PLEASE NOTE:


*

CCNet DIGEST, 16 October 1998
-----------------------------

[Please note that I will be abroad until 27 October 1998]


(1) BRIAN MARSDEN TO SPEAK IN LIVERPOOL: INVITATION TO [BRITISH] LIST
    MEMBERS
    Benny J Peiser <b.j.peiser@livjm.ac.uk>

(2) THE TUNGUSKA EVENT AS RECORDED IN A TREE TRUNK
    Doug Keenan <doug.keenan@virgin.net>

(3) BOLIDES PRODUCED BY IMPACTS OF LARGE METEOROIDS INTO THE EARTH'S
    ATMOSPHERE
    Jiri Borovicka <borovic@sunkl.asu.cas.cz>

(4) COLLISION VELOCITIES AND COLLISION FREQUENCIES OF HILDA ASTEROIDS
    M. Dahlgren, ASTRONOMICAL OBSERVATORY UPPSALA, SWEDEN

(5) POLE COORDINATES AND SHAPE OF 30 ASTEROIDS
    C. Blanco*) & D. Riccioli, UNIVERSITY OF CATANIA

(6) HIGH-QUALITY PHOTOMETRY OF ASTEROIDS
    R.O. Redman et al., NATIONAL RESEARCH COUNCIL CANADA


===========
(1) BRIAN MARSDEN TO SPEAK IN LIVERPOOL: INVITATION TO [BRITISH] LIST
    MEMBERS

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

At the invitation of Dr Benny J Peiser and JMU's Astrophysics Research
Institute, Dr Brian Marsden, associate director of the Minor Planet
Center at the Harvard-Smithsonian Center for Astrophysics, Cambridge,
Mass., U.S.A. will give a guest lecture on the controversy about
asteroid 1997 XF11 in the context of the general impact hazard at
Liverpool John Moores University on Friday, 30 October.

The lecture, which is organised by the Astrophysics Research Institute,
will take place in JMU's Peter Jost Centre in Byrom Street at 7.00 pm.
The talk is open to the public.

For further information, please contact Dr Hugh Jones
(hraj@astro.livjm.ac.uk) or Dr Benny J Peiser (b.j.peiser@livjm.ac.uk).

-----

"2028 and All That"
Brian G. Marsden
Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., U.S.A.

The rationale for the March 11 announcement of the close approach of
1997 XF11 to the earth in 2028 is discussed, with attention paid to the
events leading up to the affair, as well as to the numerous subsequent
developments.  The validity of the claim that a collision was "not
entirely out of the question" is demonstrated, as is the significance of
the 1990 precovery observations--information that was specifically sought
in the March 11 statement.  The case of 1997 XF11 is also placed in the
general context of the problem that, from time to time, asteroids and comets
do strike the earth.

==================
(2) THE TUNGUSKA EVENT AS RECORDED IN A TREE TRUNK

From Doug Keenan <doug.keenan@virgin.net>

Hi Benny, have you seen the following?

Yonenobu, H. & Takenaka C. The Tunguska event as recorded in a tree trunk.
Radiocarbon 40: 367-371 (1998).

Basically, they present isotopic evidence that the event was not due to a
comet.

Doug Keenan

=================
(3) BOLIDES PRODUCED BY IMPACTS OF LARGE METEOROIDS INTO THE EARTH'S
    ATMOSPHERE

From Jiri Borovicka <borovic@sunkl.asu.cas.cz>

Dear Dr. Peiser,

I enclose the abstract of a published paper which could be of interest
to the subscribers of the Cambridge-Conference.

Sincerely,

Jiri Borovicka
====================================================================
Astron. Astrophys. 337, 591-602 (1998)

Bolides produced by impacts of large meteoroids into the Earth's
atmosphere: comparison of theory with observations
II. Benesov bolide spectra

J. Borovicka(1), O.P. Popova(2), A.P. Golub'(2), I.B. Kosarev(2)
and I.V. Nemtchinov(2)

(1) Ondejov Observatory, Astronomical Institute of the Academy of
Sciences, CZ-251 65 Ondejov, Czech Republic
(2) Institute for Dynamics of Geospheres, Russian Academy of Sciences,
Leninsky pr. 38, build. 6, 117979 Moscow, Russia

Received 24 February 1998 / Accepted 3 June 1998

Abstract

The unique observational spectrum of the very bright Benesov bolide EN
070591 is compared to theoretical bolide spectra. The -19.5 mag bolide
was induced by a meteoroid of an estimated initial mass of 4000 kg, a
density of 2 g cm-3 and a kinetic energy of 10^12 J (0.2 kT TNT). The
ablating piston model predicts spectra of large bolides by radiative
hydrodynamics calculations. We present examples of the calculated
H-chondrite vapor spectral opacities and of the resulting spectra for
various parameters.

Both theoretical and observed spectra show that bolide radiation is 
composed of atomic line emissions, molecular bands and continuum
radiation. The role of the continuum increases with increasing
meteoroid size and with decreasing altitude. The atomic lines are
produced under the effective excitation temperature of 4000-6000 K.

The lines of Fe I are too faint and the lines of Ca I are too bright in
the model in comparison with the observations. Also the computed
continuum level is too high. These differences can be explained by
the fact that the vapors occupy a larger volume and have lower density
than predicted. This is probably a consequence of a mutual interaction
of fragments after the meteoroid fragmentation and of a not well 
understood ablation process. Other differences between the theory and
the observation are described and possible model improvements are
discussed.

