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Cambridge Conference Digest - January 27, 1998

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CAMBRIDGE-CONFERENCE DIGEST, 27 January 1998

(1) DON'T WORRY BOB: THE PROBABILITY OF CIVILISATION THREATENING 
    IMPACTS IS VERY LOW

(2) LUNAR OBSERVATIONS CAST DOUBT ON FRANK'S SMALL-COMET THEORY

(3) MORE DOUBTS ABOUT SMALL-COMET HYPOTHESIS

(4) EVEN MORE DOUBTS ABOUT SMALL-COMETS

(5) SORRY ABOUT OUR TECHNOLOGY, BUT IMAGES ON FRANK'S FILMS ARE NOT 
    REAL OBJECTS BUT DUE TO FAULTY CAMERAS  

(6) AN ALTERNATIVE HYPOTHESIS: DO METEORIC IMPACTS IN ATMOSPHERE CAUSE 
    'BLACK SPOTS' ON LUIS FRANK'S FILMS?

(7) AND FINALLY: GREEN PROPHETS OF DOOM IN SUDDEN MOOD SWING

==========================

(1) DON'T WORRY BOB: THE PROBABILITY OF CIVILISATION THREATENING 
    IMPACTS IS VERY LOW

Commentary on the Remarks of Bob Kobres (Jan. 23, 1998)
 
From: 	Clark R. Chapman, Southwest Research Institute
	

I respect the views of people who, like Bob Kobres, believe that 
there is great urgency in dealing with the impact hazard because 
the *stakes are so high*. But I do not share his views. Other 
people, including myself, believe that because the *probability is 
so low* (of a *globally* catastrophic event), it subtracts from 
such a dramatic sense of urgency as espoused by Kobres.  This is 
particularly so because there are so many destructive things that 
truly will and do happen frequently that we can devote society's 
resources to mitigate. Mounting effective mitigation against 
cosmic impacts on the time scale of a decade or a century makes 
increasing sense over, say, a crash program in the next year, 
because the probability of catastrophe -- though still small -- 
becomes 10 or 100 times greater on those time scales. That is my 
view. On the other hand, in the face of the extreme ultimate 
consequences of such an impact, it is difficult for me to 
confidently assert that the more alarmist view of Kobres is wrong. 
 
What I respect less in Kobres' essay is the appearance that it is
funding ("green slime") that underlies his desire for us to change how 
we speak of the hazard.  If there is anything scientists and 
knowledgeable lay-people interested in this topic need to nurture, it 
is their credibility. I think it is very important that we be objective 
in reporting the scientific facts, including the stakes and the 
probabilities.  We are free, as citizens, also to comment on how we 
personally evaluate these facts, in the light of our own values, but we 
should make it clear that these are just our individual value 
judgements.  I, for one, think that Kobres' quote from Duncan Steel is 
not only "not wrong" but is a fine way to express the realities of the 
impact hazard.  
 
I want to add that I think the objective facts are clear that much
smaller impacts, like Tunguska, objectively deserve only modest 
attention because -- whatever one believes about the rates and 
constancy or inconstancy of their occurrences -- such impacts (a) lack 
the finality (with regard to human civilization) of the bigger ones and 
(b) they occur *much* less frequently than other natural disasters that 
have similar consequences, for which we are doing rather less than we 
might in cost- effective mitigation measures.
 
==================================================== 
(2) LUNAR OBSERVATIONS CAST DOUBT ON FRANK'S SMALL-COMET THEORY

J.A. Grier & A.S. McEwen: The small-comet hypothesis: An upper limit to 
the current impact rate on the Moon. GEOPHYSICAL RESEARCH LETTERS, 
1997, Vol.24, No.24, pp.3105-3108

