PLEASE NOTE:


*

CCNet DIGEST, 10 June 1999
--------------------------


     QUOTE OF THE DAY

     "We are entering a golden age of asteroid and comet research, with
     space missions to 13 such targets being conducted over the next
     dozen years. The dangerous ones, though, are those we have yet to
     discover. By the year 2020 we need 20/20 vision of our local
     cosmic environment, lest some lump of rock like that which killed
     the dinosaurs creep up on us unawares" (Duncan Steel, The
     Guardian, 10 June 1999)


(1) SCIENTISTS SEARCH FOR 2020 VISION
    The Guardian, 10 June 1999

(2) VISUAL EVIDENCE FOR THE LARGEST MULTI-RING BASIN ON EARTH?
    Michael Phillips <pm622@greenwich.ac.uk>

(3) ONE WEEK TO GO TO NAME AN ASTEROID
    Linda Wong <tps@planetary.org>

(4) ASTEROID, COMET AND METEOR CONFERENCE SET FOR JULY
    UniSci, 8 June 1999

(5) ROTATION & LIGHTCURVE OF ASTEROID 4179 TOUTATIS
    A. Kryszczynska et al., ADAM MICKIEWICZ UNIVERSITY POZNAN

(6) TWO-BODY OPTIMIZATION FOR DEFLECTING EARTH-CROSSING ASTEROIDS
    S.Y. Park and I.M. Ross, USN

(7) PROGRESS REPORT ON THE JAPANESE SPACEGUARD PROJECT
    S. Isobe, NATIONAL ASTRONOMICAL OBSERVATORY

(8) 1620 GEOGRAPHOS & 433 EROS: SHAPED BY PLANETARY TIDES?
    W.F. Bottke et al., CORNELL UNIVERSITY

(9) COMPOSITIONAL SURFACE VARIETY AMONG THE CENTAURS
    M.A. Barucci et al., PARIS OBSERVATORY

(10) DYNAMICAL EVOLUTION OF ATENS & APOLLO ASTEROIDS
     R. Dvorak and E. Pilat Lohinger, UNIVERSITY OF VIENNA

(11) DIAMETER DISTRIBUTION OF EARTH-CROSSING ASTEROIDS
     A. Poveda et al., UNIV NACL AUTONOMA MEXICO

(12) ORGANIC MATERIAL FROM SPACE
     Michael Paine <mpaine@tpgi.com.au>

(13) CORRIGENDUM: IMPACT PROBABILITY FOR ASTEROID 1998 OX4
     Benny J Peiser <b.j.peiser@livjm.ac.uk>

=============
(1) SCIENTISTS SEARCH FOR 2020 VISION

From The Guardian, 10 June 1999
http://www.newsunlimited.co.uk/science/story/0,3605,57089,00.html

DEEP Impact and Armageddon gave Hollywood's version of what would
happen if Earth was menaced by a stray asteroid. But the reality is far
more frightening. Duncan Steel reports as the Lords prepare to raise
the issue next week


THE bad news is that astronomers have identified two asteroids which 
could impact Earth within the next half century. The worse news is that
one of those rocks has been lost in space.

This was not through incompetence. Think of it like cricket. An expert 
slip fielder cannot catch every nick off the edge. Especially if he's
posted to square leg.

The Spaceguard search for such civilisation-threatening projectiles is
being conducted with only part of the sky being scanned, often the
wrong part. Sponsored by Nasa and the Pentagon, major efforts are under
way in the US, but they cannot look during daylight hours, nor can they
see the southern sky. Our front gate may be closed, but the rear door
is ajar, and our windows wide open.

Asteroid 1998 OX4 was spotted last July by a university of Arizona
team. Without collaborators able to take over the tracking, it was not
followed long enough to secure definitive information about its future
path. It is effectively lost, a needle grasped from the celestial
haystack then tossed back. Maybe to prick our metaphorical backside
rather painfully one day. Considering the spread of feasible orbits,
researchers from the university of Pisa estimated there is a one in 10
million chance that it will slam into us in 2046.

Should we be worried? Only a few hundred yards in size, most likely
1998 OX4 will zip by again in 47 years. But if it hits then it will
make an impression like a hundred hydrogen bombs.

