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Reporting Asteroid Impact Risks.

WARNINGS OF POSSIBLE IMPACTS BY COMETS OR ASTEROIDS ARE not exactly new. Edmond Halley, long before the comet that bears his name made its first predicted return, suggested that a comet might eventually strike Earth with possibly devastating results.

It wouldn't be surprising to learn that, back in 1690, press reports blew Halley's warning way out of proportion, and astronomers worried that their credibility would be undermined as a result. That often happens nowadays. Faced with this problem, some researchers attending last September's Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids in Arlington, Virginia, presented ideas about how to improve public communications surrounding the increasingly high-profile impact-prediction process.

Today's astronomers generally use one or two numerical yardsticks to designate impact risk. The Torino Scale, which was designed with the general public in mind (S&T: October 1999, page 32), ranks threatening near-Earth objects on a zero-to-10 basis. A value of zero means that the risk of a given object hitting Earth is lower than that from a random strike within a given year by an unknown object of comparable energy. Higher Torino values represent either a higher probability of impact or a more severe outcome, or both.

By contrast, the Palermo Scale was created as a more precise yardstick for orbital dynamicists' use. It assigns zero to any object that has the same probability of hitting Earth as a random object of the same size striking before the predicted impact date. Negative values indicate objects that pose less risk than a random impact of the same size, and positive values indicate objects that have a greater risk. Thus it is essentially a measure of how unusual the object's impact odds are--how it compares with the average "background" rate for a given size body.

Clark Chapman (Southwest Research Institute) and Brendan Mulligan (Queen's University, Canada) have investigated a wide variety of hazard scales used to evaluate everything from hurricanes and forest fires to nuclear war and terrorism. Most of them, like the Torino, use simple, linear scales of 1 to 5 or 10. Chapman says that by comparison the Torino Scale for impact risk seems to hold up well, and he suggested it should be used consistently in communicating with the public about new, possibly hazardous asteroids. However, he says, some of the descriptions originally given for each number on the scale need to be revised.

For example, he points out that 1 on the Torino scale denotes an object that "merits careful monitoring." That can be misleading to the public, he says. Most hazard scales are worded in terms of what the public should do, but in the case of asteroid watches, the general public isn't doing the monitoring.

Chapman also says astronomers should avoid public references to the Palermo Scale--whose usage figured prominently in the reports last July about a low-probability possible impact in 2019 by asteroid 2002 N[T.sub.7]. Some of those reports, Chapman says, were "confusing and inappropriate," since they stressed the fact that this was the first object ever to get a greater-than-zero Palermo rating. Since the scale was barely a year old, he says, that's like calling the November 2001 crash of American Airlines flight 587 in Queens, New York, "the worst transportation disaster of the century" when the century was barely a year old.

But Steven Chesley (NASA/Jet Propulsion Laboratory) says that N[T.sub.7] was indeed worthy of notice. For a few days its odds of impact rose to 1 in 60,000--for an event that, if it occurred, could have caused widespread harm. Even though, as typically occurs, the odds of impact vanished within days thanks to new observations, "it was significant," he says.

David Morrison (NASA/Ames Research Center) says that such reports help raise public awareness of the issue, but they also have "demeaned the credibility of astronomers in the public's eye." While astronomers should not suppress information, he suggests, they should make sure that a good, responsible report of the facts is quickly available on the Internet, even "in a few sentences, to give our view."

Big Binary Asteroid

Using the 10-meter Keck II telescope, William J. Merline (Southwest Research Institute) has found a companion to the asteroid 121 Hermione. It appears to be about 13 kilometers across and was 630 km from Hermione as projected on the plane of the sky. Hermione itself (drastically overexposed here) is about 210 km in diameter.

Currently 31 binary asteroids are known or suspected: 9 in the main belt, 1 in Jupiter's orbit, 7 in the Kuiper Belt, and 14 small Earth-crossers.--DAVID TYTELL
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Author:Chandler, David L.
Publication:Sky & Telescope
Geographic Code:00WOR
Date:Jan 1, 2003
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