Mind your head - falling rocks

December 21, 2000

As 2001 nears, Arthur C. Clarke warns Earthlings against space invaders during the coming century.

Thanks to one of the most remarkable events in the entire history of astronomy, the names Shoemaker and Levy are now inextricably linked. When comet Shoemaker-Levy 9, named after its co-discoverers Eugene Shoemaker and David Levy, crashed into Jupiter in the summer of 1994, it immortalised them - and reminded humankind that their planet could just as easily be bombarded from space.

Unlikely as it now seems, until this century few scientists believed there could be any direct contact between Earth and the celestial sphere. President Jefferson famously remarked after hearing reports of a meteorite fall: "I would rather believe that two Yankee professors lied, than that stones fell from the sky." Well, now we know that mountains can fall from the sky, and Shoemaker was the first to prove this awesome fact beyond doubt.

David Levy's book has three main themes - biography, geology and astronomy - neatly intertwined in a triple helix. One strand is devoted to his progress from amateur stargazer to professional comet hunter: no trade union would tolerate the exorbitant hours and the miserable pay, but if you are lucky, the outcome can be immortality in the heavens. Levy now has more than 30 comets bearing his name.

Shoemaker became interested in comets by a more roundabout route, through studying the numerous craters scattered over the face of the Earth - and later the Moon. At one time he had hoped to become an astronaut, but when medical problems ruled that out, he was able to play a key role in creating the new science of lunar geology. (Oh, very well - selenology.) He left the Apollo programme to become chairman of the California Institute of Technology's division of geological and planetary sciences - while the lunar missions were still under way, and at the start of the planning for major planetary missions. Levy describes how Shoemaker tried to balance teaching and research work with administrative responsibilities; the latter sometimes suffering from his abundant enthusiasm for the former. With a combination of fascinating insights into the Earth's past and a fondness for field investigations, he inspired a whole generation of geologists who have been at the forefront of planetary exploration for three decades.

Shoemaker's favourite field visits were to the Grand Canyon and the Meteor and Sunset craters, where he showed students the Earth's geology at its best. It had, of course, been known for millennia that volcanoes could produce splendid craters of all shapes and sizes. When the telescope was invented it was immediately observed that the Moon was covered with craters, and although many of them were far larger than any on Earth, it seemed reasonable to assume that they too were volcanic. Moreover, the so-called lunar seas could best be explained as vast outpourings of lava. There appeared no need to look for any other explanation - on the Moon, or on the Earth. Nasmyth and Carpenter's classic The Moon (1874), with its beautiful photo-replicas of lunar landscapes modelled in plaster, said the last word on the matter for almost a century: "There is a feature in the majority of the ring-mountains... that seems to stamp the volcanic character upon the crater-forms. This special feature is the central cone, so well known as a characteristic of terrestrial volcanoes."

Nevertheless, there were always a few heretics who pointed out some problems with this theory and advanced explanations of their own - some perfectly ridiculous. Perhaps the most popular alternative was the one that we now know to be correct: that they were caused by impacts from space. Yet to many, this theory appeared to have one obvious and fatal defect. As one astronomer put it, echoing Nasmyth and Carpenter: "The presence of central peaks completely rules out the meteoric hypothesis."

The debate was still raging when Percy Wilkins and Patrick Moore published their own authoritative volume The Moon in 1960. They concluded that "there is a remote possibility that the Maria may have been formed by the impact of large meteors, but it is certain that the origin of the vast majority of the lunar cavities cannot be so explained, and the volcanic theory seems to correctly apply." However, they wisely (and, for that time, rather daringly) went on to say: "Only when the first spaceships take off to the Moon, and we are able to view the surface at close quarters... will this question be finally settled."

Yet even before then, conclusive evidence had been obtained that Arizona's famous Meteor Crater was correctly named. A variety of quartz - coesite - that could be produced only by enormous pressures had been found at its rim. Although some geologists put up a spirited rearguard action in defence of volcanoes, it was finally agreed that only an impactor from space could produce the extreme conditions necessary to create this material. Indeed, coesite is now regarded as definite proof of such an event - though not necessarily on the same hemisphere, because the compressed quartz may have been hurled halfway round the Earth.

After the dawn of the space age in 1957, the very first probes to Mars showed that it was covered with impact craters, though to complicate matters, it also boasted volcanoes that dwarfed any on Earth. Later images from Mercury showed a terrain almost indistinguishable from the Moon, and we now know that all the solid bodies in the solar system received such a battering 3 to 4 billion years ago, and that some of them were literally shattered into pieces.

