[meteorite-list] Small Asteroid Impacts Less Than Expected

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:57 2004
Message-ID: <200210081625.JAA24710_at_zagami.jpl.nasa.gov>

Small Asteroid Impacts Less Than Expected
October 7, 2002

DPS Press Release

Early in the morning of June 30, 1908, in the Tunguska region of Siberia
about 1,000 km (600 miles) north of Irkutsk, an asteroid about 60 meters
(200 ft) in diameter entered the Earth's atmosphere, resulting in an immense
explosion, centered about 8 km (5 miles) above the forest below. Trees were
flattened over an area about 50 km (30 miles) in diameter, several times
larger than the area encircled by the Beltway around Washington, D.C. It
exploded with energy in the range of a modern nuclear missile warhead, about
10 megatons, or about 500 times the energy of the Hiroshima atomic bomb.
While there were few, if any, casualties from this event, if such an event
were to occur in a more populated area it would be a major natural disaster,
comparable to a major flood, earthquake or volcanic eruption. For this
reason there is considerable interest in assessing just how often such an
event might be expected to occur. Since the last one was about a century
ago, it has often be supposed that the answer is "about once a century," but
this is not necessarily so. Perhaps this "Tunguska event" was an unusually
recent eventBLE red to the expected frequency, or maybe they occur even more
often on average, and we have just be lucky in the last 94 years. The
Near-Earth asteroid surveys in progress for the last few years (LINEAR,
NEAT, LONEOS and others) are aimed mainly at discovering larger asteroids
that would cause major but far less frequent damage. They also discover many
smaller asteroids in the "Tunguska" size range, presenting an opportunity to
assess the frequency of these smaller events. Even smaller objects pose no
direct danger, as they explode higher in the atmosphere and produce little
if any ground damage.

The population of near-Earth asteroids (NEAs) down to ~1 km (half a mile) in
diameter is reasonably well determined. Planetary scientists now estimate
that there are about 1,000 such NEAS equal to or larger than 1 km in
diameter, with a resulting impact frequency of about once per half million
years. However, in the size range of the "Tunguska event" NEAS (diameter
~50-75 m), estimates of population, or equivalency of impact frequency,
range from once per 200 years to once per 10,000 years. The LINEAR survey
has now discovered ~30 NEAS in the "Tunguska" size range; thus a better
estimate is possible. However, for small NEAS, a very large simulation is
needed to obtain even a few "detections" in the computer model. Dr. Alan
Harris of the Space Science Institute in Boulder, CO., recently compiled a
new simulation for objects from about 200 m to about 0 m in diameter,
dividing them into six size bins. By comparing his relative populations with
the absolute population estimates of Stuart (Science 294, 1691-1693, 2001)
in his two smallest size bins, Harris extends Stuart's curve through the
size range of "Tunguska" objects. He finds a population of the order of half
a million objects in this size range, corresponding to an expected impact
interval of the order of once per thousand years. This estimate is uncertain
by a factor of about 3, largely due to uncertainty in the actual size of the
Tunguska event.

This new estimate of the impact frequency of "Tunguska-sized" events is
considerably less than has often been supposed. If correct, it means that
the Tunguska event having happened so recently is unusual, although not
extraordinarily so, and that the risk of such events in the future is a few
times less than has been assumed. This is not to say that there is no danger
at all. Impacts are random events, so it is not possible to say that we are
"about due" for another, or that since one happened so recently that another
won't happen soon. All we can say is that the odds are less than had been
often quoted based on the assumptions that such things happen about once a
century or even more often.

Contact info:

Alan W. Harris
Senior Research Scientist
Space Science Institute
4603 Orange Knoll Ave.
La Canada, CA, 91011
Phone: 818/790-8291
E-mail: harrisaw_at_colorado.edu
Received on Tue 08 Oct 2002 12:25:00 PM PDT