Key words: meteoroids, meteors

Send offprint requests to: J. Borovicka (borovic@asu.cas.cz)

European Southern Observatory (ESO) 1998
================
(4) COLLISION VELOCITIES AND COLLISION FREQUENCIES OF HILDA ASTEROIDS

M. Dahlgren: A study of Hilda asteroids - III. Collision velocities and
collision frequencies of Hilda asteroids. ASTRONOMY AND ASTROPHYSICS,
1998, Vol.336, No.3, pp.1056-1064

ASTRONOMICAL OBSERVATORY, BOX 515, S-75120 UPPSALA, SWEDEN

The collision velocities and collision frequencies of Hilda asteroids
have been investigated numerically. The collision probabilities and
collision velocities have been determined from a data base of close
encounters, obtained by numerical integrations of the orbits of 909
asteroids for 55000 years; The integration included all known
main-belt, Cybele, Hilda, and Trojan objects larger than 50 km in
diameter. The mean collision probability is lower for Hilda asteroids
than for main-belt, Cybele and Trojan asteroids. Out of ten collisions
involving a Hilda object, about seven are with main-belt objects, and
the other three collisions are evenly distributed among the three
outer-belt groups. The collision probabilities of individual Hilda
steroids have a strong correlation with the eccentricity of their
orbits, resulting in a wide range (about a factor of six) of collision
probabilities among the Hilda objects. The mean collision velocity of
the Hilda asteroids is 4.6 km s(-1), which is 0.5 km s(-1) lower than
the average for main-belt objects. The mean collision velocity of
individual Hilda objects range from 3.3 to 6.0 km s(-1). Copyright
1998, Institute for Scientific Information Inc.

===============
(5) POLE COORDINATES AND SHAPE OF 30 ASTEROIDS

C. Blanco*) & D. Riccioli: Pole coordinates and shape of 30 asteroids
ASTRONOMY & ASTROPHYSICS SUPPLEMENT SERIES, 1998, Vol.131,
No.3, pp.385-394

*) UNIVERSITY OF CATANIA,IST ASTRON,VIALE A DORIA 6,I-95125
   CATANIA,ITALY

To obtain a statistically reliable sample of minor planets with known
rotation axis orientation and axes ratios, a selection of photometric
lightcurves sufficiently covered to give the elements needed for
applying the computation methods of the rotational elements was made.
Using the data reported in the ''Amplitude-longitude (A - lambda) plot
catalogue of asteroids'' (Riccioli & Blanco 1995) as a starting point,
the amplitude-magnitude (AM) method (Zappala et al. 1983a) was adopted.
Due to the poor data available, it was possible to apply the (AM)
method only to 30 asteroids. For more than half of these objects no
previous determination of pole coordinates and shape exists in the
literature. Copyright 1998, Institute for Scientific Information Inc.

=============
(6) HIGH-QUALITY PHOTOMETRY OF ASTEROIDS

R.O. Redman*), P.A. Feldman, H.E. Matthews: High-quality photometry of
asteroids at millimeter and submillimeter wavelengths. ASTRONOMICAL
JOURNAL, 1998, Vol.116, No.3, pp.1478-1490

*) NATIONAL RESEARCH COUNCIL CANADA,HERZBERG INST ASTROPHYS,5071 W
   SAANICH RD,VICTORIA,BC V8X 4M6,CANADA

Photometric observations with the JCMT at millimeter and submillimeter
wavelengths have been made for the asteroids 1 Ceres, 4 Vesta, 6 Hebe,
7 Iris, 16 Psyche, 18 Melpomene, and 216 Kleopatra. The measurements
reported here include a careful estimation of the possible systematic
biases that may be present in the photometry. Whenever possible, the
flux densities have been averaged over a complete rotational light
curve to eliminate rotational phase as a source of uncertainty.
Combining our measurements with those from the literature at other
wavelengths, we present spectral energy distributions (SEDs) for the
thermal emission from these asteroids spanning the infrared and radio
ranges. The effective emissivity e(EFF) is defined as the ratio of the
observed flux density to that which would have been observed from a
nonrotating, spherical blackbody with the same size, distance from the
Earth, and distance from the Sun, as though viewed at opposition. The
physical properties that influence e(EFF) are discussed qualitatively,
using our SEDs to illustrate the importance of each effect. In this
way, the effective emissivity is demonstrated to be a useful means to
present the SED of an asteroid over the whole range of wavelengths for
which thermal emission dominates the observable flux density. The most
important physical properties that distinguish the SEDs of the
nonmetallic asteroids (Ceres, Vesta, Hebe, Iris, and Melpomene) from
each other appear to be (1) the optical depth through the layer of warm
material that has been heated by the Sun on the day side of the
asteroid; (2) the density of the surface materials; and (3) the
rotation period of the asteroid. For Ceres the warm surface layer is
partially opaque at wavelengths near 1 mm, while for Vesta it is
transparent at all wavelengths longer than 0.35 mm. We attribute the
transparency of Vesta's warm surface layer to its low density. In
contrast, Iris appears to have relatively dense materials on its
surface that transport heat effectively from its surface to its deeper
layers, reducing the infrared beaming compared to Ceres and increasing
the optical depth of its warm surface layer compared to Vesta. The
effectiveness of rotation in suppressing the infrared beaming
phenomenon is illustrated by Vesta, a rapid rotator with a weak
infrared beaming effect, and by Melpomene, a slow rotator with a strong
infrared beaming effect. The SEDs of the M-type (metallic) asteroids
(Psyche and Kleopatra) have a distinctive shape, with a steep decrease
from infrared to radio wavelengths. The effective emissivities at
wavelengths near 1 mm are too low to correspond to physical
temperatures in the asteroids' surfaces but are consistent with the
presence of large metal fractions in their surface minerals, which
would make their surfaces reflective rather than emissive at long 
wavelengths. This is the first clear mineralogical distinction that we
have been able to make based on the shape of an asteroid's SED. 
Copyright 1998, Institute for Scientific Information Inc.

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