From risk to reality: Asteroids and other near-Earth objects

Photo: NASA/Don Davis

In 2001, astronomers in Japan watched as a large asteroid hurtled past the Earth. Not only did it come within an uncomfortably close 2.5 million miles of the Earth’s orbit, but experts described it as the largest Earth-threatening asteroid since the one that struck Mexico 65 million years ago, an event that many say wiped out the dinosaurs. Could another asteroid crash on our planet? If so, could we do anything to stop it?

Interplanetary space is not entirely empty. As the Earth orbits the Sun, it encounters particles and objects ranging from microscopic dust to large asteroids and comets. The tiniest particles are numerous and harmless; they cause flashes of light, and are known as meteors or “shooting stars”. The large asteroids and comets are very rare; the chance that one might hit the Earth during our lifetimes is extremely small.

Yet some are enormous bodies: those tens of kilometres in size could exterminate most life on our planet. School books describe how the impact of a 10 or 20 km asteroid drove the dinosaurs to extinction 65 million years ago. But such once-in-100-million-years events are so rare that, despite their apocalyptic horror, they have not concerned public officials.

Yet, the hazard of more frequent impacts by somewhat smaller meteoroids, asteroids and comets should be a matter of practical concern to governments worldwide. At worst, the very unlikely case of a three km asteroid striking Earth could send civilisation into a new Dark Age; this case – with a potential death toll of a billion or more – has an annualised fatality rate comparable to other serious hazards, like earthquakes or airline crashes. At a minimum, the increasing rate of discoveries of Near-Earth Asteroids combined with media sensationalism will surely alarm the public and bring the issue of this potentially solvable hazard (such as by deflecting an approaching asteroid away from the Earth) to the desks of responsible emergency management officials.

An impact on Earth by one of the largest cosmic bodies of practical concern, asteroids and comets, from 1-3 km across, could destroy life throughout an entire continent. They are large enough to be seen by astronomers using modest telescopes in an existing, loosely co-ordinated, international programme known as the Spaceguard Survey. More than half of such Near-Earth Asteroids have already been found; none of them will strike Earth during this century. Of those that remain, most will be found during the next decade or so; probably none of them will strike us either, although there is a small chance that one will be destined to collide with Earth.

More worrisome are larger meteoroids, from metres to hundreds of metres across, but which are still smaller and more numerous than the asteroids being searched for by the Spaceguard Survey. Impact rates and consequences vary enormously across this broad size-range, but such objects share some general traits. For a start, whether they explode in the atmosphere, on the ground, or in an ocean, they can have devastating consequences for people near to (or occasionally quite far from) the impact site. Also, they are mostly too small to be readily detected or tracked by existing telescopic programmes, which can only observe a portion of the sky at any one time. And their impacts are too infrequent to be witnessed and studied in detail by scientists, so their nature and effects are not yet well characterised.

Thus, scientific uncertainties are greatest for just those objects whose sizes and impact frequencies should be of greatest practical concern to public officials. Impacts of these cosmic bodies are unfamiliar even to many of those in military agencies that routinely scan the skies for more familiar military hazards. Impacts of such bodies range, depending on their size, from annual events to extremely devastating potential impacts; a 300 metre impactor might cause one million deaths, roughly equalling the death tolls of the few largest natural disasters in the last several hundred years. One of these collisions has a few tenths of a percent chance of happening during the 21st century. Impacts of the smaller of these bodies (several metres to 50 metres) will happen, and possibly during our lifetimes, so the hazards they pose must be addressed by society’s institutions.

In order to illustrate concretely the nature of the impact hazard and what precautionary measures are possible, consider three different impact scenarios. In Case A, imagine a flying “mountain”, larger than the Vehicle Assembly Building at NASA’s Kennedy Space Center or larger than the world’s largest domed stadium (the New Orleans Superdome), crashing to Earth at a speed a hundred times faster than a jet. More probable than hitting land, such an asteroid would plunge into an ocean and explode with an energy of about 10 times the yield of the largest thermonuclear bomb ever tested. Although the effects would be very different, the released energy would be close to a magnitude of eight earthquakes, roughly equal to the annual production of electricity from all nuclear power plants in France and Japan combined.

