The Great Red Spot imaged by Pioneer 11
|Mission type||Planetary and heliosphere exploration|
|Operator||NASA / ARC|
Pioneer Project website (archived)|
NASA Archive page
|Mission duration||22 years, 7 months and 19 days (active) 49 years, 11 months and 20 days (in orbit)|
|Launch mass||258.5 kg |
|Power||155 watts (at launch)|
|Start of mission|
|Launch date||April 6, 1973, 02:11:00UTC|
|Rocket||Atlas SLV-3D Centaur-D1A Star-37E|
|Launch site||Cape Canaveral LC-36B|
|End of mission|
|Last contact||November 24, 1995|
|Flyby of Jupiter|
|Closest approach||December 3, 1974|
|Distance||43,000 kilometers (27,000 miles)|
|Flyby of Saturn|
|Closest approach||September 1, 1979|
|Distance||21,000 kilometers (13,000 miles)|
Pioneer 11 (also known as Pioneer G) is a NASA robotic space probe launched on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, solar winds, and cosmic rays.  It was the first probe to encounter Saturn, the second to fly through the asteroid belt, and the second to fly by Jupiter. Later, Pioneer 11 became the second of five artificial objects to achieve an escape velocity allowing it 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.  
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.  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.  Many elements of the mission proved to be critical in the planning of the Voyager program. : 266–8
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 259 kilograms. : 42
Pioneer 11 has one additional instrument more than Pioneer 10, a flux-gate magnetometer. 
|Helium Vector Magnetometer (HVM)|
|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.
|Quadrispherical Plasma Analyzer|
|Peer through a hole in the large dish-shaped antenna to detect particles of the solar wind originating from the Sun.
|Charged Particle Instrument (CPI)|
|Detects cosmic rays in the Solar System.
|Cosmic Ray Telescope (CRT)|
|Collects data on the composition of the cosmic ray particles and their energy ranges.
|Geiger Tube Telescope (GTT)|
|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.
|Trapped Radiation Detector (TRD)|
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. 
|Twelve panels of pressurized cell detectors mounted on the back of the main dish antenna record penetrating impacts of small meteoroids.
|Asteroid/Meteoroid Detector (AMD)|
|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.
|Ultraviolet light is sensed to determine the quantities of hydrogen and helium in space and on Jupiter and Saturn.
|Imaging Photopolarimeter (IPP)|
|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.
|Provides information on cloud temperature and the output of heat from Jupiter and Saturn.
|Triaxial Fluxgate Magnetometer|
|Measures the magnetic fields of both Jupiter and Saturn. This instrument is not carried on Pioneer 10.
|Timeline of travel|
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, with a Star-37E propulsion module. Its twin probe, Pioneer 10, had been launched a year earlier on March 3, 1972.
Pioneer 11 was launched on a trajectory directly aimed at Jupiter without any prior gravitational assists.  In May 1974, Pioneer was retargeted to fly past Jupiter on a north–south trajectory, enabling a Saturn flyby in 1979. The maneuver used 17 pounds of propellant, lasted 42 minutes and 36 seconds, and increased Pioneer 11's speed by 230 km/h.  It also made two mid-course corrections, on April 11, 1973 and November 7, 1974.  
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 and gain velocity. On April 16, 1975, following the Jupiter encounter, the micrometeoroid detector was turned off. 
The Great Red Spot imaged by Pioneer 11
Beginning polar gravity assist
Jupiter polar region from 1,079,000 km
Io imaged from 756,000 km
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 were 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.
On February 25, 1990, Pioneer 11 became the 4th man-made object to pass beyond the orbit of the planets. 
By 1995, Pioneer 11 could no longer power any of its detectors, so the decision was made to shut it down.  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.
NASA terminated routine contact with the spacecraft on September 30, 1995, but continued to make contact for about 2 hours every 2 to 4 weeks.  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.  
As of January 20, 2023, Pioneer 11 is estimated to be 111.678 AU (1.67068×1010 km; 1.03811×1010 mi) from the Earth and 110.764 AU (1.65701×1010 km; 1.02962×1010 mi) from the Sun; and traveling at 11.176 km/s (40,230 km/h; 25,000 mph) (relative to the Sun) and traveling outward at about 2.36 AU per year.   The spacecraft is heading in the direction of the constellation Scutum near the current position (January 2023) RA 18h 54m dec -8° 59' ( J2000.0), close to Messier 26. In 928,000 years, it will pass within 0.25pc of the K dwarf TYC 992-192-1,  and will pass near the star Lambda Aquilae in about four million years. 
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 and 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 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".  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. 
In 1991, Pioneer 11 was honored on one of 10 United States Postage Service stamps commemorating unmanned 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". 
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.