Worried About Asteroid-Ocean Impacts? Don't Sweat The Small Stuff

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 (March 17) 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."

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.