Cover image for Voyage to the milky way : the future of space exploration
Title:
Voyage to the milky way : the future of space exploration
Author:
Goldsmith, Donald.
Personal Author:
Publication Information:
New York : TV Books, [1999]

©1999
Physical Description:
255 pages, 16 unnumbered pages of plates : color illustrations ; 24 cm
Language:
English
ISBN:
9781575000466
Format :
Book

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Summary

Summary

Throughout history, humans have dreamt of colonizing other worlds. Explorers have eyed the heavens and have devised ingenious ways to travel into the skies, first to the Earth's moon, then to the windswept plains of Mars, now to the ice-covered ocean of Jupiter's moon Europa and the outer confines of the solar system -- and even beyond, to the boundaries of our Milky Way galaxy.

As a new generation of adventures prepares to cast its lot into space, challenging previously accepted models of space exploration, Voyage to the Milky Way explores the future of space travel from both the scientific and cultural points of view. It discusses not only our boldest and most romantic visions to date, but also the myths and illusions that have informed them and the technologies we will utilize to fulfill them.

Award-winning astronomer and author Donald Goldsmith discusses and analyzes our impulse to colonize the solar system; early space-flight visionaries and rocket builders; space colony designs; rocket ship designs; robots and cyborgs; the role of private entrepreneurs in the future of space travel and development; faster-than-light travel; and the strong possibility of a twenty-first-century flight to a nearby star.


Reviews 2

Publisher's Weekly Review

Goldsmith, an astronomer and science writer (The Ultimate Einstein), takes an exciting leap into the next century in this levelheaded survey of what space exploration may hold in store. By 2008, he predicts, a dozen astronauts should be able to live in a low earth orbit for as long as they like on the International Space Station. Buoyed by the 1998 discovery that ice lies beneath craters near the moon's poles, he envisages future lunar colonies run by solar or nuclear power and perhaps functioning independently of Earth. Goldsmith steers a middle course between space zealotry and skepticism in this lucidly written, intelligent probe. Appalled at would-be space colonizers who want to immediately begin "terraforming" Mars so that the red planet will more closely resemble Earth, he points out the enormous risks of contamination posed by such an undertaking. Nevertheless, he believes that an expedition to Mars is very likely within the next 50 years. Other projects he considers include mining asteroids for minerals and visiting Europa, one of Jupiter's four larger moons, to determine whether its covering of ice conceals a global ocean. Running through his text is the raging debate over whether government or private enterprise furnishes the best means of advancing space exploration. Goldsmith ponders the uncharted social and political terrain any space colonists will face, and he remains open to the possibility that the cosmos teems with life, including advanced civilizations. His coverage of controversies over whether cosmic wormholes and faster-than-light particles exist, and his speculative overview of futuristic interstellar propulsion systems, round out an enlightening report from the cosmological frontier. 16 pages of color photos not seen by PW. (July) (c) Copyright PWxyz, LLC. All rights reserved


Choice Review

Goldsmith's excellent book, written in a thoughtful and engaging manner, is intended for the general reader with an interest in space exploration. The chapters on exploring and settling the Moon remind us of the voyage of Apollo 11, whose anniversary was celebrated recently. Trips to Mars and even to the more distant bodies in the solar system appear to be feasible in the next century. The controversy over the role of manned and unmanned probes in the exploration of space is examined. The principles of rocketry and space flight are described clearly, and the explanation of the "twin paradox," in which travelers age at a different rate than those left behind, is especially lucid. In an interesting digression from the science of spaceflight, the author introduces the question of the ethics of space exploration, beginning with the efforts to date in the UN and going forward to the question of mining asteroids. The discussion of the exploration of the solar system is a well-reasoned extrapolation of the current space program. Understandably, the contemplation of the exploration of stellar systems beyond the solar system is more speculative, but Goldsmith usefully emphasizes the vast distances involved, and the difficulties which these distances present. General readers. D. E. Hogg; National Radio Astronomy Observatory


Excerpts

Excerpts