A contrary example is 1999 AN10, discovered in January by a search team
using a US Air Force telescope in New Mexico. It didn't seem important
until, after re-emerging from the sun's glare, it was observed again by
an American amateur astronomer living in Australia. (There is no
professional tracking team in the southern hemisphere.)

The new data indicate this asteroid is particularly dangerous. In every
loop about the sun it passes twice very close by Earth. This behaviour
will continue for several centuries, unless it runs into us first.
Present estimates of the likelihood of that occurring in 2039
[actually 2044, BJP] are about one in 500,000.

Those are long odds, but this object is about a mile in size, big
enough to kill a large fraction of humankind.. We are talking about a
million megaton explosion. There is something else to consider.

Most asteroids seem not to be single lumps, but rubble piles, 
agglomerations of smaller boulders and town-sized blocks of rock and
metal. 1999 AN10 is expected to pass very close by Earth several times,
within a few tens of thousands of miles, much closer than the Moon. It
is possible that our gravitational field will tear it apart, just like
Jupiter tore comet Shoemaker-Levy 9 apart a decade ago, resulting in
the string of impacts on that planet in mid-1994.

Maybe much of a broken-up asteroid like 1999 AN10 would miss a small
target like Earth, but it's obvious that a million fragments each with
a one-in-500,000 chance of blasting us is a dangerous proposition. Such
things have surely happened before, many pairs of craters - and even
chains containing six or eight impact scars - now having been
identified around the globe.

What is clear is that because the US has charged ahead in its
sky-scouring, turning up dozens of near-Earth objects every month, a
can of worms has been opened. There are two vital requirements of any
rational response.

First, other nations must co-operate in tracking these cosmic bullets,
or we will lose more of the dangerous ones. Imagine if 1998 OX4 had
been bigger, and with a 1% chance of hitting Earth say in 2010, but
we'd let it slip through our grasp. That would be a recipe for alarm
and trepidation, maybe mass suicides. Here, a little knowledge can
certainly be a dangerous thing. We need to be sure. Second, it is in
the national interest to have a domestic source of definitive
information, to advise the government, media and appropriate
authorities on a case-by-case basis. We cannot depend upon second-hand
accounts from others.

Even if no other country does anything constructive, the US search
projects will discover asteroids having uncomfortable miss distances at
an accelerating rate. The relevant forward projections are intricate,
but this is an area in which the UK has recognised expertise. A
National Spaceguard Centre is essential if panics are to be avoided.

Realising this, the government responded favourably to a House of
Commons debate in March, sparked by Lembit Opik, LibDem MP for
Montgomeryshire, whose grandfather was an astronomer who pioneered
research on the asteroid impact threat. Next Tuesday in the upper house
Lord Tanlaw is scheduled to take the question further. A positive
response will put the UK at the forefront of the global initiative to
tackle this surprising menace.

We are entering a golden age of asteroid and comet research, with space
missions to 13 such targets being conducted over the next dozen years.
The dangerous ones, though, are those we have yet to discover. By the
year 2020 we need 20/20 vision of our local cosmic environment, lest
some lump of rock like that which killed the dinosaurs creep up on us
unawares. We have been warned. Britain is ready to lead the way in
Europe.

Duncan Steel is currently at Armagh Observatory. His book Rogue
Asteroids and Doomsday Comets (Wiley, Chichester and New York, 1995)
was the first popular-level account of the impact hazard, leading to
movies like Deep Impact and Armageddon.

Guardian Newspapers Limited 1999

============
(2) VISUAL EVIDENCE FOR THE LARGEST MULTI-RING BASIN ON EARTH?

From Michael Phillips <pm622@greenwich.ac.uk>

The Vredefort Structure is an old (~2.02 Ga), highly eroded large
impact scar, situated  approximately 120km south-west of Johannesburg,
South Africa. The central structure consists of Archaean granite-gneiss
basement with exposures of mafic greenstone complexes, surrounded by a
semicircular ring of ~2.7 Ga vertical and overturned Witwatersrand
Supergroup sediments (1). This section of the structure is
approximately 70km in diameter (2), but the overall geometry and size
are still unresolved. Some workers have used modelling based on
cratering processes to show  that the original diameter could have been
at least 250-300km or more (3). Also, previous studies of the Vredefort
Structure have indicated that a number of circular or semi-circular
features, at varying radial distances, could be rings related to the
original Vredefort event (3-7).

