Flight to Jupiter

Two identical Pioneer spacecraft were designed and fabricated by TRW Systems Group at their Redondo Beach, California facility. Each weighed only about 260 kilograms, yet carried eleven highly sophisticated instruments capable of operating unattended for many years in space. Data systems on board controlled the instrumentation, received and processed commands, and transmitted information across the vast distance to Earth.

The second Pioneer was to wait more than a year before launch. By following far behind Pioneer 10, its trajectory—in particular how deeply it penetrated the radiation belts during Jupiter flyby—could be modified, depending on the fate of the first spacecraft. At dusk on April 5, 1973, Pioneer 11 blasted from the launch pad at Cape Canaveral and followed its predecessor on the long, lonely journey into the outer solar system.

Within a few hours of launch, each Pioneer spacecraft shed the shroud that had protected it and unfurled booms supporting the science instruments and the RTG power generators. After each craft had been carefully tracked and precise orbits calculated, small onboard rockets were commanded to fire to correct its trajectory for exactly the desired flyby at Jupiter. Pioneer 10 was targeted to fly by the planet at a minimum distance of 3 Jupiter radii (RJ) from the center, or 2 RJ(about 140 000 kilometers) above the clouds. This close passage, inside the orbit of Io, allowed the craft to pass behind both Io and Jupiter as seen from Earth, so that its radio beam could probe both the planet and its innermost large satellite. Pioneer 11 was intended to fly even closer to Jupiter, but the exact targeting options were held open until after the Pioneer 10 encounter.

On Pioneer 10, all instruments appeared to be working well as the craft passed the orbit of Mars in June 1972, just 97 days after launch. At this point, as it headed into unexplored space, it truly became a pioneer. In mid-July it began to enter the asteroid belt, and scientists and engineers anxiously watched for signs of increasing particulate matter.

Pioneer 10 carried two instruments designed to measure small particles in space. One, with an effective area of about 0.6 square meters, measured the direct impact of dust grains as small as one-billionth of a gram. The other looked for larger, more distant grains by measuring sunlight reflected from them. To the surprise of many, there was little increase in the rate of dust impacts recorded as the craft penetrated more and more deeply into the belt. At about 400 million kilometers from the Sun, near the middle of the belt, there appeared to be an increase in the number of larger particles detected optically, but not to a level that posed any hazard. In February 1973 the spacecraft emerged unscathed from the asteroid belt, having demonstrated that the much-feared concentration of small debris in the belt did not exist. The pathway was open to the outer solar system!

On November 26, 1973, the long-awaited encounter with Jupiter began. On that date, at a distance of 6.4 million kilometers from the planet, instruments on board Pioneer 10 detected a sudden change in the interplanetary medium as the spacecraft crossed the point—the bow shock—at which the magnetic presence of Jupiter first becomes evident. At the bow shock, the energetic particles of the solar wind are suddenly slowed as they approach Jupiter. At noon the next day, Pioneer 10 entered the Jovian magnetosphere at a distance of 96 RJfrom the planet.

As the spacecraft hurtled inward toward regions of increasing magnetic field strength and charged plasma particles, the instruments designed to look at Jupiter began to play their role. A simple line-scan camera that could buildup an image from many individual brightness scans (like a newspaper picture transmitted by wire) obtained its first pictures of the planet, and ultraviolet and infrared photometers prepared to observe it also. By December 2, when the spacecraft had crossed the orbit of Callisto, the outermost of the large Galilean satellites, the line-scan images were nearly equal in quality to the best telescopic photos taken previously, and as each hour passed they improved in resolution. Near closest approach, Pioneer 10 transmitted partial frames of Jupiter that represented a threefold improvement over any Earth-based pictures ever taken.

The asteroid belt is a region between Mars and Jupiter that is populated by thousands of minor planets, most only a few kilometers in diameter. Before 1970, some theorists suggested that large quantities of abrasive dust might damage spacecraft passing through the asteroid belt. Pioneer 10 proved that this danger was not present, thus opening the way to the outer solar system.

Tension increased as the spacecraft plunged deeper into the radiation belts of Jupiter. Would it survive the blast of x-rays and gamma-rays induced in every part of the craft by the electrons and ions trapped by the magnetic field of Jupiter? Several of the instruments measuring the charged particles climbed to full scale and saturated. Others neared their limits but, as anxious scientists watched the data being sent back, the levels flattened off. Meanwhile, the spacecraft itself began to feel the effects of the radiation, and occasional spurious commands were generated. Several planned high-resolution images of Jupiter and its satellites were lost because of these false signals. But again the system stabilized, and no more problems occurred as, just past noon on December 3, 1973, Pioneer 10 reached its closest point to Jupiter, 130 000 kilometers above the Jovian cloud tops. Pioneer had passed its most demanding test with flying colors, and at a news conference at Ames, NASA Planetary Program Director Robert Kraemer pronounced the mission “100 percent successful.” He added, “We sent Pioneer off to tweak a dragon’s tail, and it did that and more. It gave it a really good yank, and it managed to survive.” The Project Science Chief pronounced it “the most exciting day of my life,” and most of the hundreds of scientists and engineers who participated in the encounter probably agreed with him.

