PLEASE NOTE:


*

CCNet DIGEST, 23 April 1999
---------------------------

     QUOTE OF THE DAY

     "Then [the impact survivors] went to a derelict holiday camp which
     looked as if it had been a lot nastier when it was in full working
     order. [...] there were some unexpected touches, such as a
     character eating a miniature Manchester United strip, a sure sign
     of desperation" (review in The Times, 23 April, on the
     current ITV impact disaster series 'The Last Train')


(1) SPORADIC BACKGROUND COMPONENT IN METEOR ACITIVY
    Jonathan Shanklin <jdsh@mail.nerc-bas.ac.uk>

(2) COMET IMPACTS WITH VENUS & TSUNAMIS FROM IMPACTS ON EARTH
    (NOT RELATED)
    Michael Paine <mpaine@tpgi.com.au>

(3) NEW RESEARCH ON THE SUPERNOVA OF 1054 AD
    http://xxx.lanl.gov/abs/astro-ph/9904285

(4) IMPACT SURVIVOR EATS MANCHESTER UNITED STRIP: SURE SIGN OF
    DESPERATION
    THE TIMES, 23 April 1999

(5) CRATER COUNT REVEAL SIGNIFICANT DIFFERENCES FOR IMPACTS ON
    EARTH & MARS
    J. Caers et al., STANFORD UNIVERSITY

(6) RADAR OBSERVATIONS OF ASTEROID 7822 (1991 CS)
    L.A.M. Benner et al., CALTECH, JPL

(7) IMAGING OBSERVATIONS OF ASTEROIDS WITH HUBBLE SPACE TELESCOPE
    A. Storrs et al., SPACE TELESCOPE SCIENCE INSTITUTE

(8) RECOVERY OF LOST ASTEROIDS
    A. Milani, UNIVERSITY OF PISA

(9) SECULAR INTERACTIONS OF COORBITING OBJECTS
    F. Namouni, UNIVERSITY OF LONDON QUEEN MARY & WESTFIELD COLL

(10) EXTERNAL SOURCES OF WATER FOR MERCURY'S PUTATIVE ICE DEPOSIT
     J.I. Moses et al., LUNAR & PLANETARY INSTITUTE

(11) ORIGIN OF POLYMICT BRECCIAS ON ASTEROIDS
     K. Saiki et al., AKITA UNIVERSITY


===========
(1) SPORADIC BACKGROUND COMPONENT IN METEOR ACITIVY

From Jonathan Shanklin <jdsh@mail.nerc-bas.ac.uk>

Re: CCNet Digest, 22 April 1999

The NASA article at
http://science.nasa.gov/newhome/headlines/ast21apr99_1.htm
is quite informative, however NASA seem to be unaware that there is a
sporadic background component to meteor activity and that there are
also a number of minor showers active during the first four months of
the year. Meteor enthusiasts will actually see 5 - 10 meteors an hour
most clear mornings from the sporadic component. The Virginid minor
shower is active throughout March and April and in total this will
produce more meteors than the much shorter lived Lyrid shower, though
the peak rate is only around 5 meteors an hour. The only exception to
this is when the Lyrids produce their occasional outbursts and I was
fortunate enough to see the 1982 outburst from the South Atlantic. This
produced a ZHR of around 100 but the observed rate was barely half
this. I've seen 8 Lyrids in one watch so far in April but it looks as
if cloud will obscure the maximum.  UK observers can start looking any
time after midnight, but the moon will interfere a bit until around 03
BST and it begins to get light around 04 BST.

Jon Shanklin
j.shanklin@bas.ac.uk
British Antarctic Survey, Cambridge, England
http://www.nbs.ac.uk/public/icd/jds

============
(2) COMET IMPACTS WITH VENUS & TSUNAMIS FROM IMPACTS ON EARTH
    (NOT RELATED)

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

Dear Benny

1. Comet impact with Venus

I have received a response from one of the authors of the Scientific
American article "Climate change on Venus", to my suggestion that a
comet impact may have caused obliteration of craters some 800 million
years ago:

"Mr. Paine makes an excellent point about the potential for a large
impactor on Venus to have altered the planetary climate.  David
Grinspoon  and I have calculated that the largest comet that one would
expect to have impacted Venus in the last billion years (from
statistical arguments) would have increased the atmospheric water
inventory 10 - 100 fold. Such a comet would have been smaller than
100's of km in diameter -- perhaps 40 km or so, but could certainly
have caused some kind of lithospheric disruption.  A 40 km comet would
not have put a pre-lunar like ring around Venus, but would certainly
have been capable of precipitating volcanic events and climate change.
Investigating the effects of impact-induced climate change on the
terrestrial planets is currently a major subject of research at NASA's
Astrobiology Institute.