The purpose of the work undertaken (8) was to look for any semicircular
features related to the central structure, out to a radial distance of
approximately 125 km. The main study area is within the Transvaal
System and Pretoria Group located to the north-west, north and
north-east. The southern regions of the structure are overlain and
buried beneath Permian sediments of the Karoo Supergroup where older
geological features are more difficult to observe.

The best way to search for large geological features on a regional
scale is to use satellite remote sensing. Detailed remote sensing
studies using Landsat Thematic Mapper (TM) have not been undertaken
previously at Vredefort. By using TM imagery, impact-related features
reported by past research or previously undocumented features
associated with the structure could be tested for, identified and
assessed. TM imagery is more useful to this study than previously used
Landsat MSS data, due to the wider range of spectral wavebands
available (visible, near infra-red, mid infra-red and thermal infra-red
bands). Spatial resolution is good enough for the discrimination of
large geological features (30 metres for reflective bands and 120
metres for the thermal band).

Two Landsat TM scenes of the Vredefort area from 1986 were acquired by
the School of Earth Sciences, University of Greenwich, mosaicked
together and reduced to an area of approximately 150 km2.  Image
enhancing techniques were applied, including false colour composites
and principal components analysis. Features previously found to be
indicators of  impact-affected areas, or subsidiary to impacts, were
used as search criteria. Previous workers (9, 10) have suggested using
remote sensing to look for:

i. Circular or arcuate shaped morphologic-topographic characteristics
(e.g. lakes).
ii. Circular or arcuate shaped geological formations.
iii. Concentric fault-zones.
iv. Sub-circular extensional structures in the terraced rim area.
v. Ring scarps.
vi. Radial fractures crossing ring features.
vii. Uplifted rings of (meta-) sediments.
viii. Signature and distribution of vegetation and water bodies.

All of these criteria were searched for on the TM imagery. A false
colour composite displaying band 6 (thermal) as red, band 4 (infra-red)
as green and band 1 (visible) as blue, was particularly useful for
revealing arcuate ridges at radial distances ~50km, 70km, 80km and
125km from the centre of the structure. Close inspection of the
enhanced imagery show that the ridges at ~70km contain a number of
features that appear to be faults, radial to the centre of the
Vredefort Structure. North of the central ring collar, the natural
drainage pattern is also arcuate. Radial faults and arcuate drainage
patterns have not been documented previously at Vredefort. If the
arcuate features seen at this distance are related to the Vredefort
impact event, then the minimum original diameter would have been 250km.
An impact feature on this scale may be considered to be a peak ring
crater or a multi-ring basin - so of which type is Vredefort?.

Multi-ring basins are large circular structures with not just one rim
but an additional raised ring or rings and a system of radial furrows.
Additionally, there are usually at least two asymmetric, scarped rings,
one of which may be the original rim (11). Research by other workers
using crater modelling has suggested at least one scarped ring, beyond
the central structure (5). The arcuate ring-like scarps seen on the
Landsat TM imagery is consistent with previous interpretations. These
findings may confirm other research that the original impact event was
large enough to have formed a multi-ring basin, the largest so far
confirmed on the Earth.

Further research into more distal regions from Vredefort (i.e. beyond
125km) is still required, possibly by using additional satellite
imagery. A useful study for Vredefort (and all large terrestrial impact
structures) would be to combine geophysical data, satellite imagery and
a digital elevation model. This would result in the production of
detailed 3-D subsurface and surface expressions of crater morphologies.
In the long term, this integrated, digital approach may help to resolve
some of the outstanding questions regarding the cratering history of
the Earth. 

Further details of this work are described in an abstract to be
presented at the 62nd Annual Meeting of the Meteoritical Society, July
11-16 1999, Johannesburg.

Authors:
(1) M.E.Phillips, (1) M.A.Bussell, (1) I.McDonald, (2) R.J.Hart and
(3) M.A.G.Andreoli. 1999. A Remote Sensing and Geological Investigation
of the Vredefort Impact Structure (South Africa) using Landsat TM
Imagery.

(1) School of Earth & Environmental Sciences, University of Greenwich,
    Chatham, UK.
(2) Council for Geoscience, P.O.Box 112, Pretoria 0001, South Africa.
(3) Atomic Energy Corporation of South Africa, P.O.Box 582, Pretoria
    0001, South Africa.