Pioneer 11 continued to follow steadily, emerging from the asteroid belt in March 1974. Based on the performance and findings ofPioneer 10, it was decided to send Pioneer 11 still closer to Jupiter, but on a more inclined trajectory. On April 19 thrusters on the spacecraft fired to move the Pioneer 11 aimpoint just 34 000 kilometers above the clouds of Jupiter.

In using the Pioneer 10 data to assess the hazard to Pioneer 11, scientists had to consider three aspects of the charged particle environment. First was the energy distribution of the particles: The most energetic presented the most danger. Second was the flux, the rate at which particles struck the craft. Third was the total radiation dose. One can make an analogy with a boxing match. The energy distribution tells you how hard the blows of your opponent are. The flux is a measure of how many times a minute he hits you, and the total dose measures how many blows land. The spacecraft reacts just like a boxer; the crucial question is how much total dose it absorbs. Enough radiation blows, and the system is knocked out. The trajectory chosen for Pioneer 11 resulted in higher flux, since the craft probed more deeply into Jupiter’s inner magnetosphere than had Pioneer 10. But by moving at a high angle across the equatorial regions where the flux is highest, the total dose could be kept below that experienced by Pioneer 10.

Pioneer 11 entered the Jovian magnetosphere on November 26, 1974, just a year after its predecessor. Closest approach took place on December 2. As with Pioneer 10, the radiation dose taxed the spacecraft to its limit. Again spurious commands were issued, this time affecting the infrared radiometer more than the imaging system. The craft flew at high latitude over the north polar region of Jupiter, an area never seen from Earth, and returned several excellent high-resolution pictures. Once more the little Pioneers had succeeded against the odds in opening the way to the giant planets.

Pioneer 10 and 11 encounters with Jupiter are shown as viewed from the celestial North Pole. Pioneer 10 swung around the giant planet in the counterclockwise direction, while Pioneer 11 followed a clockwise approach. In this view, Jupiter rotates counterclockwise.

Following their encounters with Jupiter, both Pioneer spacecraft returned to their normal routine of measuring the interplanetary medium. Pioneer 10 had gained speed from the gravity field of Jupiter and became the first craft to achieve the velocity needed to escape from the solar system. Pioneer 11, however, had used the pull of Jupiter to bend its trajectory inward, aiming it across the solar systemtoward Saturn. Following the successes at Jupiter, NASA announced that Pioneer 11 would be targeted for a close flyby of Saturn five years later, which was successfully carried out in September 1979. In early 1980, far beyond their design lifetimes, both spacecraft were still performing beautifully.

In this view of Jupiter, the Great Red Spot is prominent and the shadow of Io traverses the planetary disk. The gross morphology of the belts and zones, with structures showing turbulence and convective cells in the middle latitudes, is clearly seen. The small white spots surrounded by dark rings, seen mainly in the southern hemisphere, indicate regions of intense vertical convective activity, somewhat similar to cumulonimbus or thunderclouds.

The scientific results of the Pioneer flybys of Jupiter were many and varied. As is always the case, some old questions were answered and new problems were raised by the spacecraft data. Highlights of these results are summarized below.

The line-scan imaging systems of Pioneer returned some remarkable pictures of the planet during the two encounters, showing individual features as small as 500 kilometers across. In addition, the Pioneers were able to look at Jupiter from angles never observable from Earth.

One of the discoveries made from these pictures was the great variety of cloud structures near the boundaries between the light zones and dark belts. Many individual cloud patterns suggested rising and falling air. The convoluted swirls evident in these regions appeared to be the result of dynamic motions; unfortunately, with only the few “snapshots” obtained during the hours of the flyby, the actual motions of these clouds could not be followed.

In the year the Pioneer Project was begun, astronomers on Earth had measured that Jupiter emitted more heat than it absorbed from the Sun. From the Earth these measurements could be made only of the sunlit part of the planet; neither the night side nor thepoles could be seen. One of the main objectives of the Pioneer flybys was to determine the heat budget accurately from temperature measurements at many points on both the sunlit and the night sides.

The Pioneer data confirmed the presence of a heat source in Jupiter and supplied a quantitative estimate of its magnitude. The global effective temperature was found to be -148° C, to a precision of ±3 degrees. This temperature implies that Jupiter radiates 1.9 times as much heat as it receives from the Sun. The corresponding internal heat source is 10¹⁷ watts. Surprisingly, the poles were as warm as the equator; apparently, the atmosphere is very efficient at transferring solar heat absorbed near the equator up to high latitudes, or perhaps the internal component of the heat comes preferentially from the polar regions.

The Pioneer infrared experiment made the first measurement of the amount of helium on Jupiter. The ratio of the number of helium atoms to the number of hydrogen atoms was found to be He/H₂ = 0.14 ± 0.08. This is consistent with the known solar ratio of He/H₂ = 0.11. Measurements of helium in the upper atmosphere were also made by the ultraviolet experiment.

