Pioneer 11

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
OperatorUnited States NASA / ARC
NASA Archive page
Mission duration22 years, 5 months, 25 days
Spacecraft properties
ManufacturerUnited States 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
RocketAtlas SLV-3D Centaur-D1A Star-37E
Launch siteCape 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]

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:

  • 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.[4]

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

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 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 maintains a constant spin-rate of 4.8 rpm, pair two controls the forward thrust, pair three controls attitude. Information for the orientation is 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 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.[1]:43


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.[1]:44–45


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

Helium Vector Magnetometer (HVM)
Pioneer 10-11 - P50 - fx.jpg 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.[11]
Quadrispherical Plasma Analyzer
Pioneer 10-11 - P51b - fx.jpg Peer 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 Detects cosmic rays in the Solar System.[14]
Cosmic Ray Telescope (CRT)
Pioneer 10-11 - P52b - fx.jpg Collects data on the composition of the cosmic ray particles and their energy ranges.[15]
Geiger Tube Telescope (GTT)
Pioneer 10-11 - p53 - fx.jpg
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.[16]
Trapped Radiation Detector (TRD)
Pioneer 10-11 - P54 - fx.jpg

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.[17]

Meteoroid Detectors
Pioneer 10-11 - P56 - fx.jpg Twelve panels of pressurized cell detectors mounted on the back of the main dish antenna record penetrating impacts of small meteoroids.[18]
Asteroid/Meteoroid Detector (AMD)
Pioneer 10-11 - P55b - fx.jpg 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.[19]
Ultraviolet Photometer
Pioneer 10-11 - P57a - fx.jpg Ultraviolet light is 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 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.[21]
Infrared Radiometer
P58 - fx.jpg Provides 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 Measures the magnetic fields of both Jupiter and Saturn. This instrument is not carried on Pioneer 10.[23]