Small But Deadly Asteroid Threat Downgraded
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SMALL BUT DEADLY ASTEROID THREAT DOWNGRADED
By Jeff Hecht
Wednesday, November 20, 2002
A new analysis of data from US military satellites shows that locally
devastating impacts by small asteroids are likely only about once in a
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.
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
However, a key uncertainty remains, says Brown: whether large and dangerous
meteors are concentrated in streams, like this week's unthreatening Leonid
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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