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:
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.[2]
Pioneer 11 during the installation of its protective shroud
Spacecraft design
The Pioneer 11 bus measures 36 centimeters (14in) deep and with six 76-centimeter-long (30in) 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 259kilograms.[6]
Attitude control and propulsion
Orientation of the spacecraft was maintained with six 4.5-N,[7]hydrazinemonopropellant 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,[8] a star sensor able to reference Canopus,and two Sun sensors.[9]
Communications
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.[10]
Power
SNAP-19 RTG on a Pioneer 10/11 replica
Pioneer 11 uses four SNAP-19 radioisotope thermoelectric generators (RTGs) (see diagram). They are positioned on two three-rod trusses,each 3m (9ft 10in) 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.[11]
Computer
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.[12]
Scientific instruments
Pioneer 11 has one additional instrument more than Pioneer 10,a flux-gate magnetometer.[13]
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.[14]
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.[19]
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.[20]
Principal investigator: R. Fillius / University of California San Diego[16]
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.[22]
Principal investigator: Robert Soberman / General Electric Company[16]
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 (5800 to 7000 Å) and blue (3900 to 4900 Å) light. These strips are then processed to build up a visual image of the planet.[24]
Principal investigator:Tom Gehrels / University of Arizona[16]
Pioneer11 was launched on a trajectory directly aimed at Jupiter without any prior gravitational assists.[27] In May 1974,Pioneer was retargeted to fly past Jupiter on a north–south trajectory,enabling a Saturn flyby in 1979. The maneuver used 17lb (7.7kg) of propellant,lasted 42 minutes and 36 seconds,and increased Pioneer11's speed by 230km/h.[28] It also made two mid-course corrections,on April 11,1973 and November 7,1974.[1]
NASA map showing trajectories of the Pioneer 10,Pioneer 11,Voyager 1,and Voyager 2 spacecraft.
Animation of Pioneer 11's trajectory from April 6,1973 to December 31,1980 Pioneer 11·Earth·Jupiter·Saturn
Animation of Pioneer 11's trajectory around Jupiter from November 30, 1974 to December 5, 1974 Pioneer 11·Jupiter·Io·Europa·Ganymede·Callisto
Animation of Pioneer 11 around Saturn Pioneer 11·Saturn·Epimetheus·Janus·Mimas·Enceladus
Pioneer 11 flew past Jupiter in November and December 1974. During its closest approach, on December 2, it passed 42,828km (26,612mi) above the cloud tops. 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.[1]
Pioneer 11 passed by Saturn on September 1, 1979, at a distance of 21,000km (13,000mi) from Saturn's cloud tops.[27]
By this time, Voyager 1 and Voyager 2 had already passed Jupiter and were en route to Saturn, so it was decided Pioneer 11 would pass through the Saturn ring plane at the same position Voyager 2 would later have to fly through in order to reach Uranus and Neptune. If there were faint ring particles capable of damaging a probe in that area, mission planners felt it was better to learn about it via Pioneer.[27] Thus, Pioneer 11 was acting as a "pioneer" in a true sense of the word; if danger were detected, then Voyager 2 could be redirected further away from the rings but miss the opportunity to visit the ice giants in the process.
Pioneer 11 imaged—and nearly collided with—one of Saturn's small moons, passing at a distance of no more than 4,000km (2,500mi). 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 14:52UTC, at a distance of 2,500km (1,600mi). At 16:20UTC the same day, Pioneer11 encountered Mimas at a distance of 103,000km (64,000mi).
Besides Epimetheus, instruments located another previously undiscovered small moon and an additional ring, charted Saturn's magnetosphere and magnetic field, and found its planet-sized 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.
Pioneer 11 image of Saturn taken on 1979/08/26
Pioneer 11 image of Saturn taken on 1979/09/01
Pioneer 11 image of Saturn taken on 1979/09/01
Outgoing Pioneer 11 image of Saturn taken on 1979/09/03
On February 25, 1990, Pioneer 11 became the fourth human-made object to pass beyond the orbit of the planets.[29]
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 two hours every two to four 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.[1][32]
Timeline
Pioneer 10 and 11 speed and distance from the SunHeliocentric 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.
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.[3][1]
As of June 24, 2024, Pioneer 11 is estimated to be 113.121AU (16.9227×10^9km; 10.5153×10^9mi) from the Earth and 114.089AU (17.0675billionkm; 10.6052billionmi) from the Sun. It was traveling at 11.155km/s (40,160km/h; 24,950mph) relative to the Sun and traveling outward at about 2.35AU per year.[35][36] The spacecraft is heading in the direction of the constellation Scutum near the current position (June 2024) RA 18h 54m dec -8° 46' (J2000.0), close to Messier 26. In 928,000 years, it will pass within 0.25 parsecs (0.82 light-years) of the K dwarf TYC 992-192-1[37] and will pass near the star Lambda Aquilae in about four million years.[38]
Pioneer 11 has been overtaken by the two Voyager probes launched in 1977. Voyager 1 has become the most distant object built by humans and will remain so for the foreseeable future, as no probe launched since Voyager has the speed to overtake it.[39]
Analysis of the radio tracking data from the Pioneer 10 and 11 spacecraft at distances between 20 and 70AU from the Sun had 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 was suspected that there was a systematic origin to the effect, none was found. As a result, there has been sustained interest in the nature of this so-called "Pioneer anomaly".[40] Extended analysis of mission data by Slava Turyshev and colleagues 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.[41][42]
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.[43]
Commemoration
In 1991, Pioneer 11 was honored on one of 10 United States Postage Service stamps commemorating uncrewed 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".[44]
Gallery
Pioneer 11 and Saturn rings on September 1, 1979 (artist concept)
↑ S. Kronish (October 27, 1991). "Space Launches are Featured". The Index Journal. South Carolina, USA. p.21. Retrieved December 5, 2017– via Newspapers.com.
Missions are ordered by launch date. Sign † indicates failure en route or before intended mission data returned. ‡ indicates use of the planet as a gravity assist en route to another destination.
Payloads are separated by bullets ( · ), launches by pipes ( | ). Crewed flights are indicated in underline. Uncatalogued launch failures are listed in italics. Payloads deployed from other spacecraft are denoted in (brackets).
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