PLEASE NOTE:


*

CCNet DIGEST, 12 June 1998
--------------------------

(1) SUMMARY ON THE HAZARD ASSOCIATED WITH ASTEROID 1997XF11
    David Morrison <dmorrison@arc.nasa.gov>

(2) SORRY, DAVID, BUT YOU FAIL TO ADDRESS THE REAL ISSUE
    Benny J Peiser <b.j.peiser@livjm.ac.uk>

(3) CLARK CHAPMAN'S NEO ACTION PLAN
    Clark Chapman <cchapman@boulder.swri.edu>

(4) REQUEST FOR CLARIFICATION
    Benny J Peiser <b.j.peiser@livjm.ac.uk>

(5) CLARIFICATION BY CLARK CHAPMAN REGARDING FUNDING FOR MPC
    Clark Chapman <cchapman@boulder.swri.edu>

(6) END THIS DEBATE
    David H. Levy <dhlevy@LPL.Arizona.EDU>

(7) COMETS: CREATORS AND DESTROYERS
    David H. Levy <dhlevy@LPL.Arizona.EDU>



=================
(1) SUMMARY ON THE HAZARD ASSOCIATED WITH ASTEROID 1997XF11

From David Morrison <dmorrison@arc.nasa.gov>

Dear Benny:

In your 6/9/98 issue of CC Digest, you continue to support the idea
that asteroid 1997XF11 is dangerous, and you suggest that "only Clark
Chapman has responded to Brian [Marsden]'s challenge. But his reaction
is somewhat contradictory and often difficult to follow."  The reason
that many of us have not chosen to respond is that we consider this a
dead issue technically, and we see little to be gained by additional
public debate and recriminations. However, it seems that you and some
others are not satisfied, so maybe it would be useful to briefly
summarize the scientific consensus, as follows.  Please note that this
is a summary document, not a detailed technical treatise. In this
sense it is not a reply to the voluminous comments you have posted from
Brian Marsden, Clark Chapman, and others. Our intent is to distill the
essence of the situation in a non-controversial way, so as to provide
an authoritative reference for the many people who are interested in
XF11 and the impact hazard but who are not experts in celestial
mechanics or hazard analysis.

David Morrison
--------------------------------------------------------------------------------

SUMMARY ON THE HAZARD ASSOCIATED WITH ASTEROID 1997XF11

* Asteroid 1997XF11 is among the NEOs with the smallest known MOID
(Minimum Orbital Intersection Distance); that is, its orbit approaches
that of the Earth very closely in three-dimensional space.  As such it
will repeatedly come close to the Earth and may well eventually strike
our planet. It is therefore of great interest, both scientifically and
as a long-term threat. For NEOs like this that make such close
planetary encounters, we cannot reliably calculate detailed orbits for
more than a century in the future. XF11 will undoubtedly be closely
tracked over the next decades and centuries, as well as being an
excellent target for scientific studies such as radar imaging.

* Asteroid 1997XF11 never presented a significant hazard to the Earth
at its close passage in 2028. Initially any calculated orbit is of
course uncertain, but once a few weeks of observations were available
it would have been clear, had anyone done the calculations, that XF11
could not strike the Earth in 2028. While there was considerable
uncertainty in the miss distance, ranging from about 25,000 km up to
more than 700,000 km, all the calculated points of closest approach,
projected into the plane of intersection, missed the Earth by a
substantial margin. Later, as new observations were made in March and
then pre-discovery observations were located extending the observed arc
to several years, the uncertainty in the position quickly shrank. 
However, these observations did not significantly change the
probability of impact in 2028, which was (and is) essentially
zero.

* Part of the initial confusion associated with public and media
comments on XF11 resulted from the fact that the Minor Planet Center
(MPC) did not calculate the impact probability, so their statement that
"the chance of an actual collision [in 2028] is small, but one is not
entirely out of the question" was largely subjective.  When JPL
scientists made the first formal calculation of the impact probability,
they realized within an hour of addressing the issue that the
probability of impact by XF11 in 2028 was essentially zero, a
conclusion since verified by several further calculations using
different approaches, and confirmed in IAU Circular 6879.

* Because asteroid 1997XF11 has an unusually small MOID, even among the
one-hundred-odd known Potentially Hazardous Asteroids (PHAs), it
warrants continued attention from a hazard perspective.  XF11 will make
several close passes by the Earth in the next century; however, none of
these appears to represent a hazard, and the risk of impact in 2037,
which is the only post-2028 close approach so far investigated in
quantitative detail, is effectively zero. Further near-term
improvements in the orbit of XF11 (the asteroid is easily radar
detectable in 2002) will lead to even more certain results concerning
future impact risks.

