The Voyager Planetary
spacecraft Voyager 1 and Voyager 2 were launched by NASA in
separate months in the summer of 1977 from Cape Canaveral,
Florida. As originally designed, the Voyagers were to conduct
closeup studies of Jupiter and Saturn, Saturn's rings, and
the larger moons of the two planets.
their two-planet mission, the spacecraft were built to last
five years. But as the mission went on, and with the successful
achievement of all its objectives, the additional flybys of
the two outermost giant planets, Uranus and Neptune, proved
possible -- and irresistible to mission scientists and engineers
at the Voyagers' home at the Jet Propulsion Laboratory
in Pasadena, California.
spacecraft flew across the solar system, remote-control reprogramming
was used to endow the Voyagers with greater capabilities than
they possessed when they left the Earth. Their two-planet
mission became four. Their five-year lifetimes stretched to
12 and more.
between them, Voyager 1 and 2 would explore all the giant
outer planets of our solar system, 48 of their moons, and
the unique systems of rings and magnetic fields those planets
Voyager mission ended after the Jupiter and Saturn flybys
alone, it still would have provided the material to rewrite
astronomy textbooks. But having doubled their already ambitious
itineraries, the Voyagers returned to Earth information over
the years that has revolutionized the science of planetary
astronomy, helping to resolve key questions while raising
intriguing new ones about the origin and evolution of the
planets in our solar system.
of the Voyager Mission
mission was designed to take advantage of a rare geometric
arrangement of the outer planets in the late 1970s and the
1980s which allowed for a four-planet tour for a minimum of
propellant and trip time. This layout of Jupiter, Saturn,
Uranus and Neptune, which occurs about every 175 years, allows
a spacecraft on a particular flight path to swing from one
planet to the next without the need for large onboard propulsion
systems. The flyby of each planet bends the spacecraft's flight
path and increases its velocity enough to deliver it to the
next destination. Using this "gravity assist" technique, first
demonstrated with NASA's Mariner 10 Venus/Mercury mission
in 1973-74, the flight time to Neptune was reduced from 30
years to 12.
the four-planet mission was known to be possible, it was deemed
to be too expensive to build a spacecraft that could go the
distance, carry the instruments needed and last long enough
to accomplish such a long mission. Thus, the Voyagers were
funded to conduct intensive flyby studies of Jupiter and Saturn
only. More than 10,000 trajectories were studied before choosing
the two that would allow close flybys of Jupiter and its large
moon Io, and Saturn and its large moon Titan; the chosen flight
path for Voyager 2 also preserved the option to continue on
to Uranus and Neptune.
NASA Kennedy Space Center at Cape Canaveral, Florida,
Voyager 2 was launched first, on August 20, 1977; Voyager
1 was launched on a faster, shorter trajectory on September
5, 1977. Both spacecraft were delivered to space aboard Titan-Centaur
Voyager mission to Jupiter and Saturn brought Voyager 1 to
Jupiter on March 5, 1979, and Saturn on November 12, 1980,
followed by Voyager 2 to Jupiter on July 9, 1979, and Saturn
on August 25, 1981.
1's trajectory, designed to send the spacecraft closely past
the large moon Titan and behind Saturn's rings, bent the spacecraft's
path inexorably northward out of the ecliptic plane -- the
plane in which most of the planets orbit the Sun. Voyager
2 was aimed to fly by Saturn at a point that would automatically
send the spacecraft in the direction of Uranus.
Voyager 2's successful Saturn encounter, it was shown that
Voyager 2 would likely be able to fly on to Uranus with all
instruments operating. NASA provided additional funding to
continue operating the two spacecraft and authorized JPL to
conduct a Uranus flyby. Subsequently, NASA also authorized
the Neptune leg of the mission, which was renamed the Voyager
Neptune Interstellar Mission.
2 encountered Uranus on January 24, 1986, returning detailed
photos and other data on the planet, its moons, magnetic field
and dark rings. Voyager 1, meanwhile, continues to press outward,
conducting studies of interplanetary space. Eventually, its
instruments may be the first of any spacecraft to sense the
heliopause -- the boundary between the end of the Sun's magnetic
influence and the beginning of interstellar space.
Voyager 2's closest approach to Neptune on August 25, 1989,
the spacecraft flew southward, below the ecliptic plane and
onto a course that will take it, too, to interstellar space.
Reflecting the Voyagers' new transplanetary destinations,
the project is now known as the Voyager Interstellar Mission.