One of the best Pioneer images of Jupiter was obtained at a range of 545 000 kilometers by Pioneer 11. Structure within the Great Red Spot and the surrounding belts and zones can be seen. There was much less turbulent cloud activity round the spot at the time of the Pioneer flybys than was seen five years later by the Voyager cameras.

Pioneer 10 confirmed theoretical models of Jupiter that suggest the planet is nearly all liquid, with a very small core and an extremely deep atmosphere. The liquid interior seethes with internal heat energy, which is transferred from deep within the planet to its outer regions. The temperature at the center may be 30 000 K. Since the temperature at the cloud tops is around -123° C, there is a large range of temperatures within the planet.

Several Pioneer investigations yielded information on the variation of atmospheric temperature and pressure in the regions above the ammonia clouds. Near the equator, at a level where the atmospheric pressure is the same as that on the surface of the Earth (1 bar), the temperature is -108° C. About 150 kilometers higher, where the pressure drops to 0.1 bar, is the minimum atmospheric temperature of about -165° C. Above this point the temperature rises again, reaching about -123° C near a pressure level of 0.03 bar. Presumably this temperature rise is due to absorption of sunlight by a thin haze of dust particles in the upper atmosphere of Jupiter.

The measurements of the amount of helium, of the gravitational field, and of the size of the internal heat source on Jupiter greatly clarified scientists’ understanding of the deep interior of the planet. Calculations showed that the core of Jupiter must be so hot that hydrogen cannot become solid, but must remain a fluid throughout the interior. Even at great depths, therefore, Jupiter does not have a solid surface. The theory that the Great Red Spot was the result of interactions with a surface feature below the clouds thus became untenable. Whatever its exact nature, the Red Spot must be a strictly atmospheric phenomenon.

Pioneer data showed that the magnetic field of Jupiter has a dipolar nature, like that of the Earth, but 2000 times stronger. The calculated surface fields measured about 4 gauss, compared to a field of about 0.5 gauss on the Earth. The axis of the magnetic field was tilted 11 degrees with respect to the rotation axis, and it was offset by about 10 000 kilometers (0.1 RJ) from the center of the planet.

Pioneers 10 and 11 did not obtain very detailed pictures of the satellites of Jupiter. The best view was of Ganymede, which showed a surface of contrasting light and dark spots of unknown nature.

A less detailed image of Europa clearly reveals the illuminated crescent but supplies little information about the surface of this ice-covered satellite.

Two experiments yielded exciting new information on possible atmospheres of the Galilean satellites. First was an occultation, in which the Pioneer 10 spacecraft was targeted to pass behind Io as seen from Earth. At the moments just before the spacecraft disappeared, and just after reemergence from behind the satellite, the radio signal was influenced by a thin layer of ionized gas, in which the electrons had been strippedfrom the atoms by absorbed sunlight or by other processes. The ionosphere thus discovered had a peak density of about 60 000 electrons per cubic centimeter. In addition, a very extended far-ultraviolet glow, probably due to atomic hydrogen, was found near the orbit of Io by the ultraviolet photometer.

Precise radio tracking of the Pioneers as they coasted past Jupiter and its satellites revealed that Jupiter is about one percent heavier than had been anticipated, and several satellites were found to have masses that differ by more than ten percent from values determined previously. These improvements in knowledge of the masses were required to achieve the close satellite flybys being planned for later missions.

The Pioneer spacecraft carried this plaque on the journey beyond the solar system, bearing data that tell where and when the human species lived and that convey details of our biological form. When Pioneer 10 flew by Jupiter it acquired sufficient kinetic energy to carry it completely out of the solar system. Some time between one and ten billion years from now, the probe may pass through the planetary system of a remote stellar neighbor, one of whose planets may have evolved intelligent life. If the spacecraft is detected and then inspected, Pioneer’s message will reach across the eons to communicate its greeting.

A great deal of the scientific emphasis of the Pioneer missions was directed at characterizing the particles and fields in the inner magnetosphere, the region in which charged particles are trapped in stable orbits. The Pioneers found that it extended to about 25 RJ, well beyond the orbit of Callisto. Within this region, instruments on the spacecraft recorded the numbers and energy of electrons, protons, and ions. The electrons reached a maximum concentration near 3 RJ, and their numbers remained almost constant from there in toward the planet. The maximum concentration of protons observed by Pioneer 10 was at 3.4 RJ, a little inside the orbit of Io. Pioneer 11 penetrated deeper and found another maximum, about twenty times higher, at 1.9 RJ; at this distance, 10 million energetic protons hit eachsquare centimeter of the spacecraft every second. It was believed that the gap between these two peaks was due to tiny Amalthea, the innermost satellite, which orbits at 2.5 RJ. Apparently this satellite sweeps up the particles as it circles Jupiter. Another large dip in the proton distribution was attributed to sweeping by Io, with smaller effects seen near the orbits of Europa and Ganymede. There was an additional small effect at 1.8 RJ, later found by Voyager to be due to Jupiter’s ring and its fourteenth satellite.