* Any discussion of impact hazard from a known NEO should be placed in
the context of the background hazard we all live with due to
undiscovered NEOs. In any year there is a probability of roughly one in
100,000 of the Earth being hit, with little or no warning, by an
unknown object 1 km or greater in diameter. In any year the chances may
be as high as one in 100 of being hit by an undiscovered object similar
in magnitude to the Tunguska event of 1908. As a larger and larger
fraction of the NEOs are discovered, this risk from unknown NEOs
declines; that is in part the purpose of searching for these objects. 
No known NEO, including XF11, poses a threat of striking the Earth
within the next century that is nearly as high as this background risk
due to unknown objects.

Rick Binzel
Ted Bowell
Paul Chodas
Paolo Farinella
Al Harris
Andrea Milani
David Morrison
Steve Ostro
Don Yeomans

+++++++++++++++++++++++++++++++++++++++++++

David Morrison, Director of Space
NASA Ames Research Center, MS 200-7
Mountain View CA 94035-1000
Tel 650 604 5094; Fax 650 604 1165
david.morrison@arc.nasa.gov or dmorrison@mail.arc.nasa.gov
website: http://space.arc.nasa.gov
website: http://astrobiology.arc.nasa.gov
website: http://impact.arc.nasa.gov

===========================
(2) SORRY, DAVID, BUT YOU FAIL TO ADDRESS THE REAL ISSUE

From Benny J Peiser <b.j.peiser@livjm.ac.uk>

Dear David

I appreciate the your effort and that of your cosigners to respond to
Brian Marsden's new research findings. However, to say that your views
represent "the scientific consensus" is gilding the lily a little.

It appears to me that the main problems raised by Brian's new
calculations have not been addressed in your summary. The claim that
"the risk of impact in 2037" is "effectively zero" makes no sense since
this "zero-risk" assessment is only valid ever since the 1990 precovery
observations were found. After all, Brian DID find an impact scenario
for 2037. Without the 1990 data, Brian Marsden and David Asher argue,
there was a small, but real chance of an impact in the year 2037 (I
guess that before 12 March, XF11 should have been placed somewhere
between 1 and 2 on Richard Binzel's Hazard Index). If you and your
cosigners now seriously claim that XF11 has always been in category 0
(=zero risk), we would certainly be better off in the future without
such a nonsensical hazard index.

As long as you fail to disprove Brian Marden's scientific findings, I
doubt whether many NEO researchers will be ready to accept your general
"zero-risk" claim for XF11 prior to the precovery of the 1990 data. 

Benny J Peiser

P.S. May I take this opportunity to protest against your accusation
that I would "continue to support the idea that asteroid 1997XF11 is
dangerous." I deplore this sort of false accusations which are not
helpful to overcome a serious and real scientific problem among
colleagues. Can I remind you of my comment which I made with regards to
Brian Marsden's new findings:

> "In yesterday's CCNet Digest, Brian Marsden presented astronomical data
> which show that, prior to Helin's and Lawrence's discovery of the 1990
> films of asteroid 1997 XF11, there was "a small, but real" possibility
> that this PHO could have collided with earth in the next century. I am
> in no position to comment on whether or not Brian's sums add up. But
> one thing seems obvious to me: Should Brian's data and calculations be
> verified, this new twist in the XF11 controversy would prove his rather
> outspoken "zero-risk" critics wrong" (CCNet 9 June 1998).

========================
(3) CLARK CHAPMAN'S NEO ACTION PLAN

From Clark Chapman <cchapman@boulder.swri.edu>

Benny, I have posted an HTML version of the Action Plan:
www.boulder.swri.edu/clark/actnea.html

Clark

----------------
Response to Congress for an NEO Action Plan

Last updated: 11 June 1998

Action Plan Statement to: House Subcommittee on Space & Aeronautics

                      Clark R. Chapman, 9 June 1998

In response to the request of Chairman Rohrabacher at the end of the
May 21st hearings, I offer these recommendations for an "Action Plan"
to implement the Spaceguard Survey. From the hearings, I take it that
the Subcommittee's consensus is to implement the goal presented in the
June 1995 Shoemaker Report ("Report of the Near-Earth Object Survey
Working Group") and recently adopted by NASA, according to Dr.
Pilcher's testimony before this Subcommittee. That goal is:

"To extend the discovery of Near-Earth Asteroids larger than 1 km
diameter to 90% completeness within 10 years."

