In preparing this talk, I have been fortunate in knowing that much of the introductory work has already been covered in the excellent, wide-ranging POST report (POST # 126, April 1999 - PDF format from www.parliament.uk) prepared by Dr Vivienne Moore and Professor David Cope.
As the report makes clear, Near-Earth Objects - or NEOs for short - represent one of the most rapidly developing branches of astronomy. The subject has links not only to the formation of our solar system and the origin and evolution of the Earth, but also - through the many interdisciplinary connections implicit in the subject - raises some of the most profound scientific questions of the late 20th century.
In this talk, I want briefly to introduce NEOs - what they are and where they come from - and discuss their likely rate of collision with the Earth. This, and the effects of hypervelocity impacts, raises the important issue of the impact hazard and the actuarial risk that such events present.
In recent years, many studies have addressed the issue of the actuarial risk - from several perspectives. Notable, in my view, are the contributions by G. Canavan (Hazards book, 1994) and N. Holloway (RGO Spaceguard Meeting, Cambridge 1997). Despite the scientific uncertainty, there is notable agreement that the potential threat to civilization - while a very rare event - is high in relation to hazards that Society routinely seems to care about. The question, then, is what should be done?
At this point, the UK is very well placed to play a leading role in European and world efforts to advance understanding and detection of NEOs, and the implications of both large and small-body impacts. In this talk, I hope to persuade you not only that the effort would be worthwhile, but also that, as we approach the end of the 20th century, our generation has a unique responsibility to progress the issue.
The first slide summarizes the physical nature of NEOs. Briefly, they are any astronomically "small" body that happens to have an orbit that passes close to the Earth. The size range is very large - from a few tens of metres up to tens of kilometres or more. The objects themselves include comets and asteroids, and fragments thereof, and "dead" or devolatilized (or temporarily inactive) comets, and occasional but very rare "giant" comets (diameters greater than 100 km). The fragments of these larger bodies, particularly if they break down to produce large numbers of sub-km sized bodies, or even submicron-sized dust, are potentially of great concern. At the lower end of the size range of NEOs, the population merges into that of meteoroids and interplanetary dust, the total mass influx of these particles on the Earth being some tens of thousands of tonnes per year (Ceplecha et al. 1998, Space Sci. Rev., 84, 327-471).
These objects are basically great lumps of rock, or rock-and-ice in the case of comets. Their significance lies in their large numbers (recently discovered) and the fact that many have orbits that, sooner or later, may intersect that of the Earth. Their high velocities - tens of kilometres per second, or many tens of thousands of miles per hour - and large sizes (weighing billions of tonnes) mean that they carry vast amounts of kinetic energy. A one-kilometre diameter asteroid, impacting at 20 km per second has the kinetic energy equivalent of some 50 thousand Megatons of TNT.
Before moving on to consider the effects of impacts, we should explain why the subject has attracted so much attention in recent years. In short, why the fuss?
The first asteroid was discovered on 1 January 1801, but for many years the only such objects known were those moving in very stable orbits between Mars and Jupiter in what became known as the main asteroid belt. The first Earth-approacher was found in 1898, and for much of the first half of the 20th century no Earth-crossers were known at all.
By 1970 about 30 such objects had been found, and by this time it was estimated that the total population of such bodies with diameters larger than 1 km was probably about 100. A few astronomers, notably Öpik, had made detailed estimates of the cratering rate on the Earth and other planets due to this new population of solar system objects, and at this time even this number of objects was very difficult to explain theoretically. This led to searches, and more theoretical work; and finally - as we now know - to the rapid discovery of many more objects than expected. Current estimates - mainly as a result of primarily US surveys aimed at discovering more about the NEO population - indicate that there are some 1500 near-Earth asteroids larger than 1 km diameter on Earth-crossing orbits.
In addition, it is important to note that there is probably at least a similar number of comets, or extinct cometary nuclei, while the number of smaller (though still potentially damaging) objects is very much larger.
The effects of an impact depend on the size, density, composition etc. of the impacting object, as well as on its impact velocity with respect to the Earth and where it strikes. Mass for mass, comets are more dangerous than asteroids, since their typical impact velocity is ~55 km per second, compared with ~16 km per second for NEAs. Apart from this, size is the most important parameter, since mass is proportional to diameter cubed.
