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(meteorobs) Excerpts from "CCNet DIGEST, 16 April 1999"




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From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST 16 April 1999
Date: Fri, 16 Apr 1999 13:34:29 -0400 (EDT)


CCNet DIGEST, 16 April 1999
---------------------------

     POEM OF THE DAY

     Kindly submitted by Ian Giblin <giblin@panix.com> 
 
     ON MAKING CERTAIN ANYTHING HAS HAPPENED
 
     I could be worse employed
     Than as a watcher of the void,
     Whose part should be to tell
     What star if any fell.
 
     Suppose some seed-pearl sun
     Should be the only one;
     Yet still I must report
     Some cluster one star short.
 
     I should justly hesitate
     To frighten church or state
     By announcing a star down
     From, say, the Cross or Crown.
 
     To make sure what star I missed
     I should have to check on my list
     Every star in sight.
     It might take me all night.
 
     (Robert Frost)


[...]

(2) SURPRISE 1998 LEONID DISPLAY WAS A LARGE BLAST FROM THE PAST
    Jacqueline Mitton <jmitton@dial.pipex.com>

[...]

(4) SPIN VECTOR EVOLUTION OF INNER SOLAR SYSTEM ASTEROIDS
    E. Skoglov, ASTRONONOMICAL OBSERVATORY UPPSALA

(5) RADAR OBSERVATIONS OF COMETS
    P. Kamoun et al., AEROSP.

[...]

(7) STATISTICAL PROPERTIES OF METEORS
    D.D. Meisel, J.E. Richardson, SUNY COLL GENESEO

[...]

(9) RADAR OBSERVATIONS OF THE PERSEID METEOR STREAM
    M. Simek*), P. Pecina, ASTRONOMICAL INSTITUTE

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

(2) SURPRISE 1998 LEONID DISPLAY WAS A LARGE BLAST FROM THE PAST

>From Jacqueline Mitton <jmitton@dial.pipex.com>

ROYAL ASTRONOMICAL SOCIETY
PRESS NOTICE

Date: 15 April 1999
For immediate release

Ref. PN 99/09

Issued by: 

Dr Jacqueline Mitton
RAS Press Officer
Office & home phone: Cambridge ((0)1223) 564914
FAX: Cambridge ((0)1223) 572892
E-mail: jmitton@dial.pipex.com

RAS Web: http://www.ras.orgdot uk/ras/

* * * * * * * * * * * * * * * * * * * * * * * * *

CONTACTS FOR FURTHER INFORMATION ON THIS RELEASE:

Dr David Asher, Armagh Observatory
Phone: (0)1861-522928; Fax: (0)1861-527174; e-mail: dja@star..arm.acdot uk

Prof. Mark Bailey, Armagh Observatory
Phone: (0)1861-522928; Fax: (0)1861-527174; e-mail: meb@star.arm.acdot uk

Mr John McFarland, Armagh Observatory (Public Relations Officer)
Phone: (0)1861-522928; Fax: (0)1861-527174; e-mail: jmf@star.arm.acdot uk

Professor Vacheslav Emel'yanenko, South Ural University, Chelyabinsk, Russia
Phone: 007 3512 399291; Fax 007 3512 655950;
E-mail: emel@termeh.tu-chel.ac.ru

* * * * * * * * * * * * * * * * * * * * * * * * *

SURPRISE 1998 LEONID DISPLAY WAS A LARGE BLAST FROM THE PAST

In the early hours of 17th November last year (1998), meteor watchers 
awaiting the Leonid shower were taken by surprise when a spectacular 
display of bright meteors occurred 16 hours before the predicted time 
for the maximum of the shower. The explanation has now been uncovered 
as a result of research by Dr David Asher and Professor Mark Bailey, of 
Armagh Observatory, and Professor Vacheslav Emel'yanenko, of South Ural 
University, Chelyabinsk, Russia. They have shown that the bright 
meteors were seen when Earth passed through a dense arc-shaped cloud of 
particles shed from Comet Tempel-Tuttle in the year 1333. By matching 
theory and observation, Dr Asher and colleagues have proved for the 
first time that meteoroid streams associated with Halley-like comets 
have complex braid-like structures within them. This work points the 
way to more precise predictions of the timing and intensity of meteor 
showers in the future.  These results are reported in the 21st April 
1999 issue of the Monthly Notices of the Royal Astronomical Society.

The Leonid meteor shower occurs between 15 and 21 November each year, 
with peak activity on the night of the 17/18 November. These meteors 
are produced when small dust particles ejected from Comet Tempel-Tuttle 
enter the Earth's atmosphere at high speed and burn up. Comet 
Tempel-Tuttle moves around the Sun in an elliptical orbit taking 
approximately 33 years for a complete revolution. Its orbit is similar 
to that of Halley's Comet, and so Comet Tempel-Tuttle is classified as 
a Halley-type short-period comet. Owing to the large angle between the 
Earth's orbit and the comet's (162 degrees), the dust grains collide 
almost head-on with the Earth, and hit the atmosphere at about 71 
kilometres per second. At this speed, a one-centimetre particle
carries the same amount of energy as a speeding truck on a motorway.

