Pale Blue Dot: A Vision of the Human Future in Space - Carl Sagan, Ann Druyan (1997)


The great floodgates of the wonder-world swung open.


Sometime coming up, perhaps just around the corner, there will be a nation—more likely, a consortium of nations—that will work the next major step in the human venture into space. Perhaps it will be brought about by circumventing bureaucracies and making efficient use of present technologies. Perhaps it will require new technologies, transcending the great blunderbuss chemical rockets. The crews of these ships will set foot on new worlds. The first baby will be born somewhere up there. Early steps toward living off the land will be made. We will be on our way. And the future will remember.

TANTALIZING AND MAJESTIC, Mars is the world next door, the nearest planet on which an astronaut or cosmonaut could safely land. Although it is sometimes as warm as a New England October, Mars is a chilly place, so cold that some of its thin carbon dioxide atmosphere freezes out as dry ice at the winter pole.

It is the nearest planet whose surface we can see with a small telescope. In all the Solar System, it is the planet most like Earth. Apart from flybys, there have been only two fully successful missions to Mars: Mariner 9 in 1971, and Vikings 1 and 2 in 1976. They revealed a deep rift valley that would stretch from New York to San Francisco; immense volcanic mountains, the largest of which towers 80,000 feet above the average altitude of the Martian surface, almost three times the height of Mount Everest; an intricate layered structure in and among the polar ices, resembling a pile of discarded poker chips, and probably a record of past climatic change; bright and dark streaks painted down on the surface by windblown dust, providing high-speed wind maps of Mars over the past decades and centuries; vast globe-girdling dust storms; and enigmatic surface features.

Hundreds of sinuous channels and valley networks dating back several billion years can be found, mainly in the cratered southern highlands. They suggest a previous epoch of more benign and Earthlike conditions—very different from what we find beneath the tenuous and frigid atmosphere of our time. Some ancient channels seem to have been carved by rainfall, some by underground sapping and collapse, and some by great floods that gushed up out of the ground. Rivers were pouring into and filling great thousand-kilometer-diameter impact basins that today are dry as dust. Waterfalls dwarfing any on Earth today cascaded into the lakes of ancient Mars. Vast oceans, hundreds of meters, perhaps even a kilometer, deep may have gently lapped shorelines barely discernible today. That would have been a world to explore. We are four billion years late.*

On Earth in just the same period, the first microorganisms arose and evolved. Life on Earth is intimately connected, for the most basic chemical reasons, with liquid water. We humans are ourselves made of some three-quarters water. The same sorts of organic molecules that fell out of the sky and were generated in the air and seas of ancient Earth, should also have accumulated on ancient Mars. Is it plausible that life quickly came to be in the waters of early Earth, but was somehow restrained and inhibited in the waters of early Mars? Or might the Martian seas have been filled with life—floating, spawning, evolving? What strange beasts once swum there?

Whatever the dramas of those distant times, it all started to go wrong around 3.8 billion years ago. We can see that the erosion of ancient craters dramatically began to slow about then. As the atmosphere thinned, as the rivers flowed no more, as the oceans began to dry, as the temperatures plummeted, life would have retreated to the few remaining congenial habitats, perhaps huddling at the bottom of ice-covered lakes, until it too vanished and the dead bodies and fossil remains of exotic organisms—built, it might be, on principles very different from life on Earth—were deep-frozen, awaiting the explorers who might in some distant future arrive on Mars.

METEORITES ARE FRAGMENTS OF OTHER WORLDS recovered on Earth. Most originate in collisions among the numerous asteroids that orbit the Sun between the orbits of Mars and Jupiter. But a few are generated when a large meteorite impacts a planet or asteroid at high speed, gouges out a crater, and propels the excavated surface material into space. A very small fraction of the ejected rocks, millions of years later, may intercept another world.

In the wastelands of Antarctica, the ice is here and there dotted with meteorites, preserved by the low temperatures and until recently undisturbed by humans. A few of them, called SNC (pronounced “snick”) meteorites* have an aspect about them that at first seemed almost unbelievable: Deep inside their mineral and glassy structures, locked away from the contaminating influence of the Earth’s atmosphere, a little gas is trapped. When the gas is analyzed, it turns out to have exactly the same chemical composition and isotopic ratios as the air on Mars. We know about Martian air not just from spectroscopic inference but from direct measurement on the Martian surface by the Viking landers. To the surprise of nearly everyone, the SNC meteorites come from Mars.

