The Pioneer Venus Orbiter Mission - Pioneer 12
The Orbiter was launched on 20 May 1978 on an Atlas-Centaur launch vehicle. On 4 December 1978, the orbiter was injected into a highly elliptical orbit around Venus. The periapsis, or low orbital point, of the orbit was about 150 km (93 miles) above the surface of the planet. The apoapsis, or highest orbital point, was 66,000 km (41,00 miles) from the planet. The orbital period was 23 hours 11 minutes.The orbit permitted global mapping of the clouds, atmosphere and ionosphere; measurement of upper atmosphere, ionosphere, and solar wind-ionosphere interaction; and mapping of the planet's surface by radar. For the first 19 months of the mission, the periapsis was maintained at about 150 km by periodic maneuvers. As propellant began to run low, the maneuvers were discontinued, and Solar gravitational effects caused the periapsis to rise to about 2,300 km. By 1986, the gravitational effects caused the periapsis to start falling again, and the orbiter instruments could again make direct measurement within the main ionosphere.
During the Orbiter's mission, opportunities arose to make systematic observations of several comets with the Ultraviolet Spectrometer (OUVS). The comets and their date of observation were: Encke April 13 through April 16, 1984; Giacobini-Zinner, September 8 through 15, 1985; Halley, December 27, 1985 to March 9, 1986; Wilson, March 13 to May 2, 1987; NTT, April 8, 1987; and McNaught, November 19 through 24, 1987. For Halley, the results showed that, near perihelion, the water evaporation rate was about 40 tons per second.
Starting in September 1992, controllers used the remaining fuel in a series of maneuvers to keep raising periapsis altitude for as long as possible. On 8 October 1992, its fuel supply exhausted, the Orbiter ended its mission as a meteor flaming through the dense atmosphere of Venus. As the Pioneer Venus Orbiter entered the Venusian atmosphere, it produced a glowing tail like a large meteorite. This artist's rendering shows the spectacular end of the Orbiter's 14-year mission.
Spacecraft Description
The main body of the spacecraft was a flat cylinder 2.5 m (8.2 ft) in diameter and 1.2 m (4 ft) high. In the upper or forward end of the cylinder was a circular equipment shelf. All the spacecraft's scientific instruments and electronic subsystems were on this shelf. Below the shelf, 15 thermal louvers controlled heat radiation from an equipment compartment that was between the shelf and the top of the spacecraft. On top of the spacecraft was a 1.09 m (3.6 ft) diameter, despun, high gain, parabolic dish antenna. The despun design allowed the antenna to me mechanically directed to continuously face the Earth from the spinning spacecraft.
The spacecraft also carried a solid-propellant rocket motor with 18,000 N (4046 lb) of thrust. This thrust would decelerate the spacecraft by 3816 km/hr (2371 mph) and place it into an orbit around Venus. The spacecraft's launch weight of 553 kg (1219 lbs) included 45 kg (100 lb) of scientific instruments and 179 kg (395 lb) of rocket propellant.
Beneath the equipment compartment were two conical hemispheric propellant tanks. These tanks stored 32 kg (70 lb) of hydrazine propellant for three axial and four radial thrusters. These thrusters were used to change the attitude, velocity, or orbital period and spin rate during the mission. Additional information about Pioneer Venus can be found at the following locations:
The National Space Science Data Center (NSSDC) has a description of the Pioneer Venus missions and science data sets. The Center for Space Research at MIT has a science data set from the Radar Mapper instrument on the Orbiter.
The Pioneer Venus Multiprobe Mission - Pioneer 13
On 8 August 1978, slightly less than three months after the Orbiter left Earth, the Multiprobe spacecraft was launched from the Kennedy Space Center on an Atlas-Centaur launch vehicle. On 16 November 1978, the Large Probe was released from the Bus toward an entry near the equator on the day side of Venus. Four days later, on 20 November 1978, the three Small Probes were released from the bus. Two of the probes were targeted to enter on the night side and one was targeted to enter on the Venus day side. On 9 December 1978 the bus with its instruments was retargeted to enter Venus' day side.