Every 33 years or so, when Comet Tempel-Tuttle passes near to the 
Earth, the intensity of the Leonid display is greatly enhanced because 
the stream of dust grains is more densely packed close to the comet. 
Meteor 'storms' have been seen many times during the past thousand 
years, notable events being those of 1799, 1833, 1866 and 1966. The 
earliest record of Leonid meteors dates back to the year 899.

November 1998 saw astronomers preparing for a possible meteor storm 
during the night of 17/18 November. Although a moderately strong peak 
was observed as predicted, the meteor shower as a whole was dominated 
by the appearance of hundreds of exceptionally bright meteors, known as 
fireballs, more than 16 hours ahead of the predicted peak.

The intensity and duration of this exceptional event indicated that the 
Earth must have passed through an extremely dense, narrow stream of 
large dust grains and particles, having sizes ranging up to several 
centimetres. The timing suggested that these particles occupied an 
orbit somewhat different from the main stream of small grains, and that 
they left the comet's nucleus many hundreds of years ago. But in that 
case, it is necessary to explain why the stream has held together so 
tightly for so long.

To solve the problem, Dr David Asher and his co-workers calculated the 
motion of large dust grains ejected from the comet at each of the last 
42 occasions when it made its closest approach to the Sun. (Comets 
release very little dust, if any, when they are far from the Sun's 
heat.) They checked each case to see whether any of the particles could 
explain the fireballs seen in 1998, and identified September 1333 as 
the time when most of the observed particles were released. These 
particles did not spread out in space because of a dynamical process 
known as a resonance. (A similar process gives rise to the fine 
structure seen in Saturn's rings.)

Many comets and asteroids swing around the Sun in orbits that are 
simple multiples of the orbital period of Jupiter, the most massive 
planet in the solar system and the biggest disturbing influence on 
cometary orbits. Comet Tempel-Tuttle is no exception to this rule, 
having entered one of these 'resonant' orbits as long ago as the 
seventh century AD. For every fourteen revolutions of Jupiter, Comet 
Tempel-Tuttle makes five, and the same relation holds true for the 
largest dust particles gently released by the comet.

The large grains therefore have average orbital periods very close to 
that of the comet, and are kept in step by the influence of Jupiter. 
Instead of spreading around the whole orbit, they occupy a rather short 
arc, leading to the formation of a dense strand of large particles, 
distinct from the 'normal' storm strands of small particles, ahead of 
and behind the comet. The structure of the meteoroid stream close to 
the comet can be visualized as rather like a telephone wire, made up of 
many separate, narrow strands. These form a complex, braided structure 
of material within the broader, ribbon-like meteoroid stream.

The calculations by David Asher and co-workers showed that in November 
1998 most of the resonant arcs missed the Earth by a wide margin, but 
the arc of particles released in 1333 cut right through the Earth's 
orbit, and the calculated time for when this happened matched the 
observed fireball maximum to the hour.

This remarkable result is the first observational demonstration of one 
of the most important dynamical features of meteoroid streams 
associated with Halley-type short-period comets. The work highlights 
the presence of fine structure *within* meteoroid streams, and suggests 
important new avenues for research. For example, by observing the 
variations in meteor rates close to the peak of a shower it may be 
possible to infer the precise distribution in space of the 
meteor-producing strands. Variations in meteor rates may be correlated 
with changes in the meteor brightness distribution to infer the history 
of mass loss by a comet over many revolutions around the Sun.

The researchers are not expecting a repeat performance of bright 
fireballs in November this year (1999). All the resonant strands in the 
meteoroid stream will be well past Earth in space. However, a strong 
'normal' display is likely, peaking at about 2 a.m. on November 18th, 
due to meteoroids ejected from Comet Tempel-Tuttle in the years 1866, 
1899 and 1932, which have not yet had time to disperse around the 
comet's orbit.

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

(4) SPIN VECTOR EVOLUTION OF INNER SOLAR SYSTEM ASTEROIDS
 
E. Skoglov: Spin vector evolution for inner solar system asteroids. 
PLANETARY AND SPACE SCIENCE, 1999, Vol.47, No.1-2, pp.11-22
 
*) ASTRONONOMICAL OBSERVATORY,BOX 515,S-75120 UPPSALA,SWEDEN
 
The spin vector evolution of 30 asteroids in the inner solar system is 
investigated using 10 different orbital integrations for each object, 
since the orbital evolutions are chaotic for all the objects examined. 
Close encounters with major planets and resonances involving Jupiter, 
the Earth or Venus are common. Frequency-related zones, chaotic or 
non-chaotic, can be found for all the spin integrations for most of the 
objects. The positions and properties of these zones are in most cases 
rather independent of which of the 10 orbital integrations is used. The 
asteroids can be classed according to orbital behaviour, and it is 
found that the frequency-related zones for objects belonging to certain 
classes have common properties. The frequency analysis shows that the 
power spectra of most objects are dominated by frequencies in the range 
[-60, -20]''/yr. A statistical analysis showed no tendencies to a 
drift to a more prograde or retrograde distribution of spin vectors. 
There were, however, clear tendencies for some spin vector directions 
to be more probable than others during the time period examined. In 
particular highly retrograde spins seem to be more probable than 
slightly retrograde ones. (C) 1999 Elsevier Science Ltd. All rights 
reserved.
 
