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At the very beginning of the twentieth century competent scientific and lay opinion held that airplanes were impossible. The end of the century, barring the dark specter of nuclear or ecological catastrophes, will probably see joint Soviet and American manned space expeditions to the nearer planets.

This is the century in which some of the oldest dreams of Man have been realized, in which mankind has sprouted wings and realized the aspirations of Daedalus and da Vinci. Air-breathing, man-carrying machines now circumnavigate our planet in less than a day; other machines, skimming above the atmosphere, carry men around our globe in ninety minutes.

There is a generation of men and women for whom, in their youth, the planets were unimaginably distant points of light, and the Moon was the paradigm of the unattainable. Those same men and women, in middle life, have seen their fellows walk upon the surface of the Moon; in their old age, they will likely see men wandering along the dusty surface of Mars, their journeys illuminated by the battered face of Phobos. There is only one generation of humans in the ten-million-year history of mankind that will live through such a transition. That generation is alive today.

This is also the moment in our history when, for the first time, the whole of our planet has been explored, when tribalism is dissipating, when great transnational groupings of states are being organized, when stunning technological advances in communications and transportation are eroding the cultural differences among the various segments of mankind.

But cultural diversity is the forge for the survival of our civilization, just as biological diversity is the forge for the survival of life.

The Earth is overcrowded. Not yet in a literal sense: Our technology is adequate to maintain comfortably a population significantly larger than our present 3.6 billion. The Earth is overcrowded in a psychological sense. For that restless and ambition-driven fraction of mankind that has blazed new paths for our species, there are no new places to go. There are places inside of ourselves, but this is not the forte of such individuals. There are the ocean basins, but we are not yet committed to exploring them seriously; and when we do, they are likely to be exploited rapidly.

At just this time in our history comes the possibility of exploring and colonizing our neighboring worlds in space. The opportunity has come to us not a moment too soon.

October 12, 1992, will be the five hundredth anniversary of the discovery of the "New World" by Christopher Columbus. Mankind will be, I think, engaged at that very moment in an enterprise similar to Columbus'. We will have advantages over him and the mariners of his time. We know precisely where we are going and how to get there. The way will have been examined by unmanned vessels going before us. The paths will have been charted exactly. There will be hazards - collisions with asteroids on voyages to the outer Solar System, for example, or mechanical failure. But there will be no fear of slipping off the edge of the world, as many of the sailors of Columbus' time truly feared. And very likely there will not be a Solar System equivalent of doldrums or sea monsters. Yet the same thrill of exploration and the same adventuresome spirit that drove Columbus will be driving us. As the discovery and exploration of the New World had a profound and irreversible effect on European civilization, exploration and colonization of the Solar System will produce permanent changes in the history and development of mankind.

The analogy with the epic sea voyages of centuries ago is, it seems to me, remarkably close. There was the initial set of sea voyages by Columbus, an Italian in the Spanish court. Our initial set of manned Apollo explorations of the Moon was motivated in significant part by a group of expatriate German engineers led by Wernher von Braun. After Columbus' four voyages, there was essentially a hiatus of a decade or so - and then a burst of further exploratory activities by the Spanish, English, French, and Dutch - vessels flying many flags, many expeditions organized by foreign nationals.

Apollo 17 marked the end of the Apollo lunar missions. It seems clear, at least in the United States, that there will be a hiatus of a decade or more before further lunar exploration and lunar bases are organized. Apollo's primary orientation was never scientific. It was conceived at a time of political embarrassment for the United States. Several historians have suggested that a principal motivation of President Kennedy in organizing the Apollo program was to deflect public attention from the stinging defeat suffered at the Bay of Pigs invasion. Several tens of billions of dollars have been expended on the Apollo program. If the objective had been scientific exploration of the Moon, it could have been carried out much more effectively, for much less money, by unmanned vehicles. The early Apollo missions went to lunar sites of little scientific interest, because the safety of the astronauts was the prime, almost the only, concern. Only toward the very end of the Apollo series did scientific considerations play a significant role.

The Apollo program ended just as the first scientist landed on the Moon. Harrison "Jack" Schmitt, a geologist, trained at Harvard, was one of the two-man crew of the Apollo 17 landing module. He was the first scientist to study the Moon from the surface of the Moon. It is ironic that just as the Apollo program became able to achieve this major advance in the scientific exploration of the Moon, it was canceled. Fittingly enough, the first scientist to land on the Moon was the last man to land on the Moon - at least in the foreseeable future. There are no plans for follow-on manned missions to the Moon either by the United States or, so far as we know, by the Soviet Union.