Chapter One Whither Go We? The year 1999, whose nines signal the coming of a new millennium, also marks the eightieth anniversary of an obscure research publication, "A Method of Reaching Extreme Altitudes." In that scientific paper, Robert Goddard, an assistant professor of chemistry at Clark University in Worcester, Massachusetts, glimpsed the future of rocketry. From Goddard's basic design for a liquid-fueled rocket came the mighty engines that have sent astronauts to the moon and space probes to all the planets in our solar system but one. Once mocked as "moon-mad" by The New York Times , Goddard did not live to see the Goddard Space Flight Center named in his honor. The National Aeronautics and Space Administration (NASA) uses this center as its prime site for the management of automated Earth satellites that have introduced new ways for humanity to view the cosmos. But Goddard's concepts opened the era of space exploration, the epoch in which humanity saw how to extend its reach through space to arrive at other celestial objects.     At a point almost halfway between 1919 and 1999, on October 4, 1957, a race into space gave Earth its first artificial satellite, the Soviet Union's Sputnik 1 . Forty years ago, at the start of 1959, the Soviet Union and the United States had sent a total of only nine satellites into orbit around Earth, and at least ten other launch attempts had ended in failure. Yet all humanity could see that far greater successes lay in the future; indeed, as 1959 opened, the Soviet Union sent a probe close to the moon--humanity's first emissary to another world--and two years later sent the first astronauts into orbit around Earth. Thirty years ago, on July 20, 1969, the United States's furiously energetic lunar landing program reached a culmination as Neil Armstrong became the first human to walk on another celestial object.     During the next three decades, space exploration entered its adolescence as a host of countries participated in creating a vast array of satellites and spacecraft capable of myriad tasks for extending our knowledge of both our own planet and its cosmic surroundings and to improve our ability to communicate with one another. Thousands of separate objects, designed and fabricated by more than three dozen countries, have now orbited Earth. Four hundred astronauts have spent a combined total of nearly ten thousand days in space, with the individual record (nearly one year and three months) set by the Russian Valerii Polyakov in 1994-95. Twelve men have walked or ridden on the lunar surface, spending as much as seven hours at a time exploring our neighboring world. We have built and launched more than one hundred probes to make close-up observations of three dozen objects in the solar system, including Mercury, Venus, Mars, Jupiter and its four large moons, Saturn and its system of rings and satellites, Uranus, Neptune, Halley's comet, and the asteroids Gaspra and Ida. Can the stars be far beyond? The Arrangement of the Cosmos The twentieth century has carried humanity from dreams of a space age into a complex, ongoing set of contacts with the cosmos, itself revealed by a host of new instruments in space and on Earth. We now know that our solar system forms a tiny part of the Milky Way galaxy, a giant assemblage of stars, shaped like a pie plate with an ice cream bulge at its center. The sun and its gravitationally bound family of planets, moons, asteroids, meteoroids, and comets reside within the outer reaches of the plateshaped Milky Way, closer to our galaxy's edge than to its center. Though we correctly call the Milky Way "our galaxy," this title belongs to all of the galaxy's three hundred billion stars. In addition to its stars and their planets (the latter discovered only during the past few years), the Milky Way contains enormous amounts of interstellar gas and dust that float among the stars, plus a host of exotic objects to which astronomers have given suggestive names: white dwarfs, red giants, black holes, neutron stars, dark molecular clouds, planetary nebulae, starforming regions, and supernova remnants.     Every probe that we launch into space travels through the Milky Way, but for only a tiny part of the galaxy's total extent. The Milky Way's diameter covers twenty-five thousand times the distance from the sun to its closest neighboring stars. These closest stars in turn lie twenty-five thousand times farther from us than the farthest planets in our solar system, the most distant objects to which we have sent spacecraft. Thus a spacecraft voyaging from one side of the Milky Way to the other would travel about 625 million times farther than the longest space journeys--measured in billions of miles--that we have yet achieved with automated spacecraft. Planning for the Future of Space Exploration Nevertheless, we have begun to explore the Milky Way, starting with our immediate environment and looking outward toward more distant objects, which will require still longer journeys if we hope for close-up inspection. An ideal society would formulate a coherent, long-term plan to explore the cosmos, modifying it as new discoveries and changed circumstances warrant. This plan would lead us from the solar system outward, eventually taking us tens of thousands of times farther than the sun's most distant planets and finally carrying us to the stars at the appropriate time, when humanity has achieved the technological and sociological insight required for such extensive voyages.     