The moon, when we got there, turned out to be a dead place from which the living earth looked wonderful. Mars, when our robots got there, seemed disconcertingly like the moon, without the compensating view: none of the primitive plants suspected by scientists; no imperial canals like those of Edgar Rice Burroughs' Barsoom stories. Just dust.
The original architects of the space age saw Mars as the obvious next destination beyond the moon, a challenge which would require fleets of spacecraft and, later, research stations, settlements, even colonies far from the earth. There were many practical and (above all) political reasons why this dream was not to be-but the disconcerting deadness of Mars itself certainly played its part, and so did the glorious sight of the earth rising over the limb of the moon. When the most wonderful thing in the solar system is the thing you already live on, why emigrate?
Barren as it looked at first sight, though, Mars continued to fascinate scientists, and they built up an increasingly complex picture. The impression of a moon-like surface provided by the first probes in the 1960s gave way, in the 1970s, to a planet with a more intriguing history. A picture of Mars emerged in which the planet had once been active and exciting: its atmosphere thicker and warmer, its surface cut by rivers and scoured by floods, and life a distinct possibility. But this period was brief and long, long ago. Mars was now the mummified husk of this vibrant child.
The idea that there was a bygone Mars more interesting than today's slowly rekindled the dream of human exploration. It suggested that there were secrets to be found beneath the dust, and that these scientific subtleties might be the sort of thing which would only ever be decipherable by people there, on the ground. The capabilities of even the most sophisticated robotic rover, enthusiasts pointed out, remain trivial compared to what a well-trained field geologist can do. Mars as a human adventure could thus be tied to the idea of Mars as an engrossing scientific subject. Long term, there was even the extraordinary dream of restoring the wet warmth of the early Mars-"terraforming" the planet so that once again it could be made suitable for life.
By the early 1990s, this view of Mars exploration had become one of the few topics which both believers in pure science and manned exploration could get enthusiastic about (the scientists, broadly speaking, hate the rest of the manned programme). They could not get politicians on board-but surely, they reassured each other, that was just a matter of time. By the 2010s, say, Mars would be the next big thing.
Unfortunately for this consensus, the absence of people on Mars did not stop progress in understanding it. Despite its well-publicised failures, the unmanned exploration which resumed in 1996 after a 20-year hiatus, along with the study of meteorites of Martian origin, has provided a wealth of new knowledge-and a picture of a planet reports of whose death may have been exaggerated. Although the suggestion that there are long dead microbes in one of the meteorites is now widely dismissed, other meteorites have yielded evidence that the planet originally had more water than was previously thought, and that it flowed through subterranean pores long after it vanished from the surface. There are signs that some of the planet's volcanoes are still capable of the occasional outburst, suggesting that internal fires could be keeping the depths moist even today. This June, new evidence suggested that, in some circumstances, water might still be breaking through to the surface.
Add to all this biology's growing appreciation of how microbes can survive in nasty places (almost solid ice, nuclear reactors, interplanetary space), and life on Mars today becomes plausible. It looks likely that earthly bugs could live there, given a chance. Perhaps Martian bugs already do. If you could land a robotic rover near one of the places where water recently broke through to the surface, you might even manage to get a robot to scoop some up and send them back to earth.
For scientists, this is great news (although getting the rovers to the sort-of-springs may prove difficult). For those who push for human exploration, though, it's more worrying. Since men first walked on the moon and looked back on their planet-indeed, to some extent, as a result of that homeward glance-environmentalism has changed the way we do science. Today, the idea of humans exploring a living planet looks less like manifest destiny and more like contamination of a delicate alien ecosystem. When Mars seemed mummified, sending astronauts there was reasonable. If it carries life, it's a different matter. It is just possible to clean and sterilise a robot; there's no chance of achieving such cleanliness in human exploration. If the case for life on Mars becomes much stronger, then it's possible that scientists will demand that it be studied only from a distance.
There may be a certain symmetry to this. If Mars is left to Martian bacteria and earthly robots, those robots will need supervision. Controlling them from earth won't work-it takes radio signals minutes to get to Mars, more minutes to get back. But Mars has two small moons - Phobos and Deimos, the dogs of war - closer to its surface than earth's communication satellites are to ours. A station on one of these would be an ideal place from which to control robots on Mars. It may be that, when next they venture beyond earth's orbit, astronauts will again find themselves on a barren moon looking down on a living planet. Only this time around it won't be earth.
