Pioneer 11

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Pioneer 11
An artist's impression of a Pioneer spacecraft on its way to interstellar space.jpg
An artist's impression of a Pioneer spacecraft on its way to interstellar space.
Mission typePlanetary and heliosphere exploration
Operator NASA / ARC
COSPAR ID 1973-019A
SATCAT no. 6421
Website Pioneer Project website (archived)
NASA Archive page
Mission duration22 years, 5 months, 25 days
Spacecraft properties
Manufacturer TRW
Launch mass259 kilograms (571 lb)
Power155 watts (at launch)
Start of mission
Launch dateApril 6, 1973, 02:11:00 (1973-04-06UTC02:11Z) UTC
Rocket Atlas SLV-3D Centaur-D1A Star-37E
Launch site Cape Canaveral LC-36B
End of mission
Last contactSeptember 30, 1995 (1995-10-01)
Flyby of Jupiter
Closest approachDecember 3, 1974
Distance43,000 kilometers (27,000 miles)
Flyby of Saturn
Closest approachSeptember 1, 1979
Distance21,000 kilometers (13,000 miles)
Pioneer 10 - Pioneer 11 - mission patch - Pioneer patch.png  

Pioneer 11 (also known as Pioneer G) is a 259-kilogram (571 lb) robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind and cosmic rays. [1] It was the first probe to encounter Saturn and the second to fly through the asteroid belt and by Jupiter. Thereafter, Pioneer 11 became the second of five artificial objects to achieve the escape velocity that will allow them 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. [2] [3]

Robotic spacecraft uncrewed spacecraft, usually under telerobotic control

A robotic spacecraft is an uncrewed spacecraft, usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to lower cost and lower risk factors. In addition, some planetary destinations such as Venus or the vicinity of Jupiter are too hostile for human survival, given current technology. Outer planets such as Saturn, Uranus, and Neptune are too distant to reach with current crewed spacecraft technology, so telerobotic probes are the only way to explore them.

Space probe unmanned robotic spacecraft that does not orbit the Earth, but, instead, explores further into outer space

A space probe is a robotic spacecraft that does not orbit Earth, but instead, explores further into outer space. A space probe may approach the Moon; travel through interplanetary space; flyby, orbit, or land on other planetary bodies; or enter interstellar space.

NASA space-related agency of the United States government

The National Aeronautics and Space Administration is an independent agency of the United States Federal Government responsible for the civilian space program, as well as aeronautics and aerospace research.


Mission background


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:

<i>Pioneer 10</i> space probe launched in March 1972

Pioneer 10 is an American space probe, launched in 1972 and weighing 258 kilograms, that completed the first mission to the planet Jupiter. Thereafter, Pioneer 10 became the first of five artificial objects to achieve the escape velocity that will allow them to leave the Solar System. This space exploration project was conducted by the NASA Ames Research Center in California, and the space probe was manufactured by TRW Inc.

Interplanetary medium material which fills the Solar System

The interplanetary medium (IPM) is the material which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move.

Atmosphere of Jupiter atmosphere of the planet Jupiter

The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide and water. Although water is thought to reside deep in the atmosphere, its directly measured concentration is very low. The nitrogen, sulfur, and noble gas abundances in Jupiter's atmosphere exceed solar values by a factor of about three.

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

Solar wind

The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma consists of mostly electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. Embedded within the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field.

Radio occultation

Radio occultation (RO) is a remote sensing technique used for measuring the physical properties of a planetary atmosphere or ring system.

Pioneer 11 was built by TRW and managed as part of the Pioneer program by NASA Ames Research Center. [5] 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.. [6] Many elements of the mission proved to be critical in the planning of the Voyager program. [7] :266–8

TRW Inc. was an American corporation involved in a variety of businesses, mainly aerospace, automotive, and credit reporting. It was a pioneer in multiple fields including electronic components, integrated circuits, computers, software and systems engineering. TRW built many spacecraft, including Pioneer 1, Pioneer 10, and several space-based observatories. It was #57 on the 1986 Fortune 500 list, and had 122,258 employees. In 1958 the company was called Thompson Ramo Wooldridge, after three prominent leaders. This was later shortened to TRW.

Pioneer program series of United States unmanned space missions

The Pioneer program was a series of United States unmanned space missions that were designed for planetary exploration. There were a number of such missions in the program, but the most notable were Pioneer 10 and Pioneer 11, which explored the outer planets and left the solar system. Pioneer 10 and Pioneer 11 carry a golden plaque, depicting a man and a woman and information about the origin and the creators of the probes, should any extraterrestrials find them someday.

Pioneer H

Pioneer H is an unlaunched unmanned space mission that was part of the US Pioneer program for a planned 1974 launch. Had this mission and spacecraft been launched, it would have been designated Pioneer 12; that designation was later applied to the Pioneer Venus Orbiter.

