DR WILLIAM REVILLE: I recently heard a striking comment made on radio: "Americans don't admire problems, they solve them." In this spirit, American physicist John Remo is researching a method to deflect incoming asteroids on collision course with Earth (Scientific American, October 2002). The results of such work may one day save life on this planet from extinction.
Asteroids are relatively small rocky objects and there are about one million of them in our solar system. They mostly measure between 10 metres and 10 kilometres in diameter, although the largest, Ceres, is 940 kilometres across. Most asteroids orbit the Sun between Mars and Jupiter in a region called the asteroid belt. Unfortunately not all asteroids are confined to the asteroid belt. Two dangerous families of asteroids were discovered in the 1930s. The Amors asteroids spend over half their orbits closer to the Sun than the main asteroid belt, and they cross the orbit of Mars and travel close to Earth's orbit before returning to the main belt.
The Apollo asteroids have a tighter but somewhat similar orbit around the Sun to the Amors, but they actually cross Earth's orbit.
Yet another group, the Atens asteroids, and the most dangerous of all, were discovered in 1971. They spend most of their orbits around the Sun inside Earth's orbit, but some of them cross Earth's orbit travelling outwards and again travelling back inwards closer to the Sun.
Comets are icy equivalents of asteroids but orbit the Sun at much larger distances, mostly in the Kuiper belt (30-150 times Earth's distance from the Sun) and the Oort cloud (2,000-20,000 times Earth's distance from the Sun). Occasionally comets are jolted out of the Kuiper belt and Oort cloud and travel inwards through the solar system.
The damage resulting from impact of an asteroid or comet with Earth depends on their size. Grain sized particles (meteors) burn up in our atmosphere and cause no damage. We see them as bright lines of light. Impacts from objects 50-100 metres in diameter cause very significant damage. Impact with a one kilometre object would have disastrous consequences and impact with a 10 kilometre object would be catastrophic.
On June 30th 1908, a 50-100 metre-wide comet exploded with the force of two million tonnes of TNT in the air over the Tunguska river in Siberia. Over 3,000 square kilometres of forest were flattened and over 1,000 reindeer were incinerated. No people were killed because the area is unpopulated but if such an event happened over a large city millions would die.
It is believed that the mass extinction of life that occurred 65 million years ago was caused when a 10 kilometre asteroid crashed into the Yucatan peninsula in Mexico. The collision of a 10 kilometre object with Earth would throw so much dust into the air that the globe would become enveloped in a massive storm of meteors that would incinerate everything.
Unburned dust would remain aloft in the atmosphere for a long time, blotting out sunlight. The planet would also rock with massive earthquakes. Photosynthesis, which supports the major food chains, would stop on land and in the upper regions of the oceans. Humanity would be wiped out.
There are large numbers of asteroids in the solar system about one kilometre in diameter. It is estimated that impact with such an asteroid would kill about one third of the population of the Earth and we can expect impacts from 100 metre objects every 100-200 years and impacts from objects one kilometre wide and bigger every 250,000 years. The probable immediate danger then is from a Tunguska-sized object. Since most of the globe is covered by water, the next impact will likely be in the ocean. This would cause a massive tsunami with disastrous consequences around the ocean rim.
This hazard is very real. Bill McGuire, Professor of Geophysical Hazards at University College London has estimated that the chances of being killed by cosmic collision are 750 times greater than the chances of winning the national lottery.
Earth is certain to be hit by a large asteroid in the future, unless we can deflect it. The first thing we need to do is to watch the entire sky with large telescopes to catalogue and track all potentially dangerous objects. This could give about 100 years warning of any possible collision. Advance warning of at least a few decades is essential if we hope to successfully deflect a killer asteroid. Such a sky-watch programme must be established as a matter of grave urgency. The second thing we need to do is develop the technology and method that will allow us to deflect any incoming asteroid that has our name on it.
John Remo's work studies the effect of exploding nuclear bombs close to an incoming asteroid. The radiation released by the explosion would boil surface material from the asteroid into gas and heat. This would effectively turn part of the asteroid surface into a rocket motor, thereby changing the orbit of the asteroid. Results are promising and Remo believes that a 25 kiloton nuclear explosion would move a one kilometre asteroid sufficiently to avoid collision with Earth.
William Reville is Associate Professor of Biochemistry and Director of Microscopy at UCC