All these discoveries attracted relatively little interest outside the astro-geological fraternity, but in the late 1970s the situation changed abruptly when the father and son team, Luis and Walter Alvarez, came across a curious anomaly. There was a wholly disproportionate concentration of the heavy metal iridium in a thin layer deposited 65 million years ago, and the fossils of micro-organisms, which were extremely common below this layer, were rare or even non-existent, above it. Some worldwide catastrophe had evidently caused a mass extinction - and it seemed more than a coincidence that the dinosaurs disappeared at around this time.

In a classic paper published in Science in 1980 ("Extraterrestrial cause for the Cretaceous-Tertiary extinction"), Alvarez and his colleagues claimed to have solved a mystery that had long baffled palaeontologists. They pointed out that iridium, though very rare in the Earth's outer crust (because most of it has sunk down to the core) is relatively common in meteorites - and presumably in asteroids, which are believed to be their parent bodies. Here, perhaps, was the "smoking gun" that had committed the crime of the aeons.

It took a decade for this theory to be generally accepted, partly because there have been many other extinctions that can be more easily attributed to local causes: Mother Earth is quite capable of large-scale infanticide without any assistance from the Cosmos. But what appeared to be the final proof came in 1991 with the location of an enormous buried crater near Mexico of just the right age and size.

Sadly, Luis Alvarez did not live to see this spectacular vindication of his theory, but he never doubted its correctness. In the last letter I received from him, he wrote: "It's no longer a theory but a fact." (Perhaps I should mention that I was privileged to join his radar team in 1943: my only non-SF novel, Glide Path , is dedicated to him. Its partly fictitious hero wins the Nobel prize - and in 1968, "Luie" obligingly fulfiled my prediction.) These discoveries were widely and understandably publicised, because they raised an awesome question. Could what had happened in the past happen again: the patient toil of amateur astronomers - often regarded with mild amusement by the man in the street - had suddenly become relevant to the survival of the human race. Few could doubt this after comet Shoemaker-Levy 9 smashed into Jupiter on July 18 1994, giving that giant world a series of black eyes as large as the Earth, and lasting for several weeks.

S-L9's cataclysmic demise was probably watched by more telescopes than any event in history, and for a while the Levy and Shoemaker families had virtually no private lives. However, the resulting fame gave them greater opportunities to continue their work on a more lavish scale - without having to waste so much time pleading for funds. Shoemaker had always been running at least a dozen projects at once (not all of them very efficiently) but now began to focus attention on the continent that he and his wife Carolyn had grown to love - Australia. Its vast deserts were the best places on Earth to look for craters that had not been erased by the ravages of time.

On July 18 1997 - exactly three years after S-L9's impact on Jupiter - Shoemaker was driving through the outback, where any approaching car could be seen a mile away by its dust cloud. Then, "out of nowhere, a Land Rover materialised in front of them". Shoemaker was killed instantly but his wife Carolyn fortunately survived, to continue working with the Levys. In January 1998, they watched Lunar Prospector lift off from Cape Canaveral, carrying an ounce of Shoemaker's ashes to the Moon.

While I was writing this review, the following email message arrived from the International Astronomical Union: "Object 2000 SG344 was discovered on September 29 2000 by David J. Tholen and Robert J. Whiteley using the Canada-France-Hawaii 3.6-meter aperture telescope... Nasa's Jet Propulsion Laboratory estimates a one-in-500 chance of the object hitting the Earth on September 21 2030."

Before anyone gets too alarmed, later observations ruled out a 2030 encounter -but improved (?) the odds for one after 2071. Even in the very unlikely event that SG344 eventually does hit the Earth, it is a rather small object and may not do much damage. But there are uncounted bigger comets and asteroids out there: sooner or later, there will be a major impact, though hopefully not on the Cretaceous-Tertiary scale.

Almost three decades ago, I described such a disaster in my novel Rendezvous with Rama (1973), and the resulting establishment of Project Spaceguard to ensure that "no meteorite large enough to cause catastrophe would ever again be allowed to breach the defences of Earth". I am indeed happy to say that the name has been widely adopted: at the request of Congress, Nasa issued The Spaceguard Survey in 1992, and Spaceguard organisations have since been established in many countries to rally government and public support for the cause. Following enlightened discussions in the House of Lords, the UK government has recently agreed to spend money on this most important defence project of all time. (Amazingly, it was Byron who first proposed, in 1822, that the human race might need to destroy comets to save itself!) Though there is little that we could do, at the present stage of our technology, to protect the home planet against a major impact, that should not be the case by the end of the 21st century. We must acquire the ability to go into space, and move threatening objects out of the way. Shortly before his own untimely death, Carl Sagan summed up the lesson of S-L9: "In the long term, all civilisations must be space-faring. The ones that aren't, die."

Sir Arthur C. Clarke is chancellor, International Space University, and chancellor, University of Moratuwa, Sri Lanka.

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