Pioneer Venus Large Probe

Description

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.

Spacecraft and Subsystems

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.

Mission Profile

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. (All times at Venus are spacecraft time, for ground received time add 3 minutes, 12 seconds.) 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 roughly 45 km, and the probe impacted the surface (4.4 N, 304 E) at 19:39:55 UT, at which point transmissions ended.