Our attitudes toward space exploration well demonstrate that we do not yet possess an ideal society. National governments continually change their attitudes toward the importance of spending money on cosmic research, especially and understandably during economic or political crises, as has occurred in the former Soviet Union, whose once vigorous space program now rests on the hope of selling rocket launches to other countries. Even in times of relative stability, the public's frame of mind can prove fickle, the more so because space exploration offers economic rewards that remain questionable and in any case lie far in the future. The inconstancy of governmental and popular opinions with respect to exploring the universe reaches a maximum when it comes to sending humans into space--the most visible, most expensive, most dramatic, and for many people the only real part of the space program.     Consider the exploration of space by humans as it exists in the year 1999. The longest journeys through space have been eight different missions to or near the moon, with six lunar landings and two flybys. These missions, the culmination of the United States's Apollo program, all occurred between 1969 and 1972. The last person to walk on the moon left its surface more than twenty-six years ago. Astronauts have instead spent increasing amounts of time in orbit around Earth at altitudes no greater than a few hundred miles above its surface. John Glenn, the first United States astronaut to orbit Earth, became a national hero after his three circuits of the globe in 1962. Thirty-six years later, he waged a successful campaign to persuade NASA to allow him a final set of orbits on the Space Shuttle. This trip provided a spectacular finish to his distinguished career as a United States senator, but the fact remains that Glenn reached the same distance from Earth in 1998 that he did in 1962. This distance amounts to only one-thousandth of the distance to the moon, to one-millionth of the distance covered by the automated probes we have sent to Mars, and to one-hundred-billionth of the distance to the nearest stars.     As surely as we must learn to walk before we can run, short journeys must precede longer ones in our exploration of the universe. But we have in effect gone backward, from trips to the moon three decades ago to simple Earth orbits today. In hindsight, this strange sequence arose because we tried to run too soon. With the cold war at its height, the United States embarked on a race with the Soviet Union to send the first astronauts to the moon. Through its heroic expenditures, and because the Soviet Union abandoned its lunar astronaut program after it experienced repeated mishaps and incurred enormous costs, the United States won that competition. After the race was won, however, there was no justification for taking further steps in human space exploration. As the cold war approached its unforeseen end, Carl Sagan and other leaders in forming public opinion promoted the concept of a joint mission to Mars by the Soviet Union and the United States, but this notion fell into the dustbin of history, at least temporarily, when the Soviet Union collapsed at the start of the 1990s.     For now, human space exploration consists of Space Shuttle flights into orbits a few hundred miles high, along with a few Russian spaceflights into similar trajectories. The spacecraft involved have been designed to go no farther than these "low Earth orbits," which are important because they already take astronauts, and any equipment they may deploy, above nearly all of Earth's atmosphere. Many of their payloads have been used to study not the cosmos above but the ground below, or to speed communications around Earth. For these latter purposes, low Earth orbits are often superior to higher ones, but for further exploration, we must dream of longer voyages.     Could space tourism soon prove to be a paying proposition? Each Space Shuttle flight costs approximately $400 million and can carry as many as eight astronauts. Thus we can assign a cost to John Glenn's 1998 spaceflight somewhere between $50 million (if we regard him as one of the eight astronauts) and zero (if we take the view that adding one more passenger costs relatively little). If we reconfigured the Space Shuttle for tourism, it might take several dozen passengers into orbit. With ticket prices set at $10 million each, these journeys might show a profit as long as we can find passengers willing to pay these prices for a tour around our planet in rather cramped quarters. As a practical matter, space tourism must wait until we find ways to reduce the per-pound cost of spaceflight by a factor of at least a hundred. Three Different Ways to Explore the Cosmos To understand the big picture of space exploration, we must realize that we have three basic ways to extend our knowledge of the vast universe in which we live. First, we can build better instruments to study the universe and place them at superior sites for observation. The most notable and significant of these sites are locations outside our planet's protective veil of atmosphere, which blocks much of the information that would otherwise reach Earth's surface and hinders the remainder. Second, we can send automated probes to relatively nearby objects, which for forty years has meant objects within the solar system. Eventually, we may hope to send similar probes to other stars and their planets, which lie tens of thousands of times farther from us than the most distant worlds we have already visited. Third, human astronauts can go into space to examine its contents, bringing their eyes and minds to bear directly on new worlds, as long as we can provide these explorers with transportation and life support systems to ensure their survival. Being human, we tend to think of this as the height of exploration achievement, the continuation of an impulse that has spread humanity around the globe, to its mountain heights, to each of its poles, and to the depths of its oceans.     