From about 25 RJoutward to its boundary near 100 RJ, the Jovian magnetosphere is a complex and dynamic place. Beyond about 60 RJ, both Pioneers found the boundary to be highly unstable, apparently blown in and out by variations in the pressure of the solar wind, which consists of charged particles flowing outward from the Sun. In this region concentrations of Jovian particles are sometimes seen that rival the inner magnetosphere in intensity. Between 60 RJand 25 RJ, a region sometimes called the middle magnetosphere, the particle motions are more ordered, and for the most part electrons and protons are carried along with the planet’s rotation by its magnetic field. Near the equatorial plane, the flow of these particles produces an electric current circling the planet, and this current in turn generates its own magnetic field, which approaches in strength that of Jupiter itself. Occasionally the outer magnetosphere collapses down to about 60 RJ, and energetic particles are squirted from the middle region into space; these bursts of Jovian particles can sometimes be detected as far away as Earth.

At the same time that Pioneer scientists were analyzing their results and developing new concepts of the Jupiter system, a new team of investigators had been selected for the next mission to Jupiter. From about 1975 on, attention shifted from Pioneer to its successor—Voyager.

The Voyager spacecraft are among the most sophisticated, automatic, and independent robots ever sent to explore the planets. Each craft has a mass of one ton and is dominated by the 3.7-meter-diameter white antenna used for radio communication with Earth. Here Voyager undergoes final tests in a space simulator chamber. [373-7162AC]

Voyager had its origin in the Outer Planets Grand Tour, a plan to send spacecraft to all the planets of the outer solar system. In 1969, the same year in which the Pioneer Project received Congressional approval, NASA began to design the Grand Tour. At the same time, the Space Science Board of the National Academy of Sciences completed a study called “The Outer Solar System,” chaired by James Van Allen of the University of Iowa, which recommended that the United States undertake an exploration program:

1. To conduct exploratory investigations of the appearance, size, mass, magnetic properties, and dynamics of each of the outer planets and their major satellites;2. To determine the chemical and isotopic composition of the atmospheres of the outer planets;3. To determine whether biologically important organic substances exist in these atmospheres and to characterize the lower atmospheric environments in terms of biologically significant parameters;4. To describe the motions of the atmospheres of the major planets and to characterize their temperature-density-composition structure;5. To make a detailed study for each of the outer planets of the external magnetic field and respective particle population, associated radio emissions, and magnetospheric particle-wave interactions;6. To determine the mode of interaction of the solar wind with the outer planets, including the interaction of the satellites with the planets’ magnetospheres;7. To investigate the properties of the solar wind and the interplanetary magnetic field at great distances from the Sun at both low and high solar latitudes, and to search for the outer boundary of the solar wind flow;8. To attempt to obtain the composition, energy spectra, and fluxes of cosmic rays in interstellar space, free of the modulating effects of the solar wind.

1. To conduct exploratory investigations of the appearance, size, mass, magnetic properties, and dynamics of each of the outer planets and their major satellites;

2. To determine the chemical and isotopic composition of the atmospheres of the outer planets;

3. To determine whether biologically important organic substances exist in these atmospheres and to characterize the lower atmospheric environments in terms of biologically significant parameters;

4. To describe the motions of the atmospheres of the major planets and to characterize their temperature-density-composition structure;

5. To make a detailed study for each of the outer planets of the external magnetic field and respective particle population, associated radio emissions, and magnetospheric particle-wave interactions;

6. To determine the mode of interaction of the solar wind with the outer planets, including the interaction of the satellites with the planets’ magnetospheres;

7. To investigate the properties of the solar wind and the interplanetary magnetic field at great distances from the Sun at both low and high solar latitudes, and to search for the outer boundary of the solar wind flow;

8. To attempt to obtain the composition, energy spectra, and fluxes of cosmic rays in interstellar space, free of the modulating effects of the solar wind.

The report also noted that “exceptionally favorable astronomical opportunities occur in the late 1970s for multiplanet missions,” and that “professional resources for full utilization of the outer-solar-system mission opportunities in the 1970s and 1980s are amply available within the scientific community, and there is a widespread eagerness to participate in such missions.”

An additional Academy study, chaired by Francis S. Johnson of the University of Texas at Dallas and published in 1971, was even more specific: “An extensive study of the outer solar system is recognized by us to be one of the major objectives of space science in this decade. This endeavor is made particularly exciting by the rare opportunity to explore several planets and satellites in one mission using long-lived spacecraft and existing propulsion systems. We recommend that [Mariner-class] spacecraft be developed and used in Grand Tour missions for the exploration of the outer planets in a series of four launches in the late 1970s.”

Thus the stage was set to initiate the Outer Planets Grand Tours. NASA’s timetable called for dual launches to Jupiter, Saturn, and Pluto in 1976 and 1977, and dual launches to Jupiter, Uranus, and Neptune in 1979, at a total cost over the decade of the 1970s of about $750 million.

