It is said that we live in a cosmic shooting gallery, under constant threat of orbital bombardment by the many objects that hurtle through the Solar System at breathtaking speeds each and every day. Near-Earth asteroids (NEAs) have collided with our planet throughout its existence; more frequently during some time periods than others. Astronomers have discovered over 10,000 NEAs of all sizes, including nearly 1,000 NEAs with a diameter greater than 1 km – the so-called extinction level event asteroids. The consequences of an impact for our civilization cannot be overstated, yet, as encounters with NEAs are so rare, no government strategy exists to deal with one.
The impact of a relatively modestly sized stony asteroid (~300-500 m in diameter) would not likely result in an Extinction Level Event (ELE) such as those portrayed in two well-known ‘disaster’ movies of the last decade. Defined as the extinction of all or part of life on Earth, ELE’s have occurred at the K-T boundary (the Cretaceous-Tertiary extinction event) 65 million years ago and during the Great Dying of the P-Tr (Permian-Triassic extinction event) 251 million years ago – in both events, upwards of 50% of all species extant at the time die out. Too often public perception, fuelled by the media, focuses on the effects of these large-scale impacts involving objects over 1 km in diameter. All too familiar are the story-lines; debris ejected into the stratosphere, global heat pulses forcing planetary biomass to burn for weeks on end, day turning to one endless night as dust and soot shroud the planet, and photosynthesis grinding to a halt causing food chains to collapse as average temperatures plummet. These events, depicted in Hollywood, only occur with asteroids of much larger proportions – a breach of a threshold diameter barrier of 1 km is required for this kind of global devastation. However, it is certain that a large impact event would severely stress the environment and would lead to drastic population reductions of both terrestrial and marine life.
The most devastating type of collision would be oceanic. This is also the statistically most probable as just over 70% of the Earth is covered by oceans. The principle effect, after several million tonnes of seawater had been flash-boiled, would be the generation of a tsunami that would wreak catastrophic destruction upon coastal cities and generate human casualties all along the affected coastlines. It has been calculated than an asteroid of roughly 300 m in diameter would generate waves carrying more than 300 times the energy of the 2004 Asian Tsunami. A 2006 paper by Chesley and Ward, entitled ‘A Quantitative Assessment of the Human and Economic Hazard from Impact-generated Tsunami’, goes further to state that such an impact in the Atlantic ocean would create a deep water wave, upon penetration of the impactor into the sea floor, of between 2-3 m high travelling at speeds of above 450 km h -1. Upon hitting the continental shelf on both sea-boards the accentuation of deep-water amplitude (due to a retardation of the leading edge over shallow water causing shortening of wavelength and growth in height) will run up a wave height of between 10-15 m depending upon coastal and ocean floor topography. About half the world’s population lives within 200 km of the oceans, and in excess of 650 million people living within 5 m of the high-tide mark along coastal regions globally would be vulnerable to such tsunami waves, although as Chesley and Ward state no single impact could affect them all. The predicted damage cost would be measured in the billions of dollars in terms of property damage and economic losses, perhaps much higher. As of 2010 the global insurance industry held just over $300 billion in reserves to cover catastrophic losses brought about by natural disasters; consider the vulnerability of $2 trillion of insured assets along the Florida coastline, and the threat posed to the insurance industry that keeps less than $½ trillion to cover all disaster losses everywhere globally in any one year. Jeremy Leggett notes all of this, and goes further to state that the global reserve could easily be entirely wiped out by one or two “mega-cats” – catastrophes striking large metropolis or economic centres – or by a series of rapid-fire smaller catastrophes. More significantly, additional recent studies show that an asteroid-induced tsunami exceeding 100 m in height would cause massive damage to low-lying areas along the US east coast and could totally submerge vast areas of Europe such as Holland and Denmark. A 100 m tsunami would travel around 22 km inland, and a 200 m tsunami would travel up to 55 km inland. Worryingly, the same study suggests that such impacts occur every few thousand years and that we are now overdue.