First, I address the technical options available for implementing the
Survey. Second, I address ways to decide on the options and get the
various players organized so that the Survey might begin, in an
appropriate international context.

TECHNICAL ELEMENTS OF THE SPACEGUARD SURVEY

Required elements of the Survey are:

(1) Completion of upgrades (to telescopes and state-of-the-art
detectors) and establishment of full-up operations of the four current
U.S. survey sites: Spacewatch, LONEOS, LINEAR, and NEAT.

(2) To build at least one more dedicated 2-meter telescope OR to
dedicate to the Survey approximately 6 of the existing USAF 1-meter
GEODSS telescopes (some are currently operational, a few are currently
mothballed).

(3) Acquire guaranteed time on ancillary telescopes, in particular
half-time on a 3- to 4-meter telescope for observations of physical
properties of a sample of near-Earth asteroids, time on telescopes
(especially in the southern hemisphere) for astrometric follow-up, and
time as required on the two major exisiting planetary radar facilities.

(4) Support a center (analogous to the Minor Planet Center), at an
adequate level for the greatly increased discovery rates anticipated,
to oversee and coordinate the Survey; to collect, disseminate, and
archive the data; to perform routine calculations to track discovered
near-Earth asteroids; and to provide ephemerides for astrometric or
physical observational follow-up. Additionally, support the research
community to evaluate the ongoing results of the Survey in the context
of the impact hazard.

(5) Coordinate with existing international efforts and broaden
international participation in the Survey.

Comment: NASA's effort (described in testimony) primarily supports
element #1, although it is not clear if immediate procurement of the
required state-of-the-art detectors is supported. NASA explicitly will
NOT support construction of any new telescopes in #2, although it
"hopes" to cooperate with the USAF to employ GEODSS facilities (the
alternative option in #2). Such cooperation (and funding for it) is not
yet fully arranged, nor is it clear that NASA or the USAF are planning
for the required level of about 6 dedicated GEODSS facilities. NASA's
budgetary plans do not appear to cover items #3 and #4, although they
need not be fully implemented until #1 and #2 are in place.

HOW TO IMPLEMENT THE SPACEGUARD SURVEY

(1) Establish the Spaceguard Survey as a Project (analogous to a small
space mission with defined goals, rather than as an open-ended ongoing
research program); lead responsibility should be by NASA but there
should be specified, supporting roles for the USAF and other
potentially relevant federal entities (e.g. DOE, FEMA).

(2) Mandate that existing contacts between NASA and the USAF be 
upgraded, with milestones, so that a detailed technical and funding 
agreement is reached by the end of CY 1998 on agency roles within the
Spaceguard Survey, including for example: (a) development, fabrication,
and provision by the Air Force of an adequate number of 
state-of-the-art detectors like that on the existing LINEAR facility
and (b) identifying the precise role of GEODSS facilities in the
Survey. The Science Committee should use its good offices (e.g. with
House and Senate National Security Committees) to foster the
cooperation. Special attention should be given to USAF costs, for which
estimates do not yet exist. If an interagency agreement cannot be
reached for the full level of Air Force participation required, it will
be necessary to proceed with the alternative option: construction of new
2-meter telescopes.

(3) Request that NASA submit a Project plan and budget for review
within 3 months.

(4) Mandate (e.g. in authorization language) that NASA fully (not
partially) implement the Survey and provide (in NASA's appropriations
bill) an appropriately funded line item for the Project, not to be
taken from existing scientific research programs.

(5) Facilitate international meetings so that U.S. scientists and
representatives of relevant U.S. agencies meet and coordinate with
counterparts from other countries (including national space agencies,
international scientific unions, etc.). These meetings should include
not only astronomers but also experts in risk management, hazard
mitigation, etc. Since the impact hazard is international in scope, the
goal of these meetings should be to establish an international
framework within which the U.S. Spaceguard Survey Project can operate
in an independent but coordinated way and to foster
augmented international participation.

===================
(4) REQUEST FOR CLARIFICATION

From Benny J Peiser <b.j.peiser@livjm.ac.uk>

Dear Clark,

Thanks for making your proposal more accessable on your website. I
intend to circulate the full text of your 'action plan' on tomorrow's
CCNet. I was wondering, however, whether you would like to clarify your
recommendations with regard to future American support for the MPC in
point (4). This paragraph is somewhat difficult to understand and could
be misinterpreted as an attempt to starve the IAU's MPC of future
American research funds. In fact, it could be read as your attempt to
set up a rival centre. This misinterpretation (?) could, in turn, lend
support to previous allegations that you have an axe to grind against
Brian.

