The Asteroid Threat

"This is the most exciting thing I've seen since Halley's comet collided with the moon."
Homer Simpson from Bart the Mother.

In the mid 1990s, a brace of Hollywood movies illustrated - with varying degrees of plausibility - the consequences of large comets or asteroids hitting the Earth. But could it really happen? What are the chances of a significantly large asteroid or comet actually colliding with our planet?

The actual probability of a large impact is extremely small, certainly not worth losing any sleep over. However, averaged over a human lifetime, the chance of actually being killed by an asteroid is close to that of dying in a plane crash. This isn't to say that air travel is dangerous - far from it - it's just that a single plane crash would kill maybe a few hundred people at most whereas a single large asteroid impact could kill thousands or millions of people or maybe even wipe out the entire human race. This is why the asteroid threat (small though it is) should not be taken lightly.

Large objects have struck our planet in the past. Even though the geologically-active Earth is effective at erasing its history, there are still plenty of scars left; from the prominent 1.2 km Meteor Crater (known officially as Barringer Crater) in Arizona to the 180 km Chicxulub crater on the Yucatán peninsula in Mexico. (The latter impact was supposedly responsible for wiping out the dinosaurs 65 million years ago, although extreme volcanic activity in western India probably had a lot to do with it as well.) More recently in 1908, an estimated 50 metre object exploded above Tunguska, western Siberia, levelling over 2000 square kilometres of largely unpopulated tundra and forest. Had it arrived a few hours later it may well have devastated a populated area in eastern Europe.

Get to know your enemy

If you're going to take on an army it helps to know how many soldiers you're up against and what weapons they're carrying. Similarly it's important to know just how many NEAs are out there, what sizes they come in, and - perhaps most crucially of all - exactly where they're going. Estimates vary, but it is believed that there are roughly 1500 objects which are one kilometre across or larger, and about 10,000 five hundred metre objects, with numbers generally increasing exponentially as the sizes get smaller. (It is generally believed, although by no means certain, that a one-kilometre diameter is about the critical size for an impactor to cause a global-scale catastrophe.)

As of the end of November 2000, 1249 Near Earth Objects had been discovered. (I try to make a distinction between "objects" and "asteroids" because a few of these are comets and also man-made objects such as expended upper stage boosters from the Apollo program which habitually return to the Earth's vicinity.) Of these 1249 objects, 280 have been classed as Potentially Hazardous Asteroids (PHAs), of which 83 are estimated to be at least a kilometre in diameter. NASA have set a goal to track down 90 percent of all the kilometre or greater sized NEAs by the year 2010.

But just how hazardous is "potentially hazardous"? It's been the source of some confusion and several false alarms in the media in recent years. To this end, astronomer Richard Binzel of MIT has created the Torino Scale (below).

Introduced as a way of quantifying the threat posed by individual asteroids, the Torino scale (left) assigns an integer to each asteroid based upon its collision probability and its kinetic energy. The scale runs from 0 (negligible or no chance of an impact) to 10 (certain collision) and is intended as a kind of easy-to-understand "Richter Scale" for astronomers and the public. In particular, it is hoped that the Torino Scale will help prevent future occurrences of "The End is Nigh" headlines that have appeared in the media in recent years, which have led to astronomers being accused (unfairly) of getting their numbers wrong (when the real cause of the uncertainty was the initial lack of data needed to refine the orbital parameters).

However, the scale is not without its share of detractors in the astronomical community, with some senior figures accusing it of "irrelevance" and "oversimplification". Whatever your opinion of the Torino Scale, until someone comes up with something better, right now it's the only yardstick we've got.

For further information on how the scale is calculated, have a look at this NASA site. Also, further discussion on the pros and cons of the Torino scale can be found at Space.Com and Spaceguard UK.

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