ENJOYING the delights of the Podkamennaya Tunguska river valley in central Siberia was not a good idea on the morning of June 30th, 1908.
On that day a tremendous explosion occurred in the sky above, sending a blast wave that flattened a half a million acres of pine forest, burning much of it, and knocking people off their feet 60 km away, about the distance from Dublin to Mullingar.
Seismic shocks usually associated with earthquakes were registered right around the world. Witnesses recorded seeing a massive fireball in the sky just before the explosion. And an unusual night sky brightness persisted for nearly two months after the event.
It was a fraught time politically for Russia so it took a full 20 years before a scientific expedition made its way to the region to study the event. Scientists had assumed that a giant meteorite had struck the earth, but to their surprise no resultant crater was found.
Most scientists now believe that the mighty cataclysm was caused by a relatively dainty piece of a passing comet, comet Encke. A chunk of dirty ice - about 40 metres across and travelling at about 27 miles per second was thought to have been sheared off of Encke, only to strike the Earth a glancing blow.
The block, superheated in the atmosphere, blew up over Tunguska with the force of up to 5 million tonnes of TNT, equivalent to quite a hefty nuclear explosion. Cometary dust thrown out by the event was assumed to have caused the unusual brightness for weeks afterwards.
Is it any surprise, then, that people in times past have looked with dread and wonder at the passage of comets, attempting to draw signs and meaning from their appearance. For us, much of the mystery has been taken out of comets - such as our current visitor, Hale-Bopp - by our technological knowledge. But it was not always so.
Ancient civilisations feared comets as harbingers of great events - and fate (or serendipity) supported their view. One of the best known comets, Halley, which approaches every 76 years or so, appeared in the night skies when Attila the Hun and his hordes were defeated by the Romans and Visigoths in 451 at Calons. It returned in 1066 as William the Conqueror invaded and subdued England, and Halley's appearance in 1456 caused a great panic among Christians who believed it was associated with the capture of Constantinople by the Turks.
Halley's namesake, the astronomer Sir Edmund Halley, put paid to much of the awe and romance associated with the arrival of comets in 1704 when he discovered their periodic motion. He was able to predict the comet's visits by calculating its orbit, which carried it along an ellipse reaching from the sun at one end to beyond the planet Neptune at the other.
Having knocked the phantasmagoric out of cometary display, scientists ever since have sought to define what they are made of, where they come from and why they do what they do. And as we come to understand more about their movements, we can contemplate the incomprehensible - human extinction brought about by an impact on Earth by a large comet.
Halley's discovery and work by countless scientists have not managed, however, to diminish the splendour of a comet as it arcs across the night sky.
In the truest sense, everything a comet is and does is dependent on the sun. Comets are tethered by the sun's massive gravity and reach our solar system at its tug. Their lacklustre mix of ice, frozen gases and space dust giving them the nickname of dirty snowballs is virtually invisible in the blackness of space until they begin their dive towards the sun. As they approach they warm up, flashing into a spectacular display as energy from the sun encourages the discharge of both gases and dust from fissures in their surface.
The heart or nucleus of the smallest comet detected is estimated at about half a kilometre (0.3 miles) across, but the largest is thought to be about 70 kilometres (42 miles) across, surely impressive if you were to stand next to it.
Yet this is nothing compared to the visible "halo" or coma surrounding the nucleus, which can measure 100,000 miles across. And the coma is in turn dwarfed by a trailing tail of dust which can stretch for 100 million miles in a comet's wake.
The sun is reflected off materials in the coma and tail, allowing us to see and marvel when comets appear. The closer to the sun the more active they generally become until they turn the solar corner and hurl back into the vastness of space, visually diminishing as they go.
They physically diminish as well, leaving behind a trail of dust and pebble-like bits. Estimates suggest they lose material from the surface to a depth of 40 inches per complete orbit, so after many orbits a loop of debris is created, marking out the comet's path.
This material is too small to be seen unless the earth's orbit carries us through these dusty trails. Then comets offer yet another night-time spectacle, intense showers of shooting stars. The dust and stones burn up as they enter the atmosphere high above, giving us the Perseids in August (thanks to comet Swift-Tuttle which passes every 120 years) and the Leonids in November (as we pass through the dust left by comet Tempel-Tuttle which orbits every 33 years).