=======================
(5) RADAR OBSERVATIONS OF COMETS
 
P. Kamoun*), D. Campbell, G. Pettengill, I. Shapiro: Radar observations 
of three comets and detection of echoes from one: P/Grigg-Skjellerup
PLANETARY AND SPACE SCIENCE, 1999, Vol.47, No.1-2, pp.23-28
 
*) AEROSP, CANNES, FRANCE
 
Radar echoes from the nucleus of the short-period comet 
Grigg-Skjellerup were obtained using the 12.6 cm wavelength radar of 
the Arecibo Observatory during the apparition of 20 May-2 June 1982. 
Dual circularly-polarized receiving channels were employed. The 
receiving mode orthogonally polarized to the transmission yielded an 
echo equal to nearly 10 times the standard deviation of the 
accompanying noise; no echo was detected in the polarization of the 
same sense as transmitted. The observations give a radar cross-section 
of 0.50 +/- 0.13 km(2), and an upper limit to the width of the 
(unresolved) echo spectrum of less than 1 Hz. These results are 
consistent with specular reflection from a solid nucleus having a 
radius of less than 0.4 km. The upper limit depends on the unknown 
rotation rate and scattering law. Two other comets, comets Austin and 
P/Churyumov-Gerasimenko have also been probed but have not returned any 
detectable echo, thus allowing one to put only an upper limit on the 
sizes of their nuclei. (C) 1999 Published by Elsevier Science Ltd. All 
rights reserved.
 
================

(7) STATISTICAL PROPERTIES OF METEORS
 
D.D. Meisel, J.E. Richardson: Statistical properties of meteors from a 
simple, passive forward scatter system. PLANETARY AND SPACE SCIENCE, 
1999, Vol.47, No.1-2, pp.107-124
 
*) SUNY COLL GENESEO,DEPT PHYS & ASTRON,GENESEO,NY,14454
 
A project to determine some long-term statistical properties of the 
terrestrial meteor flux is described. A passive, 55 MHz (US - TV 
channel 2), forward-scatter radiometeor detection and simple (Apple 
IIe) computer logging system is used to record the epoch, duration and 
peak power of all echoes with durations greater than 0.09 s and less 
than 2 min. The system has been in 'continuous' routine operation from 
March 1993 to March 1998 with occasional interruptions due to sporadic 
E, troposcatter and equipment outages. Over 2,000,000 events have been 
recorded and logging is continuing on a 24-h basis. Grouping by echo 
type is done statistically at the data processing stage. Using only a 
small fraction of the available data, we show that this system produces 
valid and useful scientific results in spite of the fact that meteor 
velocities are not obtained. Through computer-aided statistical 
analysis, some of the milestones achieved are: (a) automatic separation 
of events into overdense, transition and underdense categories, and 
compilation of various statistics on these groups or some combination 
of them; (b) confirmation of a diurnal effect on average underdense 
trail durations; (c) confirmation of an apparent ozone depletion effect 
on the durations of overdense meteors; (d) interpretation of an 
apparent peak-power and duration correlation for overdense echoes in 
terms of the Manning forward scattering model and (e) confirmation 
(through a power spectrum analysis of hourly rates) of the presence of 
sporadic radiant sources of approximately 20 degrees of 
half-intensity-halfwidth which exhibit a 20-30% 16 day variability. 
(C) 1999 Elsevier Science Ltd. All rights reserved.
 
====================

(9) RADAR OBSERVATIONS OF THE PERSEID METEOR STREAM
 
M. Simek*), P. Pecina: Perseid meteor stream: cross and longitudinal 
structures from radar observations. PLANETARY AND SPACE SCIENCE, 1999, 
Vol.47, No.1-2, pp.281-289
 
*) ASTRONOMICAL INSTITUTE,ONDREJOV 25165,CZECH REPUBLIC
 
Results of 23 years of observations of the Perseid meteor shower by the 
Ondiejov meteor radar in the period 1958-1996 are presented. Activity 
profiles in four categories of echo durations differ slightly in the 
position of the activity peak and other characteristics of the central 
core in the range of solar longitudes (Eq. 1950.0) 138.0 degrees less 
than or equal to L. less than or equal to 140.5 degrees. Maximum 
activity of the central core is allocated at L. = 139.17 degrees +/- 
0.06 degrees in accordance with previous studies. The variation ratio 
of the year-to-year shower activity along the stream within the above 
L. limits was found to be 2.2:1. (C) 1999 Elsevier Science Ltd. All 
rights reserved.

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