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:
The Pioneer 11 bus measured 36 centimeters deep and with six 76-centimeters-long 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.:42
Orientation of the spacecraft was maintained with six 4.5-N,hydrazinemonopropellant 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, a star sensor able to reference Canopus, and two Sun sensors.:42–43
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) 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.: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.:38
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.
Principal investigator: R. Fillius / University of California San Diego
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.
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. 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.
Pioneer 11 and Saturn rings on September 1, 1979 (artist's impression)
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 was 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.
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 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." Besides announcing the end of operations, the dispatch provided a historical list of Pioneer 11 mission achievements.
Simulated view of the position of Pioneer 11 as of 8 February 2012 showing spacecraft trajectory since launch
On July 19, 2015, Pioneer 11 was 90.716 AU (1.35709×1010 km; 8.4326×109 mi) from the Earth and 91.672 AU (1.37139×1010 km; 8.5214×109 mi) from the Sun; and traveling at 11.376 km/s (25,450 mph) (relative to the Sun) and traveling outward at about 2.400 AU per year. The spacecraft is heading in the direction of the constellation Scutum.
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.
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". 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, and in July 2012 the group of researchers published their results in the Physical Review Letters scientific journal.
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.
^Britt, Robert Roy (October 18, 2004). "The Problem with Gravity: New Mission Would Probe Strange Puzzle". Space.com. Retrieved 2011-06-07. The discrepancy caused by the anomaly amounts to about 248,500 miles (399,900 kilometres), or roughly the distance between Earth and the Moon. That's how much farther the probes should have traveled in their 34 years, if our understanding of gravity is correct.
Payloads are separated by bullets ( · ), launches by pipes ( | ). Manned flights are indicated in bold text. Uncatalogued launch failures are listed in italics. Payloads deployed from other spacecraft are denoted in (brackets).