A necessary step was to obtain from the scientific community the best possible set of instruments to fly on the spacecraft. Following its initial internal studies, NASA turned for its detailed scientific planning to an open competitionin which any scientist or scientific organization was invited to propose an investigation. In October 1970 NASA issued an “Invitation for Participation in the Mission Development for Grand Tour Missions to the Outer Solar System,” and a year later it had selected about a dozen teams of scientists to formulate specific objectives for these missions. At the same time, an advanced spacecraft engineering design was carried out by the Caltech Jet Propulsion Laboratory (JPL), and studies were also supported by industrial contractors. In fiscal year 1972, plans called for an appropriation by Congress of $30 million to fund these developments, leading toward a first launch in 1976.

Even as the scientific and technical problems of the Grand Tour were being solved, however, political and budgetary difficulties intervened. The Grand Tour was an ambitious and expensive concept, designed in the enthusiasm of the Apollo years. In the altered national climate that followed the first manned lunar landings, the United States began to pull back from major commitments in space. The later Apollo landings were canceled, and in fiscal year 1972 only $10 million of the $30 million needed to complete Grand Tour designs was appropriated. It suddenly became necessary to restructure the exploration of the outer planets to conform to more modest space budgets.

The original plan for the Outer Planets Grand Tour envisaged dual launches to Jupiter, Saturn, and Pluto in the mid-1970s, and dual launches to Jupiter, Uranus, and Neptune in 1979. However, political and budgetary constraints altered the plan, and the Voyager mission to Jupiter and Saturn, with an optional encounter with Uranus, was formulated to replace it. Here the original Grand Tour trajectories from Earth to the outer planets are shown. [P-10612AC]

The new mission concept that replaced the Grand Tour dropped the objectives of exploring the outer three planets—Uranus, Neptune, and Pluto. In this way the lifetime of the mission was greatly shortened, placing less stringent demands on the reliability of the millions of components that go into a spacecraft. Limiting the mission to Jupiter and Saturn also relieved problems associated with spacecraft power, and with communicating effectively over distances of more than 2 billion kilometers. The total cost of the new mission was estimated at $250 million, only a third of that previously planned for the Grand Tour. Because it was based on the proven Mariner spacecraft design, the new mission was initially named Mariner Jupiter Saturn, or MJS; in 1977 the name was changed to Voyager. In January 1972 the President’s proposed fiscal year 1973 budget included $10 million specifically designated for Voyager; after authorization andappropriation by Congress, the official beginning of Voyager was set for July 1, 1972.

With approval of the new mission apparently assured in the Congress, NASA issued an “Announcement of Flight Opportunity” to select the scientific instruments to be carried on Voyager. Seventy-seven proposals were received; 31 from groups of scientists with designs for instruments, and 46 from individuals desiring to participate in NASA-formed teams. Of these 77 proposals, 24 were from NASA laboratories, 48 were from scientists in various U.S. universities and industry, and 5 were from foreign sources. After extensive review, 28 proposals were accepted: 9 for instruments and 19 for individual participation. The newly selected Principal Investigators and Team Leaders met for the first time at JPL just before Christmas, 1972. To coordinate all the science activity of the Voyager mission, NASA and JPL selected Edward Stone of Caltech, a distinguished expert on magnetospheric physics, to serve as Project Scientist.

The team assembled in 1972 by JPL and its industrial contractors included more than a thousand highly trained engineers, scientists, and technical managers who assumed responsibility for the awesome task of building the most sophisticated unmanned spacecraft ever designed and launching it across the farthest reaches of the solar system. At the head of the organization was the Project Manager, Harris (Bud) Schurmeier. Later, Schurmeier was succeeded by John Casani, Robert Parks, and Ray Heacock. This team had only four years to turn the paper concepts into hardware, ready to deliver to Kennedy Space Center for launch in the summer of 1977.

Project Scientist Edward C. Stone

Project Manager Harris (Bud) Schurmeier

Voyager is one of the most ambitious planetary space missions ever undertaken. Voyager 1, which encountered Jupiter on March 5, 1979, was to investigate Jupiter, its large satellites Io, Ganymede, and Callisto, and tiny Amalthea; Saturn, its rings, and several of its satellites—including Titan, the largest satellite in the solar system. Voyager 2, which arrived at Jupiter on July 9, 1979, was to examine Jupiter, Europa, Ganymede, Callisto, and Saturn and several of its satellites, after which it was to be hurled on toward an encounter with the Uranian system in 1986. Both spacecraft were also designed to study the interplanetary medium and its interactions with the solar wind.

