Fragments from a colossal collision that took place in the inner asteroid belt 500 million years ago reached Earth in just 100,000 years - the quickest travel time on record, according to a new study.
The research supports models showing that asteroid smash-ups at certain places in the Solar System can shoot debris on a fast track towards Earth.
Many meteorites found over the past few decades show signs of suffering a massive shock - as if from a single impact - 500 million years ago. Astronomers believe a titanic crash between two asteroids occurred then, about 300 million kilometres from the Sun in the Solar System's inner asteroid belt.
Most debris from this smash is expected to take about 30 million years to reach Earth. But meteorites freshly unearthed from ancient rocks in a Swedish quarry show that the first detritus from the collision arrived on Earth in only 100,000 years.
Such speedy deliveries are predicted by computer models if meteorite-producing collisions occur in particular zones around the Sun, called resonant orbits. Objects in these zones are thought to be boosted by gravitational kicks from the planets - mainly Jupiter and Saturn.
The models suggest that, if an impact occurs close to the edge of such a zone, some debris may get injected into a resonant orbit, causing it to speed towards Earth. But fragments that initially avoid the zone, and take a more leisurely route through space, arrive on Earth many millions of years later.
Philipp Heck, a graduate student at the Swiss Federal Institute of Technology in Zurich, Switzerland, and colleagues studied nine meteorites from different layers in the quarry. To determine the time it took them to travel to Earth, the researchers measured neon and helium trapped in the space rocks.
These gases are produced when energetic particles called cosmic rays hit silicon atoms in the meteorites. But cosmic rays can penetrate only the top two metres of rock. So before the collision, most of the asteroid would have been shielded from the particles.
After the break-up, however, cosmic rays were able to zap the smaller fragments. This left behind different amounts of neon and helium depending on how long the rocks spent travelling to Earth.
Meteorites at the greatest depths within the quarry showed transit times of just 100,000 years. This is 100 times faster than the leisurely 10 million years taken by meteorites on average.
"Near these places, the shape of the meteorites' orbits gets changed rather quickly, and these fragments can quickly hit Earth," Heck told New Scientist.
William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, says the new data are "dead on" with predictions. "For the first time, over timescales of hundreds of millions of years, we really have some positive evidence our models are working the way they should," he told New Scientist.
Nearly all of the asteroids that pass by Earth have been redirected by these resonant orbits, but only about one per cent of those objects will ever hit Earth, says Bottke. The chance of an asteroid collision happening near a zone - similar to the violent crash 500 million years ago - is also quite slim.
"But it would be quite spectacular," Heck told New Scientist. "If humans were still here, they would probably see 100 times more bright fireballs, some visible in daylight. The meteorite rain would last a million years."
Journal reference: Nature (vol 430, p 323)
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Mon May 18 21:54:42 BST 2009 by Yeymi
Hi people i was wondering what the speed of an asteroid would be.
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