History

Approved in February 1969, Pioneer 11 and its twin probe, Pioneer 10, were the first to be designed for exploring the outer Solar System. Yielding to multiple proposals throughout the 1960s, early mission objectives were defined as:

• Explore the interplanetary medium beyond the orbit of Mars
• Investigate the nature of the asteroid belt from the scientific standpoint and assess the belt's possible hazard to missions to the outer planets.
• Explore the environment of Jupiter.

Subsequent planning for an encounter with Saturn added many more goals:

• Map the magnetic field of Saturn and determine its intensity, direction, and structure.
• Determine how many electrons and protons of various energies are distributed along the trajectory of the spacecraft through the Saturn system.
• Map the interaction of the Saturn system with the solar wind.
• Measure the temperature of Saturn's atmosphere and that of Titan, the largest satellite of Saturn.
• Determine the structure of the upper atmosphere of Saturn where molecules are expected to be electrically charged and form an ionosphere.
• Map the thermal structure of Saturn's atmosphere by infrared observations coupled with radio occultation data.
• Obtain spin-scan images of the Saturnian system in two colors during the encounter sequence and polarimetry measurements of the planet.
• Probe the ring system and the atmosphere of Saturn with S-band radio occultation.
• Determine more precisely the masses of Saturn and its larger satellites by accurate observations of the effects of their gravitational fields on the motion of the spacecraft.
• As a precursor to the Mariner Jupiter/Saturn mission, verify the environment of the ring plane to find out where it may be safely crossed by the Mariner spacecraft without serious damage.^[5]

Pioneer 11 was built by TRW and managed as part of the Pioneer program by NASA Ames Research Center.^[6] A backup unit, Pioneer H, is currently on display in the "Milestones of Flight" exhibit at the National Air and Space Museum in Washington, D.C.^[7] Many elements of the mission proved to be critical in the planning of the Voyager program.^{[8]: 266–8}

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  Pioneer 10 and Pioneer 11 spacecraft diagram
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  Reconstructed full-scale mock-up Pioneer 10 / 11 spacecraft at the National Air and Space Museum
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  Side view of the spacecraft
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  Pioneer 11 during the installation of its protective shroud

Spacecraft design

The Pioneer 11 bus measures 36 centimeters (14 in) deep and with six 76-centimeter-long (30 in) panels forming the hexagonal structure. The bus houses propellant to control the orientation of the probe and eight of the twelve scientific instruments. The spacecraft has a mass of 259 kilograms.^[2]: 42

Power

SNAP-19 RTG

Pioneer 11 uses four SNAP-19 radioisotope thermoelectric generators (RTGs) (see diagram). They are positioned on two three-rod trusses, each 3 meters (9 feet 10 inches) 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.^{[2]: 44–45}

Scientific instruments

Pioneer 11 has one additional instrument more than Pioneer 10, a flux-gate magnetometer.^[11]

