[Prev][Next][Index][Thread]

(meteorobs) Fwd: CCNet, 23 November 1999




[Some repeats in here of previous 'meteorobs' posts, but I'm too
tired to edit this large, interesting issue! Here it all is... -Lew]

------- Forwarded Message

From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 23 November 1999
Date: Tue, 23 Nov 1999 11:22:16 -0500 (EST)

CCNet, 23 November 1999
- -----------------------


(1) THE LION ISN'T DEAD YET
    Jim Bedient <bedient@amsmeteors.org> 

(2) LEONID STRIKES THE MOON
    BBC Online News, 23 November 1999

(3) NEODyS ANNOUNCEMENT
    NEO Dynamic Site <neodys@newton.dm.unipidot it> 

(4) SIZE & PHYSICAL PROPERTIES OF COMET HALE-BOPP
    V.G. Kruchinenko & K.I. Churyumov, KYIV NATL UNIVERSITY

(5) NUCLEAR SIZE OF COMET HALE-BOPP
    Z. Sekanina, CALTECH,JET PROP LAB

(6) DETECTION OF A SATELLITE ORBITING THE NUCLEUS OF COMET 
    HALE-BOPP
    Z. Sekanina, CALTECH,JET PROP LAB

(7) CHEMICAL ABUNDANCE IN COMETS
    W.F. Huebner & J. Benkhoff, SW RES INSTITUTE

(8) GAS AND DUST RELEASE FROM COMET HALE-BOPP
    D. Prialnik, TEL AVIV UNIVERSITY

(9) IS COMET HALE-BOPP A JUVENILE COMET?
    J. Benkhoff & H. Rauer, DLR,

(10) MONTE CARLO SIMULATIONS OF OORT CLOUD COMETS
     P. Nurmi et al., UNIVERSITY OF TURKU

==============
(1) THE LION ISN'T DEAD YET

>From Jim Bedient <bedient@amsmeteors.org> 

Dear Benny,

Amongst all the accounts of Leonid activity, I note many comments along 
the line of "Well, that's it 'til 2033".  It is important to note that 
there are several other storm-level returns of the Leonid meteor shower 
possible during 2001-2002.

David J. Asher and Robert H. McNaught, whose model of the Leonid stream 
was so spectacularly vindicated on November 18 also predict encounters 
with additional filaments or trails in the Leonid stream.  The table 
below shows some of their predictions:

     Time (UT)      Estimated ZHR           Visible from 
2001 Nov 18, 10:01      1,500?       N. & Central America
2001 Nov 18, 17:31     15,000        Australia, E. Asia
2001 Nov 18, 18:19     15,000        W. Australia, E., SE & Central Asia
2002 Nov 19, 04:00     15,000        W. Africa, W. Europe, N. Canada, NE 
S.America
2002 Nov 19, 10:36     25,000        N. America

So the Lion is certainly not dead yet, and it won't be 33 years, but 
hopefully less than two before we'll be showered again with Leonids.

Jim Bedient
American Meteor Society

==============
(2) LEONID STRIKES THE MOON

>From The BBC Online News, 23 November 1999
http://news.bbc.codot uk/hi/english/sci/tech/newsid_532000/532448.stm

By BBC News Online Science Editor Dr David Whitehouse 

Astronomers think they have witnessed a meteor striking the Moon. Brian 
Cudnik from Houston, Texas, captured the event, which may be the first
such confirmed observation. 

He was looking at the Moon during last week's Leonid meteor storm in 
the hope that he might see something interesting. It had been suggested 
that skywatchers might see a Leonid strike the lunar surface about 
three hours after the storm's peak on Earth. 

Cudnik saw a brief flash near the centre of the Moon's dark side at 
about 0446 GMT. 

Observing through a telescope, he estimated that the flash, lasting 
only a fraction of a second, was at least as bright as some nearby 
stars. It was also seen by astronomer David Dunham observing near
Washington DC, who made a video recording of the event. 

Astronomers are now appealing for anyone else who may have seen the 
event to come forward. According to Dunham, analysis of the images will
permit a reasonably good determination of the brightness and location 
of the impact flash. 