Mark Bullock"

This might get published in SciAm's "Letters to the Editor"

2. Tsunami from asteroid impacts

In have created a temporary web page with the abstracts of the Tsunami
Symposium, to be held in Hawaii during May (the abstracts will
eventually reside on The Tsunami Society's website). They make very
interesting reading!

Also I have reviewed (yet again) my "Tsunami from asteroid impacts" web
page: http://www1.tpgi.com.au/users/tps-seti/tsym.html.html based on
some excellent papers published by the Tsunami Society (see links on
the above page).

Regards
Michael Paine

P.S. My father's name is Thomas! :)

=================
(3) NEW RESEARCH ON THE SUPERNOVA OF 1054 AD

Preprint from http://xxx.lanl.gov/abs/astro-ph/9904285

A Re-interpretation of Historical References to
the Supernova of 1054 AD

Authors: George W. Collins, II, William P. Claspy, John C. Martin
Comments: Figures included in text (18 pages). Accepted for publication
in July 1999 PASP.

Abstract

In this paper we re-examine historical references to the supernova
event of 1054 AD with a view to establishing a sequence astronomical
events which minimizes apparent conflicts between the various sources.
We find that the explosion of the supernova is likely to have occurred
weeks to months before the commonly accepted date of July 4th 1054.
This view is strongly supported by a number of European references to
events visible in the evening sky during the spring which are likely to
be associated with the appearance of the supernova. We find the best

fit to the light curve based on Chinese observations and a maximum
visible apparent magnitude for a supernova located at the distance to
the Crab Nebula also confirms the earlier explosion date.

=================
(4) IMPACT SURVIVOR EATS MANCHESTER UNITED STRIP: SURE SIGN OF
    DESPERATION

From THE TIMES, 23 April 1999
http://www.sunday-times.co.uk/news/pages/Times/frontpage.html?999

Meteorite drama fails to make an impact [review of the ITV series "The
Last Train"]

By Paul Hoggard

They were stumbling across a bleak, boggy moor in The Last Train (ITV).
Just watching from the sofa felt damp and chilly. Science fiction or
"survivor" fantasies filmed outside Britain usually take place in vast
wildernesses or boundless deserts.

But options are more limited here. "How about a nice bit of heath?" the
location manager asks the producer, or "I've found this brilliant bit
of moorland, dead Wuthering!" It's been this way since King Lear. If
Doctor Who had a Cyborg to escape from he ran across a moor. They were
up there again in Invasion Earth last year. Never mind urbanisation or
erosion of habitat by people of a woolly-hat-and-Thermos-of-soup
disposition; our landscape is in peril from overactive film-crews.

I joined this post-apocalyptic fantasy late, but I gather that
civilisation has been destroyed by an enormous meteorite (sic). By
coincidence, on Wednesday night the Discovery Channel had shown Three
Minutes to Impact, a documentary about the catastrophic meteor strikes
which have probably devastated the Earth in the past.

It's certainly a bit parky among the tussocks in The Last Train and the
acid rain was enough to dissolve the balls off a brass monkey. But it
certainly wasn't an ash-choked landscape of postvolcanic devastation.

The episode opened with Jandra (Amita Dhiri) lying injured on the floor
of a factory where she had fallen last week. They had to knock up a
stretcher and get her out with an improvised winch, so it began to feel
like one of those films about Outward Bound leadership courses
which go horribly wrong.

Then they went to a derelict holiday camp which looked as if it had
been a lot nastier when it was in full working order. This turned out
to be the base of the rival troupe of baddie survivors with guns who
invariably turn up in these stories.

I don't want to be too cynical. The cast is strong and plausible.
Jandra's death was rather moving and they all had a good warming round
her funeral pyre, which produced so much dense black smoke that she
must have been made of rubber tyres. And there were some unexpected
touches, such as a character eating a miniature Manchester United
strip, a sure sign of desperation. But I felt that I had walked this
moor before. [...]

Copyright 1999, The Times Newspapers Ltd.

