by Sir Martin Rees, Astronomer Royal

In 1900, a French foundation offered the Guzman Prize of 100,000 francs for the first contact with an extra-terrestrial species; but prudence led them to exclude Mars -- detecting Martians was then thought to be too easy!

NASA Is there life on Mars - the idea has always fascinated us.

How life began, and whether it exists elsewhere remains one of the most fascinating questions in the whole of science -- indeed, you don't need to be a scientist to wonder about this. But we still don't know the answer. We're less optimistic about Mars than our forbears were a hundred years ago. Even if there is life there, it would be nothing more than microscopic 'bugs' of the kind that existed on Earth early in its history--- there is certainly nothing on Mars like the 'Martians' of popular fictions.

Indeed, nobody now expects 'advanced life' on any of the planets or moons in our Solar System. But our Sun is just one star among billions. And in the vastness of space far beyond our own Solar System we can rule out nothing. Astronomers have discovered, just within the last five years, that many stars have their own retinue of planets. There are millions of other Solar Systems. And there would surely, among this vast number, be many planets resembling our Earth.

Could some of these planets, orbiting other stars, harbour life-forms far more interesting and exotic than anything we might find on Mars? Could they even be inhabited by beings that we could recognise as intelligent?

If intelligent aliens were common, shouldn’t they have visited us already? Some people, of course, claim that aliens have indeed visited us. But the evidence for UFOs is no better than that for ghosts, and I'm personally quite unconvinced. Some astronomers cite this as evidence that aliens are rare. They note that some stars are billions of years older than our Sun, and point out that, if life were common, its emergence should have had a 'head start' on planets around these ancient stars.

But the fact that we haven't been visited doesn't, in my view, imply that aliens don't exist -- the question remains open. It would be far harder to traverse the mind-boggling distances of interstellar space than to send a radio signal. That's perhaps how aliens would reveal themselves first. The nearest stars are so far away that signals would take many years in transit. For this reason alone it makes sense to 'listen' rather than transmit --- if a signal were detected, there would be time to send a measured response, but no scope for quick repartee! (Aliens equipped with large radio antennae could in any case pick up the combined output of all our TV transmitters -- if they could decode them, it's hard to think what they might conclude about 'intelligent' life on Earth!).

Attempts to search for such signals have had a hard time getting public funding (even at the level of the tax revenues from a single science fiction movie) because the topic is encumbered by 'flakey' associations with UFOs, and so forth. But there’s a serious effort in California, backed by hefty donations from some silicon-valley millionaires.

We have no idea what intelligent aliens would look like -- it would depend on the habitat that their 'home planet' offered. They could be balloon-like creatures floating in dense atmospheres; they could be the size of insects, on a big planet where gravity pulled strongly. Or they may be freely-floating is space. They could even, as some science fiction reminds us, be super-intelligent computers, created by a race of alien beings that had already died out.

Even if intelligent aliens existed, they may not be transmitting any signals; and their brains and senses may be so different from ours that we couldn't recognise them. There may be a lot more life out there than we could ever detect -- absence of evidence wouldn't be evidence of absence. There are heavy odds against such searches succeeding.

But I'm enthusiastic about these searches, because of the import of any manifestly artificial signal. Even if we couldn't make much sense of it, we'd have learnt that 'intelligence' wasn't unique and had emerged elsewhere. Our cosmos would seem far more interesting; we would look at a distant star with renewed interest if we knew it was another Sun, shining on a world as intricate and complex as our own.

If we ever established contact with aliens, what could we discuss with them? I've argued in a new book that we're assured one common interest. We'd belong to the same universe of stars and planets, all made of similar atoms and governed by universal laws. We'd all trace our origins back to a single 'genesis event' -- the so-called 'big bang', which happened about 12 billion years ago.