The first of these lines of attack, the quest to create better instruments and to place them at superior observation sites, has revolutionized our knowledge of the cosmos. Mammoth telescopes on high mountains, such as the Keck telescopes that peer at the universe from the summit of the Mauna Kea volcano in Hawaii, have joined spaceborne instruments such as the Hubble Space Telescope in securing sharper, deeper, and more precise views of the universe than any previous ones. Instruments in space have obtained images and made measurements that are simply impossible to perform even from high mountaintops. The Hubble Space Telescope, for example, studies the cosmos not only in familiar visible light but also in ultraviolet radiation, which cannot penetrate the atmosphere.     The second approach, carried out by automated probes throughout the solar system, has likewise yielded a flood of new results, though they refer directly only to our local set of objects and only by inference to the situation in other planetary systems throughout the Milky Way and beyond. In 1999 the Galileo spacecraft continues to sail on long, looping trajectories of observation around Jupiter, the largest planet in the solar system, obtaining detailed images of Jupiter and its four large satellites, especially of Europa, considered a likely site for life because it apparently maintains a liquid ocean beneath its icy surface. After August 1999, the Cassini-Huygens spacecraft, having received a "gravity boost" from its close passage by Earth during the summer, will spend four years on a voyage out to Saturn, nearly ten times as far as Earth is from the sun. There the spacecraft will detach a probe that will descend through the thick, murky atmosphere of Saturn's satellite Titan, where life may float in lakes of ethane and other hydrocarbons. Three spacecraft are now on their way to Mars to build on the success of the Viking, Mars Pathfinder , and Mars Global Observer missions; one of them will shoot a probe into the ground near Mars's south polar cap to measure the temperature, density, and soil composition.     In addition to these planetary missions, NASA's Near-Earth Asteroid Rendezvous (NEAR) spacecraft has made its first rendezvous with the asteroid Eros, a battered hunk of rock a few dozen miles across, left over from the formation of the solar system. In May 2000 this probe will match its orbit precisely to Eros's, with the spacecraft separated from the asteroid by only a few miles, so that NEAR can survey the pocked surface of Eros for clues to how the solar system formed. Another NASA probe, the Stardust spacecraft, left Earth in February 1999 on a series of orbits around the sun that will lead to a close encounter with the comet Wild 2 in January 2004. Stardust will not only make close-up observations of the comet but also acquire the first samples of cometary material, to be captured softly in an aerogel trap and returned to Earth in a re-entry capsule in the year 2006.     The spacecraft now at or on their way to Mars, Jupiter, Saturn, Eros, and Wild 2 have been designed and engineered to survive the hostile environment of interplanetary space, in which harmful radiation of different types creates a life-threatening danger to any unshielded astronaut. This fact emphasizes that the third leg of our triad of space investigation possibilities, human space travel, poses far greater problems--hence far greater costs--than the first two. Humans in space require much more than automated space probes do. Among these needs we should count not only air, water, food, and shielding against harmful radiation but also the much greater amounts of fuel required by a spacecraft with sufficient room and shielding for human activities, together with extra amounts of otherwise redundant strength and supplies to ensure a high probability of astronaut survival. Furthermore, if we expect the astronauts to return to Earth (and most of us, including most astronauts, will insist on this aspect of human spaceflight), all these requirements must be substantially increased to allow for a trip out and a trip back. Automated space probes make no such insistence; their loss, should it occur, may prove mysterious and highly frustrating to scientists who staked their careers on success but produces no human bereavement. Who Will Choose the Best Ways to Explore? In view of the imperious exigencies of human spaceflight, we cannot be overly surprised that astronauts have made only the first, tentative human voyages in space. A burning question remains before us: How should we distribute our efforts in cosmic exploration among the three possibilities--improved instruments for observation, space probes for close-up study within the solar system, and human investigation of celestial objects?     