Spacecraft design

The Pioneer 11 bus measured 36 centimeters (14 in) deep and with six 76-centimeter-long (30 in) panels forming the hexagonal structure. The bus housed propellant to control the orientation of the probe and eight of the twelve scientific instruments. The spacecraft had a mass of 260 kilograms. [1] :42

Attitude control and propulsion

Pioneer 10 - Pioneer 11 - thrusters - p43.jpg
Orientation of the spacecraft was maintained with six 4.5-N, [8] hydrazine monopropellant thrusters: pair one maintained a constant spin-rate of 4.8 rpm, pair two controlled the forward thrust, pair three controlled attitude. Information for the orientation was provided by performing conical scanning maneuvers to track Earth in its orbit, [9] a star sensor able to reference Canopus, and two Sun sensors. [1] :42–43


The space probe included a redundant system transceivers, one attached to the high-gain antenna, the other to an omni-antenna and medium-gain antenna. Each transceiver was 8 watts and transmitted 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 used a convolutional encoder to allow correction of errors in the received data on Earth. [1] :43


SNAP-19 RTG Two of four SNAP 19 RTGs of a Pioneer space probe.jpg
Pioneer 11 used four SNAP-19 radioisotope thermoelectric generators (RTGs) ( see diagram ). They were 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 required 100 W to power all systems. [1] :44–45


Much of the computation for the mission was performed on Earth and transmitted to the probe, where it was able to retain in memory, up to five commands of the 222 possible entries by ground controllers. The spacecraft included two command decoders and a command distribution unit, a very limited form of processor, to direct operations on the spacecraft. This system required that mission operators prepare commands long in advance of transmitting them to the probe. A data storage unit was included to record up to 6,144 bytes of information gathered by the instruments. The digital telemetry unit would then be used to prepare the collected data in one of the thirteen possible formats before transmitting it back to Earth. [1] :38

Scientific instruments

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

Helium Vector Magnetometer (HVM)
Pioneer 10-11 - P50 - fx.jpg Measured the fine structure of the interplanetary magnetic field, mapped the Jovian magnetic field, and provided magnetic field measurements to evaluate solar wind interaction with Jupiter. [11]
Quadrispherical Plasma Analyzer
Pioneer 10-11 - P51b - fx.jpg Peered through a hole in the large dish-shaped antenna to detect particles of the solar wind originating from the Sun. [12]
Charged Particle Instrument (CPI)
Pioneer 10-11 - P52a - fx.jpg Detected cosmic rays in the Solar System. [14]
Cosmic Ray Telescope (CRT)
Pioneer 10-11 - P52b - fx.jpg Collected data on the composition of the cosmic ray particles and their energy ranges. [15]
Geiger Tube Telescope (GTT)
Pioneer 10-11 - p53 - fx.jpg
Surveyed the intensities, energy spectra, and angular distributions of electrons and protons along the spacecraft's path through the radiation belts of Jupiter and Saturn. [16]
Trapped Radiation Detector (TRD)
Pioneer 10-11 - P54 - fx.jpg

Included an unfocused Cerenkov counter that detected 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. [17]

Meteoroid Detectors
Pioneer 10-11 - P56 - fx.jpg Twelve panels of pressurized cell detectors mounted on the back of the main dish antenna recorded penetrating impacts of small meteoroids. [18]
Asteroid/Meteoroid Detector (AMD)
Pioneer 10-11 - P55b - fx.jpg Meteoroid-asteroid detector looked into space with four non-imaging telescopes to track particles ranging from close-by bits of dust to distant large asteroids. [19]
Ultraviolet Photometer
Pioneer 10-11 - P57a - fx.jpg Ultraviolet light was sensed to determine the quantities of hydrogen and helium in space and on Jupiter and Saturn. [20]
Imaging Photopolarimeter (IPP)
Pioneer 10-11 - P60 - fx.jpg The imaging experiment relied 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 were then processed to build up a visual image of the planet. [21]
Infrared Radiometer
P58 - fx.jpg Provided information on cloud temperature and the output of heat from Jupiter and Saturn. [22]
  • Principal investigator: Andrew Ingersoll / California Institute of Technology [13]
Triaxial Fluxgate Magnetometer
Triaxial Fluxgate Mars Global Surveyor Magnetometer.gif Measured the magnetic fields of both Jupiter and Saturn. This instrument was not carried on Pioneer 10. [23]

