THE GOLDEN AGE
OF PLANETARY
EXPLORATION

MUCH OF HUMAN HISTORY can, I think, be described as a gradual and sometimes painful liberation from provincialism, the emerging awareness that there is more to the world than was generally believed by our ancestors. With awesome ethnocentrism, tribes all over the Earth called themselves “the people” or “all men,” relegating other groups of humans with comparable accomplishments to subhuman status. The high civilization of ancient Greece divided the human community into Hellenes and barbarians, the latter named after an uncharitable imitation of the languages of non-Greeks (“Bar Bar …”). That same classical civilization, which in so many respects is the antecedent of our own, called its small inland sea the Mediterranean—which means the middle of the Earth. For thousands of years China called itself the Middle Kingdom, and the meaning was the same: China was at the center of the universe and the barbarians lived in outer darkness.

Such views or their equivalent are only slowly changing, and it is possible to see some of the roots of racism and nationalism in their pervasive early acceptance by virtually all human communities. But we live in an extraordinary time, when technological advances and cultural relativism have made such ethnocentrism much more difficult to sustain. The view is emerging that we all share a common life raft in a cosmic ocean, that the Earth is, after all, a small place with limited resources, that our technology has now attained such powers that we are able to affect profoundly the environment of our tiny planet. This deprovincialization of mankind has been aided powerfully, I believe, by space exploration—by exquisite photographs of the Earth taken from a great distance, showing a cloudy, blue, spinning ball set like a sapphire in the endless velvet of space; but also by the exploration of other worlds, which have revealed both their similarities and their differences to this home of mankind.

We still talk of “the” world, as if there were no others, just as we talk about “the” Sun and “the” Moon. But there are many others. Every star in the sky is a sun. The rings of Uranus represent millions of previously unsuspected satellites orbiting Uranus, the seventh planet. And, as space vehicles have demonstrated so dramatically in the last decade and a half, there are other worlds—nearby, relatively accessible, profoundly interesting, and not a one closely similar to ours. As these planetary differences, and the Darwinian insight that life elsewhere is likely to be fundamentally different from life here, become more generally perceived, I believe they will provide a cohesive and unifying influence on the human family, which inhabits, for a time, this unprepossessing world among an immensity of others.

Planetary exploration has many virtues. It permits us to refine insights derived from such Earth-bound sciences as meteorology, climatology, geology and biology, to broaden their powers and improve their practical applications here on Earth. It provides cautionary tales on the alternative fates of worlds. It is an aperture to future high technologies important for life here on Earth. It provides an outlet for the traditional human zest for exploration and discovery, our passion to find out, which has been to a very large degree responsible for our success as a species. And it permits us, for the first time in history, to approach with rigor, with a significant chance of finding out the true answers, questions on the origins and destinies of worlds, the beginnings and ends of life, and the possibility of other beings who live in the skies—questions as basic to the human enterprise as thinking is, as natural as breathing.

Interplanetary unmanned spacecraft of the modern generation extend the human presence to bizarre and exotic landscapes far stranger than any in myth or legend. Propelled to escape velocity near the Earth, they adjust their trajectories with small rocket motors and tiny puffs of gas. They power themselves with sunlight and with nuclear energy. Some take only a few days to traverse the lake of space between Earth and Moon; others may take a year to Mars, four years to Saturn, or a decade to traverse the inland sea between us and distant Uranus. They float serenely on pathways predetermined by Newtonian gravitation and rocket technology, their bright metal gleaming, awash in the sunlight which fills the spaces between the worlds. When they arrive at their destinations, some will fly by, garnering a brief glimpse of an alien planet, perhaps with a retinue of moons, before continuing on farther into the depths of space. Others insert themselves into orbit about another world to examine it at close range, perhaps for years, before some essential component runs down or wears out. Some spacecraft will make landfall on another world, decelerating by atmospheric friction or parachute drag or the precision firing of retrorockets before gently setting down somewhere else. Some landers are stationary, condemned to examine a single spot on a world awaiting exploration. Others are self-propelled, slowly wandering to a distant horizon which holds no man knows what. And still others are capable of remotely acquiring rock and soil—a sample of another world—and returning it to the Earth.

