Over The Horizon

Cosmic Collisions -- Who's Next?

Statistically, you are in more danger of dying in the aftermath of a major asteroid collision with Earth than from an airplane crash, and New Zealanders are particularly vulnerable, according to visiting astronomer Dr Duncan Steel.

The statistics are based on the assumption that an asteroid of one kilometre in diameter is likely to hit the Earth once every 100,000 years. The likely death toll from such a collision is on the order of 100 million.

"New Zealand sits in the middle of the biggest target -- the Pacific Ocean," Steel explains. If such an asteroid exploded in the atmosphere above the Pacific, we'd be likely to see a tsunami sweep across the country, to be stopped only by the Southern Alps. Combine that with a coastal-dwelling population, and the outlook seems grim.

Add to that the huge amount of rock and debris thrown up in such an impact, and you're looking at a "cosmic winter" potentially cooling the Earth for a thousand years.

"It really makes no difference where something this size hits -- we'd all cop it," Steel says.

Steel is quick to add that driving a car remains far more dangerous for the individual, but avers that a major asteroid impact is the worst natural catastrophe possible. The Earth has seen such impacts in the past, with nine incidents of massive widespread extinctions associated with cosmic collisions over the past 250 million years. It has only been in the last two decades that astronomers, geologists and evolutionary biologists have come together to realise the part such collisions have played in Earth's history.

In addition, there have been many smaller impacts causing localised damage and death. One of the best known was the 1908 impact in Tunguska, Siberia, where over 2,000 square kilometres of forest was flattened in the virtually deserted region. Steel notes that that was the best possible place for an asteroid or comet fragment to hit -- had the collision occurred a few hours later, it would have been over densely populated areas in Europe.

Around 130 terrestrial impact craters have now been identified, ranging from hundreds of millions of years old to only a few thousand. Nineteen impact craters dot Australia, where Steel is based, showing up clearly in the arid Outback; four of these are less than 6,000 years old and are recalled in Aboriginal tales. There may well be craters waiting to be recognised in New Zealand, Steel says, but this country's active geology and climate has probably helped to eliminate most traces.

Many asteroids and comets cross over the Earth's orbit in their journey around the Sun. The largest Earth-crossing asteroid is eight kilometres in size, but for every big one there are many smaller ones, warns Steel.

One of the best known, and best studied, recurring comets in Earth skies is Halley's Comet. With a mass of 50 million tonnes, travelling at a speed of 66 kilometres per second, a collision with Halley would release 10²³ joules of energy -- equivalent to 20 million megatons of TNT or 300,000 times the largest nuclear weapon ever tested.

"That is a big bang in anyone's book."

Steel points out that a Tunguska-type event is likely to happen every 50-100 years, but that random occurrences mean there could be a 400-year gap with suddenly three impacts turning up in the space of a year.

Preparation is the answer to such threats, Steel believes. Steel is promoting the Spaceguard Survey, designed to systematically chart potentially dangerous objects. At the present rate of study, it would take over 500 years to produce an accurate survey -- Steel hopes to see support for the survey enable its completion within 20 years or so.

The survey would enable long-term identification of which are the most dangerous objects, enabling accurate prediction as far out as 200 years.

"Then we can leave it to future technology [to come up with an answer]."

There have already been some suggestions, ranging from "catching" rogue objects in a form of space net to blowing them up with unemployed nuclear missiles. Steel is not keen on the latter, pointing out that all that would do is "turn a cannonball into a shotgun". He believes that the technology already exists to provide the tiny change in velocity required to have an Earth-crosser pass harmlessly in front of or behind our planet -- such changes would be a matter of a few centimeters per second either way.

Given that the "economics of Armageddon" show massive potential losses from a cosmic collision, Steel believes that investing in the Spaceguard Survey is the only prudent thing to do.