(10) Global Catastrophes

What to do if an asteroid comes our way

Study Determines Which Effects Of An Asteroid Strike Would Result In Most Casualties

What would happen if that massive asteroid zipping by actually hit us? Study reveals exactly how millions would die


The Threat from Space

Asteroid and Comet Impacts

The astronomical event of the century

In 1993 a discovery by Carolyn Shoemaker, wife of the late and greatly lamented planetary scientist Eugene Shoemaker, and colleague David Levy, was to change forever our perception of the Earth as a safe and cosy haven insulated from the whizzes and bangs of a violent and capricious universe. The Shoemaker team had spotted 21 huge chunks of rock that had once been part of a comet torn apart by the enormous gravitational field of the planet Jupiter - a giant sphere mainly made up of hydrogen and helium gas that is large enough to contain over 1,300 Earths. Instead of orbiting the Sun, like most comets, however, this one had been captured by Jupiter’s gravity and the rocky fragments now orbited the King of Planets himself. As Jupiter already had a large retinue of moons, the addition of a few more would have been mildly interesting, if not surprising. What was extraordinary, however, was that these new ‘moons’ were very much ephemeral. The following year they would end their lives by crashing into the surface of Jupiter, providing scientists on Earth with a grandstand view of just what happens when a planet is struck by large hunks of space debris.

On 16 July 1994 - appropriately the 25th anniversary of the launch of Apollo 11, the first manned lunar landing mission - the first fragment of Comet Shoemaker-Levy struck the planet, sending up a gigantic plume of gas and debris and blasting outwards a rapidly expanding shock wave. As fragment after fragment hammered into the planet, spectacular images were gathered by the Hubble Space Telescope in Earth orbit and by the unmanned Galileo probe on its way to Jupiter. Two days after the initial impact, a chunk of rock 4 kilometres across and rather unromantically named fragment G smashed into the planet with the force of 100 million tones of TNT - roughly the equivalent of eight billion Hiroshima-sized atomic bombs. The flash generated by the collision was so brilliant that many infra-red telescopes trained on the event were temporarily blinded. The glare soon faded, however, to reveal an enormous dark impact scar wider than the Earth. Inevitably, everyone who saw this awesome image had the same thought. What would have happened if fragment G had struck the Earth instead of Jupiter?

Almost overnight our planet seemed a much more vulnerable place and the hold of our race upon it that much more tenuous. Suddenly both scientists and the public, and even politicians began to take the threat from space seriously. Two Hollywood blockbuster films fed growing interest in impact events by showing - with various degrees of scientific rigour - what we might all be in for if a comet or asteroid headed our way.

In 1996, just two years after the Jupiter impacts, an international body is known as the Space-guard Foundation was formed, with the dedicated aims of promoting the search for potentially dangerous asteroids and comets and raising the general level of awareness of the impact threat. In the United States, NASA and the Department of Defence began, albeit at a low level, to fund Space guard-related projects and the UK government established a task force to examine the risk of asteroids and comets hitting the Earth. All of a sudden everyone wanted to know what the chances were of the Earth being struck at some point in the future and what effect such a collision would have on our planet and our race. The answer to the first question is easy: the chances are 100 per cent. The Earth has been bombarded by space debris throughout its long history, and although such collisions are now far less common than they were billions of years ago, our planet will be struck again. The vital question is - when? And as regards how bad this will be for the human race: that depends largely on how big a chunk of rock hits us.

The cosmic sandstorm

To get a better idea of how frequently the Earth is likely to be hit, we need to find out how many rocks are hurtling around our solar system and, in particular, how many of these come close enough to the Earth to start us worrying. Although a vast amount of debris was swept up by the embryonic planets during the early solar system, countless dregs remain, ranging upwards in size from tiny specks a few millimetres across to gigantic rocks, such as the minor planet Ceres, over 1,000 kilometers in diameter. Like someone battling through a desert sandstorm, the Earth is constantly bombarded in the course of its journey through the solar system.

Fortunately for us, most of the billions of colliding fragments are tiny and flash into oblivion as soon as they come into contact with our planet’s atmospheric shield. Every now and again, however, the Earth collides with something larger.

