by Vic Lloyd ©

(Investigator 44, 1995 September)

In spite of assorted social ills and injustices, and a fairly regular spate of natural disasters, we are, as a race, an optimistic lot. We plan ahead, blithely predict what we may expect during the next century and anticipate — take for granted in fact — an even greater flow of technological marvels than those we already have.

Which is not only as it should be, it is just as well because apart from those hazards of living already mentioned, and with possible mayhem looming in the shape of such adversities as planet-wide pollution, the greenhouse effect and the depletion of the ozone layer, we all live on the very brink of potential cosmic-scale destruction. Our planet is fragile and vulnerable beyond belief and it is only the maintenance of an exquisitely arranged system of checks and balances that life on Earth survives at all.

Precarious as that may sound however, it doesn't end there. Lethal radiation from a supernova as far away as say 10,000 light years could, for instance, completely sterilise everything on Earth. And for all we know, the leading edge of such an overwhelming wave of destruction could well be on its way, our telescopes and instruments innocently still informing us that all is well in the case of a given star, but with the convulsion having occurred 9999 years ago.

Or the Sun might flare up — or cool down. Our galaxy could even become a Seyfert, exploding and wiping out everything, out to the very limits of its thinly populated edge. But more tenable than any of these as a candidate for devastation is the likelihood of a collision between Earth and an asteroid or comet nucleus.

During the first half billion years or so after the formation of the solar system meteorite strikes on the surface of the Earth were conmonplace. Interplanetary space was thick with debris not yet swept up by planetary gravitation and the planets were heavily bombarded by a deluge of cosmic missiles. Were it not for the erosive effect of weather produced by our unique atmosphere (mostly wind and rain) the surface of Earth would almost certainly resemble that of Mercury or the Moon. Some authorities point to Hudson's Bay, the Sea of Japan and the Gulf of Mexico as being possible remnants of giant meteorite craters.

But although things have improved since those perilous times there really is no room for complacency. Less than 50,000 years ago the meteorite that formed the Barringer Crater in Arizona generated a half megaton blast that devastated an area with a radius of 160km. And as recently as 1908, at Tunguska, Siberia, a million ton meteorite levelled nearly 3000 square km of forest land with a 30 megaton explosion sending an atmospheric shock wave twice around the Earth.

Informed opinion puts the probability of our planet becoming the victim of a disastrous meteoritic impact as once in every 100,000 years. This being the case we are safe for a long time to come ... but don't count on it! Should the worst happen however, what would be the consequences? How would we fare?

Let's conjure up a theoretical scenario and imagine a hypothetical, 1000 cubic kilometre comet, the mass of its nucleus in the order of a trillion tons. This imaginary comet has a sideal period of 100,000 years and has reached its perihelion some 2000 times over the last 200 million years. Each encounter with the Sun has deprived it of a little more of its volatile substance and it is now on the point of disintegration, its rocky, dusty core held in situ only by the tenuous grip of the final reserves of its ices and its tiny gravity.

Let us further conjecture that its next Perihelion, due next week, will mark the comet's final journey as a cohesive whole. As it swings out from behind the Sun on its predestined journey to the far reaches of the solar system its ices reach exhaustion point and its nucleus fragments into a shower of separate pieces. These vary in size from grains as fine as flour to an awesome two-cubic-kilometre sized boulder. This phenomenon is not unique. In 1846, comet Biela shattered, and in 1976 Comet West broke into at least four large pieces. As a general rule, obeying the laws of celestial mechanics, such fragments continue to loosely cling together and travel the comet's orbit as an agglomerate.

That is, unless it falls under the influence of external forces. And with our fictitious comet, such is the ease. By coincidence, Mercury, Venus and Earth are in alignment with the Sun and the trajectory of the clump carries it on a path which just grazes the outer limits of the gravitational fields of the inner two planets. Not enough force is exerted to drag the whole cluster of fragments to their respective surfaces, but it is sufficient to attract the largest segment, the two-cubic-kilometre-flying-mountain (which, to provide an identification we shall now call 'Azar'). Nevertheless, the immense boulder bypasses both inner planets, missing them by a few thousand kilometres in each case. Travelling at almost 50km per second Azar is now on a collision course with Earth.

What happens next is a bewildering and appalling series of events. As Azar hurtles past Venus, astronomers calculate its velocity and plot its course, verifying at once that it will certainly strike the surface of Earth within the next few days. However, it is impossible either to establish a precise impact point, or take any practical measure designed to alleviate what is obviously going to be a disaster of monumental proportions. World authorities are warned accordingly.

A land strike would be catastrophic enough, especially if a heavily populated area were involved, but in this event most of the energy generated would be radiated back into space fairly quickly. However, an ocean strike would have much more serious and far-reaching effects

Azar, which has a mass of two thousand million tonnes, powered now by Earth's gravitational pull, builds its velocity to 80kps. It is in Earth's lower atmosphere for only two seconds and punches through the final 20km of ever-increasingly dense air resistance in a blink of the eye.