Scientific objectives as well as the orbital positions of the planets and satellites influenced the choice of spacecraft trajectories, which were designed to provide close flybys of all four of Jupiter’s Galilean satellites and six of Saturn’s satellites—featuring a very close approach to Titan—and occultations of the Sun and Earth by Jupiter, Saturn, Titan, and the rings of Saturn. However, all these objectives could not be accomplished by a single spacecraft. No single path through the Jupiter system, for instance, can provide close flybys of all four Galilean satellites. Specifically, a trajectory for Voyager 1 that included a close encounter with Io precluded a close encounter with Europa and did not allow the spacecraft the option of being targeted from Saturn to Uranus without having to travel through the rings of Saturn. On the other hand, a trajectory that would send Voyager 2 on to Uranus precluded not only a close encounter with Io, but also a close encounter with Titan and with Saturn’s rings. The alignment of the planets and their satellites was such that encounter with Jupiter on or before April 4, 1979 was necessary for optimum investigation of Io, but encounter with Jupiter after June 15, 1979 was mandatory for a trajectory that would maintain the option to go on to Uranus. The latter trajectory also allowed Voyager 2 to maintain a healthier distance from Jupiter, avoiding the full dose of radiation that would be experienced by Voyager 1.

Although the two Voyager trajectories do have certain fundamental differences, they also have a great deal in common: Both spacecraftwere designed to study the interplanetary medium, and both would investigate Jupiter and Saturn and have close flybys with several of their major satellites. Also, if it were decided not to send Voyager 2 on to Uranus, this spacecraft could be retargeted to a Saturn trajectory similar to that of Voyager 1, providing a close flyby of Titan and a closer look at the rings. The flight paths of Voyager 1 and Voyager 2 complement each other—allowing the planets and some of the satellites to be viewed from a number of angles and over a longer period of time than would be possible with only one spaceprobe—yet the trajectories were also designed to be more or less capable of duplicating each other’s scientific investigations. This redundancy helps to ensure, so far as is possible, the success of the mission.

The redundancy built into the mission is evident, not only in the design of the trajectories, but in the spacecraft themselves. Voyagers 1 and 2 are identical spacecraft. In addition, many crucial elements are duplicated on each: For example, the computer command subsystem (CCS), the flight data subsystem (FDS), and the attitude and articulation control subsystem (AACS)—which function as onboard control systems—each have multiple reprogrammable digital computers. In addition, the CCS, which decodes commands from ground control and can instruct other subsystems from its own memory, contains duplicates for all its functional units. The AACS, which controls the spacecraft’s stabilization and orientation, has duplicate star trackers and Sun sensors. The communications system contains two radio receivers and four transmitters, two each to transmit both S-band (frequency of about 2295 megahertz) and X-band (frequency of about 8418 megahertz).

Project Manager John Casani

The Voyager spacecraft are more sophisticated, more automatic, and more independent than were the Pioneers. This independence is important because the giant planets are so far away that the correction of a malfunction by engineers on Earth would take hours to perform. Even at “nearby” Jupiter, radio signals take about forty minutes to travel in one direction between the spacecraft and Earth. Saturn is about twice as far away as Jupiter, and Uranus is twice as far as Saturn, slowing communication even more.

About the same size and weight of a subcompact car, the Voyager spacecraft carry instruments for eleven science investigations of the outer planets and their satellites. Power to operate the spacecraft is provided by three radioisotope thermoelectric generators mounted on one boom; other booms hold the science scan platform and the dual magnetometers.

Perched atop the Centaur stage of the launch rocket, each Voyager spacecraft has a mass of 2 tons (2066 kilograms), divided about equally between the spacecraft proper and the propulsion module used for final accelerationto Jupiter. The Voyager itself, with a mass of 815 kilograms and typical dimensions of about 3 meters, is almost the size and weight of a subcompact car—but enormously more complex. A better analogy might be made with a large electronic computer—but no terrestrial computer was ever asked to supply its own power source and to operate unattended in the vacuum of space for up to a decade.

Each Voyager spacecraft carries instruments for eleven science investigations covering visual, infrared, and ultraviolet regions of the spectrum, and other remote sensing studies of the planets and satellites; studies of radio emissions, magnetic fields, cosmic rays, and lower energy particles; plasma (ionized gases) waves and particles; and studies using the spacecraft radios. Each Voyager has a science boom that holds high and low resolution TV, photopolarimeter, plasma and cosmic ray detectors, infrared spectrometer and radiometer, ultraviolet spectrometer, and low energy charged particle detector; in addition, each spacecraft has a planetary radio astronomy and plasma wave antenna and a long boom carrying a low- and a high-field magnetometer.

Communication with Earth is carried out via a high-gain antenna 3.7 meters in diameter, with a smaller low-gain antenna as backup. The large white dish of the high-gain antenna dominates the appearance of the spacecraft, setting it off from its predecessors, which were able to use much smaller antennas to communicate over the more modest distances that separate the planets of the inner solar system. Although the transmitter power is only 23 watts—about the power of a refrigerator light bulb—this system is designed to transmit data over a billion kilometers at the enormous rate of 115 200 bits per second. Data can also be stored for later transmission to Earth; an onboard digital tape recorder has a capacity of about 500 million (5 × 10⁸) bits, sufficient to store nearly 100 Voyager images.

