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(meteorobs) Excerpts from "CCNet DIGEST, 27 April 1999"


Pretty rich one this time! -Lew

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From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST 27 Apirl 1999
Date: Tue, 27 Apr 1999 10:12:50 -0400 (EDT)

CCNet DIGEST, 27 April 1999
---------------------------

[...]

(4) TUNING IN TO APRIL METEOR SHOWERS
    NASA Science News <expressnews@sslab.msfc.nasadot gov>=20

(5) THE RATE OF NAKED-EYE COMETS FROM 101 BC TO 1970 AD
    A.L. Licht, UNIVERSITY OF ILLINOIS

(6) COMET HYAKUTAKE: DYNAMICS OF FRAGMENTS OF ITS COMETARY NUCLEI
    E. Desvoivres et al., UNIVERSITY OF GRENOBLE

(7) POLARIMETRIC OBSERVATIONS OF COMET HALE-BOPP
    Y.N. Gnedin et al., RUSSIAN ACADEMY OF SCIENCE

(8) DUST FLOW IN A POROUS COMET NUCLEUS
    Y. Shoshany et al., TEL AVIV UNIVERSITY

(9) RADAR OBSERVATIONS OF THE GIACOBINID METEOR STREAM
    M. Simek et al., ACADEMY OF SCIENCE OF THE CZECH REPUBLIC


============================================================

(4) TUNING IN TO APRIL METEOR SHOWERS

>From NASA Science News <expressnews@sslab.msfc.nasadot gov>=20

NASA Space Science News for April 27, 1999
=20
Tuning in to April meteor showers:  Last week's Lyrid meteor shower was =

a bit of a disappointment visually, but it put on quite a show for=20
radio observers. In this story you can learn about the basics of radio=20
meteors and listen to radar echoes from a bright shooting star.=20
FULL STORY at =
http://science.nasadot gov/newhome/headlines/ast27apr99_1.htm

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
(5) THE RATE OF NAKED-EYE COMETS FROM 101 BC TO 1970 AD

A.L. Licht: The rate of naked-eye comets from 101 BC to 1970 AD.=20
ICARUS, 1999, Vol.137, No.2, pp.355-356

UNIVERSITY OF ILLINOIS,DEPT PHYS MC 273,845 W TAYLOR ST,
ROOM 2236,CHICAGO,IL,60607

The number of comets that are bright enough and that come close enough=20
to Earth to be seen with the unaided eye fluctuates randomly from=20
century to century. The mean number seen per century, R, is a parameter =

determined by the distribution of short-period comets and by the escape =

of new, near parabolic comets into the inner Solar System from the Oort =

Cloud (J. H. Oort, 1963, The Solar System, Univ, of Chicago Press,=20
Chicago, London) and the Kuiper Belt (H. F. Levison and M. J. Duncan,=20
1997, Icarus 127, 13-32). A measurement of R provides a constraint on=20
possible escape mechanisms. In the following it is shown that R can be=20
determined by a comparison of the number of comets reported from the=20
east and west with those reported from both regions, An analysis of the =

reports compiled by I. Hasegawa (1980, Vistas in Astronomy, Pergamon,=20
Great Britain) shows that R =3D 86.0 +/- 6.7 comets/century and =
moreover=20
R has been remarkably constant over the past two millennia. One could
conclude from this that the mean rate at which all comets, visible and=20
invisible, enter the inner Solar System has also been constant over=20
this period. (C) 1999 Academic Press.

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
(6) COMET HYAKUTAKE: DYNAMICS OF FRAGMENTS OF ITS COMETARY NUCLEI

E. Desvoivres*), J. Klinger, A.C. LevasseurRegourd, J. Lecacheux, L.=20
Jorda, A. Enzian, F. Colas, E. Frappa, P. Laques: Comet C/1996 B2=20
Hyakutake: observations, interpretation and modelling of the dynamics=20
of fragments of cometary nuclei. MONTHLY NOTICES OF THE ROYAL=20
ASTRONOMICAL SOCIETY, 1999, Vol.303, No.4, pp.826-834

