NEO News 4 April 2001

NEO News (04/04/01) Miscellaneous items

Dear Friends & Students of NEOs:

This edition of NEO News contains four short news items or follow-up to previous stories.

David Morrison

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TUMBLING STONE: NEW NEO WEB-MAGAZINE

Our Italian colleagues have started publication of a new web-based magazine called Tumbling Stone, aimed at a broad public readership. Andrea Milani, one of the co-founders, writes that "Tumbling Stone, the online newsletter, is a joint initiative of NEODyS (me and my coworkers at the University of Pisa and elsewhere) and the Spaceguard Foundation (A. Carusi and his coworkers at CNR Roma and elsewhere). As explained by Andrea Carusi in the first edition, "It is dedicated to a better understanding, in the field of Near-Earth-Objects, of the meaning and importance of announcements concerning these bodies, their relationships with our planet, and the degree of hazard they may represent for mankind. We feel that such an information tool is needed for non-specialists."

The address for Tumbling Stone is http://spaceguard.ias.rm.cnr.it/tumblingstone/ It will also be linked directly from my impact hazard webpage.

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MORE ON GERVASE OF CANTERBURY

In the March 16 edition of NEO News I reported on recent work indicating that the lunar crater Giordano Bruno could not very well have been formed in 1178, which is an interpretation often quoted for the report by Gervase of Canterbury that five men witnessed an event on the Moon that might have been an impact. I concluded with the comment that discounting the identification of this event with the crater Giordano Bruno undercuts the reality of the medieval report, or at least its relevance to NEO impacts and the Taurid meteor stream.

Alan Harris of JPL wrote that "I am pleased to see yet another "debunking" of this old Hartung chestnut". But he notes that a similar argument was made 8 years ago, that if a large lunar impact had occurred in 1178 there would have been an associated meteor storm when the ejecta reached the Earth. Harris continues: Regarding the Giordano Bruno claim, I pointed out (Harris, JGR 98, 9145-9149, 1993) that "our final conclusion must be, even in the extraordinarily unlikely event that such a compact clump of ejecta were injected into heliocentric orbit from the Moon, perturbations from the Earth would have quickly dispersed the clump [so that] annual showers of more or less constant intensity should be expected. . . Either by initial dispersion in the velocity or by subsequent perturbations, an event such as Hartung proposes [the Giordano Bruno impact] should have the result that the sky would really light up every midsummer for a week or two." The bottom line is, I agree completely with Withers that there should have been an "apocalyptic" level meteor storm in the days following the claimed event, but furthermore it should have been followed by a pretty spectacular meteor storm on each anniversary since then.

Benny Peiser replied to my note in CCNet for March 19 as follows: "Not so fast, my friend. While I agree that the Giordano Bruno crater appears to be too large indeed to be convincingly associated with a hypothesised lunar impact in 1178, neither Gervase's report nor the possibility of an observed impact on the Moon - and not even the speculation about a link with the Taurid meteor stream have been "debunked." As Ed Vega rightly stresses, it's Carl Sagan's (and others') association of the eyewitness report with the Giordano Bruno crater that has been "debunked", not the report and its impact-interpretation itself. BJP"

In reply to Peiser, I believe that in the absence of physical evidence (such as a young lunar crater), the report from Gervase of Canterbury has little weight. The report is already inconsistent internally with an impact on the Moon, since it says that "this phenomenon was repeated a dozen times or more, the flame assuming various twisted shapes and then returning to normal. Then, after these transformations, the Moon from horn to horn, along its whole length, took on a blackish appearance". The above does not sound to me like the description of an impact. The story held some credibility only because of the claimed association with crater Giordano Bruno. Each person makes his or her own judgement about the veracity of ancient documents, but for me the debunking of the association of this 1178 report with any physical evidence on the Moon effectively undercuts the entire connection of Gervase's report with NEO impacts.

David Morrison

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WAS TUNGUSKA AN ASTEROID OR A COMET?

This question is still often raised. Old astronomy books almost always assert that the Tunguska impactor was cometary. However, two independent analyses published almost simultaneously in 1993 provided strong arguments in favor of the asteroidal hypothesis. The publications were:

Chyba, C.F., P.J. Thomas, and K.J. Zahnle: The 1908 Tunguska explosion: atmospheric disruption of a stony asteroid. Nature 361:40-44 (1993)

Hills, J.G. and M.P. Goda: The fragmentation of small asteroids in the atmosphere. Astronomical J. 105:1114-1144 (1993)

In both cases these teams argued that models of the rapid deceleration and fragmentation of the impacting body in the atmosphere showed that a comet would have disintegrated at a much higher altitude, while only a projectile of rocky strength and density could have penetrated to within 8 km of the surface, the height at which the Tunguska explosion occurred.