1 is now leaving the solar system, rising above the ecliptic
plane at an angle of about 35 degrees at a rate of about 520
million kilometers (about 320 million miles) a year. Voyager
2 is also headed out of the solar system, diving below the
ecliptic plane at an angle of about 48 degrees and a rate
of about 470 million kilometers (about 290 million miles)
will continue to study ultraviolet sources among the stars,
and the fields and particles instruments aboard the Voyagers
will continue to search for the boundary between the Sun's
influence and interstellar space. The Voyagers are expected
to return valuable data for two or three more decades. Communications
will be maintained until the Voyagers' nuclear power sources
can no longer supply enough electrical energy to power critical
of the Voyager 1 and 2 missions -- including launch, mission
operations from launch through the Neptune encounter and the
spacecraft's nuclear batteries (provided by the Department
of Energy) -- is $865 million. NASA budgeted an additional
$30 million to fund the Voyager Interstellar Mission for two
years following the Neptune encounter.
1 and 2 are identical spacecraft. Each is equipped with instruments
to conduct 10 different experiments. The instruments include
television cameras, infrared and ultraviolet sensors, magnetometers,
plasma detectors, and cosmic-ray and charged-particle sensors.
In addition, the spacecraft radio is used to conduct experiments.
travel too far from the Sun to use solar panels; instead,
they were equipped with power sources called radioisotope
thermoelectric generators (RTGs). These devices, used on other
deep space missions, convert the heat produced from the natural
radioactive decay of plutonium into electricity to power the
spacecraft instruments, computers, radio and other systems.
are controlled and their data returned through the Deep
Space Network (DSN), a global spacecraft tracking system
operated by JPL for NASA. DSN antenna complexes are located
in California's Mojave Desert; near Madrid, Spain; and in
Tidbinbilla, near Canberra, Australia.
project manager for the Interstellar Mission is George P.
Textor of JPL. The Voyager project scientist is Dr. Edward
C. Stone of the California Institute of Technology. The assistant
project scientist for the Jupiter flyby was Dr. Arthur L.
Lane, followed by Dr. Ellis D. Miner for the Saturn, Uranus
and Neptune encounters. Both are with JPL.
Voyager 1 made its closest approach to Jupiter on March 5,
1979, and Voyager 2 followed with its closest approach occurring
on July 9, 1979. The first spacecraft flew within 206,700
kilometers (128,400 miles) of the planet's cloud tops, and
Voyager 2 came within 570,000 kilometers (350,000 miles).
is the largest planet in the solar system, composed mainly
of hydrogen and helium, with small amounts of methane, ammonia,
water vapor, traces of other compounds and a core of melted
rock and ice. Colorful latitudinal bands and atmospheric clouds
and storms illustrate Jupiter's dynamic weather system. The
giant planet is now known to possess 16 moons. The planet
completes one orbit of the Sun each 11.8 years and its day
is 9 hours, 55 minutes.
astronomers had studied Jupiter through telescopes on Earth
for centuries, scientists were surprised by many of the Voyager
Red Spot was revealed as a complex storm moving in a counterclockwise
direction. An array of other smaller storms and eddies were
found throughout the banded clouds.
of active volcanism on the satellite Io was easily the greatest
unexpected discovery at Jupiter. It was the first time active
volcanoes had been seen on another body in the solar system.
Together, the Voyagers observed the eruption of nine volcanoes
on Io, and there is evidence that other eruptions occurred
between the Voyager encounters.
from the volcanoes extend to more than 300 kilometers (190
miles) above the surface. The Voyagers observed material ejected
at velocities up to a kilometer per second.
are apparently due to heating of the satellite by tidal pumping.
Io is perturbed in its orbit by Europa and Ganymede, two other
large satellites nearby, then pulled back again into its regular
orbit by Jupiter. This tug-of-war results in tidal bulging
as great as 100 meters (330 feet) on Io's surface, compared
with typical tidal bulges on Earth of one meter (three feet).
that volcanism on Io affects the entire jovian system, in
that it is the primary source of matter that pervades Jupiter's
magnetosphere -- the region of space surrounding the planet
influenced by the jovian magnetic field. Sulfur, oxygen and
sodium, apparently erupted by Io's many volcanoes and sputtered
off the surface by impact of high-energy particles, were detected
as far away as the outer edge of the magnetosphere millions
of miles from the planet itself.
displayed a large number of intersecting linear features in
the low-resolution photos from Voyager 1. At first, scientists
believed the features might be deep cracks, caused by crustal
rifting or tectonic processes. The closer high-resolution
photos from Voyager 2, however, left scientists puzzled: The
features were so lacking in topographic relief that as one
scientist described them, they "might have been painted on
with a felt marker." There is a possibility that Europa may
be internally active due to tidal heating at a level one-tenth
or less than that of Io. Europa is thought to have a thin
crust (less than 30 kilometers or 18 miles thick) of water
ice, possibly floating on a 50-kilometer-deep (30-mile) ocean.
turned out to be the largest moon in the solar system, with
a diameter measuring 5,276 kilometers (3,280 miles). It showed
two distinct types of terrain -- cratered and grooved -- suggesting
to scientists that Ganymede's entire icy crust has been under
tension from global tectonic processes.
has a very old, heavily cratered crust showing remnant rings
of enormous impact craters. The largest craters have apparently
been erased by the flow of the icy crust over geologic time.