Helium Vector Magnetometer (HVM)
	Measures the fine structure of the interplanetary magnetic field, mapped the Jovian magnetic field, and provides magnetic field measurements to evaluate solar wind interaction with Jupiter.[12] Principal investigator: Edward Smith / JPL Data: PDS/PPI data catalog, NSSDC data archive
Quadrispherical Plasma Analyzer
	Peer through a hole in the large dish-shaped antenna to detect particles of the solar wind originating from the Sun.[13] Principal investigator: Aaron Barnes / NASA Ames Research Center (archived website)[14] Data: PDS/PPI data catalog, NSSDC data archive
Charged Particle Instrument (CPI)
	Detects cosmic rays in the Solar System.[15] Principal investigator: John Simpson / University of Chicago[14] Data: NSSDC data archive
Cosmic Ray Telescope (CRT)
	Collects data on the composition of the cosmic ray particles and their energy ranges.[16] Principal investigator: Frank B. McDonald / NASA Goddard Space Flight Center[14] Data: PDS/PPI data catalog, NSSDC data archive
Geiger Tube Telescope (GTT)
	Surveys the intensities, energy spectra, and angular distributions of electrons and protons along the spacecraft's path through the radiation belts of Jupiter and Saturn.[17] Principal investigator: James A. Van Allen / University of Iowa (website)[14] Data: PDS/PPI data catalog, NSSDC data archive, NSSDC Jupiter data archive
Trapped Radiation Detector (TRD)
	Includes an unfocused Cerenkov counter that detects the light emitted in a particular direction as particles passed through it recording electrons of energy, 0.5 to 12 MeV, an electron scatter detector for electrons of energy, 100 to 400 keV, and a minimum ionizing detector consisting of a solid-state diode that measured minimum ionizing particles (<3 MeV) and protons in the range of 50 to 350 MeV.[18] Principal investigator: R. Fillius / University of California San Diego[14] Data: NSSDC hourly data archive, NSSDC Saturn data archive
Meteoroid Detectors
	Twelve panels of pressurized cell detectors mounted on the back of the main dish antenna record penetrating impacts of small meteoroids.[19] Principal investigator: William Kinard / NASA Langley Research Center[14] Data: NSSDC data archive list
Asteroid/Meteoroid Detector (AMD)
	Meteoroid-asteroid detector looks into space with four non-imaging telescopes to track particles ranging from close by bits of dust to distant large asteroids.[20] Principal investigator: Robert Soberman / General Electric Company[14] Data: NSSDC data archive list
Ultraviolet Photometer
	Ultraviolet light is sensed to determine the quantities of hydrogen and helium in space and on Jupiter and Saturn.[21] Principal investigator: Darrell Judge / University of Southern California[14] Data: PDS/PPI data catalog, NSSDC data archive
Imaging Photopolarimeter (IPP)
	The imaging experiment relies upon the spin of the spacecraft to sweep a small telescope across the planet in narrow strips only 0.03 degrees wide, looking at the planet in red and blue light. These strips are then processed to build up a visual image of the planet.[22] Principal investigator: Tom Gehrels / University of Arizona[14] Data: NSSDC data archive list
Infrared Radiometer
	Provides information on cloud temperature and the output of heat from Jupiter and Saturn.[23] Principal investigator: Andrew Ingersoll / California Institute of Technology[14]
Triaxial Fluxgate Magnetometer
	Measures the magnetic fields of both Jupiter and Saturn. This instrument is not carried on Pioneer 10.[24] Principal investigator: Mario Acuna / NASA Goddard Space Flight Center Data: NSSDC data archive list

Launch and trajectory

The Pioneer 11 probe was launched on April 6, 1973, at 02:11:00 UTC, by the National Aeronautics and Space Administration from Space Launch Complex 36A at Cape Canaveral, Florida aboard an Atlas-Centaur launch vehicle, with a Star-37E propulsion module. Its twin probe, Pioneer 10, had been launched a year earlier on March 3, 1972.

Pioneer 11 was launched on a trajectory directly aimed at Jupiter without any prior gravitational assists.^[25] In May 1974, Pioneer was retargeted to fly past Jupiter on a north–south trajectory, enabling a Saturn flyby in 1979. The maneuver used 17 pounds of propellant, lasted 42 minutes and 36 seconds, and increased Pioneer 11's speed by 230 km/h.^[26] It also made two mid-course corrections, on April 11, 1973 and November 7, 1974.^[4][27]

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  NASA map showing trajectories of the Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 spacecraft.
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  Animation of Pioneer 11's trajectory from April 6, 1973 to December 31, 1980
     Pioneer 11  ·   Earth  ·   Jupiter ·   Saturn
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  Animation of Pioneer 11's trajectory around Jupiter from November 30, 1974 to December 5, 1974
     Pioneer 11 ·   Jupiter ·   Io ·   Europa  ·   Ganymede  ·   Callisto
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  Animation of Pioneer 11 around Saturn
     Pioneer 11 ·   Saturn ·   Epimetheus ·   Janus ·   Mimas ·   Enceladus

Encounter with Jupiter

Pioneer 11 flew past Jupiter in November and December 1974. During its closest approach, on December 2, it passed 42,828 kilometers (26,612 mi) above the cloud tops.^[28] The probe obtained detailed images of the Great Red Spot, transmitted the first images of the immense polar regions, and determined the mass of Jupiter's moon Callisto. Using the gravitational pull of Jupiter, a gravity assist was used to alter the trajectory of the probe towards Saturn and gain velocity. On April 16, 1975, following the Jupiter encounter, the micrometeoroid detector was turned off.^[4]

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  Pioneer 11 Jupiter encounter
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  Approach on Jupiter
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  The Great Red Spot imaged by Pioneer 11
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  The Great Red Spot prior to closest approach
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  Cloud bands along the edge of Jupiter
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  Beginning polar gravity assist
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  Jupiter polar region from 1,079,000 km
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  Io imaged from 756,000 km

Media related to Pioneer 11 Jupiter encounter at Wikimedia Commons

Saturn encounter

Pioneer 11 passed by Saturn on September 1, 1979, at a distance of 21,000 km from Saturn's cloud tops.

