Asteroids Behaving Badly

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Once called boring 'vermin of the skies', asteroids are now star attractions for astronomers around the world. This attention is worthy of their name, which is Greek for 'starlike', but it has more to do with their sheer number and their destructive power than their cosmic beauty. These pockmarked giant peanut-like rocks are in fact leftovers from the formation of the planets.

The largest three asteroids are Ceres, Pallas and Vesta, with average diameters 930, 520 and 500 kilometres respectively. About 200 asteroids are larger than 100 kilometres across; 800 larger than 30 kilometres. About a million are 1 kilometre or more in diameter; and billions are of boulder or pebble size. Most of the asteroids orbit within a vast, doughnut-shaped ring between Mars and Jupiter, known as the main belt.

Occasionally, a collision may kick an asteroid out of the belt, sending it onto a dangerous path that crosses Earth's orbit. These stray asteroids take up an orbit that loops past Earth, and are called 'Earth-crossers'. This knowledge frightens astronomers. What if one of them comes too close to Earth? What cataclysm would such a rogue rock cause if it slams into Earth? The number of asteroids is very large, but the space they occupy is enormous. Most asteroids stay millions of kilometres apart. It's not like Star Wars or Star Trek spaceships weaving their way through flying rocks. But real collisions are possible with a spacecraft or Spaceship Earth.

There are believed to be about 1,800 Earth-crossers, or near-Earth asteroids, a kilometre or greater in width. Only 500 or so have been discovered so far, but astronomers hope to identify almost all of them by the end of this decade. The largest presently known is 1036 Ganymed, with a width of about 41 kilometres. There may be as many as a million near-Earth asteroids 50 metres and larger. The chances of these rocks hitting Earth are small, but even one of the smaller asteroids could destroy a large city.

Don't panic - none of them is on a collision course yet. On 14 June 2002 a football-field-sized asteroid came within 120,000 kilometres of Earth. It was the biggest asteroid in decades to get this close to us. For you and me, 120,000 kilometres is, well, a long way away. For astronomers who coin words like 'Earth-crosser' it is a hair's-breadth; for us, a hair-raiser indeed. The errant asteroid was discovered three days after it sped by Earth at 38,000 kilometres per hour. If it had struck Earth, it would have caused a Tunguska-like explosion. Did such a strike indeed take place on 30 June 1908?

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