Almost no topographic relief is apparent in the ghost remnants
of the immense impact basins, identifiable only by their light
color and the surrounding subdued rings of concentric ridges.
dusty ring of material was found around Jupiter. Its outer
edge is 129,000 kilometers (80,000 miles) from the center
of the planet, and it extends inward about 30,000 kilometers
small satellites, Adrastea and Metis, were found orbiting
just outside the ring. A third new satellite, Thebe, was discovered
between the orbits of Amalthea and Io.
rings and moons exist within an intense radiation belt of
electrons and ions trapped in the planet's magnetic field.
These particles and fields comprise the jovian magnetosphere,
or magnetic environment, which extends three to seven million
kilometers toward the Sun, and stretches in a windsock shape
at least as far as Saturn's orbit -- a distance of 750 million
kilometers (460 million miles).
magnetosphere rotates with Jupiter, it sweeps past Io and
strips away about 1,000 kilograms (one ton) of material per
second. The material forms a torus, a doughnut-shaped cloud
of ions that glow in the ultraviolet. The torus's heavy ions
migrate outward, and their pressure inflates the jovian more
energetic sulfur and oxygen ions fall along the magnetic field
into the planet's atmosphere, resulting in auroras.
as an electrical generator as it moves through Jupiter's magnetic
field, developing 400,000 volts across its diameter and generating
an electric current of 3 million amperes that flows along
the magnetic field to the planet's ionosphere.
The Voyager 1 and 2 Saturn flybys occurred nine months apart,
with the closest approaches falling on November 12 and August
25, 1981. Voyager 1 flew within 64,200 kilometers (40,000
miles) of the cloud tops, while Voyager 2 came within 41,000
kilometers (26,000 miles).
is the second largest planet in the solar system. It takes
29.5 Earth years to complete one orbit of the Sun, and its
day was clocked at 10 hours, 39 minutes. Saturn is known to
have at least 17 moons and a complex ring system. Like Jupiter,
Saturn is mostly hydrogen and helium. Its hazy yellow hue
was found to be marked by broad atmospheric banding similar
to but much fainter than that found on Jupiter. Close scrutiny
by Voyager's imaging systems revealed long-lived ovals and
other atmospheric features generally smaller than those on
the greatest surprises and the most puzzles were found by
the Voyagers in Saturn's rings. It is thought that the rings
formed from larger moons that were shattered by impacts of
comets and meteoroids. The resulting dust and boulder- to
house-size particles have accumulated in a broad plane around
the planet varying in density.
shapes of Saturn's eight smallest moons indicates that they
too are fragments of larger bodies. Unexpected structure such
as kinks and spokes were found in addition to thin rings and
broad, diffuse rings not observed from Earth. Much of the
elaborate structure of some of the rings is due to the gravitational
effects of nearby satellites. This phenomenon is most obviously
demonstrated by the relationship between the F-ring and two
small moons that "shepherd" the ring material. The variation
in the separation of the moons from the ring may the ring's
kinked appearance. Shepherding moons were also found by Voyager
2 at Uranus.
spoke-like features in the broad B-ring were found by the
Voyagers. The features are believed to be composed of fine,
dust-size particles. The spokes were observed to form and
dissipate in time-lapse images taken by the Voyagers. While
electrostatic charging may create spokes by levitating dust
particles above the ring, the exact cause of the formation
of the spokes is not well understood.
blow at extremely high speeds on Saturn -- up to 1,800 kilometers
per hour (1,100 miles per hour). Their primarily easterly
direction indicates that the winds are not confined to the
top cloud layer but must extend at least 2,000 kilometers
(1,200 miles) downward into the atmosphere. The characteristic
temperature of the atmosphere is 95 kelvins.
holds a wide assortment of satellites in its orbit, ranging
from Phoebe, a small moon that travels in a retrograde orbit
and is probably a captured asteroid, to Titan, the planet-sized
moon with a thick nitrogen-methane atmosphere. Titan's surface
temperature and pressure are 94 kelvins (-292 Fahrenheit)
and 1.5 atmospheres. Photochemistry converts some atmospheric
methane to other organic molecules, such as ethane, that is
thought to accumulate in lakes or oceans. Other more complex
hydrocarbons form the haze particles that eventually fall
to the surface, coating it with a thick layer of organic matter.