All these spacecraft have sensors that extend astonishingly the range of human perception. There are devices that can determine the distribution of radioactivity over another planet from orbit; that can feel from the surface the faint rumble of a distant planetquake deep below; that can obtain three-dimensional color or infrared images of a landscape like none ever seen on Earth. These machines are, at least to a limited degree, intelligent. They can make choices on the basis of information they themselves receive. They can remember with great accuracy a detailed set of instructions which, if written out in English, would fill a good-sized book. They are obedient and can be reinstructed by radio messages sent to them from human controllers on Earth. And they have returned, mostly by radio, a rich and varied harvest of information on the nature of the solar system we inhabit. There have been fly-bys, crash-landers, soft-landers, orbiters, automated roving vehicles, and unmanned returned sample missions from our nearest celestial neighbor, the Moon—as well as, of course, six successful and heroic manned expeditions in the Apollo series. There has been a fly-by of Mercury; orbiters, entry probes and landers on Venus; fly-bys, orbiters and landers to Mars; and fly-bys of Jupiter and Saturn. Phobos and Deimos, the two small moons of Mars, have been examined close up, and tantalizing images have been obtained of a few of the moons of Jupiter.

We have caught our first glimpses of the ammonia clouds and great storm systems of Jupiter; the cold, salt-covered surface of its moon, Io; the desolate, crater-pocked, ancient and broiling Mercurian wasteland; and the wild and eerie landscape of our nearest planetary neighbor, Venus, where the clouds are composed of an acid rain that falls continuously but never patters the surface because that hilly landscape, illuminated by sunlight diffusing through the perpetual cloud layer, is everywhere at 900°F. And Mars: What a puzzle, what a joy, enigma and delight is Mars, with ancient river bottoms; immense, sculpted polar terraces; a volcano almost 80,000 feet high; raging windstorms; balmy afternoons; and an apparent initial defeat of our first pioneering effort to answer the question of questions—whether the planet harbors, now or ever, a home-grown form of life.

There are on Earth only two spacefaring nations, only two powers so far able to send machines much beyond the Earth’s atmosphere—the United States and the Soviet Union. The United States has accomplished the only manned missions to another body, the only successful Mars landers and the only expeditions to Mercury, Jupiter and Saturn. The Soviet Union has pioneered the automated exploration of the Moon, including the only unmanned rovers and return sample missions on any celestial objects, and the first entry probes and landers on Venus. Since the end of the Apollo program, Venus and the Moon have become, to a certain degree, Russian turf, and the rest of the solar system visited only by American space vehicles. While there is a certain degree of scientific cooperation between the two spacefaring nations, this planetary territoriality has come about by default rather than by agreement. There have in recent years been a set of very ambitious but unsuccessful Soviet missions to Mars, and the United States launched a modest but successful set of Venus orbiters and entry probes in 1978. The solar system is very large and there is much to explore. Even tiny Mars has a surface area comparable to the land area of the Earth. For practical reasons it is much easier to organize separate but coordinated missions launched by two or more nations than cooperative multinational ventures. In the sixteenth and seventeenth centuries, England, France, Spain, Portugal and Holland each organized on a grand scale missions of global exploration and discovery in vigorous competition. But the economic and religious motives of exploratory competition then do not seem to have their counterparts today. And there is every reason to think that national competition in the exploration of the planets will, at least for the foreseeable future, be peaceful.

