The risk of raining stones

Before we go any further, I had better declare an interest - in fact, several interests. Although I have never met Walter Alvarez, his father Luis was a good friend, and I dedicated my 1963 novel Glide Path to him. This work of barely disguised fiction was based on my experiences as an RAF officer when I took over the Ground Control Approach radar blind-landing system which "Luie" had invented at the Massachusetts Institute of Technology. The main protagonist was modelled on him.

Luie's subsequent adventures included using cosmic rays to "X-ray" the Great Pyramid to see if it contained any hidden chambers (it did not). Later, his analysis of the Kennedy assassination tapes showed that only one gunman was involved. And as scientific adviser on official UFO investigations, he was able to demonstrate the sad scarcity of intelligent life on Earth, by showing how easily honest observers could be fooled. (I am in no position to criticise - a combination of unusual circumstances once had me baffled for several days, and was only cleared up after several phone calls to the Pentagon.) I cannot recall what turned my attention to the possible danger of asteroid impacts. It was quite an old idea, but my 1973 novel Rendezvous with Rama (which opened with the obliteration of northern Italy) did introduce a new concept. I argued that as soon as the technology permitted, we should set up powerful radar and optical search systems to detect oncoming cosmic missiles. The name I suggested was Spaceguard, which, together with Spacewatch, has now been widely adopted.

Seven years later, in 1980, Luis and hisgeologist son Walter published their famous paper "Extraterrestrial cause for the Cretaceous-Tertiary extinction" in Science. It advanced the theory that the reign of the dinosaurs had been ended 65 million years ago by an asteroid impact - a suggestion received with downright derision or resounding silence by the geological establishment.

Over the years, however, more and more evidence accumulated in support of the asteroid extinction theory. In the last letter I received from him, Luie said it was "no longer a theory - but a fact". Today, I suspect that 95 per cent of geologists would agree with him, though there are still some distinguished advocates of alternative explanations, such as vulcanism or climate change.

Anyone who thinks that geology is an even duller science than economics may be surprised at the passions it can arouse. Perhaps the classic case is Wegener's theory of continental drift, for years universally regarded as nonsense by the "experts". Yet the basic truth of Wegener's heresy (now more accurately known as plate tectonics) was established in one revolutionary decade.

The very existence of impact craters on any celestial body is another case in point. With the knowledge acquired by space probes and the Apollo missions, it now seems incredible that, right up to the 1950s, it was widely believed that the lunar craters were volcanic. One British astronomer stated: "The presence of central peaks completely rules out the meteoric hypothesis."

That was a perfectly reasonable argument - for who would have dreamed that the splash which occurs when one drops a lump of sugar into a cup of tea can happen in solid rock, on a countrywide scale? In fact, the presence of central peaks is one of the best proofs of the meteoric hypothesis.

Even the existence of meteorites themselves was denied right through the "Age of Enlightenment", despite the fact that they had been known from time immemorial. Thomas Jefferson (no amateur of science) once said: "I would sooner believe that two Yankee professors lied, than that stones fell from the sky." As a result, during the 18th century, the collections in many museums were thrown out as worthless relics of superstition. It is also almost unbelievable that for decades Arizona's accurately named Meteor Crater was "explained" as a purely terrestrial formation by virtually all geologists. But once they removed their mental blindfolds, they started finding impact craters all over the world. Over 100 have now been identified, and there must be many more hidden in the ocean depths.

Although these two books by Alvarez and Gerrit Verschuur inevitably cover much the same ground, the Alvarez is more authoritative because of its "I was there" element: it is an unfolding story told by its leading protagonist. However, I must accuse Walter (or his editor) of petty larceny: the title is an obvious rip-off from Steven Spielberg's film Indiana Jones and the Temple of Doom.

Very clearly and entertainingly written, and illustrated with fascinating colour plates, it is accessible even to nonspecialists. The treatment of the dwindling, but extremely vocal cadre of opponents to the meteoric hypothesis also seems to be fair and good natured.

The general reader, however, may prefer Verschuur's book, because it covers a much wider field and is full of fascinating asides. I did catch him out on one curious error - he gives a knighthood to the distinguished American astronomer Simon Newcomb. Though he made colossal contributions to planetary theory, Newcomb's astrodynamics was better than his aerodynamics, because he may now be best remembered for his conclusive proof that heavier-than-air flight was impossible. (Still worth reading; you will never set foot in an airplane again.) Now that we are aware of the danger from space, it is time to look more carefully at both the historical and geological records. It is foolish to ignore widespread legends, except when they are obviously absurd. And perhaps even when they are: for centuries there have been stories of ice falling from the sky: many of these were collected by that indefatigable researcher of the strange and mysterious, Charles Fort, in his book Lo! Well, within the past few months, evidence has accumulated that tens of thousands of snowballs the size of houses hit the earth every day! I must confess to a certain Jeffersonian scepticism - not because I doubt nature's ability to sustain such a bombardment, but because it seems astonishing that our satellites have survived it.