The chemistry in Titan's atmosphere may strongly resemble
that which occurred on Earth before life evolved.
active surface of any moon seen in the Saturn system was that
of Enceladus. The bright surface of this moon, marked by faults
and valleys, showed evidence of tectonically induced change.
Voyager 1 found the moon Mimas scarred with a crater so huge
that the impact that caused it nearly broke the satellite
magnetic field is smaller than Jupiter's, extending only one
or two million kilometers. The axis of the field is almost
perfectly aligned with the rotation axis of the planet.
In its first solo planetary flyby, Voyager 2 made its closest
approach to Uranus on January 24, 1986, coming within 81,500
kilometers (50,600 miles) of the planet's cloud tops.
is the third largest planet in the solar system. It orbits
the Sun at a distance of about 2.8 billion kilometers (1.7
billion miles) and completes one orbit every 84 years. The
length of a day on Uranus as measured by Voyager 2 is 17 hours,
is distinguished by the fact that it is tipped on its side.
Its unusual position is thought to be the result of a collision
with a planet-sized body early in the solar system's history.
Given its odd orientation, with its polar regions exposed
to sunlight or darkness for long periods, scientists were
not sure what to expect at Uranus.
2 found that one of the most striking influences of this sideways
position is its effect on the tail of the magnetic field,
which is itself tilted 60 degrees from the planet's axis of
rotation. The magnetotail was shown to be twisted by the planet's
rotation into a long corkscrew shape behind the planet.
of a magnetic field at Uranus was not known until Voyager's
arrival. The intensity of the field is roughly comparable
to that of Earth's, though it varies much more from point
to point because of its large offset from the center of Uranus.
The peculiar orientation of the magnetic field suggests that
the field is generated at an intermediate depth in the interior
where the pressure is high enough for water to become electrically
belts at Uranus were found to be of an intensity similar to
those at Saturn. The intensity of radiation within the belts
is such that irradiation would quickly darken (within 100,000
years) any methane trapped in the icy surfaces of the inner
moons and ring particles. This may have contributed to the
darkened surfaces of the moons and ring particles, which are
almost uniformly gray in color.
layer of haze was detected around the sunlit pole, which also
was found to radiate large amounts of ultraviolet light, a
phenomenon dubbed "dayglow." The average temperature is about
60 kelvins (-350 degrees Fahrenheit). Surprisingly, the illuminated
and dark poles, and most of the planet, show nearly the same
temperature at the cloud tops.
found 10 new moons, bringing the total number to 15. Most
of the new moons are small, with the largest measuring about
150 kilometers (about 90 miles) in diameter.
Miranda, innermost of the five large moons, was revealed to
be one of the strangest bodies yet seen in the solar system.
Detailed images from Voyager's flyby of the moon showed huge
fault canyons as deep as 20 kilometers (12 miles), terraced
layers, and a mixture of old and young surfaces. One theory
holds that Miranda may be a reaggregration of material from
an earlier time when the moon was fractured by an violent
large moons appear to be ice-rock conglomerates like the satellites
of Saturn. Titania is marked by huge fault systems and canyons
indicating some degree of geologic, probably tectonic, activity
in its history. Ariel has the brightest and possibly youngest
surface of all the Uranian moons and also appears to have
undergone geologic activity that led to many fault valleys
and what seem to be extensive flows of icy material. Little
geologic activity has occurred on Umbriel or Oberon, judging
by their old and dark surfaces.
previously known rings were studied by the spacecraft and
showed the Uranian rings to be distinctly different from those
at Jupiter and Saturn. The ring system may be relatively young
and did not form at the same time as Uranus. Particles that
make up the rings may be remnants of a moon that was broken
by a high-velocity impact or torn up by gravitational effects.
When Voyager flew within 5,000 kilometers (3,000 miles) of
Neptune on August 25, 1989, the planet was the most distant
member of the solar system from the Sun. (Pluto once again
will become most distant in 1999.)
orbits the Sun every 165 years. It is the smallest of our
solar system's gas giants. Neptune is now known to have eight
moons, six of which were found by Voyager. The length of a
Neptunian day has been determined to be 16 hours, 6.7 minutes.