References

1. Hart, R.J. 1990. Chemical Geology, 83, 233-248.
2. Kamo, S.L. et al. 1996. Earth and Planetary Science Letters, 144, 369-388.
3. Henkel, H. and Reimold, W.U. 1998. Tectonophysics, 287, 1-20.
4. Andreoli, M.A.G. 1991. Structural Map, Atomic Energy Corporation, Pretoria.
5. Brink, M.C. et al. 1997. Tectonophysics, 270, 83-114.
6. Pilkington, M. and Grieve, R.A.F. 1992. Review of Geophysics, 30, 2, 161-181.
7. Rondot, J. 1994. Earth Science Reviews, 35, 331-365.
8. Phillips, M.E. et al. 1999. (abstract)  62nd Ann.Meeting of the Meteoritical
   Society, Johannesburg. 
9. Pesonen, L.J. et al. 1998. (abstract) Impacts and the Early Earth, Cambridge, 44.
10. Zumsprekel, H. et al. 1998. (abstract) Impacts and the Early Earth, Cambridge, 44.
11. Melosh, H.J. 1989. Impact Cratering, a Geologic Process. Oxford University Press.

=============
(3) ONE WEEK TO GO TO NAME AN ASTEROID

From Linda Wong <tps@planetary.org>

NEWS RELEASE

The Planetary Society
65 N. Catalina Avenue, Pasadena, CA 91106-2301 (626) 793-5100 Fax
(626) 793-5528 E-mail: tps@mars.planetary.org  Web:
http://planetary.org

For Immediate Release: June 9, 1999

Contact: Susan Lendroth

One Week to Go to Name An Asteroid

What's in a name?  That could be up to you in the case of Asteroid
1992 KD. But you have to act fast  there is only one week left to
enter the Planetary Society's Name the Asteroid Contest, which
ends June 15, 1999.

Asteroid 1992 KD is a target for Deep Space 1, a mission designed
to test cuttingedge technologies. The spacecraft will fly by the
asteroid on July 28, 1999, traveling closer to its surface than a
jet flies above the Earth when it is at cruising altitude.

Deep Space 1 will take images of the asteroid and will measure its
basic physical properties, including mineral composition, size,
shape, surface features, and brightness.

A key technology being tested on Deep Space 1 is its ion
propulsion system. This type of engine is 10 times more efficient
than a conventional liquid or solid fuel rocket.  Deep Space 1 is
the first planetary probe to use this advanced type of propulsion
system.

During Eleanor Helin's longrunning Palomar PlanetCrossing Asteroid
Survey, she and Ken Lawrence discovered 1992 KD on May 27, 1992 at
Palomar Observatory. NASA JPL funded the Palomar PlanetCrossing
Asteroid Survey. An asteroid's discoverers traditionally have the
right to name it, and they have asked the Planetary Society to
solicit suggestions through this contest. The winning suggestion
will be submitted by the discoverers to the International
Astronomical Union Small Bodies Naming Committee, which has
official approval over the naming of asteroids.

Helin said, "It is exciting to visit and examine one of our
discoveries from seven years ago. What seemed so remote then will
now become almost a handson experience. This will be the first
close look at a near-earth object, and from it a clearer picture
of one nearearth asteroid will emerge."

Since the Deep Space 1 mission is designed to test technology, the theme
for names is INVENTORS, living or deceased. Each suggestion for a
name should be accompanied by a short explanation (50 words or
less) of why the name is appropriate. Also include your name and
address.

The contest winner will receive a $50 gift certificate for the
Planetary Society Store.

Entries may be submitted by mail, fax or email.  Log into the
Society's web site at http://planetary.org <http://planetary.org/>
for additional information.  Entries must be RECEIVED by June 15,
1999.
 
Mail:  Asteroid 1992 KD Contest
       The Planetary Society
       65 North Catalina Avenue
       Pasadena, CA 91106

       Fax:            (626) 7935528 (Att: Linda Wong)
       Email:  tps@mars.planetary.org

Please contact Susan Lendroth for additional information at
(626)7935100 or by email at tps.sl@mars.planetary.org.

Carl Sagan, Bruce Murray and Louis Friedman founded the Society in
1979 to advance the exploration of the solar system and to
continue the search for extraterrestrial life.  With 100,000
members in over 140 countries, the Society is the largest space
interest group in the world.