Project Manager Ray Heacock

During the early assembly stage, technicians at Caltech’s Jet Propulsion Laboratory equip Voyager’s extendable boom with low- and high-field magnetometers that measure the intensity and direction of the outer planets’ magnetic fields. [373-7179BC]

Radioisotope thermoelectric generator (RTGs), rather than solar cells, provide electricity for the Voyager spacecraft. The RTG use radioactive plutonium oxide for this purpose. As the plutonium oxide decays, it gives off heat which is converted to electricity, supplying a total of about 450 watts to the spacecraft at launch. This power slowly declines as the plutonium is used up, with less than 400 watts expected at Saturn flyby five years after launch. Hydrazine fuel is used to make mid-course corrections in trajectory and to control the spacecraft’s orientation.

Since the Voyagers must fly through the inner magnetosphere of Jupiter, it was imperative that the hardware systems be able to withstand the radiation from Jovian charged particles. The electronic microcircuits that form the heart and brain of the spacecraft and its scientific instruments are especially susceptible to radiation damage. Three techniques were used to “harden” components against radiation:

1. Special design using radiation-resistant materials;2. Extensive testing to select those electronic components which come out of the manufacturing process with highest reliability; and3. Spot shielding of especially sensitive areas with radiation-absorbing materials.

1. Special design using radiation-resistant materials;

2. Extensive testing to select those electronic components which come out of the manufacturing process with highest reliability; and

3. Spot shielding of especially sensitive areas with radiation-absorbing materials.

VOYAGER’S GREETINGS TO THE UNIVERSEThe Voyager spacecraft will be the third and fourth human artifacts to escape entirely from the solar system. Pioneers 10 and 11, which preceded Voyager in outstripping the gravitational attraction of the Sun, both carried small metal plaques identifying their time and place of origin for the benefit of any other spacefarers that might find them in the distant future. With this example before them, NASA placed a more ambitious message aboard Voyager 1 and 2—a kind of time capsule, intended to communicate a story of our world to extraterrestrials.The Voyager message is carried by a phonograph record—a 12-inch gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on Earth. The contents of the record were selected for NASA by a committee chaired by Carl Sagan of Cornell University. Dr. Sagan and his associates assembled 115 images and a variety of natural sounds, such as those made by surf, wind and thunder, birds, whales, and other animals. To this they added musical selections from different cultures and eras, and spoken greetings from Earthpeople in sixty languages, and printed messages from President Carter and U.N. Secretary General Waldheim.Each record is encased in a protective aluminum jacket, together with a cartridge and needle. Instructions, in symbolic language, explain the origin of the spacecraft and indicate how the record is to be played. The 115 images are encoded in analog form. The remainder of the record is in audio, designed to be played at 16⅔ revolutions per second. It contains the spoken greetings, beginning with Akkadian, which was spoken in Sumer about six thousand years ago, and ending with Wu, a modern Chinese dialect. Following the section on the sounds of Earth, there is an eclectic 90-minute selection of music, including both Eastern and Western classics and a variety of ethnic music.Once the Voyager spacecraft leave the solar system (by 1990, both will be beyond the orbit of Pluto), they will find themselves in empty space. It will be forty thousand years before they come within a light year of a star, called AC + 79 3888, and millions of years before either might make a close approach to any other planetary system. As Carl Sagan has noted, “The spacecraft will be encountered and the record played only if there are advanced spacefaring civilizations in interstellar space. But the launching of this bottle into the cosmic ocean says something very hopeful about life on this planet.”LANGUAGES ON VOYAGER RECORDSumerianAkkadianHittiteHebrewAramaicEnglishPortugueseCantoneseRussianThaiArabicRoumanianFrenchBurmeseSpanishIndonesianKechuaDutchGermanBengaliUrduHindiVietnameseSinhaleseGreekLatinJapanesePunjabiTurkishWelshItalianNguniSothoWuKoreanArmenianPolishNetaliMandarinGujoratiIla (Zambia)NyanjaSwedishUkrainianPersianSerbianLuganadaAmoy (Min dialect)MarathiKannadaTeluguOriyaHungarianCzechRajasthaniSOUNDS OF EARTH ON VOYAGER RECORDWhalesPlanets (music)VolcanoesMud potsRainSurfCrickets, frogsBirdsHyenaElephantChimpanzeeWild dogFootsteps and heartbeatsLaughterFireToolsDogs, domesticHerding sheepBlacksmith shopSawingRiveterMorse codeShipsHorse and cartHorse and carriageTrain whistleTractorTruckAuto gearsJetLift-off Saturn 5 rocketKissBabyLife signs—EEG, EKGPulsartVOYAGER RECORD PHOTOGRAPH INDEXCalibration circleSolar location mapMathematical definitionsPhysical unit definitionsSolar system parametersThe SunSolar spectrumMercuryMarsJupiterEarthEgypt, Red Sea, Sinai Peninsula and the NileChemical definitionsDNA structureDNA structure magnifiedCells and cell divisionAnatomy (eight)Human sex organsDiagram of conceptionConceptionFertilized ovumFetus diagramFetusDiagram of male and femaleBirthNursing motherFather and daughter (Malaysia)Group of childrenDiagram of family agesFamily portraitDiagram of continental driftStructure of EarthHeron Island (Great Barrier Reef of Australia)SeashoreSnake River and Grand TetonsSand dunesMonument ValleyForest scene with mushroomsLeafFallen leavesSequoiaSnowflakeTree with daffodilsFlying insect with flowersDiagram of vertebrate evolutionSeashell (Xancidae)DolphinsSchool of fishTree toadCrocodileEagleWaterholdJane Goodall and chimpsSketch of BushmenBushmen huntersMan from GuatemalaDancer from BaliAndean girlsThailand craftsmanElephantOld man with beard and glasses (Turkey)Old man with dog and flowersMountain climberCathy RigbySprintersSchoolroomChildren with globeCotton harvestGrape pickerSupermarketUnderwater scene with diver and fishFishing boat with netsCooking fishChinese dinner partyDemonstration of licking, eating and drinkingGreat Wall of ChinaHouse construction (African)Construction scene (Amish country)House (Africa)House (New England)Modern house (Cloudcroft, New Mexico)House interior with artist and fireTaj MahalEnglish city (Oxford)BostonUN Building, dayUN Building, nightSydney Opera HouseArtisan with drillFactory interiorMuseumX-ray of handWoman with microscopeStreet scene, Asia (Pakistan)Rush hour traffic, IndiaModern highway (Ithaca)Golden Gate BridgeTrainAirplane in flightAirport (Toronto)Antarctic expeditionRadio telescope (Westerbork, Netherlands)Radio telescope (Arecibo)Page of book (Newton,System of the World)Astronaut in spaceTitan Centaur launchSunset with birdsString quartet (Quartetto Italiano)Violin with music score (Cavatina)MUSIC ON VOYAGER RECORDBach Brandenberg Concerto Number Two, First Movement“Kinds of Flowers” Javanese Court GamelanSenegalese PercussionPygmy girls initiation songAustralian Horn and Totem song“El Cascabel” Lorenzo Barcelata“Johnny B. Goode” Chuck BerryNew Guinea Men’s House“Depicting the Cranes in Their Nest”Bach Partita Number Three for Violin; Gavotte et RondeausMozart Magic Flute, Queen of the Night (Aria Number 14)ChakruloPeruvian Pan PipesMelancholy BluesAzerbaijan Two FlutesStravinsky, Rite of Spring, ConclusionBach Prelude and Fugue Number One in C Major from the Well Tempered Clavier, Book TwoBeethoven’s Fifth Symphony, First MovementBulgarian Shepherdess Song “Izlel Delyo Hajdutin”Navajo Indian Night ChantThe Fairie Round from Pavans, Galliards, AlmainsMelanesian Pan PipesPeruvian Woman’s Wedding Song“Flowing Streams”—Chinese Ch’in music“Jaat Kahan Ho”—Indian Raga“Dark Was the Night”Beethoven String Quartet Number 13 “Cavatina”