Impact site 

The next step would be to use high-power telescopes to scrutinise the 
impact site to see if there have been any changes. This is unlikely as 
the impact must have been by a small meteor, probably less than a 
kilogram in mass. But astronomers will want to look, nonetheless. 

It would certainly help to establish if this is the first, confirmed 
lunar impact observation. A probable lunar meteor impact was  
photographed on 15 November, 1953, but was not confirmed by other 
observations. 

There is also a very old historical account that could also be 
explained by a meteor hitting the Moon. This was recorded by Gervase of 
Canterbury who, in 1178, along with five other monks, saw a very bright 
flash on the Moon: 

"There was a bright New Moon, and as usual in that phase its horns were 
tilted towards the east. Suddenly, the upper horn split in two. From 
the midpoint of the division, a flaming torch sprang up, spewing out 
fire, hot coals and sparks." 

Some astronomers believe that the crater Bruno, one of the youngest on 
the lunar surface, may have been formed in this event. 

Copyright 1999, BBC

===============
(3) NEODyS ANNOUNCEMENT

>From NEO Dynamic Site <neodys@newton.dm.unipidot it> 

The Near-Earth Object Dynamics Site (NEODyS) has been significantly
improved and expanded. 

NEODyS is an online information service for near-Earth asteroids 
(NEA's), available since March 1999. At its core, NEODyS is a database 
of orbital and observational information for each NEA; however, the 
distinguishing feature of the system is the degree to which the 
information is made accessible and searchable. NEODyS can be accessed
via the WWW at <http://newton.dm.unipidot it/neodys/>.

In NEODyS every NEA has its own "home page" containing sections devoted 
to the object's orbit, observations, planetary close encounters, 
additional services, and physical information. Also every observatory 
has its own page with links to tables of all NEA observations from that 
station.

One of the most valuable features is the database query facility. One 
may of course look up an asteroid by name or number, but it is also 
possible to search for all NEA's possessing some desired orbital or 
observational characteristics, or to search for close approaches
within a given distance.

The new features available since 20 November 1999 are the following:

1) There is now a daily ephemeris (position on the sky) for each object 
in the database, and the ephemerides are also searchable. This allows 
an observer to search for NEA follow up targets based on observability 
and other factors such as brightness, uncertainty, arc length, or the 
number of days unobserved. Alternatively one can search for all NEODyS 
objects in a particular region.

2) Radar observations are now incorporated into our orbital solutions. 
For those objects with radar observations, this results in 
substantially improved orbits, with far less uncertainty.

3) The orbits are now computed with statistical weighting of optical 
astrometry according to the historical performance of the observatory.
Radar observations are also weighted in a rigorous way. The information 
used to compute the weights are available on the page of each 
observatory.

4) We are now using simpler and constant URL's. This means that you can 
put a link on your own web pages to your favorite asteroid or 
observatory, for example,

1999 AN10: 
<http://newton.dm.unipidot it/cgi-bin/neodys/neoibo?objects:1999AN10;main> 
(433) Eros: <http://newton.dm.unipidot it/cgi-bin/neodys/neoibo?objects:Eros;main>
Catalina Sky Survey: <http://newton.dm.unipidot it/cgi-bin/neodys/neoibo?sites:703;
main>.

5) We have significantly improved the close approach table for each 
object, adding calendar date (most users are unfamiliar with the 
previous Modified Julian Day) and the minimum possible distance of 
approach according to the linear theory (which is less than the nominal 
approach distance, computed on the basis of the best fit orbit), and 
other parameters useful to assess the circumstances of the encounter.

6) Another feature, which is not new now but was added after the first
announcement of NEODyS, is the Impact Risk Page, available at

http://newton.dm.unipidot it/neodys/risk.html

In this page we list all the asteroids for which possible impact 
solutions, compatible with the existing observations, are known. In
all these cases we have been able to compute a very small, but 
definitely non-zero, probability of collision.  Only one rogue asteroid 
is currently listed on this page (and the probability of impact are 
extremely low).

The database is automatically updated on a daily basis as new 
observations are released from the Minor Planet Center. All of the data 
files needed to reproduce the NEODyS results are freely available, even 
the OrbFit software used for orbit determination 
<http://newton.dm.unipidot it/~asteroid/orbfit>.