Originally, they were rocks that had melted and refrozen. Radioactive dating of all the SNC meteorites shows their parent rocks condensed out of lava between 180 million and 1.3 billion years ago. Then they were driven off the planet by collisions from space. From how long they’ve been exposed to cosmic rays on their interplanetary journeys between Mars and Earth, we can tell how old they are—how long ago they were ejected from Mars. In this sense, they are between 10 million and 700,000 years old. They sample the most recent 0.1 percent of Martian history.

Some of the minerals they contain show clear evidence of having once been in water, warm liquid water. These hydro-thermal minerals reveal that somehow, probably all over Mars, there was recent liquid water. Perhaps it came about when the interior heat melted underground ice. But however it happened, it’s natural to wonder if life is not entirely extinct, if somehow it’s managed to hang on into our time in transient underground lakes, or even in thin films of water wetting subsurface grains.

The geochemists Everett Gibson and Hal Karlsson of NASA’s Johnson Space Flight Center have extracted a single drop of water from one of the SNC meteorites. The isotopic ratios of the oxygen and hydrogen atoms that it contains are literally unearthly. I look on this water from another world as an encouragement for future explorers and settlers.

Imagine what we might find if a large number of samples, including never melted soil and rocks, were returned to Earth from Martian locales selected for their scientific interest. We are very close to being able to accomplish this with small roving robot vehicles.

The transportation of subsurface material from world to world raises a tantalizing question: Four billion years ago there were two neighboring planets, both warm, both wet. Impacts from space, in the final stages of the accretion of these planets, were occurring at a much higher rate than today. Samples from each world were being flung out into space. We are sure there was life on at least one of them in this period. We know that a fraction of the ejected debris stays cool throughout the processes of impact, ejection, and interception by another world. So could some of the early organisms on Earth have been safely transplanted to Mars four billion years ago, initiating life on that planet? Or, even more speculative, could life on Earth have arisen by such a transfer from Mars? Might the two planets have regularly exchanged life-forms for hundreds of millions of years? The notion might be testable. If we were to discover life on Mars and found it very similar to life on Earth—and if, as well, we were sure it wasn’t microbial contamination that we ourselves had introduced in the course of our explorations—the proposition that life was long ago transferred across interplanetary space would have to be taken seriously.

IT WAS ONCE THOUGHT that life is abundant on Mars. Even the dour and skeptical astronomer Simon Newcomb (in his Astronomy for Everybody, which went through many editions in the early decades of this century and was the astronomy text of my childhood) concluded, “There appears to be life on the planet Mars. A few years ago this statement was commonly regarded as fantastic. Now it is commonly accepted.” Not “intelligent human life,” he was quick to add, but green plants. However, we have now been to Mars and looked for plants—as well as animals, microbes, and intelligent beings. Even if the other forms were absent, we might have imagined, as in Earth’s deserts today, and as on Earth for almost all its history, abundant microbial life.

The “life detection” experiments on Viking were designed to detect only a certain subset of conceivable biologies; they were biased to find the kind of life about which we know. It would have been foolish to send instruments that could not even detect life on Earth. They were exquisitely sensitive, able to find microbes in the most unpromising, arid deserts and wastelands on Earth.

One experiment measured the gases exchanged between Martian soil and the Martian atmosphere in the presence of organic matter from Earth. A second brought a wide variety of organic foodstuffs marked by a radioactive tracer to see if there were bugs in the Martian soil who ate the food and oxidized it to radioactive carbon dioxide. A third experiment introduced radioactive carbon dioxide (and carbon monoxide) to the Martian soil to see if any of it was taken up by Martian microbes. To the initial astonishment of, I think, all the scientists involved, each of the three experiments gave what at first seemed to be positive results. Gases were exchanged; organic matter was oxidized; carbon dioxide was incorporated into the soil.

But there are reasons for caution. These provocative results are not generally thought to be good evidence for life on Mars: The putative metabolic processes of Martian microbes occurred under a very wide range of conditions inside the Viking landers—wet (with liquid water brought from Earth) and dry, light and dark, cold (only a little above freezing) to hot (almost the normal boiling point of water). Many microbiologists deem it unlikely that Martian microbes would be so capable under such varied conditions. Another strong inducement to skepticism is that a fourth experiment, to look for organic chemicals in the Martian soil, gave uniformly negative results despite its sensitivity. We expect life on Mars, like life on Earth, to be organized around carbon-based molecules. To find no such molecules at all was daunting for optimists among the exobiologists.