I hope that this is not the case and that you are not, in effect,
asking the American Government to stop funding the work of the MPC in
order to set up your own MPC. I am sure you will be quite aware that
such a personal proposal would lead to an outcry among both American
and international colleagues.

I would prefer it if you could defuse this issue by making your
intention perfectly clear and look forward to your response.

Best wishes, Benny

> (4) Support a center (analogous to the Minor Planet Center), at an
> adequate level for the greatly increased discovery rates anticipated,
> to oversee and coordinate the Survey; to collect, disseminate, and
> archive the data; to perform routine calculations to track discovered
> near-Earth asteroids; and to provide ephemerides for astrometric or
> physical observational follow-up. Additionally, support the research
> community to evaluate the ongoing results of the Survey in the
> context of the impact hazard.


===============
(5) CLARIFICATION BY CLARK CHAPMAN REGARDING FUNDING FOR MPC

From Clark Chapman <cchapman@boulder.swri.edu>


                        M E M O R A N D U M

To:        Benny Peiser

From:      Clark R. Chapman
           Southwest Research Inst.
           1050 Walnut, Suite 426
           Boulder, Colorado 80302  USA
           [Phone: 303-546-9670; FAX: 303-546-9687]
           [E-mail: cchapman@swri.edu]; home phone: 303-642-1913

Date:      11 June 1998

Subject:   MPC

Benny, you will notice that in my statement of June 8, printed in CCNet
Digest, I referred to my intention to recommend higher funding "for the
MPC (or some equivalent entity)". I meant nothing different by my
phrase "...center (analogous to the Minor Planet Center)." The change
was motivated solely by the need to keep my wording as brief as
possible, yet the Congress would need to know what "MPC" stood for.

Obviously, whatever entity oversees the Spaceguard Survey will have to
be different from the MPC, at least in the sense that it must be
augmented in scale and will have additional responsibilities. I think
that the current MPC would be the obvious front-runner in proposing to
take on these responsibilities. But it is not the only option. There
was discussion at the Spaceguard Workshop in Volcano in 1995 that the
Spaceguard Foundation might oversee the project. One can imagine other
bidders.  Notwithstanding recent mistakes by the MPC, I think it has
generally performed its function very well and with dedication. Brian
Marsden's operation seems to me to be the obvious choice to take on
these new responsibilities. But, then, I haven't seen other proposals
nor, indeed, have bids even gone out!

====================
(6) END THIS DEBATE

From David H. Levy <dhlevy@LPL.Arizona.EDU>

I fully agree with Paolo that Benny should not have editorialized Clark
Chapman's comments before readers had a chance to read them. I also
believe that this debate is no longer about XF 11 but has gone into
personalities.  Although Clark does not have the weight and the
prestige of the Central Bureau for Astronomical Telegrams to back him,
he is  highly regarded and knows this field intimately. His comments do
not need editorializing. 

David H. Levy

==================
(7) COMETS: CREATORS AND DESTROYERS

From David H. Levy <dhlevy@LPL.Arizona.EDU>

Dear Benny,

I am sending along a copy of the New York Times Book Review of my
recent book Comets: Creators and Destroyers, published this month by
Touchstone/SImon&Schuster. You may use it in your list if you like: I
hope the book will help increase public understanding of the issues
that we write about so often!

Sincerely
David

------
It will be this summer's cinematic disaster du jour. Hollywood is now
launching a barrage of comets and asteroids on movie theater screens
across the country. If viewers' interests are piqued (or if their
nerves need soothing), ''Comets'' is a handy digest to put these
celestial visitors into perspective.

It would be hard to find a writer better suited to the task. David H.
Levy has been the discoverer or co-discoverer of 21 of them, including
Comet Shoemaker-Levy 9, the series of icy chunks that crashed into
Jupiter so spectacularly four years ago. But as a former graduate
student in English literature, Levy is also able to transform his
scientific facts into a charming and accessible story. His book is
liberally sprinkled with personal accounts, historical anecdotes and
literary references from John Keats to J. D. Salinger. His telescope, a
16-inch reflector, is named Miranda, for the Shakespeare character who
spoke of a ''brave new world.''

Comets have been an obsession for Levy since childhood, starting with a
sixth-grade assignment. That's when he learned a comet was a miles-wide
blackened snowball, a conglomeration of ices mixed with dustlike
particles. Some have short periods, like Comet Encke, which returns
every three and a third years. Others have a looser tether to the Sun,
like Comet Hale-Bopp, which appeared last year and will travel entirely
out of the solar system before returning in a few thousand years to
glow once again as it is bathed by the Sun's radiation.

