Pioneer 11 (also known as Pioneer G) is a NASA robotic space probe launched on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, the solar wind, and cosmic rays. It was the first probe to encounter Saturn, the second to fly through the asteroid belt, and the second to fly by Jupiter. Later, Pioneer 11 became the second of five artificial objects to achieve an escape velocity allowing it to leave the Solar System. Due to power constraints and the vast distance to the probe, the last routine contact with the spacecraft was on September 30, 1995, and the last good engineering data was received on November 24, 1995.
Mission background
History Approved in February 1969, Pioneer 11 and its twin spacecraft, Pioneer 10, were the first probes designed to explore the outer Solar System. Early mission objectives, developed from proposals made throughout the 1960s, were to:
Explore the interplanetary medium beyond the orbit of Mars. Investigate the asteroid belt and assess its potential hazard to missions to the outer planets. Explore Jupiter and its environment. After a Saturn encounter was added to the mission plan, additional objectives included:
Map Saturn's magnetic field and determine its strength, direction, and structure. Measure the distribution of electrons and protons of different energies along the spacecraft's trajectory through the Saturnian system. Study the interaction between Saturn's magnetosphere and the solar wind. Measure the temperatures of Saturn's atmosphere and of Titan, Saturn's largest moon. Investigate the structure of Saturn's upper atmosphere, including its ionosphere. Map the thermal structure of Saturn's atmosphere using infrared observations and radio occultation measurements. Obtain two-color spin-scan images of the Saturnian system and polarimetric measurements of Saturn. Study Saturn's rings and atmosphere using S-band radio occultation. Improve estimates of the masses of Saturn and its larger moons by measuring their gravitational effects on the spacecraft's trajectory. Characterize the environment of Saturn's ring plane in preparation for the planned Voyager program, helping determine whether it could be crossed safely without serious damage to a spacecraft. Pioneer 11 was built by TRW Inc. and managed by NASA Ames Research Center as part of the Pioneer program. A backup spacecraft, Pioneer H, is on display in the "Milestones of Flight" exhibit at the National Air and Space Museum. Many aspects of the mission informed the planning and design of the Voyager program.
Spacecraft design The Pioneer 11 spacecraft bus is a hexagonal structure 36 centimeters (14 in) deep with six panels, each 76-centimeter-long (30 in) long. It houses the propellant system used for attitude control and eight of the spacecraft's 12 scientific instruments. At launch, the spacecraft had a mass of 259 kg (571 lb). The spacecraft's orientation was controlled by six 4.5 N hydrazine monopropellant thrusters arranged in three pairs. One pair maintained the spacecraft's spin rate of 4.8 rpm, another provided trajectory correction maneuvers, and the third controlled attitude adjustments. Attitude information was provided by conical scanning maneuvers that tracked Earth, a star tracker that used Canopus as a reference, and two Sun sensors.
Power and communications
Pioneer 11 uses four SNAP-19 radioisotope thermoelectric generators (RTGs). They are positioned on two three-rod trusses, each 3 m (9 ft 10 in) in length and 120 degrees apart. This was expected to be a safe distance from the sensitive scientific experiments carried on board. Combined, the RTGs provided 155 watts at launch, and decayed to 140 W in transit to Jupiter. The spacecraft requires 100 W to power all systems. Multilayer capsules of plutonium-238, shielded by graphite heat shields, served as the fuel source for the RTGs. The space probe includes a redundant system transceivers, one attached to the high-gain antenna, the other to an omni-antenna and medium-gain antenna. Each transceiver is 8 watts and transmits data across the S-band using 2110 MHz for the uplink from Earth and 2292 MHz for the downlink to Earth with the Deep Space Network tracking the signal. Prior to transmitting data, the probe uses a convolutional encoder to allow correction of errors in the received data on Earth. Much of the computation for the mission was performed on Earth and transmitted to the probe, where it is able to retain in memory, up to five commands of the 222 possible entries by ground controllers. The spacecraft includes two command decoders and a command distribution unit, a very limited form of a processor, to direct operations on the spacecraft. This system requires that mission operators prepare commands long in advance of transmitting them to the probe. A data storage unit is included to record up to 6,144 bytes of information gathered by the instruments. The digital telemetry unit is then used to prepare the collected data in one of the thirteen possible formats before transmitting it back to Earth.
Scientific instruments Pioneer 11 has one additional instrument more than Pioneer 10, a flux-gate magnetometer.
Mission profile
Launch and trajectory
Pioneer 11 was launched on April 6, 1973, at 02:11:00 UTC from Cape Canaveral Launch Complex 36A in Florida aboard an Atlas-Centaur launch vehicle with a Star-37E upper stage. Its twin spacecraft, Pioneer 10, had been launched on March 3, 1972. Pioneer 11 was initially launched on a direct trajectory to Jupiter without any gravity assists. In May 1974, mission controllers adjusted its trajectory to pass Jupiter on a north-to-south path, enabling a flyby of Saturn in 1979. The maneuver used 17 lb (7.7 kg) of propellant, lasted 42 minutes and 36 seconds, and increased the spacecraft's speed by 230 km/h (140 mph). Pioneer 11 also performed two mid-course correction maneuvers, on April 11, 1973, and November 7, 1974.
Encounter with Jupiter
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