OVERALL
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Our understanding of the risk to the Earth of impacts from Near Earth Objects depends in part on which of two situations obtains. For objects whose orbits we know accurately in advance, we can predict with some accuracy (for many years to come) the time and place of any potential impact on the Earth. For these objects, the future is thus largely determined, with little statistical uncertainty. However, for objects yet to be discovered (for which the orbits are by definition unknown) we must rely on a statistical approach. Here, all we can do is to estimate the average frequency of impact for objects of different size, as described in Chapter 2; for none of these can the precise time or place of impact be anticipated.

Clearly, the objective should be to move as many potentially hazardous Near Earth Objects as possible from the second category to the first: to move from a situation of statistical chance to one of certainty, in which we should be able to plan ahead. Our recommendations for an advanced observational programme, to which we give the highest scientific priority, are framed accordingly.

The present position is as follows. From measurements over recent years, made largely by groups in the United States, we know the orbits of over 400 Near Earth Objects of diameter above 1 kilometre. These measurements allow us to state with some confidence that none of these is likely to hit the Earth over the next 50 years (see Annex B-3). However, it is estimated that a similar number of objects of this size have yet to be discovered. For smaller objects - which can also cause great destruction locally or regionally - we know even less. For example, we have discovered fewer than 10 per cent of objects of diameter 300 metres, and a much smaller proportion of 100 metre objects. Specifically, we believe that the aim should be to measure over thecoming years the orbits of all objects down to diameters of 300 metres, by the use of larger telescopes than those currently employed.These observations would also improve our statistical knowledge of the diminishing population of objects as yet undiscovered.

Through this programme, and through studying, as we recommend, the consequences of impacts and of the possibilities for mitigation, we should be able to make more accurate forecasts. This would also provide a firm foundation on which to increase general understanding of the problem and to communicate intelligently with the public in the event of a real emergency.

The consequences of an impact in terms of human life are estimated in the table on page 20 for different kinds of impact, assuming no attempt at mitigation.The results are inevitably speculative, and depend on a wide range of factors including the composition of the object; this range is reflected in the spread of the numbers. For sub-global events, the number of fatalities expected in an individual impact is highly variable. Once the global threshold has been exceeded, the consequences for each event are expected to be more uniform. The material damage produced by an impact and the consequences of that damage have been less well studied.

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Impacts from very large Near Earth Objects with diameters over 10 kilometres would have global consequences that could cause the extinction of most living organisms. Such events are fortunately very rare. Impacts of objects from a few kilometres in diameter to 10 kilometres are also rare.

Impacts from objects with diameters of around 1 kilometre can also have global consequences. On average these are the most dangerous because they are much more frequent than the objects of the 10 kilometre class and give many more casualties per impact than the smaller ones. Impacts of smaller objects, with diameters of a few hundred metres, would have dramatic local consequences, but are unlikely to affect the Earth as a whole. For objects below about 50 metres in size the Earth�s atmosphere usually provides good protection.

Impacts from mid-sized Near Earth Objects are thus examples of an important class of events of low probability and high consequence. There are well- established criteria for assessing whether such risks are to be considered tolerable, even though they may be expected to occur only on time-scales of thousands, tens of thousands or even hundreds of thousands of years. These criteria have been developed from experience by organisations like the British Health and Safety Executive to show when action should be taken to reduce the risks.

Flood protection, the safety of nuclear power stations, the storage of dangerous chemicals or of nuclear waste are all examples of situations in which rare failures may have major consequences for life or the environment. Once the risk is assessed, plans can be made to reduce it from the intolerable to the lowest reasonably practical levels taking account of the costs involved.

If a quarter of the world�s population were at risk from the impact of an object of 1 kilometre diameter, then according to current safety standards in use in the United Kingdom, the risk of such casualty levels, even if occurring on average once every 100,000 years, would significantly exceed a tolerable level. If such risks were the responsibility of an operator of an industrial plant or other activity, then that operator would be required to take steps to reduce the risk to levels that were deemed tolerable.

For an island country, the risks from tsunami effects are significant because of the large target area of the surrounding ocean. The western coast of Europe, including the United Kingdom, is at risk from an impact in the Atlantic Ocean or North Sea, as also are New Zealand or Japan from impacts in the Pacific Ocean. Destruction to property could of course be on a massive scale, and might not be avoidable, and the consequent social and political consequences could be severe.

The level of the risk to life and property from Near Earth Objects is largely related to what we choose to do in the future. If we do nothing, the consequences would be as described here. But by discovering and tracking most of the dangerous objects (at the same time improving our statistical knowledge of the remainder), and by studying further the consequences of impacts and the possibilities for mitigation, we can hope to exert some control over future events.

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