Linda Wong
The Planetary Society
65 N. Catalina Ave.
Pasadena, CA 91106
(626) 793-5100 
(626) 793-5528 (fax)
tps@mars.planetary.org


============
(4) ASTEROID, COMET AND METEOR CONFERENCE SET FOR JULY

From UniSci, 8 June 1999
http://unisci.com/stories/19992/0608995.htm

The seventh International Conference on Asteroids, Comets and Meteors
(ACM) will be held at Cornell University July 26-30. The conference is
sponsored by NASA, the Applied Physics Laboratory of Johns Hopkins
University, the Jet Propulsion Laboratory (JPL) and Cornell.

A broad range of scientific sessions will present the latest
developments in all aspects of studies on asteroids, comets and
meteors, including observations, theories of origin and evolution,
discoveries and astrometry.

Scientific sessions

The 450 abstracts submitted have been organized into 28 different
sessions, for approximately 18 hours of plenary talks and 18 hours of
parallel sessions. In the past, the organization of the ACM scientific
program has tended to split sessions along the asteroid, comet, meteor
subcategories, resulting in a conference of three parallel topics with
few opportunities for cross-disciplinary discussion.

This year, an effort was made to organize the plenary sessions to be
cross-disciplinary. Session titles will be broad, including such
subjects as "Composition," "Spins and Sizes," "Collisional Processes"
and "Transitional Objects."

Sessions will include speakers from each of the asteroid, comet, and
meteor categories. In addition, poster sessions will draw participation
from all categories of ACM subjects.

The ACM '99 conference's invited speakers are at the forefront of their
fields and will be reporting on recent research in their areas of
expertise. They were chosen because the recent advances in these areas
are particularly exciting and newsworthy.

Invited speakers and their topics will include:

* Steven J. Ostro, JPL: Dramatic new asteroid results from the Arecibo
  Observatory.

* M.A. Barucci, Observatoire de Paris: Observations of objects at the
  edge of the solar system.

* Donald Brownlee, University of Washington: Stardust space mission.

* S.J. Bus, Massachusetts Institute of Technology: Findings on
  structure of asteroid families.

* W.J. Merline, Southwest Research Institute: Discovery of a satellite
  of asteroid Eugenia.

* Eberhard Gruen, Max Planck Institute: Confirmation of galactic dust
  near Earth.

(Editor's Note: For a complete list of abstracts, visit the ACM
  website.)

History of the ACM

The first ACM conference was held in 1983 in Uppsala, Sweden. Follow-up
meetings were held in Uppsala in 1985 and 1989; in Flagstaff, Ariz., in
1991; in Belgirate, Italy, in 1993; and in Versailles, France, in 1996.
The number of participants has grown steadily, from 76 in 1983 to about
500 in 1996. Attendees are drawn from all over Europe, from Australia,
New Zealand, North and South America, India, Central Asia and Japan.

The spirit of the ACM conference has been to welcome scientists and
enthusiasts of asteroid, comet and meteor studies of all ages and from
all nations to a gathering where ideas can be openly shared and
discussed. ACM '99 at Cornell will continue this tradition.

========================
(5) ROTATION & LIGHTCURVE OF ASTEROID 4179 TOUTATIS

A. Kryszczynska*), T. Kwiatkowski, S. Breiter, T. Michalowski: Relation
between rotation and lightcurve of 4179 Toutatis. ASTRONOMY AND
ASTROPHYSICS, 1999, Vol.345, No.2, pp.643-645

*) ADAM MICKIEWICZ UNIVERSITY POZNAN,ASTRON OBSERV,SLONECZNA
   36,PL-60286 POZNAN,POLAND

This paper presents results of modelling light variations of a freely
precessing asteroid, assuming its ellipsoidal shape and a geometric
light scattering law. The method is based on numerical integration of
Euler equations, combined with the explicit expression of an asteroid's
brightness as a function of Euler angles. Modelling is applied to
simulate the lightcurve of 4179 Toutatis according to its triaxial
ellipsoid shape and spin state given by Hudson & Ostro (1995). A good
agreement is obtained between the frequencies of the simulated and
observed lightcurves, The results explain some apparent discrepancies
between the periods obtained from photometric and radar observations.
Copyright 1999, Institute for Scientific Information Inc.