Broadly speaking, global effects kick in for objects with diameters greater than about 1 km. Whether the critical size for global effects is 0.5 or 2.0 km is not known, and it may be preferable to err on the safe side; it is fortunate that we have not yet had a chance to test our theories in this regime. A few years ago the Jovians had just such an opportunity - with the impact of the tidally disrupted comet Shoemaker-Levy 9 in July 1994. Morrison et al's article (1994 Hazards, p.59) gives more details on the likely effects of impacts versus size.
Partly as a result of these observations, there has been a growing tendency by some commentators to regard the "critical" size for global effects, at least so far as the survival of civilization is concerned, as probably lying closer to 0.5 km than 2.0 km.
The following links illustrate some relatively small-body impacts, namely:
Despite the significance of these events, they are of relatively small concern; we are primarily interested in the large, globally threatening objects:
Where do these dangerous objects come from?
There are two principal types: comets and asteroids. Whereas comets can in principle originate from a variety of sources in the outer solar system, the two main regions being the Edgeworth-Kuiper belt (just beyond Neptune) and the outer Oort cloud (a thousand times further away still, halfway to the nearest star), asteroids originate predominantly via collisions and subsequent dynamical evolution from the main asteroid belt.
Relatively little is known about the cometary component of the NEO population , mostly because dead or inactive comets are usually very dark and difficult to detect. There is a possibly significant number of "dead" Halley-type short-period comets amongst the NEO population, but so far only one or two objects in this class have been found.
The asteroids, originating from the main asteroid belt, form the bulk of known NEOs. These have a variety of orbits, and ultimately originate via collisions in the main belt, which scatter the fragments resulting from collisions into unstable orbits which may eventually cross that of the Earth, or even sometimes to drop into the Sun.
Vesta is the possible source not only of many of the achondrite meteorites but also of a number of apparently rather similar small asteroids, known as V-types.
Perhaps surprisingly, the best estimates of the mean collision rate of comets and asteroids on the Earth are obtained by looking down, not up. The cratering record now contains more than 155 known craters (by May 1999 some authors gave the figure as 172), many of which (~70) have dimensions larger than the canonical 10-kilometre size above which we would expect a significant impact hazard to civilization.
The terrestrial cratering record indicates that roughly one 1-kilometre-sized object runs into the Earth every 100,000 years.
Looking upwards, there are still very large uncertainties, particularly affecting the cometary component. Here, not only are cometary masses not known to better than a factor of a few or so, but the proportion of unobserved, "dark" long-period objects (LPOs) to long-period comets (LPCs) is not known, nor are the corresponding ratios for Halley-type objects (HTOs) to Halley-type comets (HTCs) and Jupiter-family objects (JFOs) to Jupiter-family comets (JFCs). The result is a large uncertainty in the predicted cometary collision rate with the Earth, that will only be resolved by instigation of the proposed all-sky Spaceguard Survey.
A further point, often mentioned, is that asteroids may dominate the cratering rate for relatively small sizes (less than a km or so), but comets may dominate at large sizes.
Rare, high-consequence events (such as burglary, house fire, aircraft accidents, pollution, health scares, radioactive leaks etc.) are regarded by different groups in totally different ways. In formulating policy, the perception of a risk (e.g. amplified by the media) is sometimes as important as the risk itself. However, in recent years the concepts of Actuarial Cost, Tolerability of Risk, and Cost-Benefit Analysis have increasingly provided a common framework for comparing various different types of hazard.
The actuarial approach is perhaps easiest to understand, illustrated above. Here, the annual cost of the hazard is estimated, and - if the hazard can be avoided (which it potentially can, in the NEO case) - compared with the cost of mitigation. On this basis, the UK alone would be justified in allocating ~£100M per year to assessing the NEO impact hazard.
An important point, at the heart of the so-called "giggle" factor, needs to be made: namely, that the risk which an individual might accept or routinely tolerate is generally much higher than that which he or she would willingly force on others. This, in turn, is much higher than the risks which Society (or government) regards as tolerable. A case in point is the level of intolerability for nuclear reactor safety, where a large release might involve the deaths of some thousands of individuals. Between the intolerable and tolerable levels of risk, a cost-benefit analysis is carried out to determine what action is justified.
