by Carl Sagan
There are eleven separate scientific investigations on board each Voyager spacecraft. Each investigation corresponds to a specific scientific instrument designed for the purpose, and each instrument has associated with it a team of scientists and engineers who, in most cases, have been working on the investigation for nearly a decade. Their names are listed in Appendix E. The degree of dedication as well as skill required for such missions is very great.
The trajectories that have been chosen through the Jupiter and Saturn systems are the result of complex and often painful compromises between competing scientific objectives. Voyager is at closest approach to Jupiter and booming through its system of inner satellites for only a few hours. In that period of time only so many scientific measurements can be made. Shall we orient our mission to fly through the Io flux tube and examine the charged particles and magnetospheric interactions and radio bursts; or shall we concentrate on flying behind some of the satellites to use radio occultation techniques to search for atmospheres; or shall we concentrate on imaging and spectroscopy of the moons; or on a study of Jupiter itself? The optimum trajectories through the Jupiter system for some of the scientific objectives will not permit us to do all we want to do in the Saturn system, such as fly behind the rings or take a close look at Titan. The best trajectory through the Saturnian system will not permit us to get to Uranus at all.
The accompanying six figures show typical trajectories through the Jovian and Saturnian systems for Voyagers 1 and 2. From these diagrams we see that close encounters will be made with a large number of satellites in both systems, with the rings of Saturn, and, of course, with the planets themselves. The table on this page and this page gives some basic data on the Jupiter and Saturn systems. The lovely and exotic names of the moons are all taken from Greek mythology. They may have been unpronounceable in the past, but some of them are shortly to be household words. Before Voyager, the best images of the moons of Jupiter were obtained by Pioneers 10 and 11, which showed disks of a few of the Galilean satellites with barely discernible smudges. But we see from the table that Voyager will—assuming there are no engineering failures—obtain photographs of the Galilean satellites with resolution (the ability to make out fine details) of a few kilometers and coverage of the satellite surfaces of several tens of percent. We will move from being vaguely aware that the satellites have surface features to being able to photograph objects the size of a small city on the planet Earth. Mariner 10 was an entire mission that obtained the first close-up photographs of the planet Mercury—at a surface resolution of a few kilometers and a coverage of some tens of percent. Voyager will obtain comparable data for two or three planets and eight or ten moons. There is little doubt that the results will be spectacular.
From about 100 days before encounter, the Voyager imaging systems will obtain photographs of Jupiter superior to the best such photographs obtained through the largest telescopes on Earth. In the following weeks, the planetary image will steadily grow, many color photographs will be taken, and motion pictures of the weather on Jupiter, rotating under our cameras, will be put together. At the closest approach to Jupiter, cloud features or other objects in the atmosphere as small as a hundred meters (three hundred feet) across will be made out. A few hours from Jupiter, a mosaic of photographs will be taken filling the Great Red Spot, thought to be an immense cyclonic weather disturbance initiated perhaps a million years ago in the earlier history of Jupiter. The half-dozen or so photographs that will on this occasion fill the Great Red Spot will each be composed of about a million dots like those in a newspaper wirephoto. When Voyager arrives in the Saturn system, we will be able to place a dozen such photographs, one after another, across the rings while looking down upon the ring plane. The rings of Saturn have engaged, fascinated and tantalized even amateur astronomers with small telescopes since the time of Galileo. The Voyager photographs of the Saturnian rings should deliver a new dimension not only of scientific but also of aesthetic imagery.
While much of the public attention on the Voyager mission may be directed to the imaging science investigations, the other experiments are of very great interest and importance. Since Jupiter, Uranus and possibly Saturn radiate more energy to space than they receive from the Sun, the study of the planetary heat budgets by infrared instruments will be interesting. But these same instruments are also capable of determining something of the chemistry, perhaps even the organic chemistry, of the atmospheres of the jovian planets and Titan; a little about the mineral and ice composition of the surfaces of the moons and the rings of Saturn. They will also study the vertical structure and weather in those objects with atmospheres. A complementary investigation will be made by an ultraviolet spectrometer to study the composition and structure of the atmospheres of Jupiter, Saturn, Uranus, Titan and the Galilean satellites (where some evidence exists for extremely diffuse atmospheres), as well as a study of the doughnut-shaped clouds of sputtered atoms in the orbits of Io and perhaps other Galilean satellites. Another instrument, called a photopolarimeter, will measure the polarization of the sunlight reflected from planets and satellites as the viewing angle from the spacecraft changes. This will permit studies of the physical and chemical properties of atmospheric aerosols and the surfaces of the satellites and the rings of Saturn.
Schematic diagram of the trajectories through the outer solar system of the Voyager 1 and Voyager 2 spacecraft. The dates given are for the positions of the planets shown in their orbits. Voyager 1 is described as “Jupiter-Saturn-Titan” and Voyager 2 as “Jupiter-Saturn-Uranus.”
