Buffer Zone Brackets Impact Risks?

Summary (Jul 31, 2003): Most estimates for the terrestrial dangers from a large asteroid collision have assumed a pancake model for what happens when the large stone first hits the Earth's atmosphere. But a recent simulation applying a fragmentation model shows a dramatically lower risk: most debris may explode in the atmosphere.

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Buffer Zone Brackets Impact Risks

based on Nature report

Researchers from Imperial College London and the Russian Academy of Sciences have built a computer simulation that predicts whether asteroids with a diameter up to one kilometer (km) will explode in the atmosphere or hit the surface.

The painting titled "K/T Hit" by artist Donald E. Davis. This impact occured 65 million years ago, ending the reign of the dinosaurs.
Image Credit: Don Davis

The results indicate that asteroids with a diameter greater than 200 meters (the length of two football fields) will hit the surface approximately once every 160,000 years - way down on previous estimates of impacts every 2,500 years.

The findings also predict that many more asteroids blow up in the atmosphere than previous estimates, which means the hazard posed by impact-generated tidal waves or tsunamis is lower than previous predictions. The researchers suggest that proposals to extend monitoring of Near Earth Objects (NEO) to include much smaller objects should be reviewed.

Dr Phil Bland of Imperial's Department of Earth Science and Engineering and a Royal Society University Research Fellow, said: "There is overwhelming evidence that impacts from space have caused catastrophes for life on Earth in the past, and will do so again.

Fragments of Comet P/Shoemaker-Levy 9 colliding with Jupiter (July 16-24, 1994).
Credit: NASA

"On the Moon it's easier to track the number, frequency and size of collisions because there is no atmosphere, so everything hits the surface. On Earth the atmosphere acts like a screen and geological activity erodes many craters too.

"Massive impacts of the type thought to have wiped out the dinosaurs leave an indelible print on the Earth but we have not been able to accurately document the effect of smaller impacts. Now, we have a handle on the size of 'rock' we really need to worry about and how well the Earth's atmosphere protects us."

When small asteroids hit the atmosphere the two forces collide like two objects smashing together, which often breaks the asteroid into fragments. Until now, scientists have relied on the 'pancake' model of asteroid impact to calculate whether the asteroid will explode in the atmosphere. This treats the cascade of fragments as a single continuous liquid that spreads out over a larger area - to form a 'pancake'. But a new model known as the 'separate fragment' (SF) model, which was developed by co-author of the study, Dr Natalya Artemieva of the Russian Academy of Science, has challenged this approach.

Impact of the Tunguska explosion is visible even after 90 years.
Credit: Galena HS

"While the pancake model can accurately predict the height from the Earth's surface at which the asteroid will break up, it doesn't give an accurate picture of how the asteroid will impact," explains Dr Bland. "The SF model tracks the individual forces acting on each fragment as it descends through the atmosphere."

An aerial view of Meteor Crater, Arizona.
Credit: Jim Hurley, 1978

To create a more accurate model of how asteroids interact with the atmosphere the researchers ran more than 1,000 simulations using both models. Objects made of either iron or stone, known as 'impactors', were used to reflect the composition of asteroids and experiments were run with varying diameters up to 1 km.

The researchers found the number of impacts for iron impactors were comparable using both models. For stone the pancake model significantly overestimated the survivability rate across the range used.

The SF simulations also allowed the researchers to define the different styles of fragmentation and impact rates for iron and stone, which correspond closely with crater records and meteorite data.

"Our data show that over most of the size range we investigated stony asteroids need to be 1,000 times bigger than the iron ones to make a similar sized crater. Much larger objects are disrupted in the atmosphere than previously thought.

"But we are not out of the woods yet," added Dr Bland "asteroids that fragment in the atmosphere still pose a significant threat to human life."

Dr Phil Bland is a member of the Meteorite and Impact Group that includes scientists from Imperial College London and the Natural History Museum.

Related Web Pages

Great Impact Debate: Part I
Great Impact Debate : Part II
Great Impact Debate: Part III
Great Impact Debate: Part IV
Great Impact Debate: Encore
Impact Hazards Website
NASA/JPL Near Earth Object Program

Note: Meteors, Asteroids and Comets: [2003-07-31]

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Thursday, July 31, 2003