The argument for cancellation of Apollo was economic. Yet the incremental cost of a given mission was in the many tens of millions of dollars, something like one thousandth the total cost of the Apollo program. It is very much as if, against the advice of my wife, I purchase a Rolls-Royce automobile. She argues that a Volkswagen could get me round just as well, but I feel that a Rolls-Royce would take my mind off the troubles of my job. I then spend so much money on the Rolls-Royce that, after driving it a little bit, I find I can drive it no more because I cannot afford the price of a tank of gas - which is about one thousandth the cost of a Rolls-Royce.

I was one of the scientists opposed to an early Apollo mission. But once the Apollo technology was in hand, I was very much for its continuing usage. I believe the wrong decision was made twice - once in opting for early manned missions to the Moon, and later in abandoning such missions. After Apollo 17, the United States is left with no program, manned or unmanned, for exploration of the Moon. The Soviet Union has developed, in its Luna series of unmanned spacecraft, a proven and versatile capability for roving exploration of the lunar surface and automatic sample return to Earth.

The example of the earliest exploration of the New World suggests that the hiatus in space will be only temporary. The linkage of Soyuz and Skylab, the orbital stations of the Soviet Union and the United States, scheduled for 1975 or 1976, is presaged as the predecessor for joint manned planetary missions.

The Solar System is much vaster than the Earth, but the speeds of our spacecraft are, of course, much greater than the speeds of the sailing ships of the fifteenth and sixteenth centuries. The spacecraft trip from the Earth to the Moon is faster than was the galleon trip from Spain to the Canary Islands. The voyage from Earth to Mars will take as long as did the sailing time from England to North America; the journey from Earth to the moons of Jupiter will require about the same time as did the voyage from France to Siam in the eighteenth century. Moreover, the fraction of the gross national product of the United States or the Soviet Union that is being expended even in the more costly manned space programs is just comparable to the fraction of the gross national product spent by England and France in the sixteenth and seventeenth centuries on their exploratory ventures by sailing ships. In economic terms and in human terms, we have performed such voyages before!

I believe we will see semipermanent bases on the Moon by the 1980s. They will initially be resupplied with material and personnel from Earth, but will become increasingly self-sustaining, utilizing lunar resources. There will be children born in such colonies. They will eventually think of the Earth as "the old country" - an old-fashioned world in many senses, set in its ways, not moving with the times, more constrained and less free than the lunar colonies, despite the rigors and technological constraints of life on the Moon.

In the comparatively near future the entire Solar System will be explored by sophisticated unmanned space vehicles. I think we will see by the 1980s and 1990s deep-entry probes into the atmospheres of Jupiter and Saturn and Titan (the biggest moon of Saturn) - places that are, I believe, far and away the most favorable in the Solar System for indigenous life; we will witness passages of small spacecraft through comets, landings on the large satellites of Jupiter and Saturn, flybys as far as Neptune and Pluto, and hardy spacecraft that plunge into the Sun, radioing back data before they sear and melt in the interior inferno of the nearest star.

Human landings on even the nearer planets, however, will not be as easy as had once been thought. The surface of Venus, far from being Eden, turns out, as we have seen, to be far more like Hell. We cannot imagine manned exploration of the Venus surface in the next few decades. Venus is a planet with fiery temperatures, noxious gases, and crushing atmospheric pressures. Yet, the clouds of Venus are in a clement environment; and a manned buoyant probe - something like a nineteenth-century balloon gondola in which the astronauts work in shirtsleeves and leather oxygen masks - is not without its charm or its scientific interest.

Mars is a vastly exciting planet, of enormous geological, meteorological, and biological interest. A manned expedition to Mars would be very desirable, except for two objections. First, the cost would be crushing. One hundred billion to two hundred billion dollars is probably a conservative estimate. I cannot bring myself to believe that such an expenditure is necessary in the next few decades - when there is so much misery on Earth that could be relieved by such expenditures. Yet in the longer term, say, in the first decades of the twenty-first century, I do not think that such cost objections will be cogent - particularly because new propulsion and life-support systems will be developed.

The second objection to manned missions to Mars is more subtle. It is equally an objection to automatically returned samples from Mars, like the Soviet Union's Luna series for automatic sample return from the Moon. This is the danger of "back contamination." Precisely because Mars is an environment of great potential biological interest, it is possible that on Mars there are pathogens, organisms which, if transported to the terrestrial environment, might do enormous biological damage - a Martian plague, the twist in the plot of H. G. Wells' War of the Worlds, but in reverse. This is an extremely grave point. On the one hand, we can argue that Martian organisms cannot cause any serious problems to terrestrial organisms, because there has been no biological contact for 4.5 billion years between Martian and terrestrial organisms. On the other hand, we can argue equally well that terrestrial organisms have evolved no defenses against potential Martian pathogens, precisely because there has been no such contact for 4.5 billion years. The chance of such an infection may be very small, but the hazards, if it occurs, are certainly very high. Wholesale exterminations of native populations in Santo Domingo and Samoa and Tahiti occurred during the early days of sailing-ship exploration for just such reasons. Among the gifts carried by Columbus to the New World was smallpox.