==================
(5) CRATER COUNT REVEAL SIGNIFICANT DIFFERENCES FOR IMPACTS ON
    EARTH & MARS

J. Caers*), J. Beirlant, M.A. Maes: Statistics for modeling heavy
tailed distributions in geology: Part II. Applications. MATHEMATICAL
GEOLOGY, 1999, Vol.31, No.4, pp.411-434

*) STANFORD UNIVERSITY,STANFORD,CA,94305

In this paper we present three diverse types of applications of extreme
value statistics in geology, namely: earthquakes magnitudes, diamond
values, and impact crater size distribution on terrestrial planets.
Each of these applications has a different perspective toward tail
modeling, yet many of these phenomena exhibit heavy or long tails which
can be modeled by power laws. It is shown that the estimation of
important tail characteristics, such as the extreme value index, is
directly linked to the interpretation of the underlying geological
process. Only the most extreme data are useful for studying such
phenomena so thresholds must be selected above which the data become
power laws. In the case of earthquake magnitudes, we investigate the
use of extreme value statistics in predicting large events on the
global scale and for shallow intracontinental earthquakes in Asia.
Large differences are found between estimates obtained from extreme
value statistics and the usually applied standard statistical
techniques. In the case of diamond deposits, we investigate the impact
of the most precious stones in the global valuation of primary
deposits. It is shown that in the case of Pareto-type behavior the
expected value of few extreme stones in the entire deposit has
considerable influence on the global valuation. In the case of impact
crater distributions, we study the difference between craters
distributions on Earth and Mars and distributions occurring on other
planets or satellites within the solar system. A striking result is
that all planets display the same distributional tail except for Earth
and Mars. In a concluding account, we demonstrate the apparent
loghyperbolic variation in all of the above-mentioned examples.
Copyright 1999, Institute for Scientific Information Inc.

===============
(6) RADAR OBSERVATIONS OF ASTEROID 7822 (1991 CS)

L.A.M. Benner*), S.J. Ostro, K.D. Rosema, J.D. Giorgini, D. Choate,
R.E. Jurgens, R. Rose, M.A. Slade, M.L. Thomas, R. Winkler, D.K.
Yeomans: Radar observations of asteroid 7822 (1991 CS). ICARUS, 1999,
Vol.137, No.2, pp.247-259

*) CALTECH,JET PROP LAB,4800 OAK GROVE DR,PASADENA,CA,91109

We report Doppler-only (cw) radar observations of near-Earth asteroid
1991 CS obtained at Goldstone at a transmitter frequency of 8510 MHz
(3.5 cm) on 1996 August 26, 27, 28, and 29. Weighted, optimally
filtered sums of cw echoes achieve signal-to-noise ratios in excess of
300 per day and thoroughly cover the asteroid in rotation phase
(synodic period = 2.39 h, obtained photometrically by P. Pravec et al.
1998, Icarus 136, 124-153). A weighted sum of all cw spectra gives an
OC radar cross section of 0.24 +/- 0.08 km(2) and a circular
polarization ratio of 0.28 +/- 0.001. Our observations place up to 50
0,98-Hz resolution cells on 1991 CS at echo powers greater than 2
standard deviations of the noise. Variations of similar to 10 Hz in the
echo's 2-sigma bandwidth are evident on each day and are consistent
with the rotation period. Inversion of echo edge frequencies yields
convex bulls of the pole-on silhouette for each day. The hulls have a
mean elongation and rms dispersion of 1.18 +/- 0.02 and place a lower
bound on the maximum pole-on dimension of 1.3 km/cos delta, where d is
the angle between the radar line-of-sight and the asteroid's apparent
equator The hulls suggest that 1991 CS has the least elongated pole-on
silhouette of any of the nine near-Earth asteroids for which similar
shape information from radar observations is available. If we assume
that the projected area of 1991 CS is the same as that of a sphere with
a diameter of 1.1 km, equal to the lower bound on the minimum breadth
of the pole-on silhouette on August 28 and 29, then 1991 CS's radar
cross section and absolute magnitude of 17.5 correspond to upper limits
on the radar and optical geometric albedos of 0.25 and 0.14 that are
consistent with its S-class taxonomy. The circular polarization ratio
and radar albedo do not vary substantially as a function of rotation
phase, suggesting that the near-surface of 1991 CS is relatively
uniform. Among the 20 S-dass asteroid radar detections previously
reported, the circular polarization ratio of 1991 CS is greater than
11, comparable to 4, and less than 5. (C) 1999 Academic Press.

