How close is the earth's destruction by an asteroid hit?

The End. Finis. Kaput. We grapple with peril, but the threats that frighten us — terrorism, epidemics, earthquakes — are not existential; none is capable of killing everyone, everywhere.

An asteroid impact, on the other hand, could render us extinct. Sixty-six million years ago, the Chicxulub asteroid killed off the dinosaurs. The Tunguska asteroid, which struck Siberia in 1908, destroyed 1287 square kilometres. Estimates suggest that a Tunguska-sized asteroid will strike every 500 years; a one-kilometre object, capable of global catastrophe, every 700,000 years.

The possibility of avoiding cataclysm has inspired the people behind Asteroid Day today, supported by scientists, astronauts and media types, including the astrophysicist and Queen guitarist Brian May, the Astronomer Royal, Lord Rees and Bill Nye, the Science Guy.

They are campaigning for a rapid hundred-fold increase in the tracking of Near Earth Objects (NEOs). “The more we learn about asteroid impacts,” argues May, “the clearer it becomes that the human race has been living on borrowed time. We are currently aware of less than one per cent of objects comparable to Tunguska, and nobody knows when the next big one will hit. It takes just ONE”. (Asteroid people are terribly fond of apocalyptic language.)

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On the surface, this seems a no-brainer. There is, potentially, no greater threat to man’s existence. Since we have the technology to find and divert asteroids, why not apply it?

But hang on, it’s essential to protect ourselves from heroic hype. There is no conclusive evidence that a meteor has ever killed anyone (although there is a vague story from 1954 about a space rock that crashed through the roof of a house in Alabama, bounced off a radio and struck Ann Hodges on her “unmentionables”).

The Earth is a big place mainly covered by water. The land surface is mostly uninhabited. All this means that the chance of any single person getting hit by a meteor is around one in 20,000,000,000,000, or the same as winning the lottery two weeks running. Granted, at issue is not a little meteor hitting a single person.

Global catastrophe

The real problem is a global catastrophe such as Chicxulub. Yet, this is where a sense of proportion is needed. According to Eric Christensen of the Catalina Sky Survey, “the asteroid impact threat is very easy to overstate. The popular conception of asteroids is that they are menacing and going to kill us all, and it’s just not true”.

Experts are confident that the really huge rocks — larger than a kilometre — have already been found and do not pose a threat for the foreseeable future.

That leaves the smaller ones — more numerous and less easy to track. The people at Asteroid Day want a huge investment to allow us to detect the smaller objects.

The question comes down to how much money we’re willing to spend to insure against a tiny risk. Space ventures are always expensive and always risky — as Sunday’s crash of a cargo rocket demonstrates. The asteroid threat is minuscule compared with the perils that trouble insurance underwriters — fire, flood and wind.

“Within the next few days the Earth will be hit by an asteroid that is maybe a few metres in size, and all it will do is provide a nice light show,” says Christensen. “The risk is so small that I’m not sure the enormous cost to efficiently detect all of those asteroids is worth it.”

Every year, falling trees kill six people in the UK, but we don’t spend billions on the safety of oaks. In her book Fear: A Cultural History, Joanna Bourke explores the terrors that have plagued us. Anxiety, she argues, reflects progress — we fear what we know. In the 19th century, people dreaded being buried alive because doctors were bad at diagnosing death. No one knew about asteroids so no one feared them.

We now have the technology to detect Near Earth Objects (NEOs) but, in truth, that technology has simply provided a new anxiety. When fear warps perception, logic gets jettisoned. Those willing to spend billions on an asteroid defence will eschew a 7kg bicycle helmet. We are still more likely to be killed by a golf ball than by an asteroid. Fear, seldom warranted, is always expensive.

— The Telegraph Group Limited, London, 2015

Professor Gerard DeGroot is the author of Dark Side of the Moon: The Magnificent Madness of the American Lunar Quest.