Neptune receives only three percent as much sunlight as Jupiter
does, it is a dynamic planet and surprisingly showed several
large, dark spots reminiscent of Jupiter's hurricane-like
storms. The largest spot, dubbed the Great Dark Spot, is about
the size of Earth and is similar to the Great Red Spot on
Jupiter. A small, irregularly shaped, eastward-moving cloud
was observed "scooting" around Neptune every 16 hours or so;
this "scooter," as Voyager scientists called it, could be
a cloud plume rising above a deeper cloud deck.
bright clouds, similar to cirrus clouds on Earth, were seen
high in Neptune's atmosphere. At low northern latitudes, Voyager
captured images of cloud streaks casting their shadows on
cloud decks below.
winds on any planet were measured on Neptune. Most of the
winds there blow westward, or opposite to the rotation of
the planet. Near the Great Dark Spot, winds blow up to 2,000
kilometers (1,200 miles) an hour.
field of Neptune, like that of Uranus, turned out to be highly
tilted -- 47 degrees from the rotation axis and offset at
least 0.55 radii (about 13,500 kilometers or 8,500 miles)
from the physical center. Comparing the magnetic fields of
the two planets, scientists think the extreme orientation
may be characteristic of flows in the interiors of both Uranus
and Neptune -- and not the result in Uranus's case of that
planet's sideways orientation, or of any possible field reversals
at either planet. Voyager's studies of radio waves caused
by the magnetic field revealed the length of a Neptunian day.
The spacecraft also detected auroras, but much weaker than
those on Earth and other planets.
the largest of the moons of Neptune, was shown to be not only
the most intriguing satellite of the Neptunian system, but
one of the most interesting in all the solar system. It shows
evidence of a remarkable geologic history, and Voyager 2 images
showed active geyser-like eruptions spewing invisible nitrogen
gas and dark dust particles several kilometers into the tenuous
atmosphere. Triton's relatively high density and retrograde
orbit offer strong evidence that Triton is not an original
member of Neptune's family but is a captured object. If that
is the case, tidal heating could have melted Triton in its
originally eccentric orbit, and the moon might even have been
liquid for as long as one billion years after its capture
thin atmosphere extends about 800 kilometer (500 miles) above
Triton's surface. Nitrogen ice particles may form thin clouds
a few kilometers above the surface. The atmospheric pressure
at the surface is about 14 microbars, 1/70,000th the surface
pressure on Earth. The surface temperature is about 38 kelvins
(-391 degrees Fahrenheit) the coldest temperature of any body
known in the solar system.
moons found at Neptune by Voyager are all small and remain
close to Neptune's equatorial plane. Names for the new moons
were selected from mythology's water deities by the International
Astronomical Union, they are: Naiad, Thalassa, Despina, Galatea,
2 solved many of the questions scientists had about Neptune's
rings. Searches for "ring arcs," or partial rings, showed
that Neptune's rings actually are complete, but are so diffuse
and the material in them so fine that they could not be fully
resolved from Earth. From the outermost in, the rings have
been designated Adams, Plateau, Le Verrier and Galle.
are continuing to return data about interplanetary space and
some of our stellar neighbors near the edges of the Milky
Voyagers cruise gracefully in the solar wind, their fields,
particles and waves instruments are studying the space around
them. In May 1993, scientists concluded that the plasma wave
experiment was picking up radio emissions that originate at
the heliopause -- the outer edge of our solar system.
is the outermost boundary of the solar wind, where the interstellar
medium restricts the outward flow of the solar wind and confines
it within a magnetic bubble called the heliosphere. The solar
wind is made up of electrically charged atomic particles,
composed primarily of ionized hydrogen, that stream outward
from the Sun.
where the heliopause is has been one of the great unanswered
questions in space physics. By studying the radio emissions,
scientists now theorize the heliopause exists some 90 to 120
astronomical units (AU) from the Sun. (One AU is equal to
150 million kilometers (93 million miles), or the distance
from the Earth to the Sun.
have also become space-based ultraviolet observatories and
their unique location in the universe gives astronomers the
best vantage point they have ever had for looking at celestial
objects that emit ultraviolet radiation.
on the spacecraft have been turned off and the ultraviolet
instrument is the only experiment on the scan platform that
is still functioning. Voyager scientists expect to continue
to receive data from the ultraviolet spectrometers at least
until the year 2000. At that time, there not be enough electrical
power for the heaters to keep the ultraviolet instrument warm
enough to operate.
are several other fields and particle instruments that can
continue to send back data as long as the spacecraft stay
alive. They include: the cosmic ray subsystem, the low-energy
charge particle instrument, the magnetometer, the plasma subsystem,
the plasma wave subsystem and the planetary radio astronomy
instrument. Barring any catastrophic events, JPL should be
able to retrieve this information for at least the next 20
and perhaps even the next 30 years.