================
(7) IMAGING OBSERVATIONS OF ASTEROIDS WITH HUBBLE SPACE TELESCOPE

A. Storrs, B. Weiss, B. Zellner, W. Burleson, R. Sichitiu, E. Wells,
C. Kowal, D. Tholen: Imaging observations of asteroids with Hubble
Space Telescope. ICARUS, 1999, Vol.137, No.2, pp.260-268

*) SPACE TELESCOPE SCI INST,3700 SAN MARTIN DR,BALTIMORE,MD,21218
   AMHERST COLL,AMHERST,MA

We present the results of two Hubble Space Telescope (HST) observing
programs, consisting of 11 imaging observations of 10 asteroids. The
primary focus of the projects was to search for faint companions
(satellites) of these asteroids, No binary systems were detected. More
specifically, no companions were found at more than 0.1 arcsec
separation of brightness down to 4 magnitudes fainter than the primary
asteroid, No companions down to 6 magnitudes fainter than the primary
asteroid were found at more than 0.5 arcsec separation. It is unlikely
that companions more than 6 magnitudes fainter than a well-exposed
primary would be detected. These nondetections place very stringent
limits on the existence of companion bodies for these asteroids, A
secondary goal was to resolve the illuminated portion of the asteroids,
Nine of the 10 asteroids were marginally resolved, with three of

these-9 Metis, 18 Melpomene, and 19 Fortuna-showing significant
extension and brightness variations up to a factor of two across the
illuminated portion of the restored image. The diameters of the
resolved asteroids are generally in good agreement with those in the
TRIAD II file (E. F. Tedesco, 1989, In Asteroids II (Binzel, Gehrels,
and Matthews, Eds,)), Diameters for 19 Fortuna and 624 Hektor (which
are not in the TRIAD file) have been measured: 225 and 370 x 195 km,
respectively. (C) 1999 Academic Press.

============
(8) RECOVERY OF LOST ASTEROIDS

A. Milani: The asteroid identification problem - I. Recovery of lost
asteroids. ICARUS, 1999, Vol.137, No.2, pp.269-292

*) UNIVERSITY OF PISA,DIPARTIMENTO MATEMAT,VIA BUONARROTI 2,I-56127
   PISA,ITALY

When an asteroid is lost, the observers need to know the portion of the
celestial sphere where it could be recovered at a given time. This
region is an image of the region, in the space of orbital elements,
where the orbit is compatible with the previous observations. The mag
between these two regions is nonlinear; therefore the classical linear
approximation can fail. Indeed it fails by a large amount when both
these regions are large, which is precisely when an asteroid has been
observed only over a short are and/or it has been lost for a long time.
The recovery, and identification, of asteroids long lost is very
difficult if the only available prediction is a single point
corresponding to the least squares solution, which could be very far
from the real state; thus the availability of an efficient algorithm to
bound the recovery region is essential, also to decide if the recovery
is worth the effort, This paper proposes three new algorithms to better
approximate the recovery region based upon approximations going beyond
linearization. It gives the results of tests based upon asteroids which
have been recovered by chance and could have been found in the recovery
region computed by the new algorithms. Free software is available now,
by means of which the new algorithms can be tested, and eventually
adopted, by the observers and by the ephemerides computation centers.
(C) 1999 Academic Press.

=============
(9) SECULAR INTERACTIONS OF COORBITING OBJECTS

F. Namouni: Secular interactions of coorbiting objects. ICARUS, 1999,
Vol.137, No.2, pp.293-314

UNIVERSITY OF LONDON QUEEN MARY & WESTFIELD COLL, ASTRONOMY UNIT,
MILE END RD, LONDON E1 4NS, ENGLAND

We study the dynamics of the three-body problem when two objects are in
the 1:1 mean motion commensurability and can experience close
encounters. Three orbit families are relevant to the dynamics:
horseshoe orbits, passing orbits, and retrograde satellite orbits. The
fatter two families are present only when the relative eccentricity of
the two objects is large in comparison to the width of their coorbital
region. In the planar case, the three families are disjoint because the
guiding center of the motion is stationary with respect to secular
perturbations; as a result, collisional orbits are unstable. Orbit
transitions between different families occur in three dimensions owing
to the secular variations of the eccentricity and argument of
pericenter omega(r). Potentially colliding horseshoe orbits transit via
eccentric retrograde satellite orbits in a secular mechanism where
omega(r) librates around +/-90 degrees, Coorbital passing orbits are
located at the Kozai resonances with 2 omega(r) librating around either
0 degrees or 180 degrees, The former resonance is a dominant
feature of the dynamics once passing orbits exist. The latter is found
only at large values of the Jacobi constant. Large amplitude librations
around the Kozai resonances involve the transition from a passing orbit
to an association of horseshoe and retrograde satellite orbits and the
secular periodic reversal of the relative semimajor axis of the passing
orbit. The stability of these structures supports the existence of
coorbiting material with the planets, especially Mercury, and favors a
coorbital origin of the retrograde natural satellites.
(C) 1999 Academic Press.