By this time, Voyager 1 and Voyager 2 had already passed Jupiter and were also en route to Saturn, so it was decided to target Pioneer 11 to pass through the Saturn ring plane at the same position that the soon-to-come Voyager probes would use in order to test the route before the Voyagers arrived. If there were faint ring particles that could damage a probe in that area, mission planners felt it was better to learn about it via Pioneer. Thus, Pioneer 11 was acting as a "pioneer" in a true sense of the word; if danger were detected, then the Voyager probes could be rerouted further away from the rings, but missing the opportunity to visit Uranus and Neptune in the process.

Pioneer 11 imaged and nearly collided with one of Saturn's small moons, passing at a distance of no more than 4,000 kilometers (2,500 mi). The object was tentatively identified as Epimetheus, a moon discovered the previous day from Pioneer's imaging, and suspected from earlier observations by Earth-based telescopes. After the Voyager flybys, it became known that there are two similarly sized moons (Epimetheus and Janus) in the same orbit, so there is some uncertainty about which one was the object of Pioneer's near-miss. Pioneer 11 encountered Janus on September 1, 1979 at 14:52 UTC at a distance of 2500 km and Mimas at 16:20 UTC the same day at 103000 km.

Besides Epimetheus, instruments located another previously undiscovered small moon and an additional ring, charted Saturn's magnetosphere and magnetic field and found its planet-size moon, Titan, to be too cold for life. Hurtling underneath the ring plane, the probe sent back pictures of Saturn's rings. The rings, which normally seem bright when observed from Earth, appeared dark in the Pioneer pictures, and the dark gaps in the rings seen from Earth appeared as bright rings.

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  Pioneer 11 image of Saturn taken on 1979/08/26
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  Pioneer 11 image of Saturn taken on 1979/09/01
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  Pioneer 11 image of Saturn taken on 1979/09/01
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  Outgoing Pioneer 11 image of Saturn taken on 1979/09/03
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  Pioneer 11 image of Saturn's moon Titan

Media related to Pioneer 11 Saturn encounter at Wikimedia Commons

Interstellar mission

On February 25, 1990, Pioneer 11 became the 4th man-made object to pass beyond the orbit of the planets.^[29]

NASA ends operations

By 1995, Pioneer 11 could no longer power any of its detectors, so the decision was made to shut it down.^[30] On September 29, 1995, NASA's Ames Research Center, responsible for managing the project, issued a press release that began, "After nearly 22 years of exploration out to the farthest reaches of the Solar System, one of the most durable and productive space missions in history will come to a close." It indicated NASA would use its Deep Space Network antennas to listen "once or twice a month" for the spacecraft's signal, until "some time in late 1996" when "its transmitter will fall silent altogether." NASA Administrator Daniel Goldin characterized Pioneer 11 as "the little spacecraft that could, a venerable explorer that has taught us a great deal about the Solar System and, in the end, about our own innate drive to learn. Pioneer 11 is what NASA is all about – exploration beyond the frontier."^[31] Besides announcing the end of operations, the dispatch provided a historical list of Pioneer 11 mission achievements.

NASA terminated routine contact with the spacecraft on September 30, 1995, but continued to make contact for about 2 hours every 2 to 4 weeks.^[30] Scientists received a few minutes of good engineering data on November 24, 1995, but then lost final contact once Earth moved out of view of the spacecraft's antenna.^[4][32]

Timeline

Pioneer 10 and 11 speed and distance from the Sun

Heliocentric positions of the five interstellar probes (squares) and other bodies (circles) until 2020, with launch and flyby dates. Markers denote positions on 1 January of each year, with every fifth year labelled.
Plot 1 is viewed from the north ecliptic pole, to scale.
Plots 2 to 4 are third-angle projections at 20% scale.
In the SVG file, hover over a trajectory or orbit to highlight it and its associated launches and flybys.