A fragment of debris the size of a pea burns up in the Earth’s atmosphere every five minutes, while a soccer-ball sized lump will light up the sky with its death throes around once a month. Larger objects may run the gauntlet of the atmosphere and reach the surface, but this is rare and only happens a few times a year. Perhaps every few centuries, the Earth collides with a rock in the 40-50-metre size range - an object large enough to obliterate a city if it scores a direct hit. The last well-documented impact of this size occurred as recently as 1908 – of which more later.

While the entire solar system teems with debris, from a hazard point of view we are only really interested in those fragments that threaten to end their existence through collision with our planet. The majority of these Earth-threatening objects are rocky asteroids that have orbits around the Sun that approach or intersect the Earth’s. The true numbers of these Near Earth Asteroids (NEAs) are impossible to determine, but current estimates are pretty frightening.

In all, up to 20 million pieces of rock over 10 metres across may be hurtling across or close to our planet’s path during its journey around the Sun. Up to 100,000 of these are thought to be over 100 meters in diameter - big enough to obliterate London or New York gave a direct hit -and maybe 20,000 are half a kilometre across, sufficient to wipe out a small country if they strike land, or generate devastating tsunamis if they impact in the ocean. Fewer in number, but enormously more destructive if they hit, are those asteroids 1 kilometre or more in diameter, which have the potential to obliterate a country the size of England and - if 2 kilometres or more across - wreak havoc across the globe. Although barely equivalent in diameter to 20 soccer pitches laid end to end, such is the tremendous level of kinetic energy - or energy of motion - involved in the collision that a 2-kilometre objects striking land would leave a crater 40 kilometers or so across and loft sufficient pulverized debris into the atmosphere to block out the Sun’s rays and plunge the Earth into a freezing cosmic winter for years.

A range of estimates has been published for the number of NEAs in the 1 kilometre and above size range, with the most recent suggesting, there are around 1000. As of August 2005, 794 of these had been identified – perhaps three-quarters of the total – and their orbits projected forward in time to see if they pose a threat to the Earth in the medium term, and the search continues to find them all – a task that will take at least a couple more decades. Once this has been accomplished and assuming that one does not have our name on it, we can sleep a little safer in our beds. The problem does not, unfortunately, end there. We still have the comets to worry about. Comets are enormous masses of rock and ice that can be up to 100 kilometres or more across. In contrast to the near-circular orbits of the asteroids, most comets follow strongly elliptical paths that carry them from the freezing outposts of the outer solar system, or beyond, in close to the Sun and then out again. In the depths of space, comets are enigmatic objects and not easy to spot. As they enter the inner solar system, however, they undergo a remarkable transformation as sunlight starts to evaporate gas and dust particles from the central nucleus to form a spectacular tail that can stretch across space for 100 million kilometres or more. The stunning apparition of a comet’s tail was long regarded as a portent of doom and disaster, and in a way, this is not too far from the mark. Comets have typical velocities of 60–70 kilometres a second - a hundred times faster than Concorde, and around three times that of the typical NEA. As a result, a collision between a comet and the Earth is hugely more energetic and therefore tremendously more destructive.

Another problem with comets is that, unlike their asteroid cousins, their orbital parameters are often poorly known and therefore difficult to project into the future to see if they pose any threat.

Halley’s Comet, undoubtedly the most famous of all, follows an orbit around the Sun that takes only 76 years to complete. Consequently, it has been observed dozens of times over thousands of years and its orbit is well enough known to make it possible to calculate its path far into the future. This shows that, at least until 3000 ads, Halley’s Comet will not even come close to threatening the Earth. Other comets, however, follow parabolic orbits that take them on immeasurably long journeys far beyond the limits of the solar system. Some of these may have been observed once or twice by our distant ancestors, but others may be making their first ever visit the inner solar system. In these circumstances, there has been no opportunity to predict their orbits on the basis of earlier visitations, and our first view of one of these objects heading our way may provide us with just six months respite before an unavoidable and calamitous collision. Furthermore, because such comets have been confined to deep space, they are huge - perhaps 100 kilometres or more across. This is because they have not suffered attrition from the solar wind, the hurricane of solar particles that evaporates parts of a comet to produce the characteristic tail, as it forges its way through the inner solar system

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