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Small But Deadly Asteroid Threat Downgraded

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    NHNE News List Current Members: 748 Subscribe/unsubscribe/archive info at the bottom of this message. ... SMALL BUT DEADLY ASTEROID THREAT DOWNGRADED By Jeff
    Message 1 of 1 , Nov 21, 2002
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      NHNE News List
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      SMALL BUT DEADLY ASTEROID THREAT DOWNGRADED
      By Jeff Hecht
      New Scientist
      Wednesday, November 20, 2002

      http://www.newscientist.com/news/news.jsp?id=ns99993093
       
      A new analysis of data from US military satellites shows that locally
      devastating impacts by small asteroids are likely only about once in a
      millennium.

      The benchmark for such impacts is a 1908 blast that levelled 2000 square
      kilometres of forest in the Tunguska area of Siberia. Scientists calculate
      that a 50- or 60-metre object exploded in the atmosphere with the force of
      10 megatons of TNT.

      But no other well-documented case is known and this size of object is too
      small to spot reliably in space, so estimates of their frequency are
      sketchy. The previous best guess suggested such blasts were likely every 200
      to 300 years.

      Now Peter Brown of the University of Western Ontario says the odds are more
      in our favour. The evidence comes from military satellites that were looking
      out for nuclear weapons tests. During eight years of observations, the
      satellites were able to record 300 meteors exploding in the atmosphere.

      Taking advantage of acoustic data on 19 events, Doug ReVelle of the Los
      Alamos National Laboratory for the first time was able to calibrate how much
      energy each blast released, so Brown could plot how many objects of
      different sizes hit over the period.

      Ground damage

      They found that the frequency of objects in the one- to 10-metre range
      decreases with size. The mathematical function that describes this decrease
      is same as that determined for near-Earth asteroids larger than 50 metres,
      from astronomical observations.

      Brown says this means the same function should hold for the difficult 10- to
      50-metre range. On this basis, a one-megaton blast should occur on average
      every 130 years, while a 50-kiloton blast will occur about every decade. A
      26-kiloton explosion -- like the one recorded over the Mediterranean in June
      -- will hit about once every three to four years.

      The ground damage caused depends on both the size of the explosion and its
      altitude. "Somewhere in the hundreds of kilotons range you start getting
      effects on the ground, and certainly in the megaton range," Brown told New
      Scientist.

      However, a key uncertainty remains, says Brown: whether large and dangerous
      meteors are concentrated in streams, like this week's unthreatening Leonid
      showers.

      US military officials only started recording all meteor data after a
      50-kiloton blast in February 1994, and this interval is short enough to miss
      streams like the Leonids that peak about once a century.

      Journal reference: Nature (vol 420, p 294)

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