[Illustration: Pioneer Saturn spacecraft.]
Pioneer Saturn has given us our first close view of the spectacular
ringed planet Saturn and its system of moons. The spacecraft began its
journey to the giant planets Jupiter and Saturn on April 5, 1973, as
Pioneer 11. It reached Jupiter on December 2, 1974, passing within
42,760 km of the Jovian cloud tops and taking the only existing pictures
of Jupiter’s polar regions. Jupiter’s massive gravitational field was
used to swing Pioneer 11 back across the solar system toward Saturn.
Additional maneuvers were executed in 1975 and 1976 to place the
spacecraft on a suitable trajectory, with the final aimpoint selected in
1977.
From the many possible targeting options for the first Saturn flyby, two
aimpoints were considered, both of which would result in a
near-equatorial flyby that would give the best mapping of the
high-energy particles and the magnetic field near the planet. The
difference between these two aimpoints, which came to be known as the
“inside” and “outside” options, was their relationship to Saturn’s
unique ring system first discovered by Galileo in 1610. The “outside”
option was finally selected because it was considered to be of less risk
to the spacecraft and more valuable in planning the subsequent encounter
of Saturn by Voyager 2, which will reach Saturn in 1981. Final targeting
was completed during early 1978, when a series of timed rocket thrusts
locked Pioneer into the desired trajectory.
[Illustration: Pioneer Saturn voyage.]
[Illustration: Encounter trajectory.]
On September 1, 1979, the spacecraft, now designated Pioneer Saturn,
reached Saturn after 6 years in flight. It passed through the ring plane
outside the edge of Saturn’s A-ring and then swung in under the rings
from 2,000 to 10,000 km below them. At the point of closest approach, it
attained a speed of 114,100 km/h (71,900 mi/h) and came within 21,400 km
of the planet’s cloud tops. While it was approaching, encountering, and
leaving Saturn, the spacecraft took the first closeup pictures of the
planet, showing 20 to 30 times more detail than the best pictures taken
from Earth, and made the first close measurements of its rings and
several of its moons, including the largest moon, the planet-sized
Titan. Titan, along with Mars, has been considered by many scientists to
be the most likely place to find life in the solar system.
Pioneer Saturn unraveled many mysteries. It determined that Saturn has a
magnetic field and trapped radiation belts, measured the mass of Saturn
and some of its moons, and studied the character of Saturn’s interior.
It confirmed the presence and determined the magnitude of an internal
heat source for Saturn. Its instruments studied the temperature
distribution, composition, and other properties of the clouds and
atmospheres of Saturn and Titan, and took photometric and polarization
measurements of Iapetus, Rhea, Dione, and Tethys. Pioneer may also have
discovered a previously unknown moon of Saturn. The spacecraft measured
the mass, structure, and other characteristics of Saturn’s rings, and
passed safely through the outer E-ring, which posed a potential hazard
for Pioneer. It also discovered new rings. One of these rings, called
the F-ring by the Pioneer team, lies just outside the A-ring. The gap
between the F-ring and the A-ring has been tentatively designated the
Pioneer Division. The other new ring has been called the G-ring, which
lies well outside the F-ring.
Pioneer carries a scientific payload of 11 operating instruments;
another instrument, the asteroid/meteoroid detector, was turned off in
1975. Two other experiments, celestial mechanics and S-band occultation
of Saturn, use the spacecraft radio to obtain data. Pioneer Saturn is a
spinning spacecraft, which gives its instruments a full-circle scan 7.8
times a minute. It uses a nuclear source for electric power because the
sunlight at Jupiter and beyond is too weak for a solar-powered system.
Two booms project from the spacecraft to deploy the nuclear power source
about 3 meters from the sensitive spacecraft instrumentation. A third
boom positions the magnetometer sensor about 6 meters from the
spacecraft. Six thrusters provide velocity, attitude, and spin-rate
control. A dish antenna is located along the spin axis and looks back at
Earth throughout the mission, adjusting its view by changes in
spacecraft attitude as the spacecraft and Earth move in their orbits
around the sun.
Tracking facilities of NASA’s Deep Space Network, located at Goldstone,
California, and in Spain and Australia, supported Pioneer Saturn during
interplanetary flight and encounter. Pioneer’s radio signals, traveling
at the speed of light, took 85 minutes to reach Earth from Saturn, a
round-trip time of almost 3 hours, somewhat complicating ground control
of the spacecraft. Almost 10,000 commands were sent to the spacecraft in
the 2-week period before closest approach. Continued communications
should be possible through at least the mid 1980’s.
After the spacecraft passed Saturn, it headed out of the solar system,
traveling in the direction the solar system moves with respect to the
local stars in our galaxy and in approximately an opposite direction
from its sister spacecraft, Pioneer 10. Both spacecraft have plaques
attached to them which contain a message from Earth for any intelligent
species that may intercept the spacecraft during their endless journeys
through interstellar space.
Pioneer Saturn Scientific Instruments
Instrument Principal Investigator Experiment Objective
Helium vector Edward J. Smith Magnetic fields
magnetometer Jet Propulsion Laboratory
Fluxgate Mario Acuña Magnetic fields
magnetometer Goddard Space Flight Center
Plasma analyzer John H. Wolfe Solar plasma
Ames Research Center
Charged particle John A. Simpson Charged particle
University of Chicago composition
Cosmic ray telescope Frank B. McDonald Cosmic ray energy
Goddard Space Flight Center spectra
Geiger tube James A. Van Allen Charged particles
telescope University of Iowa
Trapped radiation R. Walker Fillius Trapped radiation
detector University of California,
San Diego
Asteroid/meteoroid Robert K. Soberman Asteroid/meteoroid
detector[1] General Electric Co. and astronomy
Philadelphia Drexel
University
Meteoroid detector William H. Kinard Meteoroid detection
Langley Research Center
Radio transmitter John D. Anderson Celestial mechanics
and DSN Jet Propulsion Laboratory
Ultraviolet Darrell L. Judge Ultraviolet
photometer University of Southern photometry
California Los Angeles
Imaging Tom Gehrels Photo imaging and
photopolarimeter University of Arizona, Tucson polarimetry
Infrared radiometer Andrew Ingersoll Infrared thermal
California Institute of structure
Technology
Radio transmitter Arvydas J. Kliore S-band occultation
and DSN Jet Propulsion Laboratory
[1]Not currently operational.