THE VOYAGE TO JUPITER—GETTING THERE
Launch
On August 20, 1977, exactly two years after the launch of the Viking
spacecraft to Mars, the first of the Voyagers—actually Voyager 2—was
boosted into space at 10:29 a.m. EDT, less than five minutes after the
launch window opened on the first day of the thirty-day launch period.
Sixteen days later, at 8:56 a.m. EDT on Labor Day, September 5, 1977,
Voyager 1 was hurled into space on a shorter, faster trajectory than its
twin, zipping past the orbit of the Moon only ten hours after launch.
Ultimately, Voyager 1 earned its title by overtaking Voyager 2 as both
spacecraft journeyed through the asteroid belt, to arrive at Jupiter
four months ahead of Voyager 2.
The Voyagers lifted off from Launch Complex 41, Air Force Eastern Test
Range, Kennedy Space Center, Cape Canaveral, Florida, atop the giant
Titan III-E/Centaur rocket. It was the last time such a launch vehicle
was scheduled to be used, as, according to plan, the Space Shuttle would
take over in the 1980s. Thus the launching of the two Voyagers signified
both an end and a beginning: a once-in-a-lifetime opportunity to
explore, in only 8½ years’ time, perhaps fifteen major bodies of the
outer solar system.
But long before even the first Voyager was to make its closest approach
to Jupiter—in fact, even before Voyager 2 was off the launch pad—there
were problems to overcome.
In early August 1977, about three weeks before launch, failures in the
attitude and articulation control subsystem (AACS) and the flight data
subsystem (FDS), two of the spacecraft’s three main computer subsystems,
prevented the VGR77-2 spacecraft, originally scheduled for launch on
August 20, from becoming Voyager 2. Instead, the “spare” spacecraft
VGR77-3 was substituted, becoming Voyager 2 upon launch August 20, and
VGR77-2, after proper repairs, became Voyager 1. Minor problems
continued right up to launch. The low energy charged particle instrument
failed and had to be replaced, and as late as T minus five minutes there
was a halt in the countdown to check on a stuck valve. Unlike a jinxed
dress rehearsal, which is said to “assure” an opening-night success,
Voyager 2’s prelaunch problems were a portent of difficulties to come.
The Voyager 2 launch was witnessed by thousands as the spacecraft
ascended gracefully into the blue Florida sky, accompanied by the
deep-throated rumblings of its rocket, echoing for miles across the
beaches and scrub forests of Cape Canaveral. The Titan-Centaur
performance was nearly flawless, and Voyager 2 quickly achieved an
accurate trajectory toward Jupiter. However, even while the engines were
still firing, the spacecraft began to experience a baffling series of
problems that would absorb the attention of hundreds of persons from
Pasadena to Washington, D.C., for the next several weeks until they were
brought under control.
During the first minutes of flight, there seemed to be two difficulties
with the AACS. The first was a problem with one of the three stabilizing
gyroscopes, but fortunately, the gyroscope began operating normally
without intervention from the ground. The other problem appeared to be
with one of the AACS computers; the spacecraft switched to a backup
computer during the Titan burn, and initial data transmissions were
incomplete. Early analysis seemed to indicate that an event during the
launch itself, rather than a faulty spacecraft computer system, was the
cause of the data loss. At first, on August 23, officials suspected that
perhaps the spacecraft had been bumped by the rocket motor one hour
after liftoff and again about seventeen hours later, when telemetry
signals indicated that the spacecraft had been jolted. However, by the
next day, flight engineers determined that electronic gyrations in the
AACS seemed to have caused the difficulty.
[Illustration: The Titan-Centaur rocket used to launch Voyager stood
as tall as a 15-story building and weighed nearly 700 tons. Here the
rocket waits for launch at Kennedy Space Center, with the Voyager
spacecraft enclosed in the white protective shroud at the top.
