PLEASE NOTE:


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LETTERS TO THE MODERATOR, 17 January 2000
-----------------------------------------


(1) IMPACT HAZARD VS. ASTEROID POPULATION
    Ed Grondine <epgrondine@hotmail.com>

(2) FIVE YEARS WARNING & PLANETARY DEFENSE
    Oliver Morton <abq72@dial.pipex.com>

(3) WW2 & PLANETARY DEFENSE
    Jon Richfield <jonr@iafrica.com>

(4) PAPERS IN NATURE ON IMPACT THREAT
    Michael Paine <mpaine@tpgi.com.au>

(5) AAS PAPERS
    Michael Paine <mpaine@tpgi.com.au>

===============
(1) IMPACT HAZARD VS. ASTEROID POPULATION

From Ed P. Grondine <epgrondine@hotmail.com>

Hello Benny -

   Unfortunately, it looks like some of the people
out at JPL are engaged in a bit of wishful thinking. 

   The number of impacts hinted at (and the only
reason they're merely "hinted at" is because they
haven't been explored yet) in the historical record
for the last 5,000 years or so is staggeringly high.
   Some of these impacts are clearly cometary, but for
the most part the exact composition of the impactors
is currently not known, due to the fact that field
surveys have not been carried out yet.
    It is important to note that the JPL survey dealt
only with the population of larger asteroids, and
further only those within the optical range of NEAT.
Others here have already pointed out that an inference
based on other, even earlier inferences about the
reflectivity of asteroids may be deceiving: dark
asteroids may simply not have showed up in the survey
with NEAT. 
    But even more simply, data about the larger
asteroids has to be very carefully analyzed before any
inferences can be drawn about the population of
smaller asteroids - little less any inference drawn
about the overall impact hazard, which includes
comets.  Perhaps the reason that fewer large asteroids
showed up than predicted is that they have simply
become small asteroids (the kind that can take out a
nation) through collisions in the population.
     Others have used other methods of inference to
try to refine the population estimates for asteroids,
some relying on models of planetary formation, others
on sampling and observer bias.  In the end, actual
field data on the impact rates for Earth, the Moon,
and Mars by both comets and asteroids will take
precendence over inferences, unless of course "wishful
thinking" prevents that research from being funded. 
    Finally, I doubt that an iron asteroid the size of
a house simply explodes in the upper atmosphere
without having any effect on the ground.  
    Perhaps some JPL researchers are worried about
NASA money for asteroid and cometary research being
diverted from NASA's Mars programs.  If they wish to
place a dead planet above the living, that's their
privilege, but it does not excuse them making
dangerously misleading statements about the overall
impact hazard. 
   If these mis-statements are the reporters' faults,
and not theirs, and their statements were mis-used,
they need to make sure that the reporters correct
their stories.
 
EP

===================
(2) FIVE YEARS WARNING & PLANETARY DEFENSE

From Oliver Morton <abq72@dial.pipex.com>

Benny

Without wanting to sound like a Pollyanna, five years is actually quite
lot of time if you have a huge amount of money and a huge amount of
motivation. Anyone who doubts this should compare the technologies of
1940 and 1945, especially with respect to computation, radar, rocketry
and nuclear explosives.

===============
(3) WW2 & PLANETARY DEFENSE

From Jon Richfield <jonr@iafrica.com>

Hi Oliver,

Thanks for the feedback.   

You are of course quite correct, as far as this goes, but without
wanting to sound like a jubilant Jeremiah (weeellll... OK! Misery *is*
fun, isnt it? :-)  ) I have a few reservations on how far it does go.

1) People certainly have done some terrific mountain-moving when they
put themselves to it and your examples are pertinent.  However, this is
a bit like drawing the targets after placing the bullet holes.  No one
knew in advance which of the wartime projects would work; many projects
simply never did work and it was mainly the promising and the successful
ones that got followed up. For instance, during that war there was
interest in detecting aircraft at night with infra red, but although IR
target acquisition and tracking is now commonplace, they never managed a
workable system at the time. Instead the English managed to implement
air-borne radar. Also, if certain objectives were never achievable,
people could accept the disaster and concentrate on alternatives. For
instance the Allied operational aircraft never did rival the German
Jets, so they simply concentrated on massive material superiority. (The
Gloster was too late to play a significant role.)

There were many other examples.

The trouble with the NEO threat is that whatever you use MUST succeed.

2) There are certain constraints in the physics. We can't just decide
to go faster and get there earlier. Not much faster and earlier
anyway. And we can't just hope to apply arbitrary force to move a huge
mass. F=MA insists on payment strictly in cash terms, whether in
Newtonian nor in Einsteinian mechanics. And if you apply your force too
abruptly, assuming that there is enough of it, you convert our
dino-killer into multiple city-busters. I know that I have argued them
to be the lesser evil, but they still are not a trivial concern. 

(BTW, if the body is particularly easy to split neatly, it strikes me
that separating it roughly into halves moving in opposite directions at
right angles to the collision course should be a particularly efficient
use of propellant power. How badly wrong am I, anyone?)

3) I did in fact mention at least one option for pre-empting the 5-year
window, namely establishing interceptor units in strategic locations in
anticipation. That is my favourite, partly because it seems to me the
most promising practical prospect, and partly because it looks like a
tempting route for development of a number of space engineering
technologies and infrastructures. It also presents interesting problems
in selecting the best locations. 

This strategy would require commitment a long time in advance, but would
give a pretty well-understood resource against the time of need.