Each Voyager spacecraft is powered by a radioisotope thermoelectric generator, and communicates all its scientific and engineering information to Earth via a large parabolic radio antenna, broadcasting at two different frequencies. But the Voyager trajectory will take it through clouds of interplanetary gas, through the charged particles in the magnetosphere of Jupiter, behind the rings of Saturn, and behind (as viewed from Earth) the atmosphere and clouds of Jupiter and Titan. Each time material passes between the spacecraft transmitter and the receiving stations on Earth, the signal fades in a characteristic way and important information can be obtained about the interposed object. For example, we expect to learn, for the first time, from the Titan occultation experiment what the pressure and temperature is at the surface of Titan, rather than up near the cloud tops to which our present instruments are restricted. In addition, the radio science investigations will determine the precise trajectory of the Voyager spacecraft as they pass close to the various planets, satellites and ring systems, and this will permit us to derive important information about the masses of these objects and—in the case of Jupiter and Saturn—about the structure of their deep interiors, data vital for understanding their origins. In addition to using the existing radio antenna for sending radio waves past planets, there are special radio-wave detectors on Voyager to study the bursts and other radio emission from Jupiter, and possibly from Saturn and Uranus, with particular attention to the influence of the satellites of Jupiter on the jovian radio bursts.
Finally, there are four investigations of the charged particles and magnetic fields in interplanetary space and around the jovian planets and their satellites. Two devices for measuring very low strength magnetic fields are attached to an extremely long girdered boom, projecting from the spacecraft so as not to be confused by the magnetic fields established by electrical circuitry in the other instruments. Among the many investigations that these instruments will carry out will be a search for the heliopause, although it is possible that the Voyager transmitters will have died long before the spacecraft traverses this magnetic boundary between the solar system and interstellar space.
The main objective of the Voyager mission is clearly this extremely rich harvest of scientific information. Voyager represents the first indepth reconnaissance of the outer solar system, and I believe it will change our view of the planetary family of the Sun forever—as well as have a profound influence on our aesthetic sense of our s
urroundings in space.
But there is something else aboard the two Voyager spacecraft. Long after its transmitters have died, far beyond the heliopause, in the remote future two phonograph records containing greetings from the planet Earth will be inexorably speeding on.
References
* * *
Gehrels, T., ed., Jupiter. University of Arizona Press, 1976.
Sagan, Carl, in The Solar System, A Scientific American Book. W. H. Freeman and Company, 1975.
—–, and Salpeter, E. E., “Particles, Environments and Hypothetical Ecologies in the Jovian Atmosphere.” Astrophysical Journal Supplement, vol. 32 (1976), 737-755.
Smith, B., et al., “Voyager Imaging Experiment.” Space Science Reviews, vol. 21 (1977), 103-128.
A dense field of stars in the constellation of the Little Bear. One of these may be AC+79 3888. Photograph courtesy of National Geographic Society—Palomar Observatory Sky Survey. Copyright, California Institute of Technology.
It was impossible to view the last tongues of flame from Voyager’s Titan booster as it departed from Cape Canaveral without contemplating the fate of the record. The record is affixed to the exterior of the spacecraft. While cosmic rays and radiation from the Sun and stars could cause some damage, the main threat to it is micrometeorites, tiny microscopic particles of fluff, probably the debris of comets, that fill interplanetary space. These microplanets are in orbit about the Sun and have their own velocities, but as the spacecraft ventures farther into the outer reaches of the solar system, those velocities become less and less. The spacecraft’s own speed of about fifteen kilometers a second as it plows through this horde of micrometeorites poses the chief hazard. The most conservative estimate of damage is based on the assumption that the spacecraft will be traveling record first. If the record were not encased in its aluminum cover, all particles that could produce tiny pits or craters larger than about half a record groove could cause damage to the sound quality. In this case, all micrometeorites heavier than about a hundredth of a microgram (equivalently, larger than about 0.007 centimeters in diameter) could cause such damage.
There are probably many more micrometeorites in the inner solar system, where comets are vaporized by the Sun’s heat and disintegrate, than in the outer solar system, where they are still in deep freeze. Again, a conservative calculation of damage might assume that micrometeorites are as abundant far beyond the orbit of Pluto as they are in the vicinity of the Earth. If this is the case, tiny pits destroying about 10 percent of the record will be accumulated by the time the spacecraft has traveled one light-year, about a quarter of the distance to the nearest star. This calculation applies only to the face of the record oriented outwards.
Ten percent damage is clearly too much even for an extraterrestrial civilization easily able to do some reasonable interpolation on the missing bits of information. It is for just this reason that the Voyager records are encased in an aluminum cover 0.08 centimeters (0.03 inches) thick. Only micrometeorites more massive than about five micrograms can penetrate the cover, and there are far fewer big micrometeorites than little ones. Employing the same conservative assumptions as before, we calculate that less than 2 percent of the record should be micropitted by the time the spacecraft reaches a distance of one light-year. This corresponds to about 4,000 tiny impacts before it leaves the cloud of cometary debris. Thereafter, in interstellar space, the abundance of micrometeorites should be much less, and the outward face of the record will degrade at the very slow rate of about 0.02 percent of its area for every fifty light-years traveled. An additional 2 percent of damage will not occur until the spacecraft has traveled an additional five thousand light-years, which is one-sixth of the distance between the Sun and the center of the Galaxy. It will take the Voyager spacecraft about a hundred million years to traverse such a distance. If Voyager were by chance to enter the planetary system of some other star, similarly endowed with comets and micrometeorites, then the record might acquire as much additional damage on the way into such a planetary system as it acquired here on the way out. But the chance of such an accidental entry is very small.
