NEO News (02/23/05) Impacts at the AAAS

Status Report From: Ames Research Center
Posted: Thursday, February 24, 2005


Update on 2004 MN4: Since we last reported on this 400-m NEA (on 01/27/05), new radar observations have further refined the orbit and shifted it closer to the Earth at its 2029 pass. Current predictions are that its closest approach on April 13, 2029 will be only 5.7 Earth radii (below the altitude of geosynchronous satellites). On that evening, it should be easily visible to the naked eye in Europe at about 3rd magnitude.

The following two stories are from the 2005 annual meeting of the American Association for the Advancement of Science (AAAS) held last weekend in Washington.

(1) Chesapeake Bay Impact

A scientific symposium at the AAAS meeting discussed the recently discovered Chesapeake Bay Impact Structure, which is the sixth largest known crater on Earth and one of the two largest craters formed since the Chicxulub impact 65 million years ago. Now buried by sediment and partly flooded by Chesapeake Bay, the underlying crater has an estimated diameter of 85 km. The age is between 35 and 36 million years, with a specific argon-argon radiometric date of 35.2 +/- 0.3 million years reported at the symposium.

The Chesapeake impact produced a large "strewn field" of melted ejecta called the North American Tektites. The impacting object was probably between 3 and 5 km in diameter. This impact was unusual in that it took place on the continental shelf beneath several hundred meters of water, in water-saturated rock. There is some evidence of a very large tsunami produced by this impact, which was in deeper water than the Chicxulub impact in Mexico.

One of the mysteries of this impact is its coincidence in time with the 100-km-diameter Popigai crater in Siberia, which also has an estimated age near 35 million years. Comparative studies of the two similar-size craters will surely be interesting, one occurring on dry land and the other underwater. There are apparently two global impact layers that have been identified in Italy separated by 10-20 cm, and perhaps these can be linked to the two craters and will provide a measure of the interval of time between the impacts.

As discussed at this symposium, there are no recognized extinctions or other indications of global environmental effects associated with either the Popigai or Chesapeake impacts. They took place at a time of global warming, but the timescale for this climate change was millions of years. These impacts were, of course, at least a factor of 10 smaller in energy than the Chicxulub impact, so we would not expect a mass extinction. On average, we experience an impact of this size on the whole planet about once per 15 million years, and on the continents about once in 40 million years. The 35-million-year-old Popigai and Chesapeake impacts are consistent with such impact-frequency expectations, but their temporal coincidence with each other seems remarkable.

(2) Judge Posner on Catastrophic Risk

Judge Richard Posner of the U.S. Court of Appeals, Seventh Circuit, presented one of the plenary AAAS Topical Lectures on "The Economic Approach to Catastrophic Risk." Much of the material was taken from his recent book "Catastrophe: Risk and Response." Posner discussed many subjects familiar to the readers of NEO News dealing with potential disasters, such as asteroid impacts, that are beyond our base of experience and could threaten the survival of civilization. He was particularly interested in how to quantify the potential risks and allocate resources to deal with them. The examples he discussed included asteroid impact, sudden global warming, nuclear terrorism, and the outbreak of lethal new pathogens.

What especially interested me about his lecture was that he used asteroid impact as his type example, since it is better understood and has received more analysis than the other catastrophic risks. Posner discussed the Tunguska and Chicxulub examples and outlined the frequency of impacts associated with various levels of environmental damage. He also explained how the concept of a threshold for global catastrophe was arrived at and the nature of the Spaceguard goal to find asteroids larger than 1 km. He commented that NASA was the only governmental agency in the United States or elsewhere that was taking the situation seriously enough to spend money, although he also noted that the NASA expenditures of $4 million per year were far less than could be justified in terms of the magnitude of the threat.

I was intrigued that Judge Posner then used the asteroid impact danger has his example of a relatively well-understood risk, which is based on sound science. In contrast, the other examples were either unknown (such as the probability of a sudden climate shift and change in ocean circulation triggered by global warming) or inherently unknowable (such as terrorism or the appearance of a new pathogen). To evaluate the economic implications of these other possible catastrophes is of course much more difficult.

Isn't it intriguing that the NEO impact hazard has gone within a few years from being a speculative and for many persons unbelievable hazard to a solidly understood risk that serves as the standard for evaluating other possible global catastrophes?

NEO News (now in its tenth year of distribution) is an informal compilation of news and opinion dealing with Near Earth Objects (NEOs) and their impacts. These opinions are the responsibility of the individual authors and do not represent the positions of NASA, the International Astronomical Union, or any other organization. To subscribe (or unsubscribe) contact For additional information, please see the website If anyone wishes to copy or redistribute original material from these notes, fully or in part, please include this disclaimer.

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