THE LEAD TIMES for planetary missions are very long. The design, fabrication, testing, integration and launch of a typical planetary mission takes many years. A systematic program of planetary exploration requires a continuing commitment. The most celebrated American achievements on the Moon and planets—Apollo, Pioneer, Mariner and Viking—were initiated in the 1960s. At least until recently, the United States has made only one major commitment to planetary exploration in the whole of the decade of the 1970s—the Voyager missions, launched in the summer of 1977, to make the first systematic fly-by examination of Jupiter, Saturn, their twenty-five or so moons and the spectacular rings of the latter.

This absence of new starts has produced a real crisis in the community of American scientists and engineers responsible for the succession of engineering successes and high scientific discovery that began in 1962 with the Mariner 2 fly-by of Venus. There has been an interruption in the pace of exploration. Workers have been laid off and drifted to quite different jobs, and there is a real problem in providing continuity to the next generation of planetary exploration. For example, the earliest likely response to the spectacularly successful and historic Viking exploration of Mars will be a mission that does not even arrive at the Red Planet before 1985—a gap in Martian exploration of almost a decade. And there is not the slightest guarantee that there will be a mission even then. This trend—a little like dismissing most of the shipwrights, sail weavers and navigators of Spain in the early sixteenth century—shows some slight signs of reversal. Recently approved was Project Galileo, a middle-1980s mission to perform the first orbital reconnaissance of Jupiter and to drop the first probe into its atmosphere—which may contain organic molecules synthesized in a manner analogous to the chemical events which on Earth led to the origin of life. But the following year Congress so reduced the funds available for Galileo that it is, at the present writing, teetering on the brink of disaster.

In recent years the entire NASA budget has been well below one percent of the federal budget. The funds spent on planetary exploration have been less than 15 percent of that. Requests by the planetary science community for new missions have been repeatedly rejected—as one senator explained to me, the public has not, despite Star Wars and Star Trek, written to Congress in support of planetary missions, and scientists do not constitute a powerful lobby. And yet, there are a set of missions on the horizon that combine extraordinary scientific opportunity with remarkable popular appeal:

Solar Sailing and Comet Rendezvous. In ordinary interplanetary missions, spacecraft are obliged to follow trajectories that require a minimum expenditure of energy. The rockets burn for short periods of time in the vicinity of Earth, and the spacecraft mainly coast for the rest of the journey. We have done as well as we have not because of enormous booster capability, but because of great skill with severely constrained systems. As a result, we must accept small payloads, long mission times and little choice of departure or arrival dates. But just as on Earth we are considering moving from fossil fuels to solar power, so it is in space. Sunlight exerts a small but palpable force called radiation pressure. A sail-like structure with a very large area for its mass can use radiation pressure for propulsion. By positioning the sail properly, we can be carried by sunlight both inwards toward and outwards away from the Sun. With a square sail about half a mile on each side, but thinner than the thinnest Mylar, interplanetary missions can be accomplished more efficiently than with conventional rocket propulsion. The sail would be launched into Earth orbit by the manned Shuttle craft, unfurled and strutted. It would be an extraordinary sight, easily visible to the naked eye as a bright point of light. With a pair of binoculars, detail on such a sail could be made out—perhaps even what on seventeenth-century sailing ships was called the “device,” some appropriate graphic symbol, perhaps a representation of the planet Earth. Attached to the sail would be a scientific spacecraft designed for a particular application.

One of the first and most exciting applications being discussed is a comet-rendezvous mission, perhaps a rendezvous with Halley’s comet in 1986. Comets spend most of their time in interstellar space and should provide major clues on the early history of the solar system and the nature of the matter between the stars. Solar sailing to Halley’s comet might not only provide close-up pictures of the interior of a comet—about which we now know close to nothing—but also, astonishingly, return a piece of a comet to the planet Earth. The practical advantages and the romance of solar sailing are both evident in this example, and it is clear that it represents not just a new mission but a new interplanetary technology. Because the development of solar-sailing technology is behind that of ion propulsion, it is the latter that may propel us on our first missions to the comets. Both propulsion mechanisms have their place in future interplanetary travel. But in the long term I believe solar sailing will make the greater impact. Perhaps by the early twenty-first century there will be interplanetary regattas competing for the fastest time from Earth to Mars.