The brief answer to this question, historically speaking, is that we look to our elected representatives, and in particular to the United States Congress, to formulate policies regarding the funding of programs to improve our knowledge of the universe. Although private foundations continue to support important projects such as the twin Keck telescopes in Hawaii, the growing expense of these enormously capable instruments suggests that national governments should distribute the tax-enforced contributions from their citizens in accordance with the principles of a democracy. Indeed much of the funding for the second Keck telescope came from NASA, which hopes to use it in part to search for more planets orbiting other stars. For both automated space probes and human spaceflight, governmental support, either from individual nations or from nations that pool their resources for such expensive undertakings, has provided the organizational and financial backing necessary for these programs to succeed.     Must this be so in the future? In succeeding chapters, we shall meet scientists and engineers who disagree, almost to the point of anger, with the concept that governments should rule space. Instead, they argue that space belongs to the people and that the people, acting as individuals or through corporations, should explore, exploit, and indeed own celestial objects. For example, Arthur C. Clarke, the famous science fiction writer and visionary, supports the notion of private missions into space. The X-Prize Foundation of St. Louis, quite independent of any government, has offered $10 million to the first private group that twice successfully sends astronauts into space, three at a time, on separate flights no more than two weeks apart. Clarke sees this prize, like the $25 thousand prize that motivated Charles Lindbergh and his competitors to fly the Atlantic seven decades ago, as a useful incentive to help us to fulfill our nature as humans, which has always been, he says, "to explore our surroundings, to push the limits of our understanding, and to turn frontiers into future homes." Although Clarke neither explains why a prize would be needed if our nature demands that we explore, nor assesses whether sooner is always better than later in turning frontiers into homes, his attitude corresponds to that of large numbers of people who want a chance to ride through space and hope that private enterprise will provide what governments cannot or will not.     The clash between those who want governments to send astronauts into space and those who believe that private enterprise can do a significantly better job should raise a heartfelt reaction in many a reader. But the reader should address an issue still deeper than this one. To me, the argument over using governmental or private means to send human expeditions throughout the solar system amounts to debating whether to go to hell in a chemically- or nuclear-powered spaceship. We should ponder long and hard before we plant ourselves too firmly on other worlds. With my prejudices clear, I hope cheerfully to present opposing views in succeeding chapters of this book. The International Space Station For now, the farthest horizon where future homes for humanity are under construction lies a few hundred miles above us, at the largest building site ever opened in space. In November and December of 1998, Russia and the United States launched the first components of the International Space Station, an immense undertaking that will cost more than $100 billion, of which the United States will contribute a bit over 85 percent, with more than a dozen other countries supplying the remainder. This project, which we will simply call the Space Station, amounts to by far the largest construction project ever undertaken in space. In inflation-adjusted dollars, its estimated cost will fall a bit below that of the Apollo program that sent astronauts to the moon. The "bit" in question currently amounts to $15 billion, but the uncertainty of future costs could reduce this amount to zero or even to a negative number. By about the year 2008, the Space Station should be completed to the point that about a dozen astronauts can live in a low Earth orbit for as long as they like.     Because the Space Shuttle can carry astronauts no more than three hundred miles above Earth's surface, the Space Station will be built just below this altitude, about 285 miles high. Like all objects that move in orbit only a few hundred miles above Earth's surface, the components of the Space Station and the station itself take about ninety minutes to complete each circuit of our planet. As they do so, they are always falling not toward but around Earth. As a result of their continuous fall, astronauts in orbit feel "weightless," even though Earth's gravity continues to pull on them. A similar feeling occurs whenever you leap from a high diving tower: During your few moments of "free fall," Earth attracts you with its gravitational force, but you feel no effects from this force because nothing brakes your fall. Similarly, when everything in a spacecraft falls in the same direction and at the same speed, the effects of the Earth's gravity go unnoticed, even though the force of gravity continues.     