Instrument Host Overview
    ========================
 
      The spacecraft was the Bus portion of the Pioneer Venus Multiprobe 
      mission. On this mission four instrumented atmospheric entry Probes 
      were carried by this Bus to the vicinity of Venus and released for 
      descent through the atmosphere to the planetary surface. Investigators 
      emphasized the study of the structure and composition of the atmosphere 
      down to the surface, the nature and composition of the clouds, the 
      radiation field and energy exchange in the lower atmosphere, and local 
      information on the atmospheric circulation pattern. A sister mission, 
      Pioneer Venus Orbiter, placed an orbiting spacecraft around Venus 5 days 
      before the Probes entered the atmosphere. Simultaneous measurements by the 
      probes and orbiter permitted relating specific local measurements to the 
      general state of the planet and its environment as observed from orbit.
 
 
 
 
    Platform Descriptions
    =====================
 
      The Multiprobe Bus had a total mass at launch of 875 kg, of which 585 kg 
      were the attached Large Probe and three Small Probes, 32 kg was hydrazine 
      fuel, and 290 kg was the Bus structure itself. The Bus was a 2.5 meter 
      diameter cylinder on top of which were mounted the Large Probe in the center 
      and the three Small Probes spaced 120 degrees apart around the Large Probe. 
      The total height of the unit from the bottom of the Bus to the top of the 
      large probe was 2.9 meters. Power was provided by a solar array around the 
      outside of the cylinder and batteries. Attitude control and knowledge was 
      provided by radial and aft thrusters and star sensors. Communications were 
      via forward and aft omnidirectional antennae and an aft medium gain horn 
      antenna. The bus was not equipped with heat shield or parachutes, it was only 
      designed to survive upper atmospheric entry. The Large Probe was designed to
      be launched from the Bus by a pyrotechnic spring-separation system. The Small 
      Probes were held by clamps which could be released by firing explosive nuts. 
      The probes would then spin off the bus tangentially due to the 48 rpm rotation. 
      The Bus carried Ion and Neutral Mass Spectrometers to study the upper atmosphere. 
      The total cost of building and operating the probes was $83 million.
      
      The Probe Bus was launched on the Pioneer Venus 2 mission on 8 August 1978 at 
      07:33:00 UT from Cape Canaveral. There was one midcourse correction on 16 August 
      1978. The trip to Venus took 123 days. The Large Probe separated from the Bus on 
      16 November and the Small Probes on 20 November. After release of the probes the 
      bus was slowed slightly to fall behind the probes as they headed towards Venus. Two 
      Small Probes entered on the nightside, and one Small Probe and the Large Probe entered 
      on the dayside of the planet. The spacecraft was spin-stabilized at about 48 rpm. The 
      Large Probe took 1-1/2 h to descend through the atmosphere, while the three smaller 
      probes reached the surface of the planet 75 min after entry. The Bus portion of the 
      spacecraft was targeted to enter the Venusian atmosphere at a shallow entry angle and 
      transmit data to Earth until the Bus was destroyed by the heat of atmospheric friction 
      during its descent. At 20:21:52 UT on 9 December 1978 the bus entered the dayside Venus 
      atmosphere (200 km altitude) at 37.9 S, 290.9 E. It returned signals until reaching an 
      altitude of 110 km one minute later at 20:22:55 UT.