Mars Rovers. Before the Viking mission, no terrestrial spacecraft had successfully landed on Mars. There had been several Soviet failures, including at least one which was quite mysterious and possibly attributable to the hazardous nature of the Martian landscape. Thus, both Viking 1 and Viking 2 were, after painstaking efforts, successfully landed in two of the dullest places we could find on the Martian surface. The lander stereo cameras showed distant valleys and other inaccessible vistas. The orbital cameras showed an extraordinarily varied and geologically exuberant landscape which we could not examine close up with the stationary Viking lander. Further Martian exploration, both geological and biological, cries out for roving vehicles capable of landing in the safe but dull places and wandering hundreds or thousands of kilometers to the exciting places. Such a rover would be able to wander to its own horizon every day and produce a continuous stream of photographs of new landscapes, new phenomena and very likely major surprises on Mars. Its importance would be improved still further if it operated in tandem with a Mars polar orbiter which would geochemically map the planet, or with an unmanned Martian aircraft which would photograph the surface from very low altitudes.

Titan Lander. Titan is the largest moon of Saturn and the largest satellite in the solar system (see Chapter 13). It is remarkable for having an atmosphere denser than that of Mars and is probably covered with a layer of brownish clouds composed of organic molecules. Unlike Jupiter and Saturn, it has a surface on which we can land, and its deep atmosphere is not so hot as to destroy the organic molecules. A Titan entry-probe and lander mission would probably be part of a Saturn orbital mission, which might also include a Saturn entry probe.

Venus Orbital Imaging Radar. The Soviet Venera 9 and 10 missions have returned the first close-up photographs of the surface of Venus. Because of the permanent cloud pall, the surface features of Venus are not visible through Earth-bound optical telescopes. However, Earth-based radar and the radar system aboard the small Pioneer Venus orbiter have now begun to map Venus surface features, and have revealed mountains and craters and volcanoes as well as stranger morphology. A proposed Venus orbital imaging radar would provide pole-to-pole radar pictures of Venus with much higher detail than can be achieved from the surface of the Earth, and would permit a preliminary reconnaissance of the Venus surface comparable to that achieved for Mars in 1971-72 by Mariner 9.

Solar Probe. The Sun is the nearest star, the only one we are likely to be able to examine close up, at least for many decades. A near approach to the Sun would be of great interest, would help in understanding its influence on Earth, and would also provide vital additional tests of such theories of gravitation as Einstein’s General Theory of Relativity. A solar probe mission is difficult for two reasons: the energy required to undo the Earth’s (and the probe’s) motion around the Sun so it can fall into the Sun, and the intolerable heating as the probe approaches the Sun. The first problem can be solved by launching the spacecraft out to Jupiter and then using Jupiter’s gravitation to fling it into the Sun. Since there are many asteroids interior to Jupiter’s orbit, this might possibly be a useful mission for studying asteroids as well. An approach to the second problem, at first sight remarkable for its naïveté, is to fly into the Sun at night. On Earth, nighttime is of course merely the interposition of the solid body of the Earth between us and the Sun. Likewise for a solar probe. There are some asteroids that come rather close to the Sun. A solar probe would approach the Sun in the shadow of a Sun-grazing asteroid (meanwhile making observations of the asteroid as well). Near the point of closest approach of the asteroid to the Sun, the probe would emerge from the asteroidal shadow and plunge, filled with a fluid that resists heating, as deeply into the atmosphere of the Sun as it could until it melted and vaporized—atoms from the Earth added to the nearest star.

