Cosmic shrapnel

The American polymath Isaac Asimov once gave this concise inventory of the solar system: "Jupiter plus debris". As Jupiter has almost three times the mass of all the other eight planets combined, his summing-up can hardly be faulted - however unflattering it may appear to the occupants of the remaining rubble.

As Galileo discovered in 1610, Jupiter is an impressive sight, even in the smallest telescope. With its four giant satellites - Io, Europa, Ganymede and Callisto - it forms a mini-solar system in its own right. For three and a half centuries, the stately gyrations of the "Galilean moons", and the ever-changing pattern of cloudscapes on the enormous world that controls them, have fascinated generations of astronomers, both professional and amateur. In fact the amateurs - particularly the members of the British Astronomical Association - have contributed more to our knowledge of Jupiter than any other group. Their modest-sized optics can do useful work, leaving the rest of the universe to the larger instruments of the professionals (who, until recently, tended to ignore the solar system entirely, while they went big-game hunting elsewhere).

It is therefore appropriate that what will certainly be the standard reference book on Jupiter for many years to come is written by the director of the British Astronomical Association's Jupiter section, John Rogers, himself an amateur, since by day he is a molecular biologist at the University of Cambridge.

C. S. Lewis once wittily remarked: "Everything is a subject about which very little can be said." This is also true of Rogers's monumental book: a completely adequate description would be: "Contains everything anyone would ever want to know about Jupiter." Its 418 large pages, and hundreds of illustrations, cover in detail not only the planet itself, its wideranging satellite system, its recently discovered rings, and the intricate circulation pattern of its atmosphere, in which cyclonic disturbances larger than earth have raged for centuries, but much, much more.

Although a great many of the images in the book are drawings or photographs made through terrestrial telescopes, which still play an essential role in studies of the planet, these cannot compare in detail - or drama - with the marvellous close-ups produced by space probes. These gave us our first glimpse of some of the most bizarre landscapes in the solar system - the surfaces of the four Galilean moons. Until the Voyager flyby in 1979, these were merely points of light in the most powerful telescope: no one had dreamed of the volcanoes of Io, the ice-covered ocean of Europa or the intricately furrowed landscape of Ganymede, which appears to have been worked over by generations of inebriated ploughmen.

Several chapters are devoted to the processes that have shaped these strange worlds - each unique, and proving once again nature's infinite capacity for surprise. In a few months' time we hope to know a great deal more about them, when the spacecraft appropriately named after Galileo begins the first extended survey of the Jovian system. It has already dropped probes that will enter Jupiter's atmosphere on December 7, and will then commence a two-year reconnaissance of the entire Jovian system. (However, it has just been announced that Galileo's on-board tape recorder may have failed, thus still further limiting the troubled and long-delayed mission - the originally planned arrival date was mid-1985.) I was particularly pleased to see Rogers's reference (with photograph) to another amateur, Major Percy B. Molesworth, who was stationed in Ceylon at the beginning of the century. He made thousands of drawings of Jupiter and Mars from his observatory in Trincomalee and I recently paid him a tribute that would have astonished him. The Russian Mars lander, now being prepared for launch, carries video messages to future colonists: mine contains these words: "MolesworthIspent much time observing Mars, and now there is a huge crater, 175km wide, named after him in your southern hemisphere. In The Hammer of God I've imagined how a New Martian astronomer might one day look back at his ancestral world, to try to see the little island from which Molesworth - and I - often gazed up at your planet."

The Great Comet Crash may be regarded as an action-packed sequel to Rogers's book. It is the record of a spectacular astronomical event - which, by what seems incredible good luck, occurred when there were methods of observing it (by Galileo and the Hubble Space Telescope) and spreading the results instantly through the global scientific community (by satellite television and the Web) which had not even existed a few years earlier.

When Comet Shoemaker-Levy 9 crashed into Jupiter in July 1994, most of the world's telescopes must have been focused on the planet, and millions who did not possess optical aids were able to download images of the event on to their computer screens. They could watch, from the comfort and safety of their armchairs, the equivalent of a nuclear exchange far more massive than Dr Strangelove could have imagined in his wildest fantasies, one producing firestorms as large as the earth.

Impact Jupiter, by codiscoverer David Levy, has all the excitement of an unfolding adventure story, and also gives a insider's view of the twin worlds of amateur and professional astronomy. It is full of engaging stories of missed opportunities, near - and real - disasters, and is also a record for the future of a time when one of the oldest of sciences is being revolutionised, by electronic and space technologies. For example, it contains some amazing images of Jupiter made with a modest (16in) telescope, which no instrument on earth could have matched a few decades ago. As it is also extremely well written, with amusing descriptions of the media frenzy surrounding the event, it perfectly complements the two more technical books.

