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Pioneer 11 (also known as Pioneer G) is a 259 kilogram (569 lb) robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere.[1] It was the first probe to encounter Saturn and the second to fly through the asteroid belt and by Jupiter. Due to power constraints and the vast distance to the probe, last contact with the spacecraft was on September 30, 1995.[2]
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:
Subsequent planning for an encounter with Saturn added many more goals:
Pioneer 11 was built by TRW and managed as part of the Pioneer program by NASA Ames Research Center.[4] 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..[5] Many elements of the mission proved to be critical in the planning of the Voyager Program.[6]:266–8
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.[1]:42
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.[15]
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.[24]
Pioneer 11 flew past Jupiter in November and December 1974. During its closest approach, on December 2, Pioneer 11 passed 42,828 kilometers (26,612 mi) above the cloud tops.[22] 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 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, Pioneer 11 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.
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".[25] 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,[26] and in July 2012 the group of researchers published their results in a scientific journal.[27]
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."[28] Besides announcing the end of operations, the dispatch provided a historical list of Pioneer 11 mission achievements.
On September 9, 2012, Pioneer 11 was 86.005 AU (1.28662×1010 km; 7.9947×109 mi) from the Earth and 86.396 AU (1.29247×1010 km; 8.0310×109 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.[29] Sunlight takes 11.92 hours to get to Pioneer 11. The brightness of the Sun from the spacecraft is magnitude -17.0.[29] Pioneer 11 is heading in the direction of the constellation Scutum.[29]
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.[30]
Pioneer 10 and Pioneer 11 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.[31]
Solar System, Saturn, Venus, Earth, Mass
Solar System, Jupiter, Uranus, Pioneer program, Neptune
Solar System, Jupiter, Earth, Venus, Neptune
Apollo program, International Space Station, Soviet Union, Mars, Space Shuttle
Jupiter, Voyager program, Nasa, Mariner program, Ranger program
Galileo (spacecraft), Jupiter, Neptune, New Horizons, Pioneer program
Mercury (planet), Venus, Messenger, List of geological features on Mercury, Mars Express
Saturn, Cassini–Huygens, Titan (moon), Enceladus, Voyager program
Pioneer program, Juno (spacecraft), Jupiter, European Space Agency, Oxygen