The apparently positive results of the life detection experiments is now generally attributed to chemicals that oxidize the soil, deriving ultimately from ultraviolet sunlight (as discussed in the previous chapter). There is still a handful of Viking scientists who wonder if there might be extremely tough and competent organisms very thinly spread over the Martian soil—so their organic chemistry could not be detected, but their metabolic processes could. Such scientists do not deny that ultraviolet-generated oxidants are present in the Martian soil, but stress that no thorough explanation of the Viking life detection results from oxidants alone has been forthcoming. Tentative claims have been made of organic matter in SNC meteorites, but they seem instead to be contaminants that have entered the meteorite after its arrival on our world. So far, there are no claims of Martian microbes in these rocks from the sky.

Perhaps because it seems to pander to public interest, NASA and most Viking scientists have been very chary about pursuing the biological hypothesis. Even now, much more could be done in going over the old data, in looking with Viking-type instruments at Antarctic and other soils that have few microbes in them, in laboratory simulation of the role of oxidants in the Martian soil, and in designing experiments to elucidate these matters—not excluding further searches for life—with future Mars landers.

If indeed no unambiguous signatures of life were determined by a variety of sensitive experiments at two sites 5,000 kilometers apart on a planet marked by global wind transport of fine particles, this is at least suggestive that Mars may be, today at least, a lifeless planet. But if Mars is lifeless, we have two planets, of virtually identical age and early conditions, evolving next door to one another in the same solar system: Life evolves and proliferates on one, but not the other. Why?

Perhaps the chemical or fossil remains of early Martian life can still be found—subsurface, safely protected from the ultraviolet radiation and its oxidation products that today fry the surface. Perhaps in a rock face exposed by a landslide, or in the banks of an ancient river valley or dry lake bed, or in the polar, laminated terrain, key evidence for life on another planet is waiting.

Despite its absence on the surface of Mars, the planet’s two moons, Phobos and Deimos, seem to be rich in complex organic matter dating back to the early history of the Solar System. The Soviet Phobos 2spacecraft found evidence of water vapor being outgassed from Phobos, as if it has an icy interior heated by radioactivity. The moons of Mars may have long ago been captured from somewhere in the outer Solar System; conceivably, they are among the nearest available examples of unaltered stuff from the earliest days of the Solar System. Phobos and Deimos are very small, each roughly 10 kilometers across; the gravity they exert is nearly negligible. So it’s comparatively easy to rendezvous with them, land on them, examine them, use them as a base of operations to study Mars, and then go home.

Mars calls, a storehouse of scientific information—important in its own right but also for the light it casts on the environment of our own planet. There are mysteries waiting to be resolved about the interior of Mars and its mode of origin, the nature of volcanos on a world without plate tectonics, the sculpting of landforms on a planet with sandstorms undreamt of on Earth, glaciers and polar landforms, the escape of planetary atmospheres, and the capture of moons—to mention a more or less random sampling of scientific puzzles. If Mars once had abundant liquid water and a clement climate, what went wrong? How did an Earthlike world become so parched, frigid, and comparatively airless? Is there something here we should know about our own planet?

We humans have been this way before. The ancient explorers would have understood the call of Mars. But mere scientific exploration does not require a human presence. We can always send smart robots. They are far cheaper, they don’t talk back, you can send them to much more dangerous locales, and, with some chance of mission failure always before us, no lives are put at risk.

“HAVE YOU SEEN ME?” the back of the milk carton read. “Mars Observer, 6’ × 4.5’ × 3’, 2500 kg. Last heard from on 8/21/93, 627,000 km from Mars.”

M. O. call home” was the plaintive message on a banner hung outside the Jet Propulsion Laboratory’s Mission Operations Facility in late August 1993. The failure of the United States’ Mars Observerspacecraft just before it was to insert itself into orbit around Mars was a great disappointment. It was the first post-launch mission failure of an American lunar or planetary spacecraft in 26 years. Many scientists and engineers had devoted a decade of their professional lives to M. O. It was the first U.S. mission to Mars in 17 years—since Viking’s two orbiters and two landers in 1976. It was also the first real post—Cold War spacecraft: Russian scientists were on several of the investigator teams, and Mars Observer was to act as an essential radio relay link for landers from what was then scheduled to be the Russian Mars ’94mission, as well as for a daring rover and balloon mission slated for Mars ’96.

The scientific instruments aboard Mars Observer would have mapped the geochemistry of the planet and prepared the way for future missions, guiding landing site decisions. It might have cast a new light on the massive climate change that seems to have occurred in early Martian history. It would have photographed some of the surface of Mars with detail better than two meters across. Of course, we do not know what wonders Mars Observer would have uncovered. But every time we examine a world with new instruments and in vastly improved detail, a dazzling array of discoveries emerges—just as it did when Galileo turned the first telescope toward the heavens and opened the era of modern astronomy.

