Instrument Host Overview
    ========================
 
      This spacecraft was the Large Probe (also called the Sounder Probe) 
      portion of the Pioneer Venus Multiprobe mission. On this mission four 
      instrumented atmospheric entry probes were carried by a spacecraft bus 
      to the vicinity of Venus and released for descent through the atmosphere 
      to the planetary surface. Two Small Probes entered on the nightside and 
      a Small Probe and this Large Probe entered on the dayside of the planet. 
      The spacecraft Bus entered the atmosphere and obtained atmospheric 
      composition data until burnup. Investigations 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 Large Probe comprised a spherical pressure vessel, a forward aeroshell 
      heat shield, and an aft cover. The probe was 1.5 m in diameter and had a mass 
      of 315 kg. The pressure vessel was built of three machined titanium parts: an 
      aft hemisphere, a flat ring section, and a forward cap. Each section was flanged 
      and bolted together and sealed with O-rings and graphoil gaskets. The interior 
      of the pressure vessel was filled with 102 kPa of nitrogen. A pressure bottle 
      held nitrogen to increase the internal pressure by 41 kPa. Two beryllium shelves 
      held the instruments and spacecraft systems. The interior of the shell was lined 
      with a 2.5 cm thick Kapton blanket. An antenna protruded from the top (aft) of 
      the sphere. Two arms on the outside of the pressure vessel held a prism and an arm 
      on the opposite side held a temperature sensor. There were 14 sealed penetrations 
      in the pressure vessel: one for the antenna, four for electrical cables, two access 
      hatches and seven for instruments. Power was provided by a 40 A-hr silver-zinc battery. 
      Radio transmission at 2.3 GHz was powered by four 10-W amplifiers. The blunt cone-shaped 
      aeroshell had an ablative carbon phenolic coating as a heat-shield. A pilot chute and 
      mortar were mounted on the aeroshell and the main chute, held by the aft cover, was 
      connected to three towers on the pressure vessel. The Large Probe carried a neutral 
      mass spectrometer, a solar flux radiometer, an atmospheric structure experiment, a 
      nephelometer, a cloud particle size spectrometer, a gas chromatograph, an infrared 
      radiometer, and radio science experiments. The total cost of building and operating 
      the probes was $83 million.
      
      The Large Probe, mounted on the Multiprobe 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 Large Probe was released from the Multiprobe Bus at 
      02:37:13 UT on 16 November 1978 and entered the Venus atmosphere (200 km altitude) on 
      9 December at 18:45:32 UT. Telemetry had been initiated 16 minutes earlier. The parachute 
      was deployed at approximately 18:46:21 UT and the heat shield jettisoned at an altitude 
      of 67 km. The parachute was jettisoned at 19:03:28 UT at an altitude of 47 km, and the 
      probe impacted the surface (4.4 N, 304 E) at 19:39:53 UT, at which point transmissions 
      ended.