Engineers involved in space projects sometimes use the misleading term "zero gravity" or "microgravity" to describe the situations in which a continuous fall produces weightlessness. But gravity acts on astronauts in near-Earth orbits as well as on those standing on Earth's surface. If a giant hand suddenly stopped the astronauts' motion in orbit and then let them go, they would fall straight down toward Earth's center. Conversely, the astronauts' momentum as they orbit would carry them straight off into deep space were it not for Earth's gravitational pull, which keeps them orbiting along a circular trajectory. The balance between gravity and momentum allows an object in orbit to continue orbiting our planet for centuries or millennia, neither escaping into deep space nor falling to Earth, until the extremely modest drag forces from the tenuous uppermost portions of the atmosphere eventually slow its motion. Eventually, any such object will enter the lower atmosphere, thus becoming subject to still stronger drag forces, and will finally strike Earth as an artificial meteorite, as NASA's Skylab space station did in 1979.     The International Space Station, which has taken more than a decade to plan and will require nearly as long for construction, has an orbit that should keep it safely above us for many centuries. To build and supply it, NASA plans more than a hundred Space Shuttle flights, eventually producing interconnected modules of laboratories and living quarters that will enclose a volume roughly twice the interior size of a Boeing 747 aircraft. (Following its own idiosyncratic accounting procedures, NASA estimates the Space Station's cost at $30 billion, rather than the actual $100 billion, by the simple technique of placing Space Shuttle flights essential to the project in a different budget.) This mammoth enterprise carries with it the possibility of accidents in space. The most significant disaster in past history was the explosion of the Space Shuttle Challenger on launch in 1986, which induced NASA to keep its other three Space Shuttles on the ground for almost three years. If such a tragedy should occur, the construction schedule for the Space Station will be similarly stretched out in time.     To build the Space Station, astronauts must overcome considerable challenges. Beyond the protective veil of Earth lies a cosmos hostile to their survival. Not only must they bring their supplies with them but they must also protect themselves against the host of particles and floods of radiation that continuously bombard our planet. Against the radiation, most of which comes from our sun (the more so during times of "solar storms"), astronauts can protect themselves with less than an inch of metal shielding--but must be sure to do so at all times, including during "space walks" (in NASA-speak, extravehicular activities, or EVAs) that are absolutely essential to assemble the Space Station. The particles that threaten a spacecraft or a space suit consist mainly of micrometeoroids, pieces of fine debris in orbit around the sun, left over from the processes that formed the planets, 45 billion years ago. Unless a particle's orbit precisely matches Earth's, its relative speed of impact on a spacecraft or astronaut will be large--as much as five miles per second, sufficient to transform even a tiny dust grain into a dangerous projectile. A micrometeoroid less than one-hundredth of an inch across can punch through the space suit of an astronaut on an EVA, and a dust grain just ten times larger can penetrate a steel bulkhead half an inch thick.     To protect astronauts against these cosmic bullets, which travel dozens of times more rapidly than the projectile from a rifle, NASA plans to reinforce the walls of the Space Station with layers of kevlar (best known for its use in bulletproof vests) and to develop quick-acting wall-patching kits to guard against a total loss of cabin air pressure. Ironically, the near-Earth environment, in which the Space Station will orbit, has become an especially dangerous locale because of the debris already left behind by previous space missions. A modest fleck of paint can damage a window on collision, while an item much larger than a micrometeoroid--a stray nut from an earlier satellite, for example--could make a sizable hole in even the strongest bulkhead the Space Station will have.     