Pioneer 11

Pioneer 11 at Saturn (artist's impression)
Pioneer 11 at Saturn (artist's impression)

Pioneer 11 was the second mission to investigate Jupiter and the outer solar system and the first to explore the planet Saturn and its main rings. Pioneer 11 (also called Pioneer G), like Pioneer 10, used Jupiter's mass in a gravitational slingshot to alter its trajectory toward Saturn. It passed close to Saturn and then it followed an escape trajectory from the solar system.

The 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 two 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.

Instruments 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 the surfaces of Jupiter, Saturn, and 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.

One of the first spacecraft views of Saturn was taken by Pioneer 11 on September 1, 1979, three days before its closest encounter. The moon Titan is visible to the upper left. The rings appear dim because they were at a shallow angle to the sun at the time of Pioneer's visit.
One of the first spacecraft views of Saturn was taken by Pioneer 11 on September 1, 1979, three days before its closest encounter. The moon Titan is visible to the upper left. The rings appear dim because they were at a shallow angle to the sun at the time of Pioneer's visit.

During its closest approach, December 4, 1974, Pioneer 11 passed to within 34,000 km of Jupiter's cloud tops. It 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.) 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 2.5 AU/year (12 km/s).

The plaque on board the Pioneer spacecraft
The plaque on board the Pioneer spacecraft

Like its sister ship, Pioneer 10, Pioneer 11 carried a plaque with a message from humankind. 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.

Pioneer anomaly

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) × 10−10 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.