-
- Vasily Dzhenkoul must have seen the fireball coming.
But on that fateful morning in 1908 there was little he could do to save
his reindeer or himself. As he watched his herd incinerate, he could hardly
have imagined that his escape would be just a temporary reprieve. In weeks
he would die an agonising death marked by all the symptoms of acute radiation
sickness.
-
- But the fact that Dzhenkoul was one of only a handful
of victims of the Tunguska explosion is perhaps most amazing of all.
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- With an estimated mass of 100 000 tons, the Tunguska
cosmic body generated a force 2 000 times that of the Hiroshima bomb, instantly
igniting 2 150km" of Siberian taiga forest and sending shock waves
around the globe. The blast was heard 1 000km away, and debris thrown up
into the earth's atmosphere resulted in months of bright nights worldwide.
-
- Ninety-one years on, most experts agree that the object
that entered our atmosphere on July 30 1908 was either a loosely packed
comet or a small, stony asteroid. It exploded in the atmosphere several
kilometres above the earth, producing a massive, destructive fireball and
leaving no crater or any substantial fragments.
-
- Most significant of all, though, is that investigations
into the Tunguska event have brought to light evidence that the frequency
with which extraterrestrial objects hit earth is much greater than previously
thought. Last week, a British government task force was set up to examine
the threat of earth impact by rogue cosmic bodies.
-
- The asteroid-impact hypothesis for the demise of the
dinosaurs has long been widely accepted as fact. Then there was the massive
3.5 megaton explosion that left the Arizona crater some 50 000 years ago.
-
- There is the Sikhote Alin impact site in Russia's Far
East and New Zealand's 800-year-old explosion at Tapanui. But who has ever
heard of an Amazonian meteorite that caused Tunguska-scale destruction
in 1930, and a comparable one in British Guyana in 1935? Who knows about
the near-miss in 1972 when an asteroid skipped through the upper atmosphere
above Wyoming and Montana before shooting back out into space above Canada?
In 1995, a suspected comet fragment hit the northeastern state of Piaui
in Brazil. Then there was the much-hyped asteroid 1997 XF11, calculated
to be on course for earth impact in 2028 until new estimates confirmed
it would miss us by 100 000km. It looked like the earth was safe.
-
- So far, major earth impacts have occurred in the most
sparsely populated parts of the globe. This may account for our underestimation
of their frequency, thinks British astronomer Dr Mark Bailey.
-
- Last century's South American impacts, when coupled with
the Tunguska event, mean largescale extraterrestrial invaders could be
up to 10 times more common than previously thought. "The earth may
be subject to three or four such events a century," warns Bailey.
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- If the Tunguska cosmic body had exploded above a city,
the casualties would have been in the hundreds of thousands. If an asteroid
were to land in the sea, it would cause global tidal waves, wiping out
coastal settlements. If 1997 XF11 had been on a collision course with earth,
the 1.6km-wide asteroid would have caused such intense earthquakes and
fierce firestorms and blasted so much debris into the atmosphere that skies
might have darkened for years, disrupting the food chain and pronouncing
a death sentence on life on earth.
-
- Despite the relatively small probability of catastrophic
earth impact, the issue is important enough to warrant near-earth object
monitoring by astronomical observatories worldwide.
-
- The Lincoln Near Earth Asteroid Research project in Massachusetts,
US, is a leader in this field. Scientists there monitor more than 2 000
near-earth objects larger than 1km in diameter - objects with the potential
to wipe out life on earth were we to come into violent contact. But a modest
near-earth object detection programme led by Dr Duncan Steel at the AngloAustralian
Observatory recently failed through underfunding. This was the only project
of its kind in the southern hemisphere, so the earth is now especially
vulnerable to undetected objects approaching from the southern sky.
-
- Dr Harry Atkinson, a New Zealander and a member of the
recently appointed British nearearth object task force, which will report
its findings to the British National Space Centre in mid-2000, admits that
"the southern hemisphere is a great weakness": near-earth objects
on course for catastrophic impact stand little chance of being detected
here.
-
- The risk of death by an errant cosmic body is, of course,
hard to quantify. But the certainty of widespread, horrific casualties
makes it a high-enough risk to demand action.
-
- How can mankind protect itself? There have been suggestions
of using something similar to the US's '80s "Star Wars" programme
to detect and destroy incoming invaders. But the use of nuclear weapons
in space is surely far too risky - and if the object were to shatter, showering
the earth with smaller projectiles, the resulting devastation could be
worse. Or we could divert an earth-bound object by attaching a vast mechanical
digger and accelerating the debris into space, thus nudging it off its
orbit - a feat beyond the capabilities of science.
-
- A fortnight into the 21st century the millennial doomsdayers
have been silenced by the steady ticking of the clock. Yet we are certain
to hear more from predictors of global cataclysm.
-
- Whether this is simply a new worry for a new millennium
or a serious threat to life as we know it, some members of our industrious,
imaginative species are already cashing in. Earth impact fever has spawned
a new trend in the Japanese lingerie market. The latest must-have item:
the meteorite detector bra.
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- At least some of us will be strapped in for impact. _____
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- Gabi Mocatta, a South African, was an interpreter on
a 1998 expedition to the Tunguska event site in Siberia. She is now working
in Australia.
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