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Fewer Earthbound Asteroids Will Hit Home

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Old July 16th 03, 08:28 PM
Ron Baalke
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Default Fewer Earthbound Asteroids Will Hit Home

Contact: Judith H Moore

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Imperial College of Science, Technology and Medicine

July 16, 2003

Fewer Earthbound asteroids will hit home

Scientists say pancake model of asteroid impact won't stick

Scientists report in Nature today that significantly fewer asteroids could
hit the Earth's surface than previously reckoned.

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 kilometre (km) will explode in the atmosphere or hit the

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

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.

"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.

"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."

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.


Notes to editors

Publication: Nature (17 July 2003)

Title: "Efficient disruption of small steroids by Earth's atmosphere"

Authors: P.A Bland (1) and N.A Artemieva (2)

(1) Department of Earth Science and Engineering, Exhibition Road, Imperial
College London, SW7 2AZ, Uk
(2) Institute for Dynamics of Geospheres, Russian Academy of Sciences,
Leninsky Prospect 38/6 Moscow, 117939 Russia.

About Imperial College London

Consistently rated in the top three UK university institutions, Imperial
College London is a world leading science-based university whose reputation
for excellence in teaching and research attracts students (10,000) and staff
(5,000) of the highest international quality.

Innovative research at the College explores the interface between science,
medicine, engineering and management and delivers practical solutions, which
enhance the quality of life and the environment - underpinned by a dynamic
enterprise culture.



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