UNIVERSITY OF ARIZONA, LUNAR & PLANETARY LAB, TUCSON,AZ,85721

Frank et al. [1986b] and Frank and Sigwarth [1993] hypothesized the 
intense bombardment of the terrestrial atmosphere by small comets. 
Their model requires that the Moon is impacted by small comets 
(10(7)-10(8) g) at a rate of almost one per minute. We calculate that 
an object of this mass, even with an exceedingly low density and 
relatively low velocity, will nevertheless produce a crater at least 50 
m in diameter. These craters will excavate immature lunar soil and 
produce a very bright spot with a diameter df at least 150 m. If 
low-density comets exist that might not create deep craters [O'Keefe 
and Ahrens, 1982], they will nevertheless disturb the regolith 
sufficiently to create detectable bright spots. If the small-comet 
hypothesis is correct then the near-global lunar imaging returned by 
Clementine in 1994 should reveal similar to 10(7) bright spots in 
locations where craters are not present in images acquired in the 
1960's and early 1970's. We find no new bright limit to the current 
cratering rate by small comets is 33/yr, similar to 10(4) below that 
expected if the small-comet hypothesis were valid. Copyright 1998, Institute
for Scientific Information Inc.

===================================
(3) MORE DOUBTS ABOUT SMALL-COMET HYPOTHESIS

B. Rizk & A.J. Dessler: Small comets: Naked-eye visibility
GEOPHYSICAL RESEARCH LETTERS, 1997, Vol.24, No.24, pp.3121-3124

UNIVERSITY OF ARIZONA, LUNAR & PLANETARY LAB, TUCSON, AZ, 85721

We investigate an obvious consequence of the small-comet hypothesis. We 
find that the 30-ton cloud of water-ice particles formed by a small 
comet would survive long enough to be an unmistakably bright object. 
The visual magnitude of such clouds would be between that of a bright 
star and the full Moon. A whole-Earth small-comet flux of 20/min 
implies the sudden appearance of at least two bright patches of light 
every five minutes. The two-hour periods after sunset and before 
sunrise ought to produce the most spectacular sightings - intermittent 
punctuations of bright rapidly-moving points of light. Because such 
events are not reported, we conclude that this class of object does not 
exist in detectable numbers. Copyright 1998, Institute for Scientific 
Information Inc.

=================
(4) EVEN MORE DOUBTS ABOUT SMALL-COMETS

T.D. Swindle & D.A. Kring: Implications of small comets for the noble 
gas inventories of Earth and Mars. GEOPHYSICAL RESEARCH LETTERS, 1997, 
Vol.24, No.24, pp.3113-3116

UNIVERSITY OF ARIZONA, LUNAR & PLANETARY LAB, 1629 E UNIV BLVD, TUCSON, 
AZ, 85721

Frank et al. [1986a,b] proposed the possibility of large numbers of 
small comets impacting the Earth. Using the parameters for the small 
comets suggested by Frank and Sigwarth [1993], we find that, over the 
lifetime of the solar system, such small comets would deliver far more 
Ar, Kr, and Xe to the atmospheres of Earth and Mars than those 
atmospheres presently contain if, as Frank and Sigwarth [1993] assumed, 
the small comets formed in very cold regions far from the Sun. Comets 
can be greatly depleted in noble gases if they form relatively close to 
the Sun (for example, near Jupiter), but this source region is 
inconsistent with other features of the proposed model. Alternatively, 
if the comets did form in very cold regions far from the Sun, the 
current flux would have to be at least a factor of 30,000 higher than 
the long-term average. Copyright 1998, Institute for Scientific 
Information Inc.

===============================
(5) SORRY ABOUT OUR TECHNOLOGY, BUT IMAGES ON FRANK'S FILMS ARE NOT 
    REAL OBJECTS BUT DUE TO FAULTY CAMERAS  

G. Parks*), M. Brittnacher, L.J.Chen, R. Elsen, M. McCarthy, 
G. Germany & J. Spann: Does the UVI on polar detect cosmic snowballs?
GEOPHYSICAL RESEARCH LETTERS, 1997, Vol.24, No.24, pp.3109-3112

