whole basin was flooded, its waters kept warm, perhaps, by the volcanic fires of Apollinaris Patera, whose lava flows reach almost to its northern rim. Slowly the basin dried up; the waters sank to ever lower levels. Now they seem completely gone–unless, perhaps, their remnants form the icy cores of the strange rounded hills in the basin’s heart. But once the waters were there, sheltering under a Martian sky.
Nathalie Cabrol has studied Gusev crater with her partner, Edmond Grin, for more than a decade. Her colleagues, especially those beyond the confines of her base at NASA Ames Research Center, say she’s obsessed; she denies it. But she rarely lets slip an opportunity to tell people that, if they want to explore a site on Mars that might once have held life, then Gusev is the place to go. As Chris McKay, another researcher at Ames, points out, liquid water is the key. On Earth, at least, where water is not, there life is not; where it is, there is life.
In the great catalog of extreme environments where life manages to persist– around volcanoes on the oceans’ floors, in the rocks of deep bore holes, in the cores of nuclear reactors, under the surfaces of
antarctic pebbles–water is the one thing always present. It may not be there all the time, but it’s there just enough. And if the same holds true on Mars, then Mars should once have been alive. Its rocks should hold fossils. If, late in 2003, a rover trundles out across the surface of Gusev, drilling into its rock and collecting samples, the smile on Cabrol’s face will be something to see. If those samples turn out to contain fossils, the joy will spread far wider.
There are two space programs currently underway that can be compared to Apollo in their ambition and complexity, if not their newsiness. One is the creation of the International Space Station, an impressive piece of engineering that will allow a small crew of astronauts to do stuff that they currently do on space shuttles, but more of it. The other, less than a tenth as costly, is the mission to bring samples of the surface of Mars back to Earth, a campaign that will go on for years and involve more than a dozen vehicles and robots. The 2003 Mars landing will be the first stage, putting down a rover at some promising site near the planet’s equator, such as Gusev. The rover will find likely looking rocks and drill samples out of them, and then load these treasures into a tiny rocket. That will lift the samples into orbit around Mars, where they will wait until a bigger spacecraft comes to scoop them up, along with fresher samples from a second site, and head back to Earth with them. The samples, now sealed safely into a re-entry capsule, will slide down Earth’s gravity well to a United States Air Force range in Utah sometime in 2008. As far as exploration goes, this mission will be the first great achievement of the next 30 years in space.
At the time of Apollo, Mars scarcely seemed worth such trou- ble. In the 1960s and 1970s all the century’s dreams of life on Mars–whether they were of grand civilizations building canals or humble lichen spreading with he seasons–were systematically dashed on the rocks of hard data. The first probes to fly past the planet revealed an almost airless world covered in craters, a place with all the desolation of Earth’s moon and much harder to get to. After landing a pair of robots that found no real signs of life, Earth gave up on the Red Planet. Between 1976 and 1996 no spacecraft was to visit it; the one attempt to do so, NASA’s Mars Observer, was apparently blown apart by an explosion en route.
Out of its ashes came a new way of exploring space, one that took lots of little steps instead of one big one. In the early 1990s NASA’s then still-new administrator, Dan Goldin, was trying to imbue the agency with a “faster, better, cheaper” ethos. As part of this it was decided that copies of the instruments from Mars Observer, a billion-dollar mission, should be sent to their destination a few at a time, on light, cheap spacecraft. The Mars Surveyor program is launching two spacecraft to Mars every two years–a previously unheard-of rate. Now the first “micromissions” are being added to the program, truly tiny spacecraft that can hitchhike into space on rockets used for other purposes. This year one Surveyor spacecraft will land near the south pole of Mars, another will go into orbit to study the climate and two basketball-sized microprobes will slap into the surface at 400 mph to see what goes on below it. Later missions include the rovers that will send back samples and a new sort of planetary explorer–an aircraft to fly through the skies of Mars 100 years to the day after the Wright brothers’ first flight at Kitty Hawk.
The Surveyor program, along with other missions by Japan and particularly France–which is also providing the spacecraft that is to bring NASA’s samples home–marks a new sort of space exploration: not looking and leaving, but staying and studying. It marks the first permanent human presence beyond Earth orbit–a disembodied presence, but still a meaningful one. For the foreseeable future there will always be something or other gathering data from Mars and sending it back to Earth. A dedicated communications link–a set of comsats for the robots of Mars like those that serve the people of Earth–is under development, and with that in place, the rate at which data return should improve, well, astronomically. The databases will wax vast; the virtual realities will get real; Mars will become a fully integrated province of cyberspace.