Comets such as Halley, which return in less than 200 years, travel in orbits that lie close to those of the planets. Like all comets that we can observe, they loop the sun then travel out varying distances some right out of the solar system past Pluto.
Hale-Bopp is a different kind of comet, originating from much further away, perhaps half the distance to the next star, in a place fancifully named the Oort Cloud. The cloud is a vast reservoir of perhaps several hundred million comets parked at the far edge of the sun's gravitational pull.
Here they hover until a passing star or the gravitational tides caused by the movement of our own sun knocks one of the comets loose. It then begins its long, high-speed passage towards us taking thousands of years to complete an orbit.
Scientists are intensely interested in these comets because they are likely to have been formed at the same time as the rest of the solar system - 4.6 billion years ago. They represent high-quality samples of the stuff from which the sun and planets were originally made. The difference is that they were too far away and, were left behind when gravity caused the bodies in our solar system to coalesce like lumps in, gravy.
SOMETIMES incoming comets pass close to the giant, planets, particularly massive Jupiter, on their way in. The powerful gravitational pull knocks the comet out of its original path, sometimes capturing it and forcing it into an orbital pattern more hake Halley.
However, comets are relatively fragile and this planetary tugging can also cause a comet to break-up. This was the fate of the Shoemaker-Levy comet. It split into a collection of chunks on one orbit that brought it too close to Jupiter, and then plunged into Jupiter itself in July, 1994.
Such an impact represents a moth, almost unimaginable release of energy, and cometary impacts with the planets - including earth - are regular if infrequent occurrences.
Scientists believe they can identify five major and comprehensive extinctions of life on Earth following direct hits by either comets or rocky asteroids. The most recent was 65 million years ago when an impact off Mexico first caused a worldwide firestorm followed by months of "night" and global climate change brought about by dust and smoke suspended in the atmosphere.
On the other hand, some theories hold that cometary impacts may have helped deliver life on Earth. It has been suggested that comets striking the Earth may have filled the oceans with water. And some have theorised that either the chemicals that supported life or some life germ itself was brought to Earth by comet.
Could a major impact happen again? This is a virtual certainty, although there is no way to predict when, whether in 100, 1,000 or a million years.
It is worth remembering Tunguska where devastation over a wide area was caused by an object just 40 metres across. Now consider Hale-Bopp, which is estimated to be 40 kms across.
Researchers argue that while the probability of the Earth being struck by a large comet is extremely small the consequences of such a collision are so profound that it is wise to calculate the risk and attempt a preventative response.
The worst-case scenario involves an impact by a body large enough to change climate on a global scale. The dust blanket would blot out the sun and cause a substantial loss of food crops. So if you survived the impact and resultant firestorm, you would be likely to freeze or die of starvation.
The minimum size of body thought capable of such damage - and which could wipe out 75 per cent of all life on earth - is between one and two kilometres across. The energy released would be greater than that delivered by a million million tonnes of TNT.
There are perhaps 2,000 near-Earth objects big enough to do this and about 300 have been charted, but researchers believe none of these have any chance of hitting us during the next century.
The US National Aeronautics and Space Administration has argued since the early 1990s for a relatively inexpensive but comprehensive programme of observation known as Spaceguard to watch for stray comets and asteroids. More fanciful plans calls for laser beams or the use of small telescopes that would scan the skies for large objects, greatly improving what we know about impact hazards.
More problematic is what to do to prevent an impact. Spaceguard might do no more than give us plenty of time to consider our ultimate doom. Some responses call for the launch of nuclear weapons to destroy or divert threatening comets or asteroids. More fanciful plans call for laser beams or the use of giant sails to divert the course of incoming bodies. All of these, however, exist in theory only. There is no actual response available to us without massive investment and so far people have been reluctant to spend even the $10 million a year needed to make Spaceguard work.
While we may not view Hale-Bopp - which might prove to be the second brightest comet in the past 600 years before it departs - with the awe of earlier civilisations, it is worth remembering that an approaching comet may one day have our collective name on it. Then we may stand and wonder at the power behind the workings of the universe.