Manned Missions. As a rule of thumb, a manned mission costs from fifty to a hundred times more than a comparable unmanned mission. Thus, for scientific exploration alone, unmanned missions, employing machine intelligence, are preferred. However, there may well be reasons other than scientific for exploring space—social, economic, political, cultural or historical. The manned missions most frequently talked about are space stations orbiting the Earth (and perhaps devoted to harvesting sunlight and transmitting it in microwave beams down to an energy-starved Earth), and a permanent lunar base. Also being discussed are rather grand schemes for the construction of permanent space cities in Earth orbit, constructed from lunar or asteroidal materials. The cost of transporting materials from such low-gravity worlds as the Moon or an asteroid to Earth orbit is much less than transporting the same materials from our high-gravity planet. Such space cities might ultimately be self-propagating—new ones constructed by older ones. The costs of these large manned stations have not yet been estimated reliably, but it seems likely that all of them—as well as a manned mission to Mars—would cost in the $100 billion to $200 billion range. Perhaps such schemes will one day be implemented; there is much that is far-reaching and historically significant in them. But those of us who have fought for years to organize space ventures costing less than one percent as much may be forgiven for wondering whether the required funds will be allocated, and whether such expenditures are socially responsible.

However, for substantially less money, an important expedition that is preparatory for each of these manned ventures could be mustered—an expedition to an Earth-crossing carbonaceous asteroid. The asteroids occur mostly between the orbits of Mars and Jupiter. A small fraction of them have trajectories that carry them across Earth’s orbit and occasionally within a few million miles of the Earth. Many asteroids are mainly carbonaceous—with large quantities of organic materials and chemically bound water. The organic matter is thought to have condensed in the very earliest stages of the formation of the solar system from interstellar gas and dust, some 4.6 billion years ago, and their study and comparison with cometary samples would be of extraordinary scientific interest. I do not think that materials from a carbonaceous asteroid are likely to be criticized in the same way that the Apollo returned lunar samples were—as being “only” rocks. Moreover, a manned landing on such an object would be an excellent preparation for the eventual exploitation of resources in space. And finally, landing on such an object would be fun: because the gravity field is so low, it would be possible for an astronaut to do a standing high jump of about ten kilometers. These Earth-crossing objects, which are being discovered at a rapidly increasing pace, are called—by a name selected long before manned spaceflight—the Apollo objects. They may or may not be the dead husks of comets. But whatever their origin, they are of great interest. Some of them are the easiest objects in space for humans to get to, using only the Shuttle technology, which will be available in another few years.

THE SORTS of missions I have outlined are well within our technological capability and require a NASA budget not much larger than the present one. They combine scientific and public interest, which very often share coincident objectives. Were such a program carried out, we would have made a preliminary reconnaissance of all the planets and most of the moons from Mercury to Uranus, made a representative sampling of asteroids and comets, and discovered the boundaries and contents of our local swimming hole in space. As the finding of rings around Uranus reminds us, major and unexpected discoveries are waiting for us. Such a program would also have made the first halting steps in the utilization of the solar system by our species, tapping the resources on other worlds, arranging for human habitation in space, and ultimately reworking or terraforming the environments of other planets so that human beings can live there with minimal inconvenience. Human beings will have become a multi-planet species.

The transitional character of these few decades is evident. Unless we destroy ourselves, it is clear that humanity will never again be restricted to a single world. Indeed, the ultimate existence of cities in space and the presence of human colonies on other worlds will make it far more difficult for the human species to self-destruct. It is clear that we have entered, almost without noticing it, a golden age of planetary exploration. As in many comparable cases in human history, the opening of horizons through exploration is accompanied by an opening of artistic and cultural horizons. I do not imagine that many people in the fifteenth century ever wondered if they were living in the Italian Renaissance. But the hopefulness, the exhilaration, the opening of new ways of thought, the technological developments, the goods from abroad, and the deprovincialization of that age were then apparent to thoughtful men and women. We have the ability and the means and—I very much hope—the will for a comparable endeavor today. For the first time in human history, it is within the power of this generation to extend the human presence to the other worlds of the solar system—with awe for their wonders, and a thirst for what they have to teach us.