Voyager 2Voyager 2 was one of a pair of spacecraft launched to explore the planets of the outer solar system
and the interplanetary environment. Each Voyager had as its major objectives at each planet to:
- investigate the circulation, dynamics, structure, and composition of the planet's atmosphere;
- characterize the morphology, geology, and physical state of the satellites of the planet;
- provide improved values for the mass, size, and shape of the planet, its satellites, and any rings; and,
- determine the magnetic field structure and characterize the composition and distribution of
energetic trapped particles and plasma therein.
Voyager 2 was launched on August 20, 1977, from Cape Canaveral. The spacecraft was launched by a
Titan IIIE-Centaur rocket.
Voyager 2 consisted of a decahedral bus, 47 cm in height and 1.78 m across from flat to flat. A
3.66 m diameter parabolic high-gain antenna was mounted on top of the bus. Most of
the science instruments were mounted on a boom extending 2.5 m from the spacecraft. At the end of
the science boom was a steerable scan platform on which were mounted the imaging and spectroscopic
remote sensing instruments. Also mounted at various distances along the science boom were the
plasma and charged particle detectors. The magnetometers were located along a separate
boom extending 13 m on the side opposite the science boom. A third boom, extending down and away from the
science instruments, held the spacecraft's radioisotope thermoelectric generators (RTGs). Two 10 m
whip antennas (used for the plasma wave and planetary radio astronomy investigations) also extended
from the spacecraft, each perpendicular to the other. The spacecraft was three-axis spin stabilized to
enable long integration times and selective viewing for the instruments mounted on the scan platform.
The Voyager spacecraft explored the planets and moons of the outer solar system.
Power was provided to the spacecraft systems and instruments through the use of three
radioisotope thermoelectric generators. The RTGs were assembled in tandem on a deployable boom hinged
on an outrigger arrangement of struts attached to the basic structure. Each RTG unit, contained in
a beryllium outer case, was 40.6 cm in diameter, 50.8 cm in length, and weighed 39 kg. The RTGs used
a radioactive source (Plutonium-238 in the form of plutonium oxide, or PuO2, in this case) which,
as it decayed, gave off heat. A bi-metallic thermoelectric device was used to convert the heat to
electric power for the spacecraft. The total output of RTGs slowly decreases with time as the radioactive
material is expended. Therefore, although the initial output of the RTGs on Voyager was approximately
470 W of 30 V DC power at launch, it had fallen off to approximately 335 W by the beginning of 1997
(about 19.5 years after launch).
Communications were provided through the high-gain antenna with a low-gain antenna for backup.
The high-gain antenna supported both X-band and S-band downlink telemetry. Voyager was
the first spacecraft to utilize X-band as the primary telemetry link frequency. Data could be stored for
later transmission to Earth through the use of an on-board digital tape recorder.
Voyager, because of its distance from Earth and the resulting time-lag for commanding, was designed to operate
in a highly-autonomous manner. In order to do this and carry out the complex sequences of spacecraft motions
and instrument operations, three interconnected on-board computers were utilized. The Computer Command Subsystem (CCS)
was responsible for storing commanding for the other two computers and issuing the commands at set times. The
Attitude and Articulation Control Subsystem (AACS) was responsible for controlling spacecraft attitude and motions
of the scan platform. The Flight Data Subsystem (FDS) controlled the instruments, including changes in
configuration (state) or telemetry rates. All three computers had redundant components to ensure continued
operations. The AACS included redundant star trackers and Sun sensors as well.
Each Voyager has a 12-inch gold-plated copper disk mounted to one of the sides of the bus. The disk has recorded on it sounds
and images of Earth designed to portray the diversity of life and culture on the planet. Each disk is encased in a
protective aluminum jacket along with a cartridge and a needle. Instructions explaining from where the spacecraft
originated and how to play the disk are engraved onto the jacket. Electroplated onto a 2 cm area on the cover is
also an ultra-pure source of uranium-238 (with a radioactivity of about 0.26 nanocuries and a half-life of
4.51 billion years), allowing the determination of the elapsed time since launch by measuring the amount of daughter
elements to remaining U238. The 115 images on the disk were encoded in analog form. The sound selections
(including greetings in 55 languages, 35 sounds, natural and man-made, and portions of 27 musical pieces) are designed
for playback at 1000 rpm. The Voyagers were not the first spacecraft designed with such messages to the future.
Pioneer 10 and Pioneer 11, LAGEOS, and the Apollo landers also included plaques with a
similar intent, though not quite so ambitious.
Each of the Voyager spacecraft carried a greeting in the form of a 12-inch gold-plated copper phonograph record containing sounds and images portraying the diversity of life and culture on Earth.
Originally planned as a Grand Tour of the outer planets, including dual launches to
Jupiter, Saturn, and Pluto in 1976 and 1977 and dual launches to Jupiter,
Uranus, and Neptune in 1979, budgetary constraints caused a dramatic rescoping of the project to
two spacecraft, each of which would go to only Jupiter and Saturn. The new mission was called
Mariner Jupiter/Saturn, or MJS. The mission was renamed Voyager about six months prior to launch.
The rescoped mission was estimated to cost $250 million (through the end of Saturn operations), only a third of
what the Grand Tour design would have cost.
Voyager 2 was the first of the two spacecraft to be launched, with liftoff occurring on August 20, 1977. Shortly
after lift-off, problems with the deployment of the science boom were detected.
Several months after launch, in April of 1978, Voyager 2's primary radio receiver failed, automatically kicking in
the backup receiver which proved to be faulty. Attempts to recover the use of the primary receiver failed and the
backup receiver was used for the remainder of the mission. Although use of the backup receiver made communication
with the spacecraft more difficult, engineers were able to find workarounds.
Voyager 2 proceeded with its primary mission and flew by Jupiter (making its closest approach on July 9, 1979)
and Saturn (on August 5, 1981). During these flybys, Voyager 2 took about 18,000 images of Jupiter and
its moons and 16,000 of Saturn and its moons.
The Voyager spacecraft explored the planets and moons of the outer solar system.
The scientists and engineers who designed Voyager and the mission, had considered the possibility of an extended
mission to Uranus and Neptune. As a result of the success of the Jupiter and Saturn flybys, approval was granted
to extend the mission, first to Uranus, then to Neptune and later to continue observations well past
Voyager 2 made successful flybys of Uranus (January 24, 1986) and Neptune (August 25, 1989).
Because of the additional distance of these two planets, adaptations had to made to accomodate the lower light
levels and decreased communications. Voyager 2 was successfully able to obtain about 8,000 images of Uranus
and its satellites. Additional improvements in the on-board software and use of image compression techniques allowed about
10,000 images of Neptune and its satellites to be taken.
Rechristened the Voyager Interstellar Mission (VIM) by NASA in 1989 after its encounter with Neptune,
Voyager 2 continued operations, taking measurements of the interplanetary magnetic field, plasma,
and charged particle environment while searching for the heliopause (the distance at which the
solar wind becomes subsumed by the more general interstellar wind). Through the end of the
Neptune phase of the Voyager project, a total of $875 million had been expended for the construction,
launch, and operations of both Voyager spacecraft. An additional $30 million was allocated for the first two
years of VIM.
Voyager 2 is currently speeding away from the Sun at a velocity of about 3.13 AU/year.