In mind if not in body, scientists and the public will explore an alien world–and see much that is strangely familiar. Carl Pilcher, science director for NASA’s solar-system-exploration program, says the Surveyor missions are already revealing a Mars that, far from being dead and dull, is “astonishingly Earthlike” in all sorts of ways. Some of its volcanoes are so young they may still be active; the planet’s crust, once thought primordial, shows signs of having been recycled, at least in part, as Earth’s has been. And though today Mars is too cold, and its air too thin, for any water to persist on its surface, in the past it was very different.
Since the ’70s it has been clear that water marked the Martian surface by flowing across it, but it was possible that such flows might have been terribly short-lived. Now it looks increasingly likely that water pooled on the surface, persisting for centuries, millenniums or more. There are lake beds like Gusev; some see traces of glaciers, which could only have grown through season after season of snowfall; and there are ever stronger hints of ancient oceans across the vast, low-lying basins of the north. There is likely to be ice in the soil round the polar caps–this year’s Surveyor mission, the polar lander, will find out how much. There may well still be liquid water sealed below the cold crust: a subterranean sea trapped in the pores of the planet’s rock.
The rocks of Mars may bear fossils. The frosts of its highlands might hold deep-frozen samples of once living beings. And it is just conceivable that the deep waters, isolated from the dead and desiccated surface by kilometers of freezing rock, might still hold not just traces of life, but life itself. If there is true alien life on Mars, then it is not unlikely that there’s life all over the place. The night sky would never look the same again.
By most standards, the solar system is pretty big. By the standards of those looking for life, it’s a little small. There’s once-wet Mars, already
the focus of the most ambitious of the robotic exploration programs. There’s Jupiter’s moon Europa–a particularly challenging target in that it is drenched by radiation and whirls around its parent planet at high speed–which seems to have a vast ocean beneath its icy crust. That is the destination of NASA’s next mission to the outer reaches of solar system. And that is just about it. While there is ice more or less all over the place, from the poles of Mercury to the wastes beyond Pluto, Earth, Mars and Europa are the only three places where there are known to be bodies of liquid water. (Other moons may also have seas beneath their surfaces, but the jury is still out.) But the planets and moons of our solar system are not the only ones there are–or even the only ones we can study. In the past four years, astronomers using Earth-based telescopes have discovered indirect but compelling evidence for about 20 large planets around other stars.
These big planets are interesting in and of themselves; but small, Earthlike ones would be much more so. And they, too, should soon be discovered, if they are out there. In the next 10 years space-based telescopes should provide indirect evidence for the existence of planets as small as the Earth or smaller. Soon after that, more ambitious instruments should be able to detect those planets directly, and perhaps to sense the presence of water and life in their atmospheres. That will mark the second great triumph of the next 30 years of exploration.
The technique that will make this possible is interferometry. An interferometer takes light from various subsidiary telescopes and combines them in such a way as to pick out details that would normally be visible only to a much larger telescope: an interferometer combining light from two telescopes 100 meters away from each other can be as sharp-eyed in this respect as a telescope with a mirror 100 meters across. Interferometers can also be used not to see things. It is possible to arrange things so that the different images of the object at the center of the telescopes’ field of view cancel each other out when the light from the different telescopes is brought together. This technique–nulling–is very hard to pull off, but very useful; it means that you can effectively blank out the brilliant light from a star when looking for the dim planets around it. Since the planets are a million or so times less bright than the star, this helps a lot.
Both America and Europe have plans to launch interferometers designed to pick out Earthlike planets some time around 2010. Everyone concerned knows that, in all likelihood, the efforts will be combined into a single mission. NASA’s current design for its Terrestrial Planet Finder (TPF) makes use of five separate spacecraft, four carrying telescopes designed to make use of infrared light (because the difference in brightness between stars and planets is less terrifying at these slightly longer wavelengths) and a fifth, the hub, containing the interferometer’s heart. To pick up the dim glow of a planet tens of light-years away each telescope will have a mirror perhaps four meters across, giving them a total surface area more than ten times that of the Hubble Space Telescope. The four telescopes would fly in a formation anything from 75 meters to a kilometer across, giving the interferometer the resolving power needed to distinguish the dim planets from the fiery stars around which they circle. The different parts of the system will need to know where each other is to within a billionth of a meter as they pass the faint traces of the planet’s light between themselves in a complex cat’s cradle of beams.