[P-19471A]]
Within an hour after launch, Voyager 2’s science scan platform boom was
to have been fully extended and locked. Instructions to deploy were
given, and the boom moved outward; however, there was no signal to
indicate that the boom was actually locked in place. Efforts to command
the boom to move into the locked position were thwarted by the
spacecraft. The first maneuver designed to try to lock the boom was
aborted by the computer command subsystem (CCS) when the AACS
erroneously indicated that it was in trouble. Three days later another
maneuver was scheduled to reprogram the faulty computer in the AACS, to
align the Sun sensors, and to try to lock the science boom. To provide a
direct check of the boom position, the scan platform was turned so that
the TV cameras could see the spacecraft. Careful measurement of these
pictures verified that the boom was within ½ degree of full deployment,
but still there was no indication that it was locked into place.
Ultimately, it was decided that the sensor to signal actuation of the
lock was at fault, and that the boom itself was almost certainly fully
extended and operational.
Because of the postlaunch problems of Voyager 2, the launch of Voyager 1
was delayed twice—from September 1 to September 3 and then to September
5—in order to inspect Voyager 1’s science boom and to try to prevent a
repetition of Voyager 2’s problems. An extra spring was attached to the
science boom to assure its full extension. Finally, as if to make up for
the troubles of the first launch, Voyager 1’s launch was both “flawless
and accurate.” All launch and postlaunch events went smoothly. The
launch window opened at 8:56 a.m. EDT, and Voyager took off promptly at
8:56:01. The booms and antennas deployed and locked in the first hours
after launch; all instruments scheduled to be on were on and working
well.
THE TITAN/CENTAUR LAUNCH VEHICLE
The two Voyager spacecraft were carried into space and accelerated
toward Jupiter by the Titan III-E Centaur rocket, the largest launch
vehicle in the NASA arsenal after the retirement of the Saturn rockets
in 1975. The Titan and Centaur vehicles were originally developed
separately and have been used with other rocket stages for many NASA
launches. They were first combined for the two Viking launches to Mars
in 1975, and this powerful four-stage launch vehicle was used again in
1977 for Voyager.
The Titan/Centaur stands nearly 50 meters tall, about the height of a
fifteen-story building. Fully fueled, it weighs nearly 700 tons. At
takeoff, the thrust of the two solid-propellant Stage-0 motors is about
10.7 million newtons. These motors, which burn for 122 seconds, use
powdered aluminum as fuel and ammonium perchlorate as oxidizer.
Together, they have a mass of 500 tons.
The first stage of the liquid propellant core of the Titan rocket
ignites about 112 seconds after takeoff. The propellant is hydrazine as
fuel and nitrogen tetroxide as oxidizer. The first stage is 3 meters in
diameter and 20 meters tall. Fueled, it has a mass of 130 tons. The
motor provides a thrust of 2.5 million newtons for a duration of 146
seconds.
About 4.3 minutes after takeoff the Titan Stage II liquid propellant
motor begins to fire, and the first stage is separated and falls back
into the Atlantic. The second stage is 3 meters in diameter and more
than 7 meters long, with a fueled mass of 35 tons. The single liquid
fuel motor burns for 210 seconds with a thrust of half a million
newtons. During the second stage burn, the shroud covering the Voyager
spacecraft is jettisoned.
The Centaur and Titan vehicles separate 8 minutes into the flight, and
the Centaur main engine begins its burn. The Centaur is nearly 20 meters
tall and 3 meters in diameter, with a mass of 17 tons. The motors have a
thrust of almost 200 000 newtons, operating on the most powerful
chemical fuels known: liquid oxygen and liquid hydrogen. The Centaur
burns for only 1 minute and 36 seconds as it attains Earth parking
orbit; the engine then shuts down as the vehicle begins a half-hour
coasting period that carries it nearly half way around the Earth. During
this time, careful tracking of the spacecraft supplies the data needed
for Earth-based computers to calculate the proper time to leave parking
orbit and start the long trip toward Jupiter.
About 50 minutes after liftoff, from a position high above the Indian
Ocean, the second burn of the Centaur main engine begins. Six minutes of
additional thrust provides enough energy to break out of Earth’s orbit.
The Voyager then separates from the Centaur for a final boost toward
Jupiter. The solid rocket motor in the spacecraft propulsion module
(acting as final stage of this five-stage launch sequence) fires for 45
seconds at a thrust of 68 000 newtons. Just an hour after liftoff, the
Voyager spacecraft is on its way, coasting on an orbit toward Jupiter at
a speed of more than 10 kilometers per second.