The brief atmospheric phase of the impact might disrupt some communications, and any ship near the impact point would be destroyed. By far the most dangerous outcome of the impact would be the resulting tsunami (“tidal wave”), which would convey perhaps 20% of the impact energy toward distant coastlines. Currently, there is a very small chance (less than 20%) that astronomers will discover such a “small” impactor in advance; if they do, there would likely be years or decades of warning, thus allowing time for space-faring nations to deflect or destroy the asteroid in question. If they don’t discover it, then there might be hours of warning before a tsunami, particularly if the impact is in the well-monitored Pacific Ocean – or maybe no warning at all if the impact is in an ocean with less advanced tsunami warning systems – before enormous waves crash ashore, threatening destruction and loss of life on a scale never seen by humanity.

For Case B, consider an enormous rock larger than any of the world’s largest buildings crashing through the Earth’s atmosphere and striking land instead of an ocean. The explosion would be as though we took one of the world’s largest thermonuclear bomb ever tested and exploded ten of them at once. An enormous crater would be created within seconds, 3-4 km across and deeper than the Grand Canyon. Everything within this city-sized zone would be killed immediately. There could be serious damage for tens of kilometres.

The chance of an asteroid over 200 metres across striking land during the 21st century is about one in a thousand. Cosmic impacts are not selective, and any one of the world’s large countries (for instance, Russia, Canada, China, the United States, Australia or Brazil) would be vulnerable. If, by luck, the object were discovered long before impact, then it would be possible to divert it so that it would miss the Earth.

For Case C, visualise an asteroid 3 km in diameter, or a somewhat smaller but higher-speed comet, crashing to Earth. It would be as though more than 1,000 of the Case A or B impacts hit the same place simultaneously. The crater alone would engulf an area comparable to one of the world’s largest cities. An impact into the ocean would penetrate the seafloor, and the resulting tsunami would be of a scale unprecedented in recorded history. Material thrown out of the Earth’s atmosphere would rain back toward the ground, filling the sky with blazing fireballs and incinerating an area perhaps as large as India or twice the size of Western Europe.

Such apocalyptic devastation nevertheless pales compared with the worldwide death and economic calamity that would be produced by sudden global climate change due to stratospheric contamination. Without advance preparation, mass starvation might result in the deaths of a large fraction of the world’s population. No nation would be spared the dramatic climate change. An asteroid over 2 km wide has a probability of striking Earth about once every two million years.

Where there are advance warnings, public communicators may be able to couch the impending disaster in familiar terms, such as protocols preparing people to flee approaching hurricanes. Education about the objective character of impacts might reduce dysfunctional reactions. If a destructive impact were to occur without warning, the management of the disaster could proceed much as though it were caused by a more prosaic natural disaster, like earthquakes or floods. The victims need not fear more esoteric after-effects, analogous to earthquake aftershocks or the lingering radiation or infection following a nuclear or biological attack.

While the generic elements of asteroid deflection technology are known – a spacecraft has already landed on one Earth-approaching asteroid, Eros – no integrated system has been designed, let alone implemented. It may be prudent or cost-effective to develop such technologies, perhaps as comparatively inexpensive add-ons to space missions conducted for other scientific purposes.

Reactions to much more deadly disasters around the world – an earthquake, typhoon, or flood – are often characterised by a subdued fatalism, i.e. they are deemed “acts of God”. While such disasters encompass countless personal tragedies and may engender massive international relief efforts, they lack the amplified and reverberating repercussions witnessed after the 11 September attacks in the US. Research in risk perception suggests that a large, unexpected asteroid impact could have an effect similar to 11 September, both in terms of public and official reactions.

In particular, many people would fear that they could be the next random casualties and may well ask, “Why can’t something be done?” Of course, the technology to discover all potential projectiles and reliably divert or destroy an incoming one, would be enormously expensive. But it is technologically feasible, and so the reason it is not being implemented must at least be an implicit political decision concerning public priorities. While much more effort is clearly required to provide a thoroughly sound foundation for decision making, we probably now know enough about near-Earth objects for those priorities to be determined objectively.


Gehrels, T. (ed.) (1994), Hazards Due to Comets and Asteroids, University of Arizona Press.

“Report of the Task Force on Potentially Hazardous Near Earth Objects”, September 2000, available online (see link below).

©OECD Observer No 237, May 2003

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