Modeling the impact physics on the computer, of course, has its uncertainties. Therefore it is worthwhile repeating a simple physical argument developed at the time by Kevin Zahnle of NASA Ames, and recently recalled by Chris Chyba of the SETI Institute. Independent of modeling details, a dense stony object (asteroid) will penetrate deeper than a less dense icy object (comet). Also, there are many more asteroids than comets in Earth-crossing orbits. Thus if Tunguska were a comet, and it exploded at 8 km, then asteroids of similar size and energy must reach the surface to produce craters. If asteroids are more common that comets (one estimate is a factor of 10), then there would be many more asteroid impacts in the 10-20 megaton range than comets, and we should see their associated craters. For every Tunguska-like airburst, we might expect ten craters the size of Meteor Crater. But these craters are not there, thus refuting the hypothesis that Tunguska was cometary. The alternative, that Tunguska was asteroidal, and that only the rare, very strong iron objects make it to the ground in the 10-20 megaton energy range, is of course entirely consistent with the rarity of features like Meteor Crater (which was produced by the impact of an iron object).

Chyba recently succinctly summarized this argument as follows: " I am impressed with Kevin Zahnle's argument that if comets of Tunguska size almost penetrate the atmosphere, then stony asteroids of that size certainly will, in which case there should be far more 1 km-size craters on Earth than in fact there are. This seems like a very difficult argument for the "Tunguska was a small comet" point of view to counter."

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MIR AND NEO IMPACTS: A RISK COMPARISON

Recent "the sky is falling" news stories have dealt with the fall from orbit of the Mir Space Station rather than NEO impacts. The press and public are clearly interested in the risk of falling objects, but sometimes risks are perceived in ways that are quite different from the hazard as calculated numerically. The following is a very rough estimate of risks, in "order of magnitude" terms only. It is intended to be illustrative, but certainly not precise. By risk I mean the chance or probability that any individual will be killed as a result of either a spacecraft atmospheric entry or the impact of a NEO.

Let's start with the risk of death as a result of being struck by a piece of Mir on the assumption that the fragments could land anywhere on Earth. Suppose 1000 large metal fragments survive to hit the ground, and that if you are within 1 meter of the impact point you will be killed. Thus 1000 square meters are at risk, out of a total surface area of the Earth of about 100 trillion (10**14) square meters (not counting the Polar Regions). This is one part in 100 billion of the Earth's surface, and that is the risk to each individual. Multiplying by the Earth's population of 6 billion, we get a chance of about 1 in 20 that one person on Earth would be killed.

In fact, the Mir atmospheric entry was far from random. It was steered to an impact point in the mid-Pacific Ocean. Unless you lived in that part of the world, the risk to you was zero (not allowing for an uncertainty in how well this controlled entry would be executed). Since the total population of the Pacific is only a few million, the chance that someone would be killed was less than 1 in 20,000. The folks who sold the Russians a $200 million insurance policy were very unlikely to have to pay off any claims.

For comparison, consider the annual risk of dying as a result of an NEO collision with the Earth. A number of studies (e.g., the paper that Clark Chapman and I published in Nature in 1994) have shown that this risk is dominated by near Earth asteroids of about 2 km diameter. There is a roughly 1 in a million chance of such an impact each year, with estimated death of 1-2 billion people. Thus the annual risk to each of us of from NEO impacts is about 1 in a few million, or more than 10,000 times greater than the risk from an uncontrolled Mir entry. (Note: This is a conservative estimate; many would argue for a NEO-impact risk that is higher by an order of magnitude.)

We see from these simple calculations that the risk (per year) to each of us from asteroid impact is thousands of times greater than from an uncontrolled Mir entry, and millions of times greater than from a controlled Mir dive into the Pacific. Yet no one is taking out insurance policies to protect from cosmic impacts, and this risk receives less news coverage than the demise of Mir. Why the disparity? For one thing, the death of Mir was a known event that provided a good story, while we have no specific prediction of any NEO impact. For another, Mir was a human-built object over which we had some control (and responsibility), while an NEO impact is considered an "act of God". But I suspect that the difference also reflects the fact that very few reporters tried to make a quantitative comparison of these risks. If they had, the results might have surprised them!

David Morrison

Wednesday, April 4, 2001
Source: Ames Research Center