[Illustration: The Voyager spacecraft is dominated by the large
3.7-meter-diameter antenna used for communication with Earth. Here
the spacecraft undergoes final tests before launch. The science
instrument scan platform is folded against the spacecraft on the
right; the three cylinders on the left are the RTG power sources.
[260-108BC]]
[Illustration: Voyager 2 was the first of the spacecraft to be
launched, on August 20, 1977, propelled into space in a
Titan/Centaur rocket. [P-19450AC]]
The First Year Is the Roughest
During the autumn of 1977 Voyager 2, and to a lesser extent Voyager 1,
continued to plague controllers with erratic actions. Thrusters fired at
inappropriate times, data modes shifted, instrument filter and analyzer
wheels became stuck, and the various computer control systems
occasionally overrode ground commands. Apparently, the spacecraft
hardware was working properly, but the computers on board displayed
certain traits that seemed almost humanly perverse—and perhaps a little
psychotic. In general, these reactions were the result of programming
too much sensitivity into the spacecraft systems, resulting in panic
over-reaction by the onboard computers to minor fluctuations in the
environment. Ultimately, part of the programming had to be rewritten on
Earth and then transmitted to the Voyagers, to calm them down so that
they would ignore minor perturbations, yet still be ready to perform
automatic sequences required to protect the spacecraft from major
threats. Meanwhile, however, more serious problems were developing.
On February 23, 1978, during a series of movements or slews, Voyager 1’s
scan platform slowed and stopped before completing the maneuver. This
failure caused a great deal of concern, since the scan platform houses
the optical instruments that are crucial to the observation of the
Jovian system—the ultraviolet spectrometer, the IRIS, the
photopolarimeter, and the two TV cameras. At JPL, tests were run on a
proof-test model—an exact copy of the Voyager spacecraft—to try to find
out why Voyager 1’s scan platform had become stuck. On March 17, Voyager
1’s scan platform was tested—JPL engineers instructed the platform to
move slowly for a short distance, and Voyager responded as ordered.
Further tests were conducted on March 23. This time the scan platform
was ordered to execute a sequence of four slews, moving away from the
part of the sky where the original failure had occurred and ending with
the position that it would be most useful to leave the platform in—just
in case the platform should become stuck again. On April 4 the scan
platform was commanded to perform a sequence of 38 slews, and fifty more
slews were performed on April 5. All were successful. Yet engineers were
still hesitant to force the platform to move through the region where it
had originally stuck, and extensive discussions were held to determine
if the Jupiter observations could be carried out without risking a
return to the danger area. It was argued, however, that full mobility of
the scan platform really was required, and on May 31 commands were sent
to maneuver the scan platform through the danger region. It moved
normally: The scan platform was operating properly again. After
additional slewing tests were run in mid-June, the scan platform was
pronounced fit for operation. Engineers suspected that the material
caught in the platform gears must have been crushed or moved out of the
way by the continued slewing, allowing the platform to move once more.
[Illustration: Voyager 1 was launched on September 5, 1977. The
launch was delayed 5 days to make last-minute adjustments to avoid
the postlaunch difficulties experienced by Voyager 2. [P-19480AC]]
An even more serious crisis soon endangered the Voyager 2 spacecraft. In
late November 1977, the S-band radio receiver began losing amplifier
power in its high-gain mode, so the solid-state amplifier was switched
to its low-power position. No further problems were noted until April 5,
1978, when Voyager 2’s primary radio receiver suddenly failed, and
shocked engineers discovered that the backup receiver was also faulty.
The trouble was detected after Voyager’s computer command subsystem
directed the spacecraft to switch from the primary radio receiver to the
backup receiver. This command was issued as part of a special protection
sequence: If the primary radio receiver receives no commands from Earth
for seven days, the backup receiver is switched on instead; if the
secondary receiver in turn receives no instructions over a twelve-hour
period, the system reverts to the main receiver. When, on April 5,
Voyager 2’s radio reception was switched from the primary to the
secondary receiver, flight engineers found that they were unable to
communicate with the spacecraft—the secondary receiver’s tracking loop
capacitor was malfunctioning. That meant that the secondary receiver
could not follow a changing signal frequency sent out from Earth. The
frequencies of signals transmitted from Earth are affected by the
Doppler effect—just as the siren on a fire engine seems first to rise in
pitch as the truck approaches, then falls as the truck speeds away, so
the frequency of signals transmitted from Earth fluctuates with the
Earth’s rotation as the Deep Space Network’s radio antennas move toward
or away from the spacecraft. The engineers had to wait until the primary
radio receiver was switched back on before they could communicate with
the spacecraft. Once the primary receiver was on, Voyager 2 began
receiving instructions from Earth, but approximately thirty minutes
later, there was an apparent power surge in the receiver. The fuses
blew. There was no recourse. The main receiver had failed; its loss was
permanent. It remained for the engineers to devise a way to communicate
with the slightly deaf spacecraft.
