San Diego

A forgotten report on ocean waves generated by nuclear explosions has surfaced, forcing researchers to rethink their theories on asteroid impacts.

The Handbook of Explosion-Generated Water Waves was completed in 1968 at the request of the US Office of Naval Research by William Van Dorn of the Scripps Institution of Oceanography in La Jolla, California. But it was not entered into an academic library catalogue until March 2002, when Scripps' library did so as part of a project to record old technical reports.

Since the report was written, planetary scientists have investigated the potential effect of small asteroids — particularly those with diameters of 100–500 metres — landing in the ocean, and have concluded that the resulting tsunamis could devastate regions over 20 kilometres inland.

But new analyses of Van Dorn's highly technical report on waves produced by nuclear explosions, which takes account of factors such as the absorption of wave energy by continental coastal shelves, indicate that tsunami damage would be limited to immediate coastal areas.

Jay Melosh, a planetary scientist at the University of Arizona in Tucson, tracked down the report last year. He plans to talk at the Lunar and Planetary Science Conference in League City, Texas, next month about his own analyses that predict limited inland damage from asteroid-generated waves. “It appears the defence community has already determined that explosion-generated waves are neither a serious threat nor a promising weapon,” writes Melosh in his abstract.

Such risk assessments are extremely important as governments consider the costs of telescopes or satellites needed to conduct asteroid surveys. Scientists agree that the impact of an asteroid larger than 1 km in diameter would be catastrophic, but there is much debate over the risk from smaller ones. If Earth is under threat only from larger asteroids, then equipment and research costs will be much lower than if scientists also have to look out for numerous smaller objects.

The US government is funding the development of the Panoramic Survey Telescope and Rapid Response System and the Large Synoptic Survey Telescope, whose brief includes surveys of asteroids of various sizes. The latter telescope is expected to cost around $200 million.

A NASA task force is to complete a report in the spring on the risk from smaller asteroids. “It is fortunate this very important report has come out now,” says Steve Chesley, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, and a member of the task force.