From: baalke@zagami.jpl.nasa.gov   
      
   Contact: Judith H Moore   
   j.h.moore@imperial.ac.uk   
   44-0-20-7594 6702   
   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   
   surface.   
      
   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   
   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.   
      
   "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.   
      
   Website: http://www.imperial.ac.uk   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)