[Illustration: Each Voyager spacecraft follows a billion-kilometer
path to Jupiter. Except for minor thruster firings to achieve small
trajectory corrections, each Voyager coasts from Earth to Jupiter,
guided by the gravitational pull of the Sun. At Jupiter, the
powerful tug of the giant planet deflects the spacecraft and speeds
them up, imparting an extra kick to send them on their way toward
Saturn.]
Voyager 1
Voyager 2
Jupiter-Saturn-Uranus
Sun
Earth 8/20/77
Earth 9/1/77
Mars 8/20/77
Jupiter 8/20/77
Jupiter 3/5/79
Jupiter 7/9/79
Saturn 8/20/77
Saturn 11/13/80
Saturn 8/27/81
Uranus 8/20/77
Uranus 1/30/86
Because the switching of the radio receivers was still controlled by the
special protection sequence discussed earlier, flight engineers would
have to wait for seven days—until April 13—before they could attempt
communication with the spacecraft again. During that week special
procedures were established and rehearsed so that commands could be sent
to Voyager in the short time that the backup receiver would be on. On
Thursday, April 13, 1978, the seven days were up and the spacecraft
should have shifted from the dead main receiver to the sick backup
system. There was just a twelve-hour “window” in which to restore
communication. At about 3:30 a.m. PST the Madrid tracking station of the
Deep Space Network sent its first order to the spacecraft, approximately
474 million kilometers away. Almost an hour later, word arrived from
Voyager that the command had been accepted. (One-way light time for a
signal to travel the distance from Earth to Voyager at that time was
almost 27 minutes.) Elated flight controllers went ahead and transmitted
nine hours of commands to the spacecraft.
Voyager 2 was successfully commanded again on April 18 and April 26. The
April 26 commands included a course change maneuver that was executed
properly on May 3. On June 23, Voyager 2 was programmed for a backup
automatic mission at Saturn in the event that the secondary radio
receiver should also fail. These backup mission instructions would
operate all the science experiments, but only a minimum amount of data
would be returned, since the scan platform would only be programmed to
move through three positions rather than thousands as it would in normal
operation. Instructions for a backup minimum automatic encounter at
Jupiter were transmitted to Voyager 2 in two segments, the second of
these on October 12, 1978.
With the backup instructions recorded on board the spacecraft, Voyager
personnel felt their fears partially allayed. If Voyager 2’s secondary
radio receiver failed, the spacecraft would still obtain some science
data at Jupiter and Saturn. But that would mean that there would be no
mission beyond Saturn; our first opportunity to explore Uranus, its
satellites, its newly discovered ring system, and possibly even to get a
look at Neptune, would not come in this century.
Another major concern affecting both Voyager spacecraft was the proper
management of hydrazine fuel reserves. Hydrazine is used by the
thrusters on the Voyagers for stabilization of the spacecraft and for
trajectory correction maneuvers (TCM). Each Voyager was loaded with 105
kilograms of hydrazine budgeted for use on the long flight to Jupiter,
Saturn, and beyond. Because of the excellent performance of the launch
rockets, both Voyagers required less hydrazine than anticipated for
their final boost into proper trajectory toward Jupiter, and at first it
looked as though both spacecraft would have plenty of propellant to
spare.