There have been three known major impacts this century (Siberia in 1908 and 1947; Brazil in 1930). And on August 10 1972, a large meteor travelled half way across the United States and was seen not only by thousands of people, but recorded by many amateur photographers, It came within a mere 58 kms of the ground; had its trajectory been just a trifle different, some US city might have emulated Hiroshima. One can imagine the consequences of such an event during the cold war! It has even been suggested that the 1871 Chicago fire was caused, not by Mrs O'Leary's cow kicking over a lantern, but by a shower of meteorites. The evidence for this is that many surrounding towns were consumed at the same time, some with great loss of life. I wonder if the 1666 fire of LondonI Other obvious candidates are Sodom and Gomorrah - a theory which recently caused heated discussions in Israel. Verschuur discusses many such possibilities and makes a good case for real historical events behind the - almost universal - flood legends, and even the story of Atlantis. Unfortunately, Emmanuel Velikovsky's Worlds in Collision (1950), which advanced the absurd theory that Mars, Venus and the Earth had performed a cosmic billiards act in historic times, made it impossible for reputable scholars to take this subject seriously, and so research was halted for decades. Ironically, Velikovsky was correct in some minor details - but both his time-scale and his physics were wrong by factors of millions.

Perhaps the event which, more than any other, made everyone take catastrophic impacts seriously was the spectacular collision of Shoemaker-Levy with Jupiter in July 1995 - one of the most observed astronomical events in history. It is a delightful coincidence that Gene Shoemaker, who for decades fought a lonely battle to prove there have been meteor impacts on earth, now has his name associated with this historic event as one of the co-discoverers of the ill-fated comet. By yet another coincidence, this time a most tragic one, the above paragraph was written on the very day - July 18 - that Shoemaker met his untimely death in a car accident, while conducting his annual search for impact craters in central Australia. The loss to science is enormous.

One result of this changing attitude was that, as early as 1990, the US House of Representatives requested Nasa to look into the matter. I am flattered that the resulting document is called the Spaceguard Survey. Since then, much has happened in this rapidly developing field. A Spaceguard Foundation has been established, with branches in the UK and Australia. A proposal will shortly be made to the European Space Agency to set up a Spaceguard Central Node, to cover all aspects of discovery, data storage, and threat identification. Perhaps the best indication of the seriousness with which the problem is now regarded is that a meeting of experts to discuss "Hazards due to Near Earth Objects" was held at the Royal Greenwich Observatory in July. Mention should also be made of Spacewatch, a devoted and badly underfinanced group of observers, which has already made a whole series of valuable discoveries.

Well, as Lenin once memorably asked, "What is to be done?" Some might argue that, in a world already nervous about global warming, poisoned oceans, DIY nuclear bombs, etc. etc., any discussion of asteroid insurance is a massive exercise in irrelevancy. Indeed, many might prefer not to know, if a killer comet or asteroid was headed this way.

Yet there is much that can - and should - be done, as is proved by the current intense debate among astronomers, space scientists, and under-employed star warriors looking for new targets. It is an old idea - going back at least to Andre Maurois's The War Against The Moon (19) - that only a threat from beyond the earth could unify the quarrelsome human species. It may indeed be a stroke of luck that such a threat has been discovered, at just the period in history when we can devise technologies to deal with it.

Although some suggested cures may sound worse than the disease (Edward Teller's proposed bodyguard of orbiting hydrogen bombs has not been received with much enthusiasm), there are several plausible alternatives. They all depend on the length of the warning time - which is why Spacewatch is so vital; it could give us decades to prepare a real Spaceguard.

Of the many defences proposed, the most elegant (and environmentally friendly) is to rendezvous with any asteroid on an orbit liable to impact earth, and to persuade it to make a slight change of course. If there was sufficient warning time, only a modest amount of rocket propulsion would be necessary.

This was the scenario I developed in The Hammer of God, and I am pleased to say Spielberg and Jaws collaborators Richard Zanuck and David Brown are filming Hammer - though they have changed the title to Deep Impact and may be incorporating elements from that classic but totally unscientific war-horse, George Pal's When Worlds Collide. (I have not yet been able to get hold of a script, so am keeping my fingers crossed.) In one of his last books, Carl Sagan pointed out that no really long-lived civilisation could survive unless it develops space travel, because major asteroid impacts will be inevitable in any solar system over the course of millennia. Larry Niven summed up the situation with the memorable phrase: "The dinosaurs became extinct because they didn't have a space programme." And we will deserve to become extinct, if we do not have one.

Arthur C. Clarke, an amateur astronomer based in Sri Lanka, was recently honoured by having the l0km-diameter asteroid 4923 (aka 1981 E0) named after him. As it is the same size as the principal suspect in the Dinosaur Murder Case, its proud absentee landlord promises that it will never come anywhere near the Earth.