Levy has lost none of his childhood wonder; he describes his vocation
as ''the world's slowest sport, in which scores are measured not in
afternoons but in lifetimes.'' He began searching in 1965 and spent
more than 900 hours at his telescope before finding his first comet 19
years later. So what keeps him and others coming back, to what seems
like a wearying endeavor? For some it is the chance to inscribe their
name on a sliver of the universe. ''For me,'' Levy answers, ''comet
hunting is a field of dreams.'' But ''it helps,'' he adds, ''to have
the perseverance of an Arctic explorer, the heart of a poet and the
patience of Job'' when facing the nighttime wind and cold.

Over the centuries, humanity has experienced a love-hate relationship
with comets. At first, comets were feared as omens of doom -- it was
counted significant that one showed up in 44 B.C., the year Julius
Caesar was assassinated. The British astronomer Edmond Halley at last
demystified them when, calculating from Newton's laws of gravity, he
confidently predicted that a comet he had seen in 1682 would reappear
in 1758. It did. Comets, he showed us, are simply planetoids in
constant, if eccentric, orbit around the Sun.

More recently, though, comets have been reclaiming their old
reputation, ever since evidence emerged that 65 million years ago some
form of monstrous meteor slammed into Earth off the coast of Yucatan
with the force of 100 million hydrogen bombs. Whatever it was, it
gouged out a crater 100 miles wide and 25 miles deep, spewing out
enough debris to darken the planet for decades and kill the dinosaurs.
No wonder Hollywood is taking note. Yet there was a silver lining to
the devastation. It gave mammals the opportunity to rule the world.

Comets and asteroids may be mere specks -- solar system trash. But Levy
aptly demonstrates that this debris has decidedly affected our lives,
starting five billion years ago. Earth's tilt is probably due to the
impact of a large object at its birth, giving us the seasons. Another
collision by a Mars-sized planetesimal tore enough material out of our
planet to forge the Moon. Meanwhile, a continual hail of smaller comets
provided Earth with both water and organic building blocks. Hubble
telescope observations of Hale-Bopp showed the comet shedding nine tons
of water each second.

Comets were multitudinous eons ago. Now they lurk either in a disk
beyond the orbit of Pluto (which is actually an oversize comet) or in a
halo farther out called the Oort cloud. Jupiter was the housekeeper: it
acted as a gravitational vacuum cleaner, either sweeping the comets
outward or consuming them. Comet Shoemaker-Levy was just the latest
example, and Levy explains how it was discovered almost by accident,
during a fitful survey on a bad-weather night. What resulted ''was a
scramble to put together the largest telescope armada ever assembled in
the history of astronomy to observe a single event,'' he writes.

HERE was the dress rehearsal for what will (not may) happen to Earth in
the future. Levy weaves a haunting tale of what would occur if a comet
like Shoemaker-Levy, with its 21 separate pieces, made a direct hit.
What's most disturbing is reading that comets are rarely found more
than a year before they enter our terrestrial neighborhood. We'd have
little warning. Given our current level of technology, there would be
nothing we could do to prevent an impact. ''Changing the orbit of a
10-mile-wide comet hurtling toward us at a high velocity is, one
scientist insists, like trying to move a tank with a popgun,'' Levy
notes. We have a better chance of keeping tabs on (and maybe even
altering) the paths of asteroids, a failed planet's rocky remains
that continually crisscross the inner solar system.

''Comets'' was obviously not written to be a definitive reference work.
Its level and pace are most appropriate to interested newcomers who
want a quick overview on topics ranging from Mars rocks to shooting
stars. Shooting stars, by the way, are really pieces of comet dust,
each no bigger than a grain of beach sand, left behind in a comet's
trail. When Earth crosses that wake, these particles vaporize in a
streak of light. Levy reports that a spectacular show, as many as
150,000 meteors per hour, will take place on Nov. 17, 1999, when Earth
travels through the litter left by Comet Tempel-Tuttle this year.
''Those who are lucky enough to witness the spectacle,'' he writes,
''will get a glimpse of what the earth was like during its primordial
age. As cometary debris rains out of the sky, the remnants of
destruction and creation of life will light up the sky just as they did
at the dawn of life on Earth.''

Readers will be convinced that comets are far more than illuminated
shuttlecocks that occasionally cross the heavens. Homo sapiens might not
have evolved without them.

----------------------------------------------------------------------------
Marcia Bartusiak is the author of ''Thursday's Universe'' and
''Through a Universe Darkly.''

(C) 1998 The New York Times Book Review


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