Charles E. Kolhase, Manager of Mission Analysis and Engineering for the
Voyager Project, later explained the situation: “Voyager 1 should have
been launched September 1. Had it been launched on September 1—and I’m
glad it wasn’t—the maneuver to correct the trajectory for a Titan flyby
would have required a change in velocity of 100-110 meters per second—an
enormous maneuver—and we would have had a propellant margin for going on
to Saturn of perhaps 4.5 kilograms. But, by launching on the fifth of
September we increased our margin to 23 kilograms. Fortunately, for
every launch date that went by, that velocity change maneuver was
shrinking at a rate of 10 meters per second per day. Now, a 1 meter per
second change uses about a pound of hydrazine [about 0.5 kilogram]. So
when we launched on the fifth of September, now we suddenly had 40
pounds of hydrazine excess over what we would have had if we had
launched on the first of September. As a result, Voyager 1 is in great
shape as far as hydrazine is concerned.”
THE DEEP SPACE NETWORK
A vital component of the Voyager Mission is the communications system
linking the spacecraft with controllers and scientists on Earth. The
ability to communicate with spacecraft over the vast distances to the
outer planets, and particularly to return the enormous amounts of data
collected by sophisticated cameras and spectrometers, depends in large
part on the transmitters and receivers of the Deep Space Network (DSN),
operated for NASA by JPL.
The original network of these receiving stations was established in 1958
to provide round-the-world tracking of the first U.S. satellite,
Explorer 1. By the late 1970s, the DSN had evolved into a system of
large antennas, low-noise receivers, and high-power transmitters at
sites strategically located on three continents. From these sites the
data are forwarded (often using terrestrial communications satellites)
to the mission operations center at JPL.
The three DSN stations are located in the Mohave desert at Goldstone,
California; near Madrid, Spain; and near Canberra, Australia. Each
location is equipped with two 26-meter steerable antennas and a single
giant steerable dish 64 meters in diameter, with approximately the
collecting area of a football field. In addition, each is equipped with
transmitting, receiving, and data handling equipment. The transmitters
in Spain and Australia have 100-kilowatt power, while the 64-meter
antenna at Goldstone has a 400-kilowatt transmitter. Most commands to
Voyager are sent from Goldstone, but all three stations require the
highest quality receivers to permit continuous recording of the data
streams pouring in from the spacecraft.
Since the mid-1960s, the DSN’s standard frequency has been S-band (2295
megahertz). Voyager introduces a new, higher frequency telemetry link at
X-band (8418 megahertz). The X-band signal can carry more information
than S-band with similar power transmitters, but it requires more exact
antenna performance. In addition, the X-band signal is absorbed by
terrestrial clouds and, especially, rain. Fortunately, all three DSN
stations are in dry climates, but during encounters the weather
forecasts on Earth become items of crucial concern if precious data are
not to be lost by storm interference.
As a result of the development of larger antennas and improved
electronics, the DSN command capabilities and telemetry data rates have
increased dramatically over the years. For example, in 1965 Mariner 4
transmitted from Mars at a rate of only 8⅓ bits of information per
second. In 1969, Mariners 6 and 7 transmitted picture data from Mars at
16 200 bits per second. Mariner 10, in 1973, achieved 117 200 bits per
second from Mercury. Voyager operates at a similar rate from Jupiter,
about six times farther away. Many of these improvements in data
transmission result from changes in the DSN rather than in the
spacecraft transmitters.
Problems with hydrazine management developed, however. Voyager 1’s first
trajectory correction maneuver achieved only 80 percent of the required
speed change. Exhaust plumes from the thrusters apparently struck part
of the spacecraft, causing a 20 percent loss in velocity. That being the
case, Voyager might require more fuel than had been expected to complete
the mission. The extra fuel requirements did not threaten Voyager 1
itself, since it held ample fuel to reach Saturn; the concern was for
Voyager 2, where the effective loss of fuel might be enough to
jeopardize the Uranus mission.
[Illustration: Project members.]
[Illustration: Project members.]
Because of the plume impingement problem on Voyager 1, Voyager 2’s first
trajectory correction maneuver was adjusted to allow for the possibility
of a 20 percent loss in thrust. The Voyager 2 maneuver was successful,
but controllers felt that additional action was required to conserve
fuel. One way to save was by reducing requirements on control of the
spacecraft orientation. Less control fuel would be needed if the already
miniscule pressure exerted on the spacecraft by the solar wind could be
reduced. Flight engineers at JPL calculated that the pressure would be
reduced if the spacecraft were tipped upside down; however, to
accomplish this, the spacecraft would have to be steered by a new set of
guide stars. By reprogramming the attitude control system it was found
possible to substitute the northern star, Deneb, in the constellation of
Cygnus, for the original reference star, Canopus, in the southern
constellation of Carina. With this change, as well as readjustment of
Voyager 2’s trajectory near Jupiter, inflight consumption of hydrazine
was reduced significantly.