An important paper was presented at the July 1997 RGO Cambridge Spaceguard meeting, written by Dr Nigel Holloway of AWE Aldermaston.
He considered the hazard posed by NEOs from the "Tolerability of Risk" (ToR) standpoint developed to assess nuclear reactor safety and serious, but rare, industrial accidents or environmental disasters. The concept that emerges is that if the risk is above some notional limit (which depends, for example, on the likely number of fatalities), and therefore "intolerable", then changes are forced on the operator to make the probability of an accident per unit time less likely, and therefore the risk more tolerable. The limit of tolerability represents a lower limit below which action, while strictly not necessary, might still be recommended to drive the risk down to a level "As Low As Reasonably Practicable". This is the ALARP principle. In nuclear reactor design, the level of intolerability is set at one event per 100,000 years (per reactor), while the limit of tolerability is sometimes set as low as one event per 10 million years. These issues were also explored in a POST document produced in June 1996 Safety in Numbers: Risk Assessment in Environmental Protection (PDF format). Viewed from this perspective, there is no question that the NEO hazard is significant, or, to put it another way: if NEOs were a business, NEO plc would not be allowed to operate!
The majority of environmental hazards are assessed in the same way, i.e. according to the principle of Tolerability of Risk (ToR) and ALARP. Application of the Precautionary Principle ensures that: "where the environmental costs of inaction are high and the financial costs of avoiding that risk are low, the precautionary principle should clearly lead to early action."
Figure 1 of N.Holloway's paper, illustrates the limits of intolerability and tolerability for UK Societal Risks; note that the limit of intolerability is taken as one event per 100,000 years for 10,000 fatalities; the NEO impact hazard is just as frequent, but would involve 10,000,000 fatalities for the UK alone.
The comet or asteroid impact hazard is unique. First, the risk to civilization is unbounded (e.g. geological evidence for impact generated mass extinctions of life); secondly, the risk is predictable, years or decades in advance, given sufficient knowledge of the NEO population; and thirdly, it is avoidable. The means exist, in principle, to mitigate the threat, provided that we have enough warning.
These features of the impact hazard highlight the urgency of developing a global Spaceguard Programme, and the importance of carrying out further, related astronomical research and assessments of the risk. Research into NEOs not only has a strong justification in terms of pure science and the Public Understanding of Science educational (and entertainment) spin-off, but it also brings fundamental indirect benefits.
The outcome of this research into the interaction of astronomically "small" bodies with planets has potentially the most profound implications for mankind. We live at a special epoch: for the first time in the history of life on Earth (3.5 billion years), the facts about impacts are at least broadly understood, and a species has developed with not only the knowledge but also the technical capacity to mitigate part of the risk.
The discovery of the impact hazard also provides a further example of "spin-off" from a discipline whose activity these days is often justified in purely cultural terms. Astronomy has its roots in culture, possibly even in religion (e.g. Hoyle 1994: The Origin of the Universe and the Origin of Religion), but from time to time (e.g. in navigation) the discipline has great practical value.
Whereas curiosity killed the cat, it also confers a huge Darwinian advantage. In this case, our species' natural curiosity may be our salvation.
Applied Astronomy, including the exploitation of natural resources in space (many of which reside in NEOs), is likely to see major growth in the 21st century, while solving the NEO impact hazard should also count as a significant economic benefit, measured in units of hundreds of £M per year.
In summary, despite continuing scientific uncertainty relating to the total number, sizes and physical structure and composition of NEOs, there is broad agreement that a baseline level of risk due to the NEO impact hazard has been established.
Tunguska-size objects run into the Earth about once every one hundred years; but the more significant kilometre-sized impactors, which run into the Earth every 100,000 years or so, cause global devastation and carry the potential to destroy billions of lives world-wide.
Current astronomical surveys have discovered less than 10% of the globally most dangerous objects, and a much smaller proportion of comets and smaller bodies. There is a clear need for further investigations into all aspects of the impact hazard, including the interdisciplinary studies relating to impacts on time-scales of thousands of years, comparable to that of the development of civilization. The UK is currently well placed to play a leading role in the Spaceguard Programme, both on the European scene and world-wide.
Last Revised: 2010 March 1st