To firm up the odds on alien life, we need to understand how life begins and evolves. An extraordinary precession of species (almost all now extinct) have swum, crawled and flown during the Earth’s 4.5 billion year history. For a billion years, primitive 'bugs' exhaled oxygen, transforming the young Earth's poisonous atmosphere and clearing the way for our eventual emergence. We know from fossils that a cornucopia of swimming and creeping things evolved during the Cambrian era 550 million years ago. The next 200 million years saw the greening of the land, offering a habitat for exotic creatures ---dragonflies as big as seagulls, millipedes a yard long, giant scorpions and squid-like sea-monsters. Then came the dinosaurs. Their sudden demise opened the way to mammals -- to the evolution of apes and us. We are the outcome of time and chance: if evolution was 're-run', there would be no humans, and we can't predict whether any other species would achieve our dominant role. So we can't lay firm odds on whether 'intelligence' would emerge on another Earth-like planet.

We know this happened on Earth, but we'd dearly like to discover a second example where even the earliest stages of life might exist. Mars remains the best place to look. Three years ago, American scientists announced evidence for fossil 'bugs' on a meteorite that had come from Mars. This claim, hyped up at a press conference attended by President Clinton himself, was dubious and premature -- NASA has been backtracking on it ever since. We'll learn more from a series of space probes that will be sent to Mars in the next decade, to study its surface, and eventually return samples to Earth. And there are longer-term plans to search elsewhere -- for instance, a submersible robot will probe the ice-covered oceans of Jupiter's moons Europa and Callisto.

All this depends on the space programme. For most of the present century, space travel was a futuristic concept, familiar from comics and corn flakes packets. But in July 1969, Neil Armstrong's 'one small step' made space travel a reality. Those of us who are now middle-aged can remember viewing 'live' the murky TV pictures of that event: it seemed a high point in a decade blighted by the arms race and the Vietnam war.

Another lasting image from the 1960s was the first photograph of the entire round Earth, taken from the Moon. Our habitat of land, oceans and clouds was revealed as a thin delicate-seeming glaze. Our home planet -- the 'third rock from the Sun' -- is very special. The beauty and vulnerability of 'spaceship Earth' contrasts with the stark and sterile moonscape on which the astronauts left their footprints.

In the 1960s, the first brief excursions to the Moon seemed just a beginning. We imagined follow-up projects: a permanent 'lunar base', rather like the one at the South Pole; or even huge 'space hotels' orbiting the Earth. Manned expeditions to Mars seemed a natural next step. But none of these has happened. The year 2001 will not resemble Arthur C. Clarke's depiction, any more than 1984 (fortunately) resembled Orwell's.

The programme, announced by President Kennedy in 1961, 'to land a man on the Moon before the end of the decade, and return him safely to earth', was lavishly funded because America wanted to beat Russia. Their pride had been badly dented in 1957, when Russia launched the first 'Sputnik', and this was a chance to recapture the lead in the space race. Reaching the Moon was an end in itself: the last lunar landing was in 1972.

Manned spaceflight now seems a rather jaded spectator-sport: the veteran senator John Glenn's recent trip in the Space Shuttle may have been a morale-booster for elderly Americans, but it didn't recapture the excitement of his pioneering flight 36 years earlier. We admired the Russian cosmonauts more for their fortitude and DIY skills than for anything else, as they coped with one malfunction after another in the decrepit Mir spacecraft.

Nationals of other countries have hitched rides into space. The British astronaut Michael Foal heroically survived the hazards of Mir, the Russian Space Station. French, Bulgarian and Mongolian astronauts have also made the trip. But none of this has recaptured public enthusiasm.

The practical case for manned spaceflight was never strong, and it gets weaker as robots and computers get more powerful. Space technology -- now funded commercially as well as by governments -- has abundantly proved its value. Thousands of small unmanned objects have been launched into orbit.

Satellites are routinely used for long distance telephones and satellite tv broadcasts. The 'global positioning satellites' allow planes or ships to navigate precisely -- and allow solo hikers or sailor to locate themselves accurately anywhere on Earth, with a pocket-sized instrument. Weather forecasts depend on pictures and data from space.