In late August 1978 both Voyagers were reprogrammed to ensure better
science results at Jupiter encounter; for example, the reprogramming
would prevent imaging (TV) photographs from blurring when the tape
recorder was operating. By early November, flight crews had begun
training exercises to rehearse for the Voyager 1 flyby of Jupiter on
March 5, 1979. A near encounter test was performed on December 12-14,
1978: a complete runthrough of Voyager 1’s 39-hour near encounter
period, which would take place March 3-5, 1979. Participants included
the flight team, the Deep Space Network tracking stations, the
scientists, and the spacecraft itself. Results: Voyager and the Voyager
team were all ready for the encounter.
Meanwhile, the spacecraft were busy returning scientific data to Earth.
Technically, the Voyagers were in the cruise phase of the mission—a
period that, for Voyager 2, would last until April 24, 1979, and for
Voyager 1, until January 4, 1979, when each spacecraft would enter its
respective observatory phase.
Cruise Phase Science
In the first few days after launch, the spacecrafts’ instruments were
turned on and calibrated; various tests for each instrument would
continue to be performed throughout the cruise phase. This period
presented a great opportunity for the Voyagers to study the
interplanetary magnetic fields, solar flares, and the solar wind. In
addition, ultraviolet and infrared radiation studies of the sky were
performed. In mid-September 1977 the television cameras on Voyager 1
recorded a number of photographs of the Earth and Moon. A photograph
taken September 18 captured both crescent Moon and crescent Earth. It
was the first time the two celestial bodies had ever been photographed
together.
In November both Voyagers crossed the orbit of Mars, entering the
asteroid belt a month later. On December 15, at a distance of about 170
million kilometers from Earth, Voyager 1 finally speeded past its slower
twin. The journey through the asteroid belt was long but uneventful:
Voyager 1 emerged safely in September 1978, and Voyager 2 in October.
Unlike Pioneers 10 and 11, the Voyagers carried no instruments to look
at debris in the asteroid belt.
By April both spacecraft were already halfway to Jupiter and, about two
months later, Voyager 1, still approximately 265 million kilometers from
Jupiter, began returning photographs of the planet that showed
considerable detail, although less than could be obtained with
telescopes on Earth. Both the imaging (TV) and the planetary radio
astronomy instruments began observing Jupiter, and, by October 2, 1978,
officials announced that “the polarization characteristics of Jupiter’s
radio emissions have been defined. In the high frequencies, there is
consistent right-hand circular polarization, while in the low
frequencies, there is a consistent left-hand circular polarization. This
was an unexpected result.” In addition to scientific studies of the
interplanetary medium and a first look at Jupiter, the plasma wave
instrument, which studies waves of charged particles over a range of
frequencies that includes audio frequencies, was able to record the
sound of the spacecraft thrusters firing, as hydrazine fuel is
decomposed and ejected into space. The sound was described as being
“somewhat like a 5-gallon can being hit with a leather-wrapped mallet.”
On December 10, 1978, 83 million kilometers from Jupiter, Voyager 1 took
photographs that surpassed the best photographs ever taken from
ground-based telescopes, and scientists were anxiously awaiting the
start of continuous coverage of the rapidly changing cloud forms. These
pictures, together with data from several ground-based observatories,
were carefully scrutinized by a team of scientists at JPL to select the
final targets in the atmosphere of Jupiter to be studied at high
resolution during the flyby. The observatory phase of Voyager 1’s
journey to Jupiter was about to begin.
[Illustration: As Voyager 1 approached Jupiter, the resolution of
the images steadily improved. In October 1978, at a distance of
about 125 million kilometers, the image was less clear than would be
obtained with an Earth-based telescope. [P-20790]]
[Illustration: By December 10, the spacecraft had moved to a
distance of 85 million kilometers, and the resolution was about 2000
kilometers, comparable to the best telescopic images. [P-20829C]]
[Illustration: On January 9, 1979 (c), at a distance of 54 million
kilometers, the image surpassed all ground-based views and
approached the resolution of the Pioneer 10 and 11 photos.