What long-term payoff will come from overcoming these dangers to build a successful Space Station? NASA once claimed that great benefits would flow to Earth's inhabitants, including new manufacturing processes carried out in the weightless conditions of space. The estimated time when these processes will prove cost-effective has receded far into the future, and NASA now stresses that the Space Station will provide increased experience in dealing with the problems and hazards confronting those who plan to spend long intervals of time in space. (On occasion, NASA's goals seem even more modest; thus in November 1998, when Russia launched the Space Station's first module, Dan Goldin, NASA's administrator, stressed its international aspect, which indeed would once have been unthinkable. Speaking in the Bronx accent that gives his utterances a street-wise knowingness, Goldin stated, "It's gonna be tough. It's not gonna be pretty. But we're going to have a real international space station." This remark epitomizes Goldin's ambivalence toward a project too far along to be stopped but which threatens to consume so much of future NASA budgets that more useful research may not go forward). Although the Space Station is mainly producing debt for the public and profits for aerospace corporations, NASA was correct on one key point. The International Space Station will provide an excellent way--the best way, in fact--to discover how humans can survive for long periods in space. To the extent that this is our goal in space exploration, the Space Station can fulfill it.     NASA also points out that when we finally create a network of outposts in the solar system, a space station will be an immensely handy item to have. Even though the space station orbits only a few hundred miles above Earth, in order to join it in orbit we must expend about half the total energy required for a journey to the moon or to Mars. This is so because of Earth's significant gravitational force, which we effectively overcome by achieving an orbit that allows us to fall around Earth. From this position, we need only a relatively modest rocket to send a spacecraft onward, plus, of course, the ability to land on other celestial objects and eventually to return--not to Earth, but to the space station For travel from the space station back to Earth, we can envision a fleet of specialized, short-hop rockets, like the switchyard engines of a long-distance railroad system. Eventually, of course, humans may travel back and forth from a near-Earth orbit to other worlds in the solar system without ever landing on our home planet--a plan that will save considerable amounts of energy but will first require the creation of a community in orbit around Earth.     However, not everyone interested in sending humans to explore the solar system favors construction of the Space Station. Many of the people who are most enthusiastic about sending astronauts back to the moon and onward to the planets find the emphasis and expense associated with the International Space Station rather ludicrous. To find out how to survive on a voyage to Mars, for example, we should create a mission to Mars, not a spaceship that orbits Earth seven thousand times a year and finds itself no farther away than when it began. Allowing for a certain willingness to risk the lives of the daredevils who might embark on the first trip to Mars, this argument makes reasonably good sense, and we shall consider it again in Chapter 5.     In assessing the difficulties of long-term human endurance in space, we must not pass lightly over the medical and psychological issues while noting the physical problems of food, water, air, fuel, and shielding. Four decades of sending humans into space have shown, unsurprisingly, that the sensation of weightlessness causes serious problems for organisms that have evolved in an environment with significant gravitational effects. Adaptation to weightlessness does not come easily. Intriguingly, some humans avoid the nausea of spaceflight far more easily than others, without any obvious advantages of youth or fitness. The weightless conditions in space move fluids from an astronaut's lower limbs into the head and also increase the amount of blood in the chest, fooling the heart until an innate regulatory mechanism decreases the plasma volume, simultaneously thinning the blood. Without gravity to fight against, muscles and bones quickly become weaker, especially in the body's major load-bearing structures. Many of these conditions can be simulated and have been experienced simply by lying horizontally in bed for several weeks without rising. With daily exercise, astronauts can avoid most of the changes to bone and muscle that would otherwise result from weightlessness.     Even more difficult than adapting to the weightless condition may be the readaptation to gravity conditions after landing, an important consideration for travel to other worlds. Astronauts returning to Earth have often found themselves disoriented for several days, like someone who arises from weeks of constant bed rest. We can anticipate, for instance, that astronauts who reach Mars will spend days or weeks recapturing their nonweightless selves before they can tolerate a planet whose surface gravitational force is about half that on Earth. The lunar environment, which the Apollo astronauts tasted only briefly, offers gravitational forces just one-sixth those on Earth--probably enough to keep the body happy on a long-term basis, and offering such new possibilities in athletic contests (once the problems of space suit bulkiness have been overcome) as the mile-long home run and the five hundred-yard football pass.     