The system is being upgraded and improved continuously, comments and 
suggestions are welcome. Among the improvement we are working on that 
we can mention are

A) Availability of NEA proper elements (in table and graphical
form)

B) Complete list of all *possible* close approaches until 2050 (This 
data is presently maintained privately as part of our recently 
implemented automatic monitoring of the impact risk for all NEA's, but
we hope to put it in a publishable form soon.)

C) Qualitative study of the orbit-to-orbit distance with respect to the 
Earth

D) Inclusion in the database of the asteroids which are not NEA 
according to the nominal, best fit orbit, but nevertheless could be
NEA.

NEODyS has been created by A. Milani and S. Chesley, Department of 
Mathematics, University of Pisa, Italy.

===============
(4) SIZE & PHYSICAL PROPERTIES OF COMET HALE-BOPP

V.G. Kruchinenko*) & K.I. Churyumov: Size and physical properties of 
the comet Hale-Bopp (C/1995 O1)nucleus. EARTH MOON AND PLANETS, 1999, 
Vol.77, No.3, pp.141-146

*) KYIV NATL UNIV, ASTRON OBSERV, STR OBSERV 3, UA-254053 KIEV 
   53,UKRAINE

It is possible to estimate the real size of the nucleus from the 
equation of the energy balance, using some parameters obtained through 
space experiments tied with the exploration of comet Halley. We have 
also taken the albedo value 0.04 (for that part of the nucleus surface 
which is covered with mineral crust and does not sublimate). 
Representing the nucleus of comet Halley as a triaxial ellipsoid a.b.c 
= 15.8.7.5 km we can calculate the parameter of its shape B = RequSF/V 
= 3.20, where R-equ = (a.b.c)(1/3), S-F - the total surface of the 
ellipsoid, V - its volume. Parameter B was also used for comet 
Hale-Bopp's nucleus. The equation of the energy balance for comet Hale-
Bopp's nucleus is analyzed for the moment when the comet passed 
perihelion and the sublimation rate (for water) was 10(31) molecules . 
s(-1) approximate to 3.10(8)g.s(-1). The energy balance equation 
contains the following components: energy coming from the Sun and 
absorbed by the comet nucleus, sublimation energy and energy of heat 
radiation of that part (1 - gamma) of the nucleus surface which does 
not sublimate and iscovered with mineral crust; the mean temperature of 
the nucleus surface (of the mineral crust) at perihelion according to 
our calculations is 330 K. As a result the dependence of the value 
R(e)qu on gamma, the fraction of the nucleus surface which is a source 
of sublimation, was obtained. The minimal value of the nuclear radius 
R-equ (for gamma = 1.0, i.e., the total surface sublimation) equals 
14.6 km; for gamma = 0.5 the value R-equ = 20.7 km. For gamma = 0.1 
(comet Halley's nucleus had approximately this value of gamma) R(e)qu = 
46.3 km.; the thickness of the mineral crust equals approximate to 1 cm 
for the heat conductivity coefficient lambda approximate to 2.10(4)
erg.cm(-1). s(-1).K-1. Copyright 1999, Institute for Scientific 
Information Inc.

===================
(5) NUCLEAR SIZE OF COMET HALE-BOPP

Z. Sekanina: A determination of the nuclear size of comet Hale-Bopp 
(C/1995 O1). EARTH MOON AND PLANETS, 1999, Vol.77, No.3, pp.147-153