TPF will not just find planets; it will also start to study them. If it can be made to point steadily at a planet for two weeks or so, it will pick up enough of the infrared light given off to start analyzing it, picking out wavelengths that correspond to the presence of water vapor, carbon dioxide, methane, ozone and the like in the planet’s atmosphere. As the British scientist James Lovelock pointed out in the late 1960s, Earth’s atmosphere is a long, slow flame. Some of its constituents–for example, oxygen and methane–continually react with each other. As the oxygen and methane use each other up, so life produces more of them. Thus Earth’s atmosphere is kept far from the chemical equilibrium it would be in if there were no life to keep it on its toes. The atmospheres of the rest of the solar system’s other planets, on the other hand, are as balanced as the scales of justice. When TPF looks at the infrared light from Earth-sized planets around other stars, it should be able to sniff out any peculiarly lively imbalances.
Will there be such an imbalance, a sign of life elsewhere? No one knows. The single relevant certainty is that life has persisted on Earth for 3.8 billion years, a fact that says nothing about its likeliness elsewhere. It may be abundant or it may be very rare. If we were to find evidence of past life on Mars, then we might begin to think life quite commonplace. But only if we were sure that the Martian fossils were not our own long-lost relatives. Meteorites can be blasted from one planet to another–a transit system that was more reliable in the early solar system than it is today–and bacteria seem to be resilient enough to survive such journeys. It is possible that our earliest ancestors crossed the gulf between Earth and Mars, that life originated on only one of the two planets and then spread to the other. That would say something new about life’s resilience in the solar system, but not about the likelihood of finding it elsewhere.
We do not know on which planet our earliest ancestors lived. We have no idea on what other planets life might now be thriving. Those are the basic justifications for the Mars missions, for TPF, for most of the interesting bits of the space program. Scientific investigations, like tennis matches, are undertaken not because we know what the result will be, but because we don’t. That’s why we play.
Thirty years ago, mankind went out into space and saw it to be dull and lifeless. Thirty years hence the prospect will look very different. Mars will have been examined up close in a number of places; chunks of its surface, and of other bodies, will have been studied in laboratories on Earth. Balloons or even aircraft will have visited the atmospheres of Mars, Venus and Saturn’s moon Titan. They may even have ventured into the gaudy cloudscapes of Jupiter itself. Midget submarines will have penetrated the permanent icecap of Jupiter’s moon Europa to sample the ocean underneath. Asteroids and comets will have been visited by the dozen, some by solar-sailing ships, gossamer spacecraft using the pressure of sunlight to fly around the solar system. And far from all this disturbance, interferometers a generation beyond TPF will have seen what there is to be seen of dozens of solar systems, picking out planets as small as little Mercury, perhaps even seeing moons around alien Jupiters. Work on tiny probes that could actually visit other stars will be underway, even if their launches will still be decades off. What was once lumped together as space will have become resolved into a vast array of places, some boring to all but the experts, some truly fascinating.
All this is true whether or not humans venture farther than their low-flying space stations. There is no need for people to actually be there for any of these things to get done. All that’s needed is a continuation of today’s robotic space-science programs at today’s budget levels. Some see that as reason enough to think that human exploration is over for the foreseeable future. As John Pike, who analyzes space policy for the Federation of American Scientists, puts it, “The Von Braun paradigm has been broken.” Wernher Von Braun, the engineer-king of Apollo, thought that robot missions would naturally lead to human ones. The programs would always get bigger and more complex; they would thus require bigger rockets and bigger spacecraft, and so there would be a need for systems big enough for bulky humans and their supplies. He did not envision “faster, better, cheaper”; given the way that space science is now done, there is no foreseeable need for any sort of booster that can take humans beyond the space station.
Nor does politics argue for a return to deep space. Apollo was justified by rivalry with Russia; so, to begin with, was the space station. After 1989, the space station–in which a great deal of money was already tied up–was reinvented as a post-cold-war project. It became, in principle, a peace-affirming collaboration with the former enemy, one with the happy side effect of giving Russia’s underemployed engineers something benign to devote their talents to.