Other options might include very powerful space guns or ablative
lasers. Such developments might be developed desperately and emerge as
unexpected last-minute winners, neither of which looks particularly
promising at present. But for instance they might be useful if deployed
in lunar Trojan orbit, for instance in case of a threat from well
outside the eliptic, for which we could hardly afford a standing patrol
of nuclear missiles.

Mmm... that should do for now.

Cheers

Jon

=============
(4) PAPERS IN NATURE ON IMPACT THREAT

From Michael Paine <mpaine@tpgi.com.au>

Dear Benny,

The  13 January 2000 issue of Nature has a feature on NEOs.
See http://www.nature.com/cgi-bin/wbsp-home.cgi
The following is an extract. The PDF files can be downloaded by
non-subscribers.
regards
Michael Paine


Tracking near-Earth asteroids

Each near-Earth asteroid has a 0.5% chance of collision with the Earth
in the next million years. Current estimates for the total number of
such asteroids range from 1,000 to 2,000, equating to a 1% chance of a
catastrophic collision with Earth in the next millennium. As detection
techniques improve, estimates of the number of Earth-threatening
asteroids are getting more accurate, and the good news is that the
latest estimate is lower, at roughly 700.

A reduced estimate of the number of kilometre-sized near-Earth
asteroids. DAVID RABINOWITZ, ELEANOR HELIN, KENNETH LAWRENCE & STEVEN
PRAVDO

Eyes wide shut
DAVID JEWITT
In 1994 NASA committed itself to discovering, within ten years, all
near-Earth asteroids greater than 1 km in diameter. Such 1-km
near-Earth objects (NEOs) could cause global devastation if they
struck the Earth. A new study suggests that the number of 1-km
NEOs is only half what we thought. But most of the threatening
NEOs (between 100 m and 1 km) have yet to be discovered.
| Full text | PDF |

Deep impact? Not half
NATURE NEWS SERVICE
| Full text |

Nature Macmillan Publishers Ltd.

=============
(5) AAS PAPERS

From Michael Paine <mpaine@tpgi.com.au>

Dear Benny,

These two items from the current American Astronomical Society meeting
may be of interest to CCNet. See also "Did bacteria survive the trip
from Mars"
http://space.com/science/solarsystem/mars_bacteria_000112_wg.html

regards
Michael Paine

AAS 195th Meeting, January 2000
--------------------------------------
Session 108. Asteroids, etc. [etc!!]
http://www.aas.org/publications/baas/v31n5/aas195/365.htm
[108.01] A Progress Report on the Lincoln Near Earth Asteroid Research
Project

R. M. Elowitz, G. H. Stokes, M. Bezpalko, M. S. Blythe, J. B.. Evans, E.
C. Pearce, R. W. Sayer, F. C. Shelly, H. E. M. Viggh (MIT Lincoln
Laboratory)

The Lincoln Near-Earth Asteroid Research (LINEAR) project is a MIT
Lincoln Laboratory effort cooperatively sponsored by the United States
Air Force Office of Scientific Research (AFOSR) and the National
Aeronautics and Space Administration (NASA). The objective of the LINEAR
project is to substantially contribute to the NASA goal of cataloging 90
percent of the Near Earth Asteroids (NEAs) with sizes larger than 1 km,
within the next 10 years.

Since March 1998, the LINEAR project has been hosted on a 1-meter
diameter telescope located at the Lincoln Laboratory Experimental Test
Site (ETS) on the White Sands Missile Range near Socorro, New Mexico.
Beginning in October 1999, the LINEAR system added a second 1-meter
telescope to routine operations, thus doubling the search capacity. Each
telescope is equipped with a large format 2560x1960 back-illuminated
frame-transfer CCD along with associated camera/processing elements
developed by MIT Lincoln Laboratory for United States Air Force space
surveillance applications. Since March of 1998, LINEAR has contributed
70% of the world wide discoveries of NEAs. As of January 1, 2000 the
LINEAR project has discovered 74 Potentially Hazardous Asteroids (also
referred to as PHAs), 22 Atens, 150 Apollos and 140 Amors type Near
Earth asteroids. In addition, LINEAR has discovered 33 comets since the
project began, and the first two asteroids with retrograde orbits that
show no indication of cometary activity. Future plans for the LINEAR
project include further automation of operations and processing
enhancements that will increase the already impressive discovery rate of
the LINEAR program.
---------------------------------------
Session 24. Extrasolar Planets and Substellar Companions
http://www.aas.org/publications/baas/v31n5/aas195/220.htm

[24.05] Transfer of Potentially Life-carrying Meteoroids from One
Planetary System to Another

M.J. Valtonen, J.Q. Zheng (Turku U.), C. Mileikowsky (KTH, Stockholm),
L. Lindegren (Lund U.), H. Rickman (Uppsala U.), F.A. Cucinotta (JSRC,
NASA), J.W. Wilson (LSRC, NASA), B. Gladman (U.Toronto), G. Horneck
(DLR, Cologne), J.Melosh (U.Arizona)

We calculate the probability that large size ejecta from a major
collision on a planet in an extrasolar planetary system find their way
to our solar system and collide with the Earth. The production rate of
ejecta is estimated from the conditions in Our Solar System,
and the evolution of the environment of the Sun is derived on the
assumption that the Sun was initially a member of a star cluster.
The maximum survival times of colonies of bacteria in the interstellar
space are estimated when the bacteria reside inside large meteoroids.
Using the maximum survival times as the maximum travel time we derive
the probability that a potentially life-carrying meteoroid from an
extrasolar system has landed on the Earth during the Earth's history.
The probability is less than unity, but not necessarily by many orders
of magnitude.


-----------------
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