==================
(6) TWO-BODY OPTIMIZATION FOR DEFLECTING EARTH-CROSSING ASTEROIDS

S.Y. Park*) and I.M. Ross: Two-body optimization for deflecting
earth-crossing asteroids. JOURNAL OF GUIDANCE CONTROL AND DYNAMICS,
1999, Vol.22, No.3, pp.415-420

*) USN,POSTGRAD SCH,DEPT AERONAUT & ASTRONAUT,MAIL CODE AA RO,699
   DYER RD,MONTEREY,CA,93943

We present a formulation of a finite-dimensional optimization problem
associated with the deflection of Earth-crossing asteroids. The
performance measure is minimizing the delta-V requirement for achieving
a minimum target separation distance. A number of astrodynamical
constraints are identified and modeled. The constrained optimization
problem is numerically, solved using a sequential quadratic programming
method. Our numerical analysis indicates that the minimum delta-V
requirement is not a monotonically decreasing function of warning time;
rather, there is a finer structure associated with the orbital period
of the colliding asteroid. The analysis tool presented here may be used
for optimizing the interceptor mission for impact mitigation. Copyright
1999, Institute for Scientific Information Inc.

==========
(7) PROGRESS REPORT ON THE JAPANESE SPACEGUARD PROJECT

S. Isobe: Japanese 0.5 m and 1.0 m telescopes to detect space debris
and near-earth asteroids. ADVANCES IN SPACE RESEARCH, 1999, Vol.23,
No.1, pp.33-35

NATIONAL ASTRONOMICAL OBSERVATORY,2-21-1 OSAWA,MITAKA,TOKYO 181,JAPAN

After our experimental activities of several years, our telescope
project to build telescopes for the detection and observation of space
debris and near-earth asteroids has been approved by the Science and
Technology Agency and the project was started from the fiscal year
1998. Two telescopes will be built : a 1.0 m telescope with a 3 degree
field of view and position resolution of 0.2 are second to observe
geosynchronous earth orbit (GEO) space debris larger than 20 - 30 cm
and near-earth objects (NEO) larger than 1.0 km, and a 0.5 m telescope
will be used for follow-up observations of GEO and NEO and for testing
observations of low earth orbit (LEO) space debris. The telescopes will
be in full operation around 2001. (C) 1999 COSPAR. Published by
Elsevier Science Ltd.

==============
(8) 1620 GEOGRAPHOS & 433 EROS: SHAPED BY PLANETARY TIDES?

W.F. Bottke*), D.C. Richardson, P. Michel, S.G. Love: 1620 Geographos
and 433 Eros: Shaped by planetary tides? ASTRONOMICAL JOURNAL, 1999,
Vol.117, No.4, pp.1921-1928

*) CORNELL UNIVERSITY,CTR RADIOPHYS & SPACE RES,ITHACA,NY,14853

Until recently, most asteroids were thought to be solid bodies whose
shapes were determined largely by collisions with other asteroids.
Recent work by Burns and others has shown that many asteroids may be
little more than rubble piles, held together by self-gravity; this
means that their shapes may be strongly distorted by tides during close
encounters with planets. Here we report on numerical simulations of
encounters between an ellipsoid-shaped rubble-pile asteroid and Earth.
After an encounter, many of the simulated asteroids develop the same
rotation rate and distinctive shape as 1620 Geographos (i.e., highly
elongated with a single convex side, tapered ends, and small
protuberances swept back against the rotation direction). Since our
numerical studies show that these events occur with some frequency, we
suggest that Geographos may be a tidally distorted object. In addition,
our work shows that 433 Eros, which will be visited by the NEAR
spacecraft in 1999, is much like Geographos, suggesting that it too may
have been molded by tides in the past. Copyright 1999, Institute for
Scientific Information Inc.

===============
(9) COMPOSITIONAL SURFACE VARIETY AMONG THE CENTAURS

M.A. Barucci*), M. Lazzarin, G.P. Tozzi: Compositional surface variety
among the Centaurs. ASTRONOMICAL JOURNAL, 1999, Vol.117, No.4,
pp.1929-1932

*) PARIS OBSERVATORY,PL JULES HUSSIEU 5,F-92195 MEUDON,FRANCE

The Centaurs are a particular family of objects with orbits whose
semimajor axes fall between those of Jupiter and Neptune. They are
likely the transition objects between the Kuiper belt population and
short-period comets. To investigate the nature of these particular
objects, we have performed optical spectroscopic observations of five
Centaurs. The results show a great diversity among the reflectances of
the five Centaurs. The colors do not seem to be related to the
perihelion distance of the objects. We looked for weak cometary
emission features, in particular the CN-band emission at 3880 Angstrom,
but no CN emission feature has been detected within 3 sigma in any of
the investigated spectra. Copyright 1999, Institute for Scientific
Information Inc.