Space exploration need not involve humans. It can be better (and far more cheaply) carried out by fleets of unmanned probes, exploiting the advances that have given us mobile phones and high-powered personal computers.

Cameras and scientific instruments have beamed back pictures from the other planets of our Solar system. And the Hubble Space Telescope has imaged stars and galaxies so deep in space that their light set out on its journey towards us billions of years before our Earth and Sun were born. The cosmos is fantastically larger and more complex than could have been imagined by the ancients who first mapped the constellations.

The cosmos confronted with huge spans of time, as well as stupendous expanses of space. Life on Earth has evolved for billions of years, but our Sun has burnt up less than half its fuel, and will keep shining for another five billion years. If life isn't prematurely snuffed out, our remote progeny will surely -- in the aeons that lie ahead -- spread far beyond this planet.

We plainly can't forecast the vastly remote future. But what might happen in the first decades of the new millennium? How long will it be before people return to the Moon, and perhaps explore still further afield?

The centerpiece of the current US programme is the new International Space Station: this will be in orbit a few hundred miles up, and the size of a football field. It will be the most expensive artifact ever constructed, costing its own weight in gold. Even if it is finished -- something that seems uncertain, given the immense and ever-rising costs, and prolonged delays -- it will be neither practical nor inspiring. Thirty years after men walked on the Moon, a new generation of astronauts will be going round and round the Earth, in more comfort than Mir can offer, but much more expensively. The astronauts will be able to do experiments, but most of those could be done more cheaply by robots in smaller free-flying satellites.

The Space Station would make somewhat more sense as a staging post on the way to other planets. But no such follow-up will materialise unless public enthusiasm revives, or unless some technical breakthrough renders space travel much cheaper and easier than it now seems.

Present launching techniques are as extravagant as air travel would be if the plane had to be rebuilt after every flight. Spaceflight will only be affordable when it adopts the same techniques as supersonic aircraft. Tourist trips into orbit may then become routine. And wealthy adventurers may boldly go further. Future Richard Bransons, for whom round-the-world ballooning seems too tame and routine, could aim for the Moon. If Bill Gates seeks a challenge that won't make his later life seem an anticlimax, he could sponsor the first expedition to Mars.

Some people use the so-called 'insurance policy argument' to advocate a manned space programme. There is an ever-present risk (though fortunately a small one) that a comet or asteroid will hit the Earth. The craters on the Moon's surface are records of these impacts. An impact on Earth - leaving a huge undersea crater near Chicxulub in the Gulf of Mexico, probably sealed the fate of the dinosaurs 65 million years ago.

There is about one chance in 10000 that, within the next 50 years, the Earth will be hit by an asteroid large enough to cause world-wide devastation -- ocean waves hundreds of feet high, tremendous earthquakes, and changes in global weather. This chance is low - but no lower than the risk (for the average person) of being killed in an air crash. Indeed, it's higher than any other natural hazards that most Europeans or North Americans are exposed to.

The ever-present risk from nature has been augmented since humankind entered the nuclear and biotechnological age. Humanity will remain vulnerable to these (probably increasing) hazards so long as it is confined here on Earth. But once self-sustaining communities exist away from the Earth -- on the Moon, on Mars, or freely floating in space -- our species would be invulnerable to any global disaster, and whatever potential it has for the 5-billion-year future could not be snuffed out.

Whether on not humans spread beyond the Earth during the next millennium, we'll still want to know whether we are alone. It would in some ways be disappointing if searches for alien intelligence were doomed to fail. On the other hand, it would boost our 'cosmic' self-esteem. If our tiny Earth were a unique abode of intelligence, we could view it in a less humble cosmic perspective than it would merit if the Galaxy already teemed with complex life. We'd have even stronger motives to cherish this 'pale blue dot' in the cosmos, and not foreclose life's future -- a future that could be even longer than the time span over which simple life has evolved into humans. That is why we should expand our cosmic vision in the new millennium.