*) UNIVERSITY OF WASHINGTON, GEOPHYS PROGRAM, BOX 351650, 
SEATTLE, WA, 98195

If 20 to 40 ton cosmic snowballs pelt Earth as claimed by Frank and 
Sigwarth [1997a], dark pixels will be produced in the 130.4 nm images 
of dayglow obtained by the Ultraviolet Imager (UVI) on the Polar 
spacecraft. Examination of the UVI images has revealed that dayglow 
images are indeed spotted with single and multiple dark pixels. But is 
a snowball the only explanation for these dark pixels? To learn more 
about the dark pixels, we have examined the calibration images obtained 
from the same camera just before the instrument was launched. We find 
that dark pixels similar to those in dayglow images also exist in 
calibration images. This strongly indicates that the source of the dark 
pixels is instrumental. For further verification, a statistical 
analysis found the dark pixels from dayglow and calibration images have 
nearly identically shaped occurrence patterns. We have also looked for 
evidence of spacecraft 'wobble' which demonstrates that the source of a 
bright or dark feature in the images is external to the camera, but 
found none for dark pixels. Finally, we studied the bright streaks that 
frequently appear in UVI images, sometimes comet-like in appearance. 
These trails are ionization tracks produced by cosmic rays or other 
penetrating energetic particles interacting with our camera. We 
conclude that the source of the dark pixels in dayglow images is 
internal to the camera system and there is no scientific evidence from 
UVI that snowballs pelt Earth. Copyright 1998, Institute for Scientific 
Information Inc.

==============================
(6) AN ALTERNATIVE HYPOTHESIS: DO METEORIC IMPACTS IN ATMOSPHERE CAUSE 
    'BLACK SPOTS' ON LUIS FRANK'S FILMS?

M.B.E. Boslough & G.R. Gladstone: An impact plume model for atmospheric 
holes in the FUV dayglow. GEOPHYSICAL RESEARCH LETTERS, 1997, Vol.24, 
No.24, pp.3117-3120

SANDIA NATIONAL LABS, MS 0820, POB 5800, ALBUQUERQUE, NM, 87185

A transient decrease in FUV dayglow emission from a similar to 10(3) 
km(2) region of Earth's upper atmosphere is one expected outcome of the 
collision of a stony meteoroid as small as 50 cm in diameter. 
Computational models of the entry of projectiles between 210 and 21,000 
kg predict the ejection of low-density plumes of air from the 
stratosphere and mesosphere at velocities of up to 5 km/s. The plumes 
reach an altitude of over 1000 km, displacing the atomic O-rich 
thermosphere, and replacing it with photoelectron-and O-poor air from 
beneath 80 km. The FUV dayglow is normally dominated by O emissions so 
the plume appears as a dark spot when viewed from above. FUV dayglow 
holes are reported at rates similar to 10(4)-10(6) larger than the 
impact flux of objects in the sie range modeled here, suggesting either 
1) the observed rate is greatly overestimated, 2) there is an 
additional mechanism, or 3) much smaller impactors (which we have not 
modeled) can also generate FUV darkening plumes. Copyright 1998, Institute
for Scientific Information Inc.

======================

(7) AND FINALLY: GREEN PROPHETS OF DOOM IN SUDDEN MOOD SWING

From: NEW SCIENTIST, 24 January 1998

THERE MAY BE A FUTURE AFTER ALL

It was about as likely as the US cutting its carbon dioxide emissions 
or the world's farmers renouncing pesticides. Lester Brown, president 
of the Worldwatch Institute in Washington, DC and one of the 
environmental movement's leading doom merchants, has surprised the 
world by lightening up.

Each year for the past 14 years, Brown has forecast gloabl disaster as 
the demand for natural resources outstripped supply. But the institutes 
latest annual audit of the Earth's ecological health, State of the 
World 1998, he claims there are signs of hope. Large corporations such 
as British Petroleum and Toyota are beginning to adopt strategies for 
sustainable development, while countries like China and Denmark are 
making major advances in energy efficiency. The world's $2-billion wind 
power business is growing by 25 per cent a year, compared with 1 per 
cent annual increases in the coal and oil markets. Brown even suggests 
that the world could be on the brink of an environmental revolution 
comparable to the overthrow of state communism in Europe in 1990.

The new-found optimism, however, does not prevent him from sounding his 
usual dire warnings. "If the world economy as it is now structured 
continues to expand, it will eventually destroy its natural support 
system," he says.

Rob Edwards. 
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The permission by the Editor of the NEW SCIENTIST to post this 
article for scholarly use on the cambridge-conference network is 
greatfully acknowledged.