=================
(10) DYNAMICAL EVOLUTION OF ATENS & APOLLO ASTEROIDS

R. Dvorak and E. Pilat Lohinger: On the dynamical evolution of the
Atens and the Apollos. PLANETARY AND SPACE SCIENCE, 1999, Vol.47, No.5,
pp.665-677

UNIVERSITY OF VIENNA,INST ASTRON,TURKENSCHANZSTR 17,A-1180
VIENNA,AUSTRIA

In this investigation we compare the dynamical evolution of the two
groups of Earth crossing asteroids namely the Atens which move-grosso
mode-inside the orbit of the Earth and the Apollos which move outside
the Earth's orbit. The main goal of this study was to compute the
encounters of these asteroids with the Earth, respectively the
collision probabilities. We also briefly analyzed their qualitative
behavior over one million years and computed the mixing of the two
groups. The approach to achieve these results was a purely numerical
one: we used extensive numerical integrations in the framework of a
dynamical model of the planetary system (consisting of the Sun and the
planets Venus through Saturn). Other interesting features of the
dynamics of Apollos and Atens are confirmed in our study-e.g. the
temporary capture into the 1:1 mean motion resonance with the Earth.
Close encounters with the inner planets (primarily with the Earth and
with Venus) are the major events which change the orbit of such an
asteroid, but the Kozai resonances and the secular resonances have to
be taken into account to understand the dynamical evolution over longer
time intervals. The results of collision probabilities with the Earth
are in good agreement with statistically derived results by other
authors: approximately 20 (50 for smaller objects) collisions for each
Aten and 10 collisions for each Apollo per 1 billion years. (C) 1999
Elsevier Science Ltd. All rights
reserved.

=================
(11) DIAMETER DISTRIBUTION OF EARTH-CROSSING ASTEROIDS

A. Poveda*), M.A. Herrera, J.L. Garcia, K. Curioca: The diameter
distribution of Earth-crossing asteroids. PLANETARY AND SPACE SCIENCE,
1999, Vol.47, No.5, pp.679-685

*) UNIV NACL AUTONOMA MEXICO,INST ASTRON,CIUDAD UNIV,MEXICO CITY
   04510,DF,MEXICO

A cumulative distribution function N(d) of diameters of ECAs is derived
by fitting an exponential function to the observed distribution of
absolute magnitudes for the brightest objects (H less than or equal to
15.5), where there is evidence of completeness. This luminosity
function can be transformed (with appropriate albedos and densities) to
the frequency distribution of diameters, masses and energies. The
distribution of masses thus found is consistent with the self-similar
theoretical distribution of masses subject to a steady-state regime of
collisions found by Dohnanyi, and by Williams and Wetherill. The
frequency of collisions with the earth of asteroids of a given diameter
or energy is calculated. (C) 1999 Elsevier Science Ltd. All rights
reserved.

================
(12) ORGANIC MATERIAL FROM SPACE

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

Dear Benny,

The July issue of Scientific American has a feature article about
comets delivering (non-biological) organic material to the surface of
planets. See http://www.sciam.com/1999/0799issue/0799bernstein.html

This topic is also covered in a recent posting at  the PSRD site:
"Martian Organic Matter in ALH84001?" See
http://www.soest.hawaii.edu/PSRdiscoveries/June99/organicsBecker.html

As Victor Noto's website proclaims "... the Big Rock giveth and taketh
away" :)

Michael Paine

==================
(13) CORRIGENDUM: IMPACT PROBABILITY FOR ASTEROID 1998 OX4

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

I would like to correct the value given in my recent CCNet editorial
(8 June 1999) for the collision probability of asteroid 1998 OX4. The
chance of impact with the Earth in the year 2046 is 10^-7, not 10^-8.
For further details see:
http://newton.dm.unipi.it/neodys/1998OX4.risk.html

Benny J Peiser

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