Instrument Host Overview
    ========================
 
      This spacecraft was the third Small Probe 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 for descent through the atmosphere to the planetary surface. 
      Two Small Probes entered on the nightside, and one Small Probe and 
      one Large Probe entered on the dayside of the planet. The spacecraft 
      Bus entered the atmosphere and obtained atmospheric composition data 
      until burnup. Investigations emphasize the study of the structural 
      composition and nature of the atmosphere down to the surface, and 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 the Orbiter permitted 
      relating specific local measurements to the general state of the planet 
      and its environment as observed from orbit.
 
      The three small probes were identical and were designated the North Probe 
      (1978-078E), the Night Probe (1978-078F), and the Day Probe (1978-078G). 
      They were all mounted on the Multiprobe Bus (1978-078A) and were released 
      on 20 November 1978. The probes were targeted for different entry points 
      in the Venus atmosphere, all entered on 9 December 1978. The total cost 
      of building and operating the Pioneer Venus probes was $83 million.
 

 
 
    Platform Descriptions
    =====================
 
      The probe consisted of a spherical pressure vessel surrounded by a forward 
      conical heat shield and an afterbody. The probes had a mass of 90 kg and a 
      diameter of 0.8 m. The pressure vessel held all the scientific instruments 
      and spacecraft systems. It was made of two precisely machined flanged titanium 
      hemispheres joined by bolts with seals between. The seals comprised O-rings and 
      graphoil flat gaskets. The vessel walls were lined on the inside with Kapton 
      blankets and the interior was filled with 102 kPa of Xenon. The instruments were 
      mounted on two beryllium shelves to absorb heat. A small hemispherical antenna 
      protruded from the top of the pressure vessel. The pressure vessel had 7 openings, 
      one for the antenna, three for electrical cables, two for scientific instruments, 
      and one for an access hatch. There were also special diamond and sapphire windows. 
      The probe did not have any thrusters, once released it was on a ballistic trajectory. 
      The aeroshell was a 45 degree blunt cone made of titanium which used a bonded 
      carbon phenolic ablative coating as a heat shield. The aeroshell was permanently 
      attached to the pressure vessel, as was the fiberglass honeycomb afterbody. The 
      small probes did not have parachutes. The small probe carried an atmospheric 
      structure experiment, a nephelometer, a net-flux radiometer, and radio science 
      experiments.
 
      The Day Probe was released from the Multiprobe Bus at 13:06:29 UT on 20 November 1978. 
      (All times are given in spacecraft time, Earth received time was approximately 3 minutes 
      later.) It was targeted for the dayside at midsouthern latitudes and reached Venus on 
      9 December 1978. The probe initiated telemetry at 18:35:27 UT and entered the atmosphere 
      (200 km altitude) at 18:52:18 UT. After a 56 minute descent, the probe touched down on the 
      surface (31.3 S, 317 E) at 19:47:59 UT. The probe continued to transmit for another 
      67 minutes, 37 seconds after landing.