According to the Commission of Inquiry, the cause of the failure was probably a rupture of the fuel tank during pressurization, gases and liquids sputtering out, and the wounded spacecraft spinning wildly out of control. Perhaps it was avoidable. Perhaps it was an unlucky accident. But to keep this matter in perspective, let’s consider the full range of missions to the Moon and the planets attempted by the United States and the former Soviet Union:

In the beginning, our track records were poor. Space vehicles blew up at launch, missed their targets, or failed to function when they got there. As time went on, we humans got better at interplanetary flight. There was a learning curve. The adjacent figures show these curves (based on NASA data with NASA definitions of mission success). We learned very well. Our present ability to fix spacecraft in flight is best illustrated by the Voyager missions described earlier.

We see that it wasn’t until about its thirty-fifth launch to the Moon or the planets that the cumulative U.S. mission success rate got as high as 50 percent. The Russians took about 50 launches to get there. Averaging the shaky start and the better recent performance, we find that both the United States and Russia have a cumulative launch success rate of about 80 percent. But the cumulative mission success rate is still under 70 percent for the U.S. and under 60 percent for the U.S.S.R./Russia. Equivalently, lunar and planetary missions have failed on average 30 or 40 percent of the time.

Missions to other worlds were from the beginning at the cutting edge of technology. They continue to be so today. They are designed with redundant subsystems, and operated by dedicated and experienced engineers, but they are not perfect. The amazing thing is not that we have done so poorly, but that we have done so well.

We don’t know whether the Mars Observer failure was due to incompetence or just statistics. But we must expect a steady background of mission failures when we explore other worlds. No human lives are risked when a robot spacecraft is lost. Even if we were able to improve this success rate significantly, it would be far too costly. It is much better to take more risks and fly more spacecraft.

Knowing about irreducible risks, why do we these days fly only one spacecraft per mission? In 1962 Mariner 1, intended for Venus, fell into the Atlantic; the nearly identical Mariner 2 became the human species’ first successful planetary mission. Mariner 3 failed, and its twin Mariner 4 became, in 1964, the first spacecraft to take close-up pictures of Mars. Or consider the 1971 Mariner 8/Mariner 9 dual launch mission to Mars. Mariner 8 was to map the planet. Mariner 9 was to study the enigmatic seasonal and secular changes of surface markings. The spacecraft were otherwise identical. Mariner 8 fell into the ocean. Mariner 9 flew on to Mars and became the first spacecraft in human history to orbit another planet. It discovered the volcanos, the laminated terrain in the polar caps, the ancient river valleys, and the aeolian nature of the surface changes. It disproved the “canals.” It mapped the planet pole to pole and revealed all the major geological features of Mars known to us today. It provided the first close-up observations of members of a whole class of small worlds (by targeting the Martian moons, Phobos and Deimos). If we had launched only Mariner 8, the endeavor would have been an unmitigated failure. With a dual launch it became a brilliant and historic success.

There were also two Vikings, two Voyagers, two Vegas, many pairs of Veneras. Why was only one Mars Observer flown? The standard answer is cost. Part of the reason it was so costly, though, is that it was planned to be launched by shuttle, which is an almost absurdly expensive booster for planetary missions—in this case too expensive for two M. O. launches. After many shuttle-connected delays and cost increases, NASA changed its mind and decided to launch Mars Observer on a Titan booster. This required an additional two-year delay and an adapter to mate the spacecraft to the new launch vehicle. If NASA had not been so intent on providing business for the increasingly uneconomic shuttle, we could have launched a couple of years earlier and maybe with two spacecraft instead of one.

But whether in single launches or in pairs, the spacefaring nations have clearly decided that the time is ripe to return robot explorers to Mars. Mission designs change; new nations enter the field; old nations find they no longer have the resources. Even already funded programs cannot always be relied upon. But current plans do reveal something of the intensity of effort and the depth of dedication.