Other recreational activities will sooner or later lead to the first children conceived in space, and eventually to the first to be born in orbit or on a celestial object other than Earth. NASA's studies on mice and rats born in space imply that the absence of gravitational clues in infancy may have serious negative effects on adult animals. Much research remains to be done in this area before we celebrate the first space-born human astronaut. But once the first has been born, can the one millionth be far behind? Are We Destined to Colonize the Cosmos? From a long-term perspective, any argument over whether or not humans will live in space seems pointless. Judging by our past behavior, once we acquire the ability to establish ourselves on other celestial objects, some of us will do so. The issue then reduces itself to whether the human race will survive sufficiently long and will therefore prove itself able to develop the necessary technological skills for these colonies to spring into existence, to flourish, and to expand to the point where they can establish new colonies of their own.     In the final chapter of this book, we examine some potential flaws in this expectation and conclusion. For now, however, let us assume it is correct and that only the long-term survival of the human species calls into question whether we shall ever send colonists to other worlds. Were it not for the fact that this colonization cannot begin tomorrow, the sharp differences of opinion over whether such colonies are a good idea would rise to the forefront of public attention. As things are, however, we can leave these debates for our grandchildren, who will be the first to decide the merits of settling humans on the moon, Mars, asteroids, or the moons of Jupiter.     Or will they? A small group of technologists, dedicated to the goal of sending humans to live in space and convinced that this project can offer the inspiration and commitment that human society so desperately needs, have called for space colonization to begin immediately. To quote Robert Zubrin, one of the leaders in this effort, "We have in hand all the technologies required for undertaking within a decade an aggressive, continuing project of Mars exploration." Zubrin and his associates plan to accelerate their plans by avoiding governmental interference and direction; instead, they hope to obtain private contributions that will allow them to send humans to Mars, first to explore, then to colonize, and eventually to transform Mars into a planet more like Earth.     Listening to or recounting these plans, I tend to recoil in horror. I may not mind so much creating garbage on Earth, where we can hope to deal with it someday, but I draw the line at contaminating other planets. Among other problems, a tremendous contamination issue arises when dealing with a celestial object that might have life beneath its surface or might carry a fossil record of life in its rocks and dust. The thought that humans might expend enormous energy to reach the red planet, only to discover that Earth-born, Mars-borne life overlays and conceals any record that indigenous life may have left on Mars seems too sad to be endured. On a still deeper level, it is questionable whether we possess the moral right not simply to explore, which we can rather easily excuse as the inevitable product of our remarkable curiosity, but also to exploit, as, for example, by strip-mining the surface of the moon or of mineral-bearing asteroids. Depressing as this prospect appears, we descend yet one step further when we contemplate the possibility that faint-hearted, namby-pamby eco-nuts will crowd Earth while their bold and dauntless brethren carry the human flag toward worlds ripe for the taking, justifying their actions not only as entirely human but even as noble expressions of the private property concept that has served our society so well.     What forum exists in which to thrash out these competing views of space exploration? What process will or should lead humanity to a common view of the cosmos in which we live? What authority will enforce any decisions arising from our discussions of space and its contents? Quite clearly, the answer is none. In Chapter 4, we consider the role that the United Nations has played and could play in regulating human activities in space, but in this arena, as in terrestrial matters, we shall see that a great deal of effort would be needed to achieve even modest unanimity, which could be overthrown by a few enthusiasts with a mission. To be sure, we have no definitive proof that unity is desirable, and we all can imagine circumstances in which individuality should triumph over communal decisions.     So onward into space! Stay with me, and I'll take you to the moon and to Mars, outward through the asteroids to the giant planets and their satellites and then onward into the depths of the Milky Way galaxy in which we live. At each stage we will examine what seems feasible, and what may become possible in the future, for sending both automated spacecraft and human adventurers to distant celestial objects. I shall attempt to keep my prejudices subject to full disclosure, and to present opposing views of human destiny as fairly as I can. My goal is to bring us to the end of our mental voyage through the Milky Way wiser and more fully informed, ready to participate in the debates that will arise about what is to be done with the cosmos. Copyright © 1999 TV Books, L.L.C.. All rights reserved.