[P-20926C]]
[Illustration: In this, taken January 24 at a distance of 40 million
kilometers, the resolution exceeded 1000 kilometers. [P-20945C]]
The Observatory Phase
The observatory phase, originally scheduled to start on December 15,
1978, eighty days before encounter, was postponed until January 4, 1979,
to provide the flight team a holiday-season break. For the next two
months, Voyager would carry out a long-term scientific study—a “time
history”—of Jupiter, its satellites, and its magnetosphere. On January
6, Voyager 1 began photographing Jupiter every two hours—each time
taking a series of four photographs through different color filters as
part of a long-duration study of large-scale atmospheric processes, so
scientists could study the changing cloud patterns on Jupiter. Even the
first of these pictures showed that the atmosphere was dynamic “with
more convective structure than had previously been thought.”
Particularly striking were the changes in the planet, especially near
the Great Red Spot, that had taken place since the Pioneer flybys in
1973 and 1974. Jupiter was wearing a new face for Voyager.
In mid-January, photos of Jupiter were already being praised for
“showing exceptional details of the planet’s multicolored bands of
clouds.” Still 47 million kilometers from the giant, Voyager 1 had by
now taken more than 500 photographs of Jupiter. Movements of cloud
patterns were becoming more obvious; feathery structures seemed painted
across some of the bands that encircle the planet; swirling features
were huddled near the Great Red Spot. The satellites were also beginning
to look more like worlds, with a few bright spots visible on Ganymede
and dark red poles and a bright equatorial region clearly seen on Io
when it passed once each orbit across the turbulent face of Jupiter.
From January 30 to February 3, Voyager sent back one photograph of
Jupiter every 96 seconds over a 100-hour period. Using a total of three
different color filters, the spacecraft thus produced one color picture
every 4¾ minutes, in order to make a “movie” covering ten Jovian “days.”
To receive these pictures, sent back over the high-rate X-band
transmitter, the Deep Space Network’s 64-meter antennas provided
round-the-clock coverage. Voyager 1 was ready for its far encounter
phase.
[Illustration: The Galilean satellites of Jupiter first began to
show as tiny worlds, not mere points of light, as the Voyager 1
observatory phase began. In this view taken January 17, 1979, at a
range of 47 million kilometers, the differing sizes and surface
reflectivities (albedos) of Ganymede (right center) and Europa (top
right) are clearly visible. The view of Jupiter is unusual in that
the Great Red Spot is not easily visible, but can just be seen at
the right edge of the planet. Most pictures selected for publication
include the photogenic Red Spot. [P-20938C]]
Far Encounter Phase
By early February Jupiter loomed too large in the narrow-angle camera to
be photographed in one piece; 2 × 2 three-color (violet, orange, green)
sets of pictures were taken for the next two weeks; by February 21,
Jupiter had grown too large even for that, and 3 × 3 sets were scheduled
to begin. When these sets of pictures are pasted together to form a
single picture, the result is called a mosaic.
[Illustration: By February 1, 1979, Voyager 1 was only 30 million
kilometers from Jupiter, and the resolution of the imaging system
corresponded to about 600 kilometers at the cloud tops of the giant
planet. At this time, a great deal of unexpected complexity became
apparent around the Red Spot, and movie sequences clearly showed the
cloud motions, including the apparent six-day rotation of the Red
Spot. [P-20993C]]
[Illustration: One of the most spectacular planetary photographs
ever taken was obtained on February 13 as Voyager 1 continued its
approach to Jupiter. By this time, at a range of 20 million
kilometers, Jupiter loomed too large to fit within a single
narrow-angle imaging frame. Passing in front of the planet are the
inner two Galilean satellites. Io, on the left, already shows
brightly colored patterns on its surface, while Europa, on the
right, is a bland ice-covered world. The scale of all of these
objects is huge by terrestrial standards; Io and Europa are each the
size of our Moon, and the Red Spot is larger than the Earth.