As it slams Earthward an immense air-cushion forms as a bow wave, so compressed that it turns to plasma, a boiling, turbulent slush of ruptured, collapsed oxygen and hydrogen atoms, their abandoned electrons glowing with the vivid blue of hard radiation. The massive air-cushion has no discernible effect on Azar's velocity and in that brief moment, generating immense heat and light, it glows like a dazzling little white dwarf star brighter than the Sun, imprinting a burning track of abused electrons in its wake.

As air rushes in to equalise the near vacuum left by the incandescent trail a thunderclap louder than anything heard on Earth for millions of years radiates out with murderous intensity, producing vibrations violent enough to powder stone. Any animals or people engulfed within the influence of its terrible resonance simply disintegrate.

Reaching the surface, and now glowing at 1000 C , Azar punches into the South West pacific Basin, 800km south of Tonga. There is no splash. Subjected to the brutal velocity of the immense boulder — nearly 300,000 kph — water has no more resistance than air. In a millisecond a vast area of ocean is vaporised and Azar plunges deep into levels where darkness has been the norm for thousands of millennia. It penetrates half a kilometre of seabed sediment and 5km of crust 1ike a hot knife through butter, finally coming to rest in the dense Mohorovicic Discontinuity, eleven kilometres beneath the surface of the ocean. The unparalleled force of the impact shatters rocks to talcum powder for hundreds of square kilometres around and releases a savage gush of white-hot lava.

No point on Earth escapes the devastating shock effect of the ferocious collision. The tectonic plates react to the outrage by tilting and buckling as a pulsating wave of energy ripples through them. Buildings topple, dams burst, bridges collapse and whole sides of mountains detach and thunder down valleys. In the United States and Europe, several nuclear power stations are demolished with frightening consequences. The abrupt cessation of Azar's colossal momentum has released energy equivalent to that of 600 thousand megatons of exploding TNT. Even at this early stage in the chain of events the turmoil is unprecedented and the toll of human life horrendous.

Meanwhile, at the site of the impact a gigantic crater, five kilometres deep and fifty kilometres across, opens for a fraction of a second. In that instant, 10,000 cubic kilometres of water (enough to cover the whole of Australia to a depth of 6.5 metres) is transformed into superheated steam, as clear as air. Behind the steam, forcing it upward, a gigantic fireball surges to the surface, expanding and spreading across the ocean at 1000 kph.

Momentarily, a circular wall of water hovers three kilometres above and around the vast oceanic crater and then, in a flood more terrifying than anything ever seen or heard, roars back into the abyss — in turn be to instantly vaporised into millions more tons of steam as the water hits molten lava at the crater's base. Nothing within an area of 1000 square kilomtres survives the onslaught and nearly all of the pacific islands vanish for ever.

As the scalding vapour shoots skywards, bearing an assortment of debris, mountain ranges of dense cloud build up, kilometres thick, and promptly begin to spread around the globe. Winds reach hurricane force in response to rapidly changing atmospheric pressures and lightning, accompanied by ear-splitting thunder is constant as electrical potential builds and discharges in wild confusion. The whole planet is soon deluged by torrential rain as the massive clouds reach into the upper atmosphere and precipitate.

On the surface of the ocean, even more mischief is afoot. Forming a circle around the impact point a wave, already ominously higher than those associated with known tsunamis, speeds outwards at 1200 kph. When it reaches the continental shelves of the land masses surrounding the Pacific Basin it begins to build until, when it strikes the coastal shal1ows, its wavelength shortening, it becomes a towering mass of energy filled water, 700 metres in height (as high as Mount Lofty). With terrible force it sweeps away whole cities, races up rivers and causes enormous destruction as it smashes inland, its energy only expending when it reaches high ground. And, as it sweeps back into the ocean with almost as much speed and vigour the water carries millions of tons of detritus and the bodies of uncountable people, clotting the seaways on an unprecedented scale.

Although the coasts of the Pacific countries bear the initial brunt of Azar's brutal assault, the whole planet is cruelly afflicted in the long term. Rain, on a world-wide basis will be incessant for months to come, bringing with it millions of tons of oceanic ooze, salt and pulverised rock, poisoning topsoil everywhere. And the kilometres thick cloud mass, temporarily rivalling that of Venus, will not only block out the Sun, its upper surface will reflect sunlight and therefore heat, back into space. Our ecological pattern will change radically; some lifeforms disappearing for ever and others proliferating. Most deserts will vanish to be replaced by barren, soggy marshes; the polar caps will increase in size and in the northern hemisphere new gigantic glaciers will slowly encroach into the continents. A new ice age is not an unreasonable assumption. It could well take a thousand years for Earth to fully recover from such an ecological outrage but in some areas things will never be the same. In order to survive, especially in food production man's ingenuity will be stretched to the very limit...

A11 of this is, of course, only a hypothetical scenario; a pretty piece of imaginative 'science fiction' based on theoretical possibilities.

However, Asteroid 1991 BA ('the size of a small house') came perilously close to producing something 1ike the foregoing in January 1991; it missed us by a mere 170,000 km. Moving at about 50km per second the Asteroid was less than an hour's travelling time away. A tiny adjustment in its trajectory might have made an enormous difference to our wel1-being; we only narrowly escaped what could well have been a terrifying catastrophe...

But what about its next orbit...?