The most obvious objective for people in space beyond the space station is a series of Mars missions; their crews would be far more capable than any rovers currently envisaged, able to dig holes and swing geological hammers with ease and grace. But, though such a set of missions would not need to be any more expensive than the space station–it might even be cheaper–it would still require a budget 10 or 20 times bigger than the shoestring on which the Surveyor program survives. Any new project on that sort of scale will, on the evidence of the past, require a justification beyond simple science. In the furor over the subsequently much-disputed signs of life in a Martian meteorite, Bill Clinton said that he would “put the full intellectual and technological might of the United States behind the search for life on Mars”; but as everyone associated with the Surveyor program will be quick to tell you, he hasn’t. Just like sending people to the moon or building the space station, missions to put humans on Mars would require a political rationale, and now that history is over, it is not clear what that might be. Bob Zubrin, the engineer whose work on the design of Mars missions can take much of the credit for making them even vaguely feasible, argues that the point of such an adventure would be to get history going again. Through his writing, and the lobbying efforts of the Mars Society that he has founded, Zubrin pushes the idea that America and the world at large require a new frontier. Mars, he says, is it. As yet, though, the idea has no major political champions. That may be because politicians are reluctant to buy into Zubrin’s belief that, without such a frontier, America’s greatness is in necessary decline.
Perhaps in time the politicians will come round to the lure of Mars. Or perhaps the next missions will be less dramatic. Unlike today’s low-flying space telescopes, the planet-finding interferometers of 10 and 20 years hence will be a long way from Earth, probably at an intriguing mathematical fiction called the earth-sun L2 point, about 1.5 million kilometers away. Because of the way the gravitational fields of Earth and sun interact, it’s possible to go into orbit around this point even though there is nothing there. As Wesley Huntress, who used to be NASA’s associate administrator for space science, has pointed out, sending people from a space station to the L2 point every year or so to look after these expensive telescopes might make sense. It would be much cheaper than a Mars mission, and the space tugs designed to go to L2 might then be modified to visit nearby asteroids. Humans could thus slowly diffuse into the solar system, rather than leaping from planet to planet in haste.
They may rush or they may dawdle. Either way, though, they will know their destination intimately before they ever get there. Robot vanguards will ensure that the first humans to set foot on Mars or anywhere else will have a powerful feeling of deja vu. Indeed this familiarity could well be the thing that makes the visits possible. Today, images of Mars and the other planets still look alien (though not as alien as the moon). This is because they are almost all taken from orbit; they are data more than they are landscapes. After many landings in many sites, after flights through the thin air to far horizons, after sunsets seen through dust storms, after Earth and moon shine as an evening double star in a purple twilight, and after it is all delivered to any desktop that wants it as streaming video, things will be very different. That difference may change things. As NASA’s Pilcher puts it, “Sooner or later we’re going to get to the point where people are saying not ‘Why should we send people to Mars?’ but ‘Why aren’t we sending people to Mars?’ The day when that question changes is the day we will be on the path.”
It is possible that the transformation wrought by robotic reconnoitering might be even deeper. The empathy people showed toward the little Mars rover Sojourner was extraordinary–it may well be the single best-loved uninhabited machine ever. As attitudes toward technologies change, as our information systems become ever more clearly extensions of our selves, it could just be that virtual realities will be our preferred tools of exploration. The robots out there sending back the data might provide all the focus that the desire for vicarious adventure requires. Their capacities are already growing apace. When NASA’s Deep Space One mission passes the asteroid 1992KD on July 29, it will be the first spacecraft to make its own decisions about how to investigate its target, rather than following a minutely choreographed routine dictated in advance by its Earthly controllers. Robots cannot do everything. But their limitations, galling to scientists, might just add to their Sojourner-like cuteness. They could be all the emissaries to the universe we require, our endless entertainment as they stumble to the stars.
But even if people are not necessary to make space interesting, they definitely help. They are far more effective at many scientific tasks than unaided robots will be in the next three decades. And on top of that, they are alive. In the coming century the science of life and the powers it brings will be central to the human experience, central to our technology, our politics, our values. Biology, not physics, will be the key science. Space exploration, born out of the physics of the rocket and the bomb, is mutating to reflect this change in its intellectual environment. It is finding a new way to matter, a new way to mean something: only by discovering life elsewhere will we truly be able to understand the life we already know.
It is not necessary to search for life with living people. But it is fitting. When the heirs to Apollo return to space, it will not be to look back at our home, but to look out at the homes of others. It will not be to show that they don’t belong there, but to show that they do.