In all of these calculations—which are mainly due to Paul Penzo of the Jet Propulsion Laboratory—the damage applies only to the outward-facing side of the record. The inward-facing side, protected by the record itself and by the spacecraft, suffers essentially no damage at all. A rough estimate of a billion years for the average lifetime of the record therefore seems reasonably safe. The records were mounted with Side 1 inwards. Therefore all of the pictorial information, human and cetacean greetings, and “The Sounds of Earth” (as well as the first third of the music—from the First Movement of the Second Brandenburg Concerto to the Partita No. 3 for Violin) will survive essentially forever.
And toward where are the Voyager spacecraft destined? Are they likely under any circumstances to encounter another planetary system? The directions in the sky toward which the spacecraft eventually will be headed depend very much on the precision of maneuvers near Jupiter, Saturn, and Uranus during the strictly scientific phase of the mission. Voyager 1 is tentatively planned to arrive at Saturn on November 13, 1980, and to leave the solar system toward a point in the sky with a decclination of 10.1 degrees and a right ascension of 260.0 degrees. It is in the constellation Ophiuchus. Voyager 2, if all goes well, will arrive at Uranus on January 30, 1986, and leave the solar system with a declination of — 14.9 degrees and a right ascension of 315.3 degrees, in the direction of the zodiacal constellation Capricornus. This Voyager 2 direction assumes that the spacecraft will not, as it is currently not planned to, encounter Neptune on its way out.
Stars have their own, so-called proper, motions. The Voyager spacecraft are moving so slowly that in many tens of thousands of years the stars in the solar neighborhood will have reassorted themselves into quite different relative positions than those they now occupy. It is a difficult computer task to calculate what stars might by chance be along the Voyager spacecraft trajectories 50,000 or 100,000 years from now. Mike Helton of the Jet Propulsion Laboratory has attempted to make such a calculation. He calls attention in particular to an obscure star called AC+79 3888, which is now in the constellation of Ursa Minor—the Little Bear, or Little Dipper. It is now seventeen light-years from the Sun. But in 40,000 years it will by chance be within three light-years of the Sun, closer than Alpha Centauri is to us now. Within that period Voyager 1 will come within 1.7 light-years of AC+79 3888 and Voyager 2 within 1.1 light-years. Two other candidate stars are DM+21 652 in the constellation Taurus and AC-24 2833 183 in the constellation Sagittarius. However, neither Voyager 1 nor Voyager 2 will come as close to these stars as to AC+79 3888.
Astronomers classify this star as a red dwarf of spectral type M4. It is substantially smaller and cooler than the Sun. It may also be much older. The nearest M dwarf star which is not a member of a double or multiple star system is called Barnard’s Star. It is about six light-years away. Our ability to detect planetary systems around other stars is at present extremely limited, although it is rapidly improving. Some preliminary evidence suggests that there are one or more planets of about the mass of Jupiter and Saturn orbiting Barnard’s star, and general theoretical considerations suggest that planets ought to be a frequent complement of most such stars.
If future studies of AC+79 3888 demonstrate that it indeed has a planetary system, then we might wish to do something to beat the odds set by the haunting and dreadful emptiness of space—the near certainty that, left to themselves, neither Voyager spacecraft would ever plummet into the planet-rich interior of another solar system. For it might be possible—after the Voyager scientific missions are completed—to make one final firing of the onboard rocket propulsion system and redirect the spacecraft as closely as we possibly can so that they will make a true encounter with AC+79 3888. If such a maneuver can be effected, then some 60,000 years from now one or two tiny hurtling messengers from the strange and distant planet Earth may penetrate into the planetary sys
tem of AC + 79 3888. Since this star is probably much older than the Sun, it may be that intelligent life evolved there long ago. But the evolution of intelligence does not proceed at a uniform pace. Perhaps in 60,000 years intelligence and technical civilizations will have only recently emerged on a planet of this system. The inhabitants will of course be deeply interested in the Sun, their nearest star, and in its retinue of planets. What an astonishing finding the Voyager record, this gift from the skies, would then represent!
They would wonder about us. They would know that 60,000 years is a long period of time in the history of civilizations. They would recognize the tentativeness of our society, its tenuous acquaintance with technology and wisdom together. Had we destroyed ourselves or had we gone on to greater things? Some of the Voyager music intentionally expresses a kind of cosmic loneliness, which would perhaps communicate itself across the expanse of light-years and the differences in evolutionary histories. We, too, were time-capsuling, searching the skies and seeking another civilization with which to communicate.