As I write this book, there are tentative plans by the United States, Russia, France, Germany, Japan, Austria, Finland, Italy, Canada, the European Space Agency, and other entities for a coordinated robotic exploration of Mars. In the seven years between 1996 and 2003, a flotilla of some twenty-five spacecraft—most of them comparatively small and cheap—are to be sent from Earth to Mars. There will be no quick flybys among them; these are all long-duration orbiter and lander missions. The United States will refly all of the scientific instruments that were lost on Mars Observer. The Russian spacecraft will contain particularly ambitious experiments involving some twenty nations. Communications satellites will permit experimental stations anywhere on Mars to relay their data back to Earth. Penetrators screeching down from orbit will punch into the Martian soil, transmitting data from underground. Instrumented balloons and roving laboratories will wander over the sands of Mars. Some microrobots will weigh no more than a few pounds. Landing sites are being planned and coordinated. Instruments will be cross-calibrated. Data will be freely exchanged. There is every reason to think that in the coming years Mars and its mysteries will become increasingly familiar to the inhabitants of the planet Earth.

IN THE COMMAND CENTER on Earth, in a special room, you are helmeted and gloved. You turn your head to the left, and the cameras on the Mars robot rover turn to the left. You see, in very high definition and in color, what the cameras see. You take a step forward, and the rover walks forward. You reach out your arm to pick up something shiny in the soil, and the robot arm does likewise. The sands of Mars trickle through your fingers. The only difficulty with this remote reality technology is that all this must occur in tedious slow motion: The round-trip travel time of the up-link commands from Earth to Mars and the down-link data returned from Mars to Earth might take half an hour or more. But this is something we can learn to do. We can learn to contain our exploratory impatience if that’s the price of exploring Mars. The rover can be made smart enough to deal with routine contingencies. Anything more challenging, and it makes a dead stop, puts itself into a safeguard mode, and radios for a very patient human controller to take over.

Conjure up roving, smart robots, each of them a small scientific laboratory, landing in the safe but dull places and wandering to view close-up some of that profusion of Martian wonders. Perhaps every day a robot would rove to its own horizon; each morning we would see close-up what had yesterday been only a distant eminence. The lengthening progress of a traverse route over the Martian landscape would appear on news programs and in schoolrooms. People would speculate on what will be found. Nightly newscasts from another planet, with their revelations of new terrains and new scientific findings, would make everyone on Earth a party to the adventure.

Then there’s Martian virtual reality: The data sent back from Mars, stored in a modern computer, are fed into your helmet and gloves and boots. You are walking in an empty room on Earth, but to you you are on Mars: pink skies, fields of boulders, sand dunes stretching to the horizon where an immense volcano looms; you hear the sand crunching under your boots, you turn rocks over, dig a hole, sample the thin air, turn a corner, and come face to face with … whatever new discoveries we will make on Mars—all exact copies of what’s on Mars, and all experienced from the safety of a virtual reality salon in your hometown. This is not why we explore Mars, but clearly we will need robot explorers to return the real reality before it can be reconfigured into virtual reality.

Especially with continuing investment in robotics and machine intelligence, sending humans to Mars can’t be justified by science alone. And many more people can experience the virtual Mars than could possibly be sent to the real one. We can do very well with robots. If we’re going to send people, we’ll need a better reason than science and exploration.

In the 1980s, I thought I saw a coherent justification for human missions to Mars. I imagined the United States and the Soviet Union, the two Cold War rivals that had put our global civilization at risk, joining together in a far-seeing, high-technology endeavor that would give hope to people everywhere. I pictured a kind of Apollo program in reverse, in which cooperation, not competition, was the driving force, in which the two leading space-faring nations would together lay the groundwork for a major advance in human history—the eventual settlement of another planet.

The symbolism seemed so apt. The same technology that can propel apocalyptic weapons from continent to continent would enable the first human voyage to another planet. It was a choice of fitting mythic power: to embrace the planet named after, rather the madness ascribed to, the god of war.

We succeeded in interesting Soviet scientists and engineers in such a joint endeavor. Roald Sagdeev, then director of the Institute for Space Research of the Soviet Academy of Sciences in Moscow, was already deeply engaged in international cooperation on Soviet robotic missions to Venus, Mars, and Halley’s Comet, long before the idea became fashionable. Projected joint use of the Soviet Mir space station and the Saturn V-class launch vehicle Energiya made cooperation attractive to the Soviet organizations that manufactured these items of hardware; they were otherwise having difficulty justifying their wares. Through a sequence of arguments (helping to bring the Cold War to an end being chief among them), then-Soviet leader Mikhail S. Gorbachev was convinced. During the December 1987 Washington summit, Mr. Gorbachev—asked what was the most important joint activity through which the two countries might symbolize the change in their relationship—unhesitatingly replied, “Let’s go to Mars together.”

