All bodies in the solar system with visible, solid surfaces show extensive evidence of cratering (with two exceptions - Io, a moon of Jupiter, which is being resurfaced by volcanically produced sulphur, and Europa, another Jovian moon, which has a surface of water ice). On Earth, we see very few obvious craters because geological processes such as volcanism and erosion soon destroy the evidence. On bodies with little or no atmosphere, such as Mercury or the Moon, craters abound, and even on Venus, with its enormously dense atmosphere, there is stark testimony to major impact events. There is strong evidence that there was a period of intense bombardment in the inner solar system that ended about 3.9 billion years ago. Since then, cratering appears to have continued at a slower, but fairly uniform rate. The cratering records, as witnessed on other Solar System bodies, should be viewed with care given the protective shield of the Earth's atmosphere, but the indications are clear. The atmosphere protects the surface from the multitude of small debris, the size of grains of sand or pebbles, about 100 tons of which impact with the planet every day. Meteors in the night sky are visible evidence of small debris burning up high in the atmosphere, while smaller particles decelerate more gently in the upper atmosphere, and settle slowly to the ground. Up to a diameter of about 100 metres, most stony meteoroids are destroyed in the atmosphere by pressure induced explosions, though some fragments can reach the ground as stony meteorites.

So, what is going to do the killing after a major impact? Immediate effects will include the obvious explosive effects at ground zero and local firestorms raised by the superheated air from the impact. A crater, about 20 times the diameter of the impacting body, will be excavated in a matter of seconds, and debris will be ejected into sub orbital trajectories. This debris will later re-enter the atmosphere – the meteor shower from hell - possibly all over the globe raising massive fires that destroy a significant proportion of the biomass. Intense acid rain would result from the ionisation of the air as the impactor entered the atmosphere, as would the production of pyrotoxins. The ozone layer would be severely damaged, and major volcanism and seismic activity can be expected as the shock wave of the impact ripples through the planet. All of this will cause a global environmental disaster of extreme severity. In addition to most or all of these effects, an impact at sea will produce a significant "tsunami," capable of travelling considerable distances, and possessing enormous energy. Such surges will pose a substantial threat to low lying and coastal areas. The United Kingdom, with much of its population and economic infrastructure located in precisely such areas, would be at particular risk from an impact anywhere in the Atlantic Ocean. Japanese research indicates that there is a one- percent chance that every major city on the Pacific Rim will receive catastrophic damage from an impact-induced tsunami at some stage during the next hundred years.

However, the main killing mechanism will be the vast amount of dust and debris injected into the upper atmosphere, combined with the smoke from the firestorms (witness recent events in Indonesia). These will obscure the Sun and cause a phenomenon similar to, but much more severe than the “nuclear winter” that became such an issue during the Cold War. It is this that is likely to pose the greatest threat to the ecosphere on a global scale as food chains collapse and darkness, cold and starvation set in.

After a few months or years the atmosphere will clear, but the surface of the Earth, now mainly white in colour, might reflect too much of the Sun's radiation to prevent a new ice age. However, there are other mechanisms at work. The atmosphere will contain a substantial excess of CO2, resulting from the global fires, the release of the gas from carbonate rocks and vulcanism. The Earth could be in for a massive overdose of greenhouse effect. The balance between sweltering and freezing is a very fine one.

Such globally threatening events can be expected on time scales of 100,000 years.

Smaller strikes, in the 50-100 metre range, though not globally threatening, have in the past caused massive damage to the area of impact, and often at considerable distance. We saw this at Tunguska in 1908 and in the Amazonian Rain Forest in 1930. The spread of human settlement, civilisation, and particularly urbanisation, makes it much more likely that a future impact, even relatively small, could result in the massive loss of human life and property. The time scale for such impacts is between 50 and 100 years.

Even much smaller impacts can have significant effects. Typically a 10-metre diameter body will have the kinetic energy of about 100 kilotons, Hiroshima was 150 kilotons, and is likely to detonate at an altitude above 10 km, causing little or no damage on the ground, but considerable alarm to those who witness it as was seen on 9 October 1997 in El Paso, Texas. Such events have been recorded by US surveillance satellites at the rate of one or two per month, and smaller Kiloton sized explosions happen every 1 to 10 days.

There are now more than 150 ring like structure structures on the Earth identified as definite impact craters. However, most of them are not obviously craters. Their identity has been masked by heavy erosion over the centuries, but the minerals and shocked rocks found nearby confirm that they were caused by massive impacts. The Ries Crater in Bavaria is a lush green basin some 25 kilometres (15 miles) in diameter with the city of Nordlingen in the middle. Fifteen million years ago a 1500 metre asteroid or comet impacted there, excavating more than a trillion tons of material and scattering it over the northern hemisphere. This sort of event is thought to occur about once every 100,000 years. A step up the threat scale is the type of impact that created the Chicxulub crater in Mexico some 65 million years ago. This is the event that destroyed up to 70% of species then living on the planet, including the dinosaurs. Impacts on this scale are thought to occur once every 50 -100 million years. Chicxulub is currently the largest known crater that has a confirmed impact origin, but there are several other larger ring-like structures about which geologists are, as yet, uncertain.

Currently there are more than 200 asteroids known to have orbits that cross that of Earth, though, being a three dimensional problem, this does not necessarily mean that a collision is inevitable. They range in diameter from a few metres up to 9 kilometres (1620 Ivar). A working group chaired by Dr. David Morrison of the NASA Ames Research Center, estimates that there are some 2,100 such asteroids larger than one kilometre and perhaps 320,000 larger than 100 metres. An impact on the Earth by one of the smaller bodies would be a local or regional catastrophe, but it would not be globally threatening. However, the working group concluded that an impact by a body larger than about two kilometres would, in addition to the immediate effects of the collision, significantly degrade the global environment to the extent that the survival of a significant proportion of the human population would be put at serious risk. An impact by an object larger than about five kilometres would inevitably lead to mass extinctions on a large scale. More recently, partly due to the results of the Shoemaker-Levy 9 impact on Jupiter, the threshold for a globally threatening body has been reduced to a diameter of about one kilometre.

An individual's chance of being killed by the effects of an asteroid or comet impact is small, but the risk increases with the size of the impacting body, with the greatest risk associated with global catastrophes resulting from impacts of objects larger than one kilometre. Calculations have been done relating to the relative probabilities of a citizen of the USA dying from a variety of causes. The results are shown in Table 1.

Table 1: Causes of Unnatural Death in the USA

These figures (after Chapman and Morrison) were accepted at the time, but a growing number of scientists feel that the figure for Asteroid Impact is too low by a likely factor of two, and should be closer to one in 10,000. The figures also fail to demonstrate the qualitative difference between individual events, such as the majority of cases in the table above, and the sudden, but massive loss of life caused by a major impact event.

Science knows of no asteroid or comet likely to collide with Earth in the short or medium term, but the number of known Earth crossing asteroids is small when compared with the total population. Even if it is assumed that the probability of a major collision is small, the global nature of the threat makes the assessment of its true nature an urgent priority.

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