Pioneer 11 (also called Pioneer G) was the second mission of the Pioneer program (after its sister probe Pioneer 10) to investigate Jupiter and the outer solar system and the first to explore Saturn and its main rings. Pioneer 11 used Jupiter's mass in a gravity assist to alter its trajectory toward Saturn. The spacecraft made a successful Saturn flyby and then followed an escape trajectory from the solar system.
Its objectives were to study the interplanetary and planetary magnetic fields; solar wind properties; cosmic rays; transition region of the heliosphere; neutral hydrogen abundance; distribution, size, mass, flux, and velocity of dust particles; Jovian aurorae; Jovian radio waves; the atmospheres of planets and satellites; and the surfaces of Jupiter, Saturn, and some of their satellites. It was developed out of NASA Ames Research Center as part of the Pioneer series (6-12) of spacecraft that include Pioneer Venus.
The unmanned spacecraft was launched from Cape Canaveral on April 6, 1973. It is 2.9 meters long and has a 2.74-meter-diameter high-gain antenna, topped with a medium-gain antenna. A low-gain, omnidirectional antenna is mounted below the high-gain dish. The spacecraft contains four SNAP-19 radioisotope thermoelectric generators (RTGs), which generated 144 W at Jupiter, but had decreased to 100 W by the time it reached Saturn.
There were three reference sensors: a star (Canopus) sensor, and two Sun sensors. Attitude position could be calculated from the reference direction to Earth and Sun, with the known direction to Canopus as backup. Pioneer 11's star sensor gain and threshold settings were modified, based on experience gained from the settings used on Pioneer 10. Three pairs of rocket thrusters provided spin-axis control (maintained at 4.8 rpm) and change the spacecraft's velocity. The thrusters could be either fired steadily or pulsed, by command.
Rather than a traditional lens camera, Pioneer used its spin to scan scenes line-by-line (spin-scan). Its imaging system (photopolarimeter) could record brightness values of one narrow spot (pixel) at a time in two wavelengths corresponding to Red and Blue light. A combination of craft rotation and gradual forward or backward movement of this "pixel" allowed color images to be reconstructed using computers back at JPL (Earth). It is roughly comparable to early mechanical television scanners.
Instruments on the Pioneer 11 probe studied the interplanetary and planetary magnetic fields; solar wind properties; cosmic rays; the transition region of the heliosphere; neutral hydrogen abundance; distribution, size, mass, flux, and velocity of dust particles; Jovian aurorae; Jovian radio waves; the atmospheres of planets and satellites; and higher layers of the atmospheres of Jupiter and Saturn, and the surfaces of some of their satellites. The instruments carried for these experiments were a magnetometer, a plasma analyzer (for solar wind), a charged-particle detector, an ion detector, non-imaging telescopes with overlapping fields of view to detect sunlight reflected from passing meteoroids, sealed pressurized cells of argon and nitrogen gas for measuring penetration of meteoroids, an ultraviolet photometer, an infrared radiometer, and an imaging photopolarimeter, which produced photographs and measured their polarization. Further scientific information was obtained from celestial mechanics and occultation phenomena.
Pioneer 11 instrument list:
During its closest approach, December 4, 1974, Pioneer 11 passed to within 34,000 km of Jupiter's cloud tops and obtained dramatic images of the Great Red Spot, made the first observation of the immense polar regions, and determined the mass of Jupiter's moon Callisto. Using Jupiter's mass in a gravitational slingshot, the probe altered its trajectory towards Saturn.
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 probe would use in order to test the route before Voyager 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 a few thousand kilometers from it. 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 amazing 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.
Outer regions of the Solar system
Pioneer 11 explored the outer regions of our Solar system, studying the Solar Wind and cosmic rays entering our portion of the Milky Way.
The spacecraft has operated on a backup transmitter since launch. Instrument power sharing began in February 1985 due to declining generator power output. Science operations and daily telemetry ceased on September 30, 1995 when the RTG power level was insufficient to operate any experiments. As of the end of 1995, when its mission ended, the spacecraft was located at 44.7 AU from the Sun at a nearly asymptotic latitude of 17.4 degrees above the solar equatorial plane and was heading outward at ~2.4 AU/year (11.6 km/s); this is the lowest velocity of the five spacecraft now escaping the Solar System (Voyagers 1 and 2, Pioneers 10 and 11, and New Horizons). 
Earth's motion has carried it out of alignment with the spacecraft antenna. As the antenna cannot be maneuvered to point back at our planet, it is no longer possible with current technology to establish further communication from Earth with the probe.
The spacecraft is headed toward the constellation of Aquila (The Eagle), Northwest of the constellation of Sagittarius. Barring incident, Pioneer 11 will pass near one of the stars in the constellation in about 4 million years.
The Pioneer plaque
Like its sister ship, Pioneer 10, Pioneer 11 carried a plaque with a message from humankind. The Pioneer plaques are identical in that they depict a Jupiter flyby. However, Pioneer 11 was redirected to Saturn making its plaque somewhat out of date. If the space probe is ever found by extraterrestrial intelligences, this message is meant to provide information about the origin of the spacecraft. It includes a drawing depicting a man, a woman, the transition of a hydrogen atom, and the location of the Sun and Earth in the galaxy.
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 an anomalous, small Doppler frequency drift. The drift can be interpreted as being due to a constant acceleration of (8.74 ± 1.33) × 10 m/s² directed towards the Sun. Although it is suspected that there is a systematic origin to the effect, none has been found. As a result, the nature of this anomaly has become of growing interest.
Published in July 2009.
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