But the Reagan Administration was not interested. Cooperating with the Soviets, acknowledging that certain Soviet technologies were more advanced than their American counterparts, making some American technology available to the Soviets, sharing credit, providing an alternative for the arms manufacturers—these were not to the Administration’s liking. The offer was turned down. Mars would have to wait.

In only a few years, times have changed. The Cold War is over. The Soviet Union is no more. The benefit deriving from the two nations working together has lost some of its force. Other nations—especially Japan and the constituent members of the European Space Agency—have become interplanetary travelers. Many just and urgent demands are levied on the discretionary budgets of the nations.

But the Energiya heavy-lift booster still awaits a mission. The workhorse Proton rocket is available. The Mir space station—with a crew on board almost continuously—still orbits the Earth every hour and a half. Despite internal turmoil, the Russian space program continues vigorously. Cooperation between Russia and America in space is accelerating. A Russian cosmonaut, Sergei Krikalev, in 1994 flew on the shuttle Discovery (for the usual one-week shuttle mission duration; Krikalev had already logged 464 days aboard the Mir space station). U.S. astronauts will visit Mir. American instruments—including one to examine the oxidants thought to destroy organic molecules in the Martian soil—are to be carried by Russian space vehicles to Mars. Mars Observer was designed to serve as a relay station for landers in Russian Mars missions. The Russians have offered to include a U.S. orbiter in a forthcoming Proton-launched multi-payload mission to Mars.

The American and Russian capabilities in space science and technology mesh; they interdigitate. Each is strong where the other is weak. This is a marriage made in heaven—but one that has been surprisingly difficult to consummate.

On September 2, 1993, an agreement to cooperate in depth was signed in Washington by Vice President Al Gore and Prime Minister Viktor Chernomyrdin. The Clinton Administration has ordered NASA to redesign the U.S. space station (called Freedom in the Reagan years) so it is in the same orbit as Mir and can be mated to it: Japanese and European modules will be attached, as will a Canadian robot arm. The designs have now evolved into what is called Space Station Alpha, involving almost all the spacefaring nations. (China is the most notable exception.)

In return for U.S. space cooperation and an infusion of hard currency, Russia in effect agreed to halt its sale of ballistic missile components to other nations, and generally to exercise tight controls on its export of strategic weapons technology. In this way, space becomes once again, as it was at the height of the Cold War, an instrument of national strategic policy.

This new trend has, though, made some of the American aerospace industry and some key members of Congress, profoundly uneasy. Without international competition, can we motivate such ambitious efforts? Does every Russian launch vehicle used cooperatively mean less support for the American aerospace industry? Can Americans rely on stable support and continuity of effort in joint projects with the Russians? (The Russians, of course, ask similar questions about the Americans.) But cooperative programs in the long term save money, draw upon the extraordinary scientific and engineering talent distributed over our planet, and provide inspiration about the global future. There may be fluctuations in national commitments. We are likely to take backward as well as forward steps. But the overall trend seems clear.

Despite growing pains, the space programs of the two former adversaries are beginning to join. It is now possible to foresee a world space station—not of any one nation but of the planet Earth—being assembled at 51° inclination to the equator and a few hundred miles up. A dramatic joint mission, called “Fire and Ice,” is being discussed—in which a fast flyby would be sent to Pluto, the last unexplored planet; but to get there, a gravity assist from the Sun would be employed, in the course of which small probes would actually enter the Sun’s atmosphere. And we seem to be on the threshold of a World Consortium for the scientific exploration of Mars. It very much looks as though such projects will be done cooperatively or not at all.

WHETHER THERE ARE VALID, cost-effective, broadly supportable reasons for people to venture to Mars is an open question. Certainly there is no consensus. The matter is treated in the next chapter.

I would argue that if we are not eventually going to send people to worlds as far away as Mars, we have lost the chief reason for a space station—a permanently (or intermittently) occupied human outpost in Earth orbit. A space station is far from an optimum platform for doing science—either looking down at the Earth, or looking out into space, or for utilizing microgravity (the very presence of astronauts messes things up). For military reconnaissance it is much inferior to robotic spacecraft. There are no compelling economic or manufacturing applications. It is expensive compared to robotic spacecraft. And of course it runs some risk of losing human lives. Every shuttle launch to help build or supply a space station has an estimated 1 or 2 percent chance of catastrophic failure. Previous civilian and military space activities have littered low Earth orbit with fast-moving debris—that sooner or later will collide with a space station (although, so far, Mir has had no failures from this hazard). A space station is also unnecessary for human exploration of the Moon. Apollo got there very well with no space station at all. With Saturn V or Energiya class launchers, it also may be possible to get to near-earth asteroids or even Mars without having to assemble the interplanetary vehicle on an orbiting space station.

