inclined orbit. The multiprobe consists of a bus, three small probes, and a large probe. All five objects will enter the Venus atmosphere and will transmit data on its characteristics directly to earth while descending to the surface. The development of these spacecraft required the solution of many difficult and unique technical problems.
Sulfur reactions on Venus are discussed. It is suggested that photochemical reactions convert SO2 to H2SO4 and remove O2 from the Venus stratosphere, while thermochemical reactions near the surface probably make the SO2, regenerate CO2 from photolytic CO, and produce free sulfur. Optical evidence for liquid sulfur on Venus is examined; the Venera data suggest a light drizzle of liquid sulfur above 18 km and the existence of several per cent of sulfur vapor below that height.
The orbiter and multiprobe components of the Pioneer Venus mission are briefly described. The orbiter was launched on May 20, 1978 and was placed into a highly eccentric near-polar orbit around Venus on December 4, 1978, while the multiprobe was launched on August 8, 1978 and reached Venus on December 9, 1978. Parameters of the orbiter orbit are presented, and modifications of the periapsis altitude are described. The time sequences of the probe entry events are reported for the large probe, north probe, day probe, and night probe, which, along with the bus, are the components of the multiprobe. The multiprobe entry and impact locations as well as related data are reported.
The first gas chromatographic analysis of the lower atmosphere of Venus is reported. Three atmospheric samples were analyzed. The third of these samples showed carbon dioxide (96.4 percent), molecular nitrogen (3.41 percent), water vapor (0.135 percent), molecular oxygen (69.3 ppm), argon (18.6 ppm), neon (4.31 ppm), and sulfur dioxide (186 ppm). The amounts of water vapor and sulfur dioxide detected are roughly compatible with the requirements of greenhouse models of the high surface temperature of Venus. The large positive gradient of sulfur dioxide, molecular oxygen, and water vapor from the cloud tops to their bottoms, as implied by Earth-based observations and these results, gives added support for the presence of major quantities of aqueous sulfuric acid in the clouds. A comparison of the inventory of inert gases found in the atmospheres of Venus, Earth, and Mars suggests that these components are due to outgassing from the planetary interiors.
A model of the diffusion of ionosheath plasma into the wake region of Venus is presented. It is shown that particle diffusion, which is assumed to be a consequence of the fluctuating magnetic field observed in the wake of Venus by Pioneer Venus and Veneras 9 and 10, can explain the plasma observations made in the wake by Veneras 9 and 10. The pressure due to these diffusing particles when balanced against the ionospheric pressure yield ionopause heights less than 1000 km for zenith angles less than about 135 deg. The model also predicts significant fluxes of low energy electrons and ions for zenith angles less than 135 deg, which are capable of producing the observed nightside ionosphere.
A model for the variability of the Venus ionopause as a function of solar wind dynamic pressure and EUV flux during quiescent solar wind conditions is presented. The radio occultation measurements of the Venus ionopause from Mariner 5, 10 and Venera 9, 10 spacecraft, as well as recent in situ Pioneer Venus measurements are interpreted in terms of this model. An ionospheric model consistent with observations in the 400-1000 km region is predominantly O(+) with densities of about 10,000/cu cm and (T sub e + T sub i) approximately equal to 4500-6500 K. For ionopause measurements below 400 km the ionosphere appears severely compressed and density and temperature profiles cannot be simply described, although a strong correlation with solar wind dynamic pressure is observed. Possible effects of IMF direction switching on the dynamics and structure of the ionosphere are also considered and compared with available ionopause data.
High data rate, high resolution observations made by the Pioneer Venus 1 Orbiter of Venus clouds in the polar region are reported. Data obtained by the Pioneer IR radiometer reveal the presence of a localized, very warm (260 K) feature at about 80 deg N, 359 deg E, indicating a cloud height differential of about 15 km between the depressed, but not totally absent, cloud level indicated by the temperature peak and the surrounding cold zones. The observations are considered to constitute new evidence for the presence of strong atmospheric subsidence near the poles, fitting a model of vortex-type polar circulation. Implications of the observations for the single cell model of the Venus stratosphere are noted.
The Pioneer Venus Orbiter and the Multiprobe spacecraft propulsion subsystems and their performance are presented. Monopropellant hydrazine subsystems on each spacecraft provided the capability to spin up the spacecraft after separation and perform all spin rate, velocity, and attitude changes required by the control subsystem to satisfy mission objectives. The propulsion subsystem provides thrust on demand by supplying anhydrous hydrazine from the propellant tanks through manifolds, filters and valves to the thrust chamber assemblies where the hydrazine is catalytically decomposed and expanded in a conical nozzle. The subsystems consist of seven 1 lbf thrusters for the Orbiter and six 1 lbf thrusters for the multiprobe which are isolated by two latch valves from the two propellant tanks so that two redundant thruster clusters are provided to ensure mission completion in the event of a single point failure. The propellant feed system is of all-welded construction to minimize weight and leakage and titanium is used as the primary material of construction. The multiprobe burned up on entering the Venus atmosphere with enough propellant left for the mission and the Orbiter was inserted into Venus orbit with enough propellant remaining for more than 2 earth years of orbital operations.
Preliminary observations of day-night variations in the ion composition of the ionosphere of Venus, obtained by the Pioneer Venus Orbiter ion mass spectrometer experiment, are reported. A remarkable abundance and extent of ionization in the deep regions of the nightside ionosphere was observed, in spite of the long Venus night. A comparison of dayside and nightside ion distributions reveals a nightside composition similar in several respects to that of the dayside, with the ions O(+) and O2(+) forming the nightside F 2 and F 1 regions, respectively, as in the dayside. Important differences include a greater abundance of low-latitude ionization in the nightside, a significant increase of H(+) and NO(+) ions with increasing solar zenith angle, and extreme dynamic variability of the nightside region above 160 km. Ion composition data support the view that the nightside ionosphere can be maintained by the transport of ionization from the dayside.
Additional plasma measurements in the vicinity of Venus are presented. It is shown that (1) there are three distinct plasma electron populations - solar wind electrons, ionosheath electrons, and nightside ionosphere electrons; (2) the plasma ion flow pattern in the ionosheath is consistent with deflected flow around a blunt obstacle; (3) the plasma ion flow velocities near the downstream wake may, at times, be consistent with the deflection of plasma into the tail, closing the solar wind cavity downstream from Venus at a relatively close distance (within 5 Venus radii) to the planet; (4) there is a separation between the inner boundary of the downstream ionosheath and the upper boundary of the nightside ionosphere; and (5) during the first 4.5 months in orbit the measured solar wind plasma speed continued to vary, showing a number of high-speed, but generally nonrecurrent, streams.