===============
(10) EXTERNAL SOURCES OF WATER FOR MERCURY'S PUTATIVE ICE DEPOSIT

J.I. Moses*), K. Rawlins, K. Zahnle, L. Dones: External sources of
water for Mercury's putative ice deposits. ICARUS, 1999, Vol.137, No.2,
pp.197-221

*) LUNAR & PLANETARY INST,3600 BAY AREA BLVD,HOUSTON,TX,77058

Radar images have revealed the possible presence of ice deposits in
Mercury's polar regions. Although thermal models indicate that watts
ice can be stable in permanently shaded regions near Mercury's poles,
the ultimate source of the water remains unclear. We use stochastic
models and other theoretical methods to investigate the role of
external sources in supplying Mercury with the requisite amount of
water. By extrapolating the current terrestrial influx of
interplanetary dust particles to that at Mercury, we find that
continual micrometeoritic bombardment of Mercury over the last 3.5 byr
could have resulted in the delivery of (3-60) x 10(16) g of water ice
to tb permanently shaded regions at Mercury's poles (equivalent to an
average ice thickness of 0.8-20 m), Erosion by micrometeoritic impact
on exposed ice deposits could reduce the above value by about a half.
For comparison, the current ice deposits on Mercury are believed to be
somewhere between similar to 2 and 20 m thick. Using a Monte Carlo
model to simulate the impact history of Mercury, we find that asteroids
and comets can also deliver an amount of water consistent with the
observations. Impacts from Jupiter-family comets over the last 3.5
billion years can supply (0.1-200) x 10(16) g of water to Mercury's
polar regions (corresponding to ice deposits 0.05-60 m thick),
Halley-type comets can supply (0.2-20) x 10(16) g of water to the poles

(0.07-7 m of ice), and asteroids can provide (0.4-20) x 10(16) g of
water to the poles (0.1-8 m of ice). Although all these external
sources are nominally sufficient to explain the estimated amount of ice
currently at Mercury's poles, impacts by a few large comets and/or
asteroids seem to provide the best explanation for both the amount and
cleanliness of the ice deposits on Mercury. Despite their low
population estimates in the inner solar system, Jupiter-family comets
are particularly promising candidates for delivering water to Mercury
because they have a larger volatile content than asteroids and more
favorable orbital and impact characteristics than Halley-type comets.
(C) 1999 Academic Press.

=========
(11) ORIGIN OF POLYMICT BRECCIAS ON ASTEROIDS

K. Saiki*), H. Takeda: Origin of polymict breccias on asteroids deduced
from their pyroxene fragments. METEORITICS & PLANETARY SCIENCE, 1999,
Vol.34, No.2, pp.271-283

*) AKITA UNIV,FAC ENGN & MAT SCI,RES INST MAT & RESOURCES,1-1
   TEGATA GAKUEN MACHI,AKITA 0108502,JAPAN

Cumulate eucrite, noncumulate eucrite, and diogenite meteorites are
considered to have come from the crust of one (or similar) parent
asteroid. Howardites are regarded as regolith breccias of eucrites and
diogenites, and polymict eucrites are regarded as polymict breccias of
eucrites. These polymict breccias show many textural and chemical
features. In order to gain a better understanding of the origin of
polymict breccias and the origin of their components, we investigated
four polymict breccias, Yamato (Y)-791439, Y-791192, Y-82009, and
Y-82049 with a scanning electron microscope (SEM) equipped with a
chemical mapping system, and by electron probe microanalysis (EPMA). We
analyzed all pyroxene grains with chemical maps, classified them by
chemical composition, and observed their chemistry and mineralogy in
detail. The characteristics of pyroxenes suggest that the polymict
breccias were generated by gathering locally ordinary eucrites and
cumulate eucrites. The chemical-evolutionary features of the pyroxenes
(such as homogenization, chemical zoning, and exsolution lamellae)
suggest that there were at least two long annealing events and one
short (or low-temperature) annealing event, separated by mixing events.
Local heterogeneity on the asteroidal crust is also suggested.
Copyright property of the Meteoritical Society, 1999.

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