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.
The asteroid that exploded over the Tunguska River in 1908 is estimated to have been 160- to 180-feet in diameter. And a similar sized asteroid is believed to have exploded over Khazakstan in the late 1940s.
The 1908 Tunguska Siberia asteroid was fairly small and yet devastated a large area when it exploded.
A notorious example occurred in 1908 when an asteroid in this size range is believed to have exploded above the uninhabited Tunguska region of Siberia, leveling trees for some 800 square miles (2,000 square kilometers) around. Astronomers have for a decade or so said so-called Tunguska events probably occur about once every hundred years, leading some to speculate that we're about due for another.
So the take-home lesson is that you can still worry about getting killed by smaller asteriods that could hit closer to where you are. You just can't expect to be killed by a sub-kilometer asteroid that hits the ocean thousands of miles away from land.
|Share |||Randall Parker, 2003 March 18 01:22 PM Dangers Natural General|