[P-21082C]]
While the imaging experiments were in the limelight, the other
scientific instruments had also begun to concentrate on the Jovian
system. The ultraviolet spectrometer had been scanning the region eight
times a day; the infrared spectrometer (IRIS) spent 1½ hours a day
analyzing infrared emissions from various longitudes in Jupiter’s
atmosphere; the planetary radio astronomy and plasma wave instruments
looked for radio bursts from Jupiter and for plasma disturbances in the
region; the photopolarimeter had begun searching for the edge of Io’s
sodium torus; and a watch was begun for the bow shock—the outer boundary
of the Jovian magnetosphere.
On February 10, Voyager 1 crossed the orbit of Sinope, Jupiter’s
outermost satellite. Yet the spacecraft even then had a long way to
go—still 23 million kilometers from Jupiter, but closing in on the
planet at nearly a million kilometers a day. A week later, targeted
photographs of Callisto began to provide coverage of the satellite all
around its orbit; similar photos of Ganymede began on February 25.
Meanwhile excitement was building. As early as February 8 and 9, delight
with the mission and anticipation of the results of the encounter in
March were already evident. Garry E. Hunt, from the Laboratory for
Planetary Atmospheres, University College, London, a member of the
Imaging Team, discussing the appearance of Jupiter’s atmosphere as seen
by Voyager during the previous month, said “It seems to be far more
photogenic now than it did during the Pioneer encounters; I’m more than
delighted by it—it’s an incredible state of affairs. There are
infinitely more details than ever imagined.”
[Illustration: The Voyager Project was operated from the Jet
Propulsion Laboratory managed for NASA by the California Institute
of Technology. Located in the hills above Pasadena, California, JPL
is the main center for U.S. exploration of the solar system.
[JB17249BC]]
[Illustration: The Voyager TV cameras do not take color pictures
directly as do commercial cameras. Instead, a color image is
reconstructed on the ground from three separate monochromatic
images, obtained through color filters and transmitted separately to
Earth. There are a number of possible filter combinations, but the
most nearly “true” color is obtained with originals photographed in
blue (480 nanometers), green (565 nanometers), and orange (590
nanometers) light. Before these can be combined, the individual
frames must be registered, correcting for any change in spacecraft
position or pointing between exposures. Often, only part of a scene
is contained in all three original pictures. Shown here is a
reconstruction of a plume on Jupiter, photographed on March 1, 1979.
The colors used to print the three separate frames can be seen
clearly in the nonoverlapping areas. For other pictures in this
book, the nonoverlapping partial frames are omitted. [P-21192]]
The pictures from Voyager are “clearly spectacular,” said Lonne Lane,
Assistant Project Scientist for Jupiter. “We’re getting even better
results than we had anticipated. We have seen new phenomena in both
optical and radio emissions. We have definitely seen things that are
different—in at least in one case, unanticipated—and are begging for
answers we haven’t got.” There was already, still almost a month from
encounter, a strong feeling of accomplishment among the scientists and
engineers; they had done a difficult task and it has been successful.
By the last week of February 1979, the attention of thousands of
individuals was focused on the activities at JPL. Scientists had arrived
from universities and laboratories around the country and from abroad,
many bringing graduate students or faculty colleagues to assist them.
Engineers and technicians from JPL contractors joined NASA officials as
the Pasadena motels filled up. Special badges were issued and reserved
parking areas set aside for the Voyager influx. Twenty-four hours a day,
lights burned in the flight control rooms, the science offices, the
computer areas, and the photo processing labs. In order to protect those
with critical tasks to perform from the friendly distraction of the new
arrivals, special computer-controlled locks were placed on doors, and
security officers began to patrol the halls. By the end of the month the
press had begun to arrive. Amid accelerating excitement, the Voyager 1
encounter was about to begin.
[Illustration: The smaller-scale clouds on Jupiter tend to be more
irregular than the large ovals and plumes. At the lower right, one
of the three large white ovals clearly shows internal structure,
with the swirling cloud pattern indicating counterclockwise, or
anticyclonic, flow. A smaller anticyclonic white feature near the
center is surrounded by a dark, cloud-free band where one can see to
greater depths in the atmosphere. This photo was taken March 1 from
a distance of 4 million kilometers. [P-21183C]]