A land impact for a similarly sized asteroid would likely result in only localised to regional damage, less widespread than an ocean impact, but equally as devastating for the affected area. Upon impact with a land surface a characteristic bowl-form crater several kilometres across and bounded by a structurally elevated rim would be excavated. The blast-wave would obliterate the immediate area, up to a radius of 75 km, and probably severely devastate a much larger area up to 200 km in diameter (the size of a small US state such as West Virginia). The energy released would probably equal or surpass the total equivalent of the world’s nuclear arsenal, and eject some millions of tons of rock and dust into the lower atmosphere. As both the impactor and target area become fragmented and vaporized, the sun would be clouded for a length of time measured in days and weeks, not months and years. If the asteroid made landfall in an area of high population density, such as the north-east corridor of the US, Los Angeles, or Tokyo millions would die instantly. Indeed, even an impact outside of an urban area but within a 75 km radius of an urban conurbation would likely result in damage and fatalities on a currently unprecedented scale of natural disasters, particularly if there was little or no warning. The associated impact hazard, known as an ‘urban fire-storm’, where ignition of combustible materials occurs spontaneously if enough heat energy is applied, would only add to the catastrophe. It is postulated that this would be similar to the effects experienced by survivors of Hiroshima. The greatest harm though would be caused if sub-micrometer dust was able to reach the stratosphere; due to its long residence time in the atmosphere it could cause a fall in global temperatures. This could begin to threaten worldwide agricultural production and supply in the short-term although, due to the small nature of this impact event, not to such an extent as to begin to seriously endanger global populations. Localised acid rain may be induced due to the reaction of nitrogen and oxygen in the atmosphere; acidifying lakes, soils, streams, and perhaps even the surface layer of the oceans for a short period. Again, however, this would not be a global event due to the size of the asteroid considered but affecting an area some hundreds, or perhaps thousands, of square kilometres around the impact site. It is important to consider that an impactor of a diameter around 300 m is going to only affect local to regional scales. Beyond a radius of around 200 km from the impact site, the event would merely be mostly a frightening experience rather than a fatal one, and would not be expected to lead to long-term climatic effects.
The third impact possibility is that of the ‘non-impact event’; a bolide exploding violently in the atmosphere above the surface of the Earth. Such episodes have been recorded in recent history, such as the 1908 bolide that exploded above Tunguska, Siberia and are traceable only through the tell-tale iridium anomaly (an unusual abundance of an element rarely found in the Earth’s crust) on the ground as no fragmentary evidence is left. Indeed – some bolide impacts remain contentious due to the lack of any remnants from the asteroid/comet that exploded and alternative theories are frequently espoused. A bolide event typically requires a significantly different asteroid composition; that of carbonaceous chondrite. Volatiles – compounds with low boiling points – under the asteroid surface would heat up as the asteroid grazed the Earth’s atmosphere, forcing any hydrogen and carbon contained within to ignite. This would vaporize the asteroid into dust and gas in the lower atmosphere, leaving a layer of carbonaceous dust, melted metal silicates, and elements not typically found in the crust. The effects would be dependent upon the overpressure of the blast but likely to be relatively wide-ranging. Potentially, these could include blast-wave acceleration of window glass, radiant ignition of fires, structural failure of buildings, eardrum rupture, and injury from the blast wave itself. The combined effects of which are highly similar to those of a nuclear detonation at a similar altitude without the associated gamma rays or neutron bursts. There is also the perception within the scientific community that there would be the associated production of an electro-magnetic pulse (EMP) with bolide explosions, with an example of such a pulse as recently as 18th January 2000 in the Yukon Territory of Canada. This raises the unthinkable related hazard of a bolide blast being misinterpreted as the explosion of a nuclear weapon and the possibility of an auto-response by global powers before clarification is possible.
