Watch the sky for asteroids

Breakthrough ideas have a way of seeming obvious in retroshy;spect, and about a decade ago, a Columbia University geophysicist named Dallas Abbott had a breakthrough idea. She had been pondering the craters left by comets and asteroids that smashed into Earth. Geologists had counted them and concluded that space strikes are rare events and had occurred mainly during the era of primordial mists. But, Abbott realised, this deduction was based on the number of craters found on land 8212; and because 70 per cent of Earth8217;s surface is water, wouldn8217;t most space objects hit the sea? So she began searching for underwater craters caused by impacts rather than by other forces, such as volcanoes. What she has found is spine-chilling: evidence that several enormous asteroids or comets have slammed into our planet quite recently, in geologic terms. If Abbott is right, then you may be here today8230; only because by sheer chance those objects struck the ocean rather than land.

Abbott believes that a space object about 300 metres in diameter hit the Gulf of Carpentaria, north of Australia, in 536 A.D. An object that size, striking at up to 50,000 miles per hour, could release as much energy as 1,000 nuclear bombs. Debris, dust, and gases thrown into the atmosphere by the impact would have blocked sunlight, temporarily cooling the planet 8212; and indeed, contemporaneous accounts describe dim skies, cold summers, and poor harvests in 536 and 5378230;

What8217;s more, the Gulf of Carpentaria object was a skipping stone compared with an object that Abbott thinks whammed into the Indian Ocean near Madagascar some 4,800 years ago8230; Researchers generally assume that a space object a kilometre or more across would cause significant global harm: widespread destruction, severe acid rain, and dust storms that would darken the world8217;s skies for decades. The object that hit the Indian Ocean was three to five kilometers across8230; and caused a tsunami in the Pacific 600 feet high 8212; many times higher than the 2004 tsunami8230;

At the start of her research8230; Abbott reasoned that if colossal asteroids or comets strike the sea with about the same frequency as they strike land8230; perhaps 100 large impact craters might lie beneath the oceans. In less than a decade of searching, she and a few colleagues have already found what appear to be 14 large underwater impact sites. That they8217;ve found so many so rapidly is hardly reassuring.

Other scientists are making equally unsettling discoveries.

Excerpted from an article by Gregg Easterbrook in the Atlantic Monthly