It took many months for the multiple bruises on Jupiter to heal, and scientists will be studying and analysing them for years. Quite apart from the intrinsic interest of the event, it is full of warnings for the future, as well as lessons from the past. For we now know that what happened before our eyes on Jupiter has occurred many - perhaps thousands - of times in the long history of our own planet, and will certainly happen again.

Even before the dramatic demonstration provided by Shoemaker-Levy 9, many scientists - and not a few science-fiction writers - had drawn attention to the potential danger of impacts from space. Until quite recently, they were in a small minority; well into this century, geologists were trying to explain Arizona's famous and accurately named Meteor Crater as a volcanic formation. (It was young Shoemaker who obtained his PhD thesis by demonstrating its true origin: little did he guessI) And before the opening of the space age, most astronomers believed that the lunar craters were also volcanic. I hope that my old friend Pat Moore won't be annoyed when I remind him that this hypothesis was taken for granted in his classic 1955 book The Moon. The alternative seemed ridiculous, if only because there are mountains at the very centres of such magnificent craters as Tycho and Copernicus. One astronomer thought the matter settled once and for all when he wrote: "The presence of central peaks completely rules out the meteoric hypothesis."

Well! Einstein's verdict that "the Good Lord is subtle but he is not malicious" seems to have been overturned in this case. For who would ever have imagined that the rebound, or splash-back, which occurs when a lump of sugar is dropped into a cup of tea could occur in solid rock, on a continental scale? The central peaks were proof, not disproof, of the meteoric hypothesis, but no one was smart enough to realise it until well into this century.

Space probes have now revealed that not only earth's moon, but almost all the planets and satellites in the solar system, bear evidence of heavy bombardment in the remote past. Such evidence once existed on earth, but has been largely obliterated by millions of years of erosion. However, now that we know impact craters must be present on our planet, we are finding them (or their fossilised remains) by the score.

What has done most to focus attention on the subject was undoubtedly the dramatic suggestion of the late Nobel prizewinning physicist Luis Alvarez and his geologist son Walter that an asteroid impact caused the extinction of the dinosaurs, circa 65 million years ago. This theory (in his last letter to me, Alvarez claimed that it was "no longer a theory, but a fact") is now widely accepted, though it seems possible that other agencies may also have contributed to the demise of the most magnificent creatures our planet has ever produced.

These discoveries prompted the United States Congress to react with mild alarm, and it requested Nasa to assess the danger of asteroid or cometary impact - and to decide what, if anything, could be done to alleviate it. A number of scientific conferences led to the 1992 Spaceguard Report - a title taken, with due acknowledgement, from my 1973 novel Rendezvous with Rama.

The problem falls into two parts: detection of potentially dangerous objects - of which there may be hundreds of thousands - and their deflection or destruction if they present a genuine hazard. The first part of the programme, Project Spacewatch, is already underway, with very modest (often private) funding. Needless to say, Levy and the Shoemakers are closely associated with this. It may take many decades before a comprehensive search can be completed: this will depend entirely upon the number of telescopes allotted to the search.

The second part of the programme is far more challenging, and many unemployed Cold Warriors have volunteered their services, offering technologies developed for Ronald Reagan's Strategic Defense Initiative - declassified portions of which have already effected a major revolution in astronomy and space research. Lasers and nuclear bombs are an obvious answer to cosmic projectiles, but there may be more elegant solutions. If the oncoming projectile is detected in time - an efficient "spacewatch" network could give warning years ahead - a gentle nudge by a rocket tug would be sufficient to divert the intruder.

Science fiction writer Larry Niven has summed up the situation very neatly: "The dinosaurs became extinct because they didn't have a space programme." Carl Sagan has also pointed out that any longlived civilisation must be a spacefaring one, for the same reason. The awesome images in The Great Comet Crash should be enough to make anyone take these ideas seriously.

We live in a dangerous universe. Though global, dinosaur-scale catastrophes may be millions of years apart, there have been at least three near-misses in this century. Had it arrived only a few hours later, the flying iceberg (part of the Beta Taurid meteor stream) that exploded several kilometres above the Siberian forest in 1908 could have wiped out a major city.

One surprising lesson from the Shoemaker-Levy story is that Jupiter may have played a beneficent role in the history of our planet, its enormous gravitational field sweeping up comets that might otherwise have impacted upon earth. Indeed, life might never have originated here in the first place, if Jupiter had not shielded our planet in the days when the newborn solar system was packed with continually colliding debris left over from its formation. If much of this had fallen upon earth, the oceans would have been kept boiling - and sterile. On the other hand, as analysis of their orbits has shown, Jupiter may occasionally divert comets towards earth, as well as away from it.

If - unlike poor Kohoutek - Comet Hale-Bopp lives up to its advance billing and dominates our night skies in the spring of 1997, the world's governments may turn their attention away from their current problems long enough to consider proper financing for Spaceguard. If they do not, the human race may suffer the same fate as the dinosaurs, when the Jovian bomb-doors open again.

Arthur C. Clarke is an amateur astronomer based in Sri Lanka.