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.
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.
The Pioneer 11 bus is 36 cm (14 in) deep and consists of six 76 cm (30 in) panels forming a hexagonal structure. The bus houses the propellant used to control the spacecraft’s orientation and eight of its 12 scientific instruments. The spacecraft has a mass of 259 kg (571 lb).