Mission profile

Pioneer 11 launching from Launch Complex 36A. Pioneer G (Pioneer 11) launch.jpg
Pioneer 11 launching from Launch Complex 36A.
Animation of Pioneer 11's trajectory from 6 April 1973 to 31 December 1980
Pioneer 11  *   Earth  *   Jupiter *   Saturn Animation of Pioneer 11 trajectory.gif
Animation of Pioneer 11's trajectory from 6 April 1973 to 31 December 1980
  Pioneer 11 ·   Earth  ·   Jupiter  ·   Saturn
Animation of Pioneer 11's trajectory around Jupiter from 30 November 1974 to 5 December 1974
Pioneer 11 *   Jupiter *   Io *   Europa  *   Ganymede  *   Callisto Animation of Pioneer 11 trajectory around Jupiter.gif
Animation of Pioneer 11's trajectory around Jupiter from 30 November 1974 to 5 December 1974
  Pioneer 11 ·   Jupiter  ·   Io  ·   Europa  ·   Ganymede  ·   Callisto
Timeline of travel
Spacecraft launched at 02:11:00.
Passage through the asteroid belt.
Start Jupiter observation phase.
Encounter with Jovian system.
Callisto flyby at 786,500 km.
Ganymede flyby at 692,300 km.
Io flyby at 314,000 km.
Europa flyby at 586,700 km.
Jupiter shadow entry.
Jupiter occultation entry.
Jupiter closest approach at 42,828 km.
Jupiter shadow exit.
Jupiter occultation exit.
Amalthea flyby at 127,500 km.
Phase stop.
Start Saturn observation phase.
Encounter with Saturnian system.
Iapetus flyby at 1,032,535 km.
Phoebe flyby at 13,713,574 km.
Hyperion flyby at 666,153 km.
Descending ring plane crossing.
Epimetheus flyby at 6,676 km.
Atlas flyby at 45,960 km.
Dione flyby at 291,556 km.
Mimas flyby at 104,263 km.
Saturn closest approach at 20,591 km.
Saturn occultation entry.
Saturn shadow entry.
Janus flyby at 228,988 km.
Saturn occultation exit.
Saturn shadow exit.
Ascending ring plane crossing.
Tethys flyby at 329,197 km.
Enceladus flyby at 222,027 km.
Calypso flyby at 109,916 km.
Rhea flyby at 345,303 km.
Titan flyby at 362,962 km.
Phase stop.
Begin Pioneer Interstellar Mission.
[1] :61–94 [24] [25]

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. Its twin probe, Pioneer 10 , had launched a year earlier on March 3, 1972. Pioneer 11 was launched on a trajectory directly aimed at Jupiter without any prior gravitational assists. [26] 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. [27] It also made two mid-course corrections, on April 11, 1973 and November 7, 1974. [3]

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. [24] 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. On April 16, 1975, following the Jupiter encounter, the micrometer detector was turned off. [3]

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.

Interstellar mission

On February 25, 1990, Pioneer 11 became the 4th man-made object to pass beyond the orbit of the planets. [28]

NASA ends operations

By 1995, Pioneer 11 could no longer power any of its detectors, so the decision was made to shut it down. [29] 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." [30] 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. [29] Scientists received a few minutes of good engineering data on 24 November 1995 but then lost final contact once Earth permanently moved out of view of the spacecraft's antenna. [3] Its signal became too faint to hear in 2002. [31]

Current status

:Pioneer 10 and 11 speed and distance from the Sun Pioneer speed and distance from Sun.svg
:Pioneer 10 and 11 speed and distance from the Sun

On January 30, 2019, Pioneer 11 was 100.84 AU (1.5085×1010 km; 9.374×109 mi) from the Earth and 100 AU (1.5×1010 km; 9.3×109 mi) from the Sun; and traveling at 11.241 km/s (40,470 km/h; 25,150 mph) (relative to the Sun) and traveling outward at about 2.37 AU per year. [32] The spacecraft is heading in the direction of the constellation Scutum near the current position (August 2017) RA 18h 50m dec -8° 39.5' (J2000.0) close to Messier 26. In 928,000 years it will pass within 0.25pc of the K dwarf TYC 992-192-1. [33]

Pioneer 11 has now been overtaken by the two Voyager probes, launched in 1977, and Voyager 1 is now the most distant object built by humans. [34]

Pioneer anomaly

Analysis of the radio tracking data from the Pioneer 10 and 11 spacecraft at distances between 20–70 AU from the Sun has consistently indicated the presence of a small but anomalous Doppler frequency drift. The drift can be interpreted as due to a constant acceleration of (8.74 ± 1.33) × 10−10 m/s2 directed towards the Sun. Although it is suspected that there is a systematic origin to the effect, none was found. As a result, there is sustained interest in the nature of this so-called "Pioneer anomaly". [35] Extended analysis of mission data by Slava Turyshev and colleagues has determined the source of the anomaly to be asymmetric thermal radiation and the resulting thermal recoil force acting on the face of the Pioneers away from the Sun, [36] and in July 2012 the group of researchers published their results in the Physical Review Letters scientific journal. [37]

Pioneer plaque

Pioneer 10 and 11 both carry a gold-anodized aluminum plaque in the event that either spacecraft is ever found by intelligent lifeforms from other planetary systems. The plaques feature the nude figures of a human male and female along with several symbols that are designed to provide information about the origin of the spacecraft. [38]


In 1991, Pioneer 11 was honored on one of 10 United States Postage Service stamps commemorating unmanned spacecraft exploring each of the then nine planets and the Moon. Pioneer 11 was the spacecraft featured with Jupiter. Pluto was listed as "Not yet explored". [39]

Pioneer 11 Saturn Rings.png
Pioneer 11 and Saturn rings on September 1, 1979
(artist concept)
An artist's impression of the encounter between Pioneer 11 and Saturn.jpg
Pioneer 11's flyby of Saturn
(artist concept)
Pio11 8feb2012.jpg
Position of Pioneer 11 as of 8 February 2012 showing spacecraft trajectory since launch (simulated view)

See also

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. Interstellar probes trajectory.svg
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.

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