*) UNIVERSITY OF GRENOBLE 1,CNRS,LGGE,54 RUE MOLIERE,BP 96,F-38402 ST=20
   MARTIN DHER,FRANCE

Comet C/1996 B2 Hyakutake was extensively observed at the Pic du Midi=20
observatory during late March of 1996, Bright condensations were=20
observed in the near-nucleus coma. We have performed a detailed data=20
analysis in order to derive the position of these features with respect =

to the nucleus. We make the hypothesis that they are induced by=20
fragments of the nucleus. Despite the frequency of fragmentation of=20
cometary nuclei, the dynamics of the fragments is not yet well=20
understood. We propose a general approach in order to study the motion=20
of the fragments in the orbital plane of the comet. An estimate of the=20
non-gravitational forces is used to describe the motion of the fragment =

and of the nucleus with respect to their centre of mass. Then the=20
equations of the theory of perturbed Keplerian motion are solved in=20
order to study the motion of the centre of mass, This approach is=20
applied to Comet C/1996 B2 Hyakutake, The results are in good agreement =

with the observations, An excellent fit is obtained for a fragment size =

of 20 m, assuming a density of 300 kg m(-3). Copyright 1999, Institute=20
for Scientific Information Inc.
=20
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
(7) POLARIMETRIC OBSERVATIONS OF COMET HALE-BOPP

Y.N. Gnedin*), T.M. Natsvlishvili, V.D. Bychkov, V.P. Romanenko:=20
Polarimetric observations of comet Hale-Bopp. ASTRONOMY LETTERS-A=20
JOURNAL OF ASTRONOMY AND SPACE ASTROPHYSICS, 1999, Vol.25, No.3,=20
pp.191-197

*) RUSSIAN ACADEMY OF SCIENCE,MAIN ASTRON OBSERV,ST PETERSBURG=20
   196140,RUSSIA

We report the results of polarimetric observations of comet Hale-Bopp=20
made at the I-m telescope of the Special Astrophysical Observatory in=20
October 1996 and in March/April 1997 using MINIPOL polarimeter. We=20
present the wavelength dependences of the degree and position angle of=20
polarization, which differ from those of other comets and characterize=20
the unusual activity of comet Hale-Bopp. We estimate the contribution=20
of molecular scattering. Strong variability in the degree and position=20
angle of polarization and in the wavelength dependences of these=20
parameters indicate both very high inhomogeneity of the dust component=20
and a presence of dust grain alignment mechanism. Copyright 1999,=20
Institute for Scientific Information Inc.

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
(8) DUST FLOW IN A POROUS COMET NUCLEUS

Y. Shoshany, M. Podolak, D. Prialnik, B. Berkowitz: A Monte Carlo model =

for the flow of dust in a porous comet nucleus. ICARUS, 1999, Vol.137,=20
No.2, pp.348-354

*) TEL AVIV UNIVERSITY,DEPT GEOPHYS & PLANETARY SCI,IL-69978=20
   RAMAT AVIV,ISRAEL

We describe a Monte Carlo model that simulates dust migration in a=20
porous cometary nucleus. We present computations for media in which the =

pore-size distributions are either random or normal; additional=20
computations indicate that media with power-law size distributions=20
behave very similarly to random distributions. We show how the average=20
time to cross the medium varies as a function of porosity and how the=20
structure of the medium varies with time. Implications for the=20
structure of the cometary nucleus are discussed. (C) 1999 Academic=20
Press.
=20
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
(9) RADAR OBSERVATIONS OF THE GIACOBINID METEOR STREAM

M. Simek*), P. Pecina: The Giacobinid meteor stream observed by radar=20
in 1998. ASTRONOMY AND ASTROPHYSICS, 1999, Vol.343, No.3, pp.L94-L96

*) ACADEMY OF SCIENCE OF THE CZECH REPUBLIC,INST ASTRON,CZ-25165=20
   ONDREJOV,CZECH REPUBLIC

The return of the 1998 Giacobinid meteor shower associated with the=20
comet 21P/Giacobini-Zinner was observed by meteor radar at Ondrejov,=20
Czech Republic. The duration of the intense part of the shower was=20
approximately 3 hours. Though the Limiting magnitude of the radar is +8 =

M-r, only overdense echoes down to +4.2 M-r were analyzed. A maximum=20
number of 82 such shower meteors was recorded in the ten minute=20
interval 13h 00m-13h 10m UT corresponding to solar longitude L. =3D=20
194dot degrees 8200 +/- 0dot degrees 0034 (J2000.0). The mass-distribution=20
index s; =3D 1.38 +/- 0.04 indicates a lack of faint meteors in the =
most=20
dense part of the stream. Copyright 1999, Institute for Scientific=20
Information Inc.


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