Although the probability of an impact, even by a medium-sized asteroid, is actually relatively small, the consequences of such an impact are enormous. It can be said that the risk to the individual is average and comparable to the risk taken when flying in an aeroplane (due to the product of the extremely low impact occurrence probability and the extremely high casualty expectancy). There is therefore a finite risk to the population of the Earth. The greatest challenge is intellectual; reconciling the exceptionally low annual probability of being killed by an asteroid impact against the almost unparalleled consequences of such an impact; it was the gradual hazard awareness of the late 80s and early 90s that prompted action to improve and expand detection of Near-Earth Objects. This was specifically outlined by Congressional reports submitted by the US House of Representatives’ Committee on Science, Space, and Technology – advocating that NASA engage in workshops to detect and intercept NEOs. The paper commissioned in 1990 by the American Institute of Aeronautics and Astronautics (AIAA) that looked at dealing with the threat of an asteroid striking the Earth put forward two disquieting points. The first was that the PCAS (Planet Crossing Asteroid Search; initiated in 1973, this was the first of two surveys by Eugene Shoemaker using the telescopes situated atop Palomar Mountain in California – now defunct) and PACS (Palomar Asteroid and Comet Survey; the second of Shoemaker’s surveys using the Palomar telescopes, this program started in 1982 – like PCAS, it was terminated in 1994) projects are estimated to have discovered half of the known Earth-crossing asteroids; the second was that they project that an object this size comes by undetected once every 2-3 years. The saying goes that 300 m asteroids are a “dime a dozen” in the Solar System.
To reiterate, the probability that a dangerous asteroid will impact by the close of this decade is negligible, as is the probability that this asteroid will impact the Earth at any point within the next century. Yet, if such an impact were to occur – there are four key potential hazard targets that would either exacerbate the effects of the actual impact or prevent effective handling of the disaster. These include (i) sensitive national security sites such as national capitols or command centres, the total destruction of which (and associated decapitation of a highly centralized government) would contribute disproportionately to the chaos; (ii) sites with hazardous materials that may be released by impact such as nuclear power plants or weapons depots, chemical plants and oilfields; (iii) geologically sensitive sites such as volcanoes and earthquake regions; (iv) sites with essential roles for food production, storage, or distribution – food interruption generates the threat of starvation and societal disorder. Whilst an international response may be effective for regional-scale impacts, there would be no recovery for a devastating larger impact; the question of just how resilient our agriculture, commerce, economy, and societal organisation might prove in the face of an unprecedented catastrophe remains unanswered. As of now, FEMA (the Federal Emergency Management Agency, a division of the U.S. Department of Homeland Security) have no hazard mitigation or contingency plans in place to deal with the threat of an asteroid impact and the associated dangers. Equally, the United States Strategic Command (USSTRATCOM, which absorbed the U.S. Space Command in 2002) makes no mention of planetary defence in its Long-Term Vision, although the ability to detect Earth-crossing objects is listed amongst its deficiencies.
Events of the size described are expected, on average, once every 100,000 years or so; whilst most astronomers would say that we are “over-due” for such a sizeable impact, we can be by no means certain when that impact will occur. There are still untold numbers of asteroids (estimates range up to some hundred thousand) that remain either undiscovered or uncharted, many of which will be of a similar size or larger. It is perhaps recklessly irresponsible that the findings and reports of the organizations whose very remit is space exploration – agencies with such spine-tingling titles as the AIAA (American Institute of Aeronautics and Astronautics) and NASA (National Aeronautics and Space Administration) – are widely ignored by the elected officials, committees, and public bodies with oversight on planetary defence. There have been continued cuts in funding since the late 1990s for both civil and government space programs like the NEAT (Near Earth Asteroid Tracking; a program begun in 1995 by Eleanor Helin as part of the Spaceguard Survey, aimed at taking over and consolidating the studies of PCAS and PACS) system under Administrations with other agendas. With intense competition between more traditional recipients of what little NASA funding is available, those bodies responsible for detection have found it harder year-on-year to continue progressively scanning the skies above for potential asteroid threats. It is more and more likely that the events of Tunguska in 1908 will be repeated, only next time with potentially devastating results. We, as a species, could find ourselves learning of an impact event only after it has occurred.