VOYAGER’S GREETINGS TO THE UNIVERSE

The Voyager spacecraft will be the third and fourth human artifacts to escape entirely from the solar system. Pioneers 10 and 11, which preceded Voyager in outstripping the gravitational attraction of the Sun, both carried small metal plaques identifying their time and place of origin for the benefit of any other spacefarers that might find them in the distant future. With this example before them, NASA placed a more ambitious message aboard Voyager 1 and 2—a kind of time capsule, intended to communicate a story of our world to extraterrestrials.

The Voyager message is carried by a phonograph record—a 12-inch gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on Earth. The contents of the record were selected for NASA by a committee chaired by Carl Sagan of Cornell University. Dr. Sagan and his associates assembled 115 images and a variety of natural sounds, such as those made by surf, wind and thunder, birds, whales, and other animals. To this they added musical selections from different cultures and eras, and spoken greetings from Earthpeople in sixty languages, and printed messages from President Carter and U.N. Secretary General Waldheim.

Each record is encased in a protective aluminum jacket, together with a cartridge and needle. Instructions, in symbolic language, explain the origin of the spacecraft and indicate how the record is to be played. The 115 images are encoded in analog form. The remainder of the record is in audio, designed to be played at 16⅔ revolutions per second. It contains the spoken greetings, beginning with Akkadian, which was spoken in Sumer about six thousand years ago, and ending with Wu, a modern Chinese dialect. Following the section on the sounds of Earth, there is an eclectic 90-minute selection of music, including both Eastern and Western classics and a variety of ethnic music.

Once the Voyager spacecraft leave the solar system (by 1990, both will be beyond the orbit of Pluto), they will find themselves in empty space. It will be forty thousand years before they come within a light year of a star, called AC + 79 3888, and millions of years before either might make a close approach to any other planetary system. As Carl Sagan has noted, “The spacecraft will be encountered and the record played only if there are advanced spacefaring civilizations in interstellar space. But the launching of this bottle into the cosmic ocean says something very hopeful about life on this planet.”

LANGUAGES ON VOYAGER RECORD

SOUNDS OF EARTH ON VOYAGER RECORD

VOYAGER RECORD PHOTOGRAPH INDEX

MUSIC ON VOYAGER RECORD

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