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WORRIED ABOUT ASTEROID-OCEAN IMPACTS? DON'T SWEAT THE SMALL STUFF
From Lori Stiles, UA News Services, 520-621-1877
March 17, 2003 

The idea that even small asteroids can create hazardous tsunamis may
at last be pretty well washed up.

Small asteroids do not make great ocean waves that will devastate
coastal areas for miles inland, according to both a recently released
1968 U.S. Naval Research report on explosion-generated tsunamis and
terrestrial evidence.

University of Arizona planetary scientist H. Jay Melosh is talking
about it today at the 34th annual Lunar and Planetary Science
Conference in League City, Texas. His talk, "Impact-Generated
Tsunamis: an Over-Rated Hazard," is part of the session, "Poking
Holes: Terrestrial Impacts." 

---------------------
Contact Information
H. Jay Melosh
520-621-2806
jmelosh{at}lpl.arizona.edu
----------------------

Given all life's worries, new evidence that asteroids smaller than a
kilometer in diameter won't generate catastrophic tsunamis is welcome
news, and not only for coast dwellers. It will save taxpayers the
cost of financing searches for small Earth-approaching asteroids, a
savings of billions of dollars, Melosh said.

(The current NASA-funded effort to search and map truly hazardous
Earth-approaching asteroids - those one kilometer or larger in
diameter - is now half done and on track to be finished by the end of
the decade, Melosh noted. NASA funds NEAT, LINEAR and the UA
Spacewatch programs in this effort.)

The idea that asteroids as small as 100 meters across pose a serious
threat to humanity because they create great, destructive ocean
waves, or tsunamis, every few hundred years was suggested in 1993 at
a UA-hosted asteroids hazards meeting in Tucson.

At that meeting, a distinguished Leiden Observatory astrophysicist
named J. Mayo Greenberg, who since has died, countered that people
living below sea level in the Netherlands for the past millennium had
not experienced such tsunamis every 250 years as the theory
predicted, Melosh noted. 

But scientists at the time either didn't follow up or they didn't
listen, Melosh added.

While on sabbatical in Amsterdam in 1996, Melosh checked with Dutch
geologists who had drilled to basement rock in the Rhine River delta,
a geologic record of the past 10,000 years. That record shows only
one large tsunami at 7,000 years ago, the Dutch scientists said, but
it coincides perfectly in time to a giant landslide off the coast of
Norway and is not the result of an asteroid-ocean impact.

In addition, Melosh was highly skeptical of estimates that project
small asteroids will generate waves that grow to a thousand meters or
higher in a 4,000-meter deep ocean.

Concerned that such doubtful information was ­ and is - being used to
justify proposed science projects, Melosh has argued that the hazard
of small asteroid-ocean impacts is greatly exaggerated.

Melosh mentioned it at a seminar he gave at the Scripps Institution
of Oceanography a few years ago, which is where he met tsunami expert
William Van Dorn.

Van Dorn, who lives in San Diego, had been commissioned in 1968 by
the U.S. Office of Naval Research to summarize several decades of
research into the hazard posed by waves generated by nuclear
explosions. The research included 1965-66 experiments that measured
wave run-up from blasts of up to 10,000 pounds of TNT in Mono Lake,
Calif. 

The experiments indeed proved that wave run-up from explosion waves
produced either by bombs or bolides (meteors) is much smaller
relative to run-up of tsunami waves, Van Dorn said in the report. "As
most of the energy is dissipated before the waves reach the
shoreline, it is evident that no catastrophe of damage by flooding
can result from explosion waves as initially feared," he concluded.

The discovery that explosion waves or large impact-generated waves
will break on the outer continental shelf and produce little onshore
damage is a phenomenon known in the defense community as the "Van
Dorn effect." 

But Van Dorn was not authorized to release his 173-page report when
he and Melosh met in 1995.

Melosh, UA planetary sciences alumnus Bill Bottke of the Southwest
Research Institute and others agreed at a science conference last
September that they needed to find the report.

Bottke found the title - "Handbook of Explosion-Generated Water
Waves" - in a Google search.

Given a title, UA science librarian Lori Critz then discovered that
the report had been published and added to the University California
San Diego library collection in March 2002. Bottke also tracked it
down, and had the report by the time Melosh requested it by
interlibrary loan. Both made several photocopies.

Melosh said, "I since found out it was actually read into the
Congressional Record as part of the MX Missile controversy."

BIOSKETCH: H. JAY MELOSH
Melosh, a professor in the UA planetary sciences department and Lunar
and Planetary Laboratory, is well known for his work in theoretical
geophysics and planetary surfaces. His principal research interests
are impact cratering, planetary tectonics, and the physics of
earthquakes and landslides. His recent research has focused on
studies of the giant impact origin of the moon, the K/T boundary
impact that extinguished the dinosaurs, the ejection of rocks from
their parent bodies, and the breakup and collision of comet
Shoemaker-Levy 9 with Jupiter. Melosh also is active in
astrobiological studies that relate chiefly to the exchange of
microorganisms between the terrestrial planets. Melosh earned his
doctorate from Caltech in 1973 and joined the UA faculty in 1982. He
is on the 12-member science team for Deep Impact, a $279 million
robotic mission that will become the first to penetrate the surface
of a comet when it smashes its camera-carrying copper probe into
Comet Tempel 1 on July 4, 2005.

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