A space station could serve inspirational and educational purposes, and it certainly can help to solidify relations among the spacefaring nations—particularly the United States and Russia. But the only substantive function of a space station, as far as I can see, is for long-duration spaceflight. How do humans behave in microgravity? How can we counter progressive changes in blood chemistry and an estimated 6 percent bone loss per year in zero gravity? (For a three- or four-year mission to Mars this adds up, if the travelers have to go at zero g.)

These are hardly questions in fundamental biology such as DNA or the evolutionary process; instead they address issues of applied human biology. It’s important to know the answers, but only if we intend to go somewhere in space that’s far away and takes a long time to get there. The only tangible and coherent goal of a space station is eventual human missions to near-Earth asteroids, Mars, and beyond. Historically NASA has been cautious about stating this fact clearly, probably for fear that members of Congress will throw up their hands in disgust, denounce the space station as the thin edge of an extremely expensive wedge, and declare the country unready to commit to launching people to Mars. In effect, then, NASA has kept quiet about what the space station is really for. And yet if we had such a space station, nothing would require us to go straight to Mars. We could use a space station to accumulate and refine the relevant knowledge, and take as long as we like to do so—so that when the time does come, when we are ready to go to the planets, we will have the background and experience to do so safely.

The Mars Observer failure, and the catastrophic loss of the space shuttle Challenger in 1986, remind us that there will be a certain irreducible chance of disaster in future human flights to Mars and elsewhere. The Apollo 13 mission, which was unable to land on the Moon and barely returned safely to Earth, underscores how lucky we’ve been. We cannot make perfectly safe autos or trains even though we’ve been at it for more than a century. Hundreds of thousands of years after we first domesticated fire, every city in the world has a service of firefighters biding their time until there’s a blaze that needs putting out. In Columbus’ four voyages to the New World, he lost ships left and right, including one third of the little fleet that set out in 1492.

If we are to send people, it must be for a very good reason—and with a realistic understanding that almost certainly we will lose lives. Astronauts and cosmonauts have always understood this. Nevertheless, there has been and will be no shortage of volunteers.

But why Mars? Why not return to the Moon? It’s nearby, and we’ve proved we know how to send people there. I’m concerned that the Moon, close as it is, is a long detour, if not a dead end. We’ve been there. We’ve even brought some of it back. People have seen the Moon rocks, and, for reasons that I believe are fundamentally sound, they are bored by the Moon. It’s a static, airless, waterless, black-sky, dead world. Its most interesting aspect perhaps is its cratered surface, a record of ancient catastrophic impacts, on the Earth as well as on the Moon.

Mars, by contrast, has weather, dust storms, its own moons, volcanos, polar ice caps, peculiar landforms, ancient river valleys, and evidence of massive climatic change on a once-Earthlike world. It holds some prospect of past or maybe even present life, and is the most congenial planet for future life—humans transplanted from Earth, living off the land. None of this is true for the Moon. Mars also has its own legible cratering history. If Mars, rather than the Moon, had been within easy reach, we would not have backed off from manned space flight.

Nor is the Moon an especially desirable test bed or way station for Mars. The Martian and lunar environments are very different, and the Moon is as distant from Mars as is the Earth. The machinery for Martian exploration can at least equally well be tested in Earth orbit, or on near-Earth asteroids, or on the Earth itself—in Antarctica, for instance.

Japan has tended to be skeptical of the commitment of the United States and other nations to plan and execute major cooperative projects in space. This is at least one reason that Japan, more than any other spacefaring nation, has tended to go it alone. The Lunar and Planetary Society of Japan is an organization representing space enthusiasts in the government, universities, and major industries. As I write, the Society is proposing to construct and stock a lunar base entirely with robot labor. It is said to take about 30 years and to cost about a billion U.S. dollars a year (which would represent 7 percent of the present U.S. civilian space budget). Humans would arrive only when the base is fully ready. The use of robot construction crews under radio command from Earth is said to reduce the cost tenfold. The only trouble with the scheme, according to reports, is that other scientists in Japan keep asking, “What’s it for?” That’s a good question in every nation.

The first human mission to Mars is now probably too expensive for any one nation to pull off by itself. Nor is it fitting that such a historic step be taken by representatives of only a small fraction of the human species. But a cooperative venture among the United States, Russia, Japan, the European Space Agency—and perhaps other nations, such as China—might be feasible in the not too distant future. The international space station will have tested our ability to work together on great engineering projects in space.