It is no use arguing that samples can be brought back safely to Earth, or to a base on the Moon, and thereby not be exposed to Earth. The lunar base will be shuttling passengers back and forth to Earth; so will a large Earth orbital station.

The one clear lesson that emerged from our experience in attempting to isolate Apollo-returned lunar samples is that mission controllers are unwilling to risk the certain discomfort of an astronaut - never mind his death - against the remote possibility of a global pandemic. When Apollo 11, the first successful manned lunar-lander, returned to Earth - it was a spaceworthy, but not a very seaworthy, vessel - the agreed-upon quarantine protocol was immediately breached. It was adjudged better to open the Apollo 11 hatch to the air of the Pacific Ocean and, for all we then knew, expose the Earth to lunar pathogens, than to risk three seasick astronauts. So little concern was paid to quarantine that the aircraft-carrier crane scheduled to lift the command module unopened out of the Pacific was discovered at the last moment to be unsafe. Exit from Apollo 11 was required in the open sea.

There is also the vexing question of the latency period. If we expose terrestrial organisms to Martian pathogens, how long must we wait before we can be convinced that the pathogen-host relationship is understood? For example, the latency period for leprosy is more than a decade. Because of the danger of back-contamination of Earth, I firmly believe that manned landings on Mars should be postponed until the beginning of the next century, after a vigorous program of unmanned Martian exobiology and terrestrial epidemiology.

I reach this conclusion reluctantly. I, myself, would love to be involved in the first manned expedition to Mars. But an exhaustive program of unmanned biological exploration of Mars is necessary first. The likelihood that such pathogens exist is probably small, but we cannot take even a small risk with a billion lives. Nevertheless, I believe that people will be treading the Martian surface near the beginning of the twenty-first century.

Beyond that, it is just possible to glimpse the outline of further exploration and colonization. The large moons of Jupiter, and Titan, the biggest moon of Saturn, are major worlds in their own right. Titan is known to have an atmosphere much thicker than that of Mars. These five moons all have large quantities of ices on their surfaces. To make these worlds more habitable, their ices can be tapped for fuel, for the production of food and for the generation of atmospheres. Schemes for similarly terraforming Mars and Venus have been suggested and will very likely be refined and implemented (see Chapter 22). More exotic possibilities, requiring much more advanced technologies, are not at all beyond the likely capabilities of mankind in the next century or two. These include the establishment of bases on and in the asteroids and the short-period comets.

In another century or so, novel forms of propulsion within the Solar System will be developed. One of the most charming of these is solar sailing, the use of the pressure of sunlight and of the protons and electrons in the solar wind for tripping through the Solar System. Enormous sails will be required for such an enterprise, but they can be extremely thin. We can imagine a spacecraft surrounded with tens of miles of golden gossamer-thin sails, delicately and exquisitely furled to catch the solar wind. Remote scientific stations will be on the lookout for the gusts produced by solar flares. Going outward from the Sun will be easy; tacking inward will be more difficult. Spacecraft using solar electric power and nuclear fusion will very likely also be developed in the next century.

In something like two or three centuries, assuming even a modest growth in our technological capability, I would imagine the entire Solar System will have been explored thoroughly - at least to the extent that the Earth has been explored at the present time, some two or three centuries after the first large-scale exploration and colonization activities were begun by European sailing vessels. After that, it is not out of the question that a more wholesale rearrangement of our Solar System will begin, first slowly and then at a more rapid rate -astroengineering projects to move the planets about, to rearrange their masses for the convenience of mankind, his descendants, and his inventions.

By then - perhaps long before then - we may have made contact with other advanced civilizations in the Galaxy. Or perhaps not. In any case, we will be ready in a few centuries for the next step. At just the point the Solar System begins to be filled in - again psychologically, rather than physically - we will be ready for interstellar voyages. That is a limitless prospect, one that can occupy the best exploratory instincts of mankind forever.

Pioneer 10 is the first interstellar spacecraft launched by mankind. It was also the fastest spacecraft launched, to the date of its departure. But it will take eighty thousand years for Pioneer 10 to reach the distance of the nearest star. Because space is so empty, it will never enter another Solar System. The little golden message aboard Pioneer 10 will be read, but only if there are interstellar voyagers able to detect and intercept Pioneer 10.

I believe that such an interception may occur, but by interstellar voyagers from the planet Earth, overtaking and heaving to this ancient space derelict - as if the Nina, with its crew jabbering in Castilian about falling off the edge of the world, were to be intercepted, somewhere off Tristan da Cunha, by the aircraft carrier John F. Kennedy.

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