The cost of sending a kilogram of something no farther away than low Earth orbit is today about the same as the cost of a kilogram of gold. This is surely a major reason we have yet to stride the ancient shorelines of Mars. Multistage chemical rockets are the means that first took us into space, and that’s what we’ve been using ever since. We’ve tried to refine them, to make them safer, more reliable, simpler, cheaper. But that hasn’t happened, or at least not nearly as quickly as many had hoped.

So maybe there’s a better way: maybe single-stage rockets that can launch their payloads directly to orbit; maybe many small payloads shot from guns or rocket-launched from airplanes; maybe supersonic ramjets. Maybe there’s something much better that we haven’t thought of yet. If we can manufacture propellants for the return trip from the air and soil of our destination world, the difficulty of the voyage would be greatly eased.

Once we’re up there in space, venturing to the planets, rocketry is not necessarily the best means to move large payloads around, even with gravity assists. Today, we make a few early rocket burns and later midcourse corrections, and coast the rest of the way. But there are promising ion and nuclear/electric propulsion systems by which a small and steady acceleration is exerted. Or, as the Russian space pioneer Konstantin Tsiolkovsky first envisioned, we could employ solar sails—vast but very thin films that catch sunlight and the solar wind, a caravel kilometers wide plying the void between the worlds. Especially for trips to Mars and beyond, such methods are far better than rockets.

As with most technologies, when something barely works, when it’s the first of its kind, there’s a natural tendency to improve it, develop it, exploit it. Soon there’s such an institutional investment in the original technology, no matter how flawed, that it’s very hard to move on to something better. NASA has almost no resources to pursue alternative propulsion technologies. That money would have to come out of near-term missions, missions which could provide concrete results and improve NASA’s success record. Spending money on alternative technologies pays off a decade or two in the future. We tend to be very little interested in a decade or two in the future. This is one of the ways by which initial success can sow the seeds of ultimate failure; and is very similar to what sometimes happens in biological evolution. But sooner or later some nation—perhaps one without a huge investment in marginally effective technology—will develop effective alternatives.

Even before then, if we take a cooperative path, there will come a time—perhaps in the first decades of the new century and the new millennium—when an interplanetary spacecraft is assembled in Earth orbit, the progress in full view on the evening news. Astronauts and cosmonauts, hovering like gnats, guide and mate the prefabricated parts. Eventually the ship, tested and ready, is boarded by its international crew, and boosted to escape velocity. For the whole of the voyage to Mars and back, the lives of the crew members depend on one another, a microcosm of our actual circumstances down here on Earth. Perhaps the first joint interplanetary mission with human crews will be only a flyby or orbit of Mars. Earlier, robot vehicles, with aerobraking, parachutes, and retrorockets, will have set gently down on the Martian surface to collect samples and return them to Earth, and to emplace supplies for future explorers. But whether or not we have compelling, coherent reasons, I am sure—unless we destroy ourselves first—that the day will come when we humans set foot on Mars. It is only a matter of when.

According to solemn treaty, signed in Washington and Moscow on January 27, 1967, no nation may lay claim to part or all of another planet. Nevertheless—for historical reasons that Columbus would have understood well—some people are concerned about who first sets foot on Mars. If this really worries us, we can arrange for the ankles of the crew members to be tied together as they alight in the gentle Martian gravity.

The crews would acquire new and previously sequestered samples, in part to search for life, in part to understand the past and future of Mars and Earth. They would experiment, for later expeditions, on extracting water, oxygen, and hydrogen from the rocks and the air and from the underground permafrost—to drink, to breathe, to power their machines and, as rocket fuel and oxidizer, to propel the return voyage. They would test Martian materials for eventual fabrication of bases and settlements on Mars.

And they would go exploring. When I imagine the early human exploration of Mars, it’s always a roving vehicle, a little like a jeep, wandering down one of the valley networks, the crew with geological hammers, cameras, and analytic instruments at the ready. They’re looking for rocks from ages past, signs of ancient cataclysms, clues to climate change, strange chemistries, fossils, or—most exciting and most unlikely—something alive. Their discoveries are televised back to Earth at the speed of light. Snuggled up in bed with the kids, you explore the ancient riverbeds of Mars.

*Although in a few places, such as the slopes of the elevation called Alba Patera, there are multibranched valley networks that by comparison are very young. Somehow, even in the most recent billion years, liquid water seems to have flowed here and there, from time to time, through the deserts of Mars.

*Short for Shergotty-Nakhla-Chassigny. You can see why the acronym is used.