CALTECH,JET PROP LAB,4800 OAK GROVE DR,PASADENA,CA,91125

The signal of the nucleus was digitally extracted from six images of 
the innermost coma of this comet, obtained with the Hubble Space 
Telescope's Wide-Field Planetary Camera 2 in the planetary mode between 
October 23, 1995 and October 17, 1996. Two different anisotropic, 
power-type laws were used to filter out the contribution from the dust 
coma: one peaking at the center of the elliptical surface brightness 
distribution (law A), the other peaking at its focus (law B). The 
nuclear R magnitudes in the Cousins system, reduced to a zero phase 
angle and to 1 AU from Earth and the Sun with a phase coefficient of 
0.035 mag/deg and an inverse square distance power law, are found to 
average 9.46 +/- 0.07 and 9.48 +/- 0.18 when law A and law B are 
applied, respectively. These results become 9.49 +/- 0.07 and 9.51 +/- 
0.17, when the nucleus signal on the October 1995 image is assumed to 
consist of a sum of the contributions from two unresolved nuclear 
components. In either scenario, no systematic variations are apparent 
in the nuclear brightness with time, which suggests the absence of any 
significant contamination of the extracted nuclear signal by the coma. 
Assuming a geometric albedo of 4 percent, the corresponding effective 
nuclear diameter amounts to 71 +/- 4 km (formal error). This result 
substantially exceeds the size estimates published by Weaver et al., 
which are based only on the October 1995 observation and which were 
obtained with the help of a different reduction method. Runs in which a 
power-type law fitting the contribution from the coma was assumed to 
hold all the way to a small fraction of a pixel from the nucleus led to 
distinctly inferior solutions and yielded spurious values much less 
than 70 km for the nuclear diameter. Copyright 1999, Institute for 
Scientific Information Inc.

============
(6) DETECTION OF A SATELLITE ORBITING THE NUCLEUS OF COMET 
    HALE-BOPP

Z. Sekanina: Detection of a satellite orbiting the nucleus of comet 
Hale-Bopp (C/1995 O1). EARTH MOON AND PLANETS, 1999, Vol.77, No.3, 
pp.155-163

CALTECH,JET PROP LAB,4800 OAK GROVE DR,PASADENA,CA,91125

This paper reports on the detection of a satellite around the principal 
nucleus of comet Hale-Bopp. As shown elsewhere, a successful 
morphological model for the comet's dust coma necessitates the 
postulation of overlapping jet activity from a comet pair. The 
satellite has been detected digitally on images taken with the Hubble 
Space Telescope's Wide Field Planetary Camera 2 in the planetary mode 
on five days in May-October 1996. An average satellite-to-primary 
signal ratio is 0.21 +/- 0.03, which implies that the satellite is 
similar to 30 km in diameter, assuming the main nucleus is similar to 
70 km across. To avoid collision, the separation distance must exceed 
50-60 km at all times. The satellite's projected distances on the 
images vary from 160 to 210 km, or 0.06 to 0.10 arcsec. The satellite 
was not detected in October 1995, presumably because of its subpixel 
separation from the primary. The radius of the gravitational sphere of 
action of the principal nucleus 70 km in diameter is 370-540 km at 
perihelion, increasing linearly with the Sun's distance: the satellite 
appears to be in a fairly stable orbit. Its orbital period at similar 
to 180 km is expected to be similar to 2-3 days, much shorter than the 
intervals between the HST observations. If the main nucleus should be 
no more than 42 km across, Weaver et al.'s upper limit, the satellite's 
orbit could become unstable, with the object drifting away from the 
main nucleus after perihelion. Potentially relevant ground-based 
detections of close companions are reported. Efforts to determine the 
satellite's orbit and the total mass of the system will get under way 
in the near future. Copyright 1999, Institute for Scientific 
Information Inc.

===========
(7) CHEMICAL ABUNDANCE IN COMETS

W.F. Huebner*) & J. Benkhoff: On the relationship of chemical 
abundances in the nucleus to those in the coma. EARTH MOON AND PLANETS, 
1999, Vol.77, No.3, pp.217-222

*) SW RES INST,PO DRAWER 28510,SAN ANTONIO,TX,78228

One of the goals of comet research is the determination of the chemical 
composition of the nucleus because it provides us with the clues about 
the composition of the nebula in which comet nuclei formed. It is well 
accepted that photo-chemical reactions must be considered to establish 
the abundances of mother molecules in the coma as they are released 
from the comet nucleus or from distributed dust sources in the coma. 
However, the mixing ratios of mother molecules in the coma changes with 
heliocentric distance. To obtain the abundances in the nucleus relative 
to those in the coma, we must turn our attention to the release rates 
of mother molecules from the nucleus as a function of heliocentric 
distance. For this purpose, we assume three sources for the coma gas: 
the surface of the nucleus (releasing mostly water vapor), the dust in 
the coma (the distributed source of several species released from dust 
particles), and the interior of the porous nucleus (the source of many 
species more volatile than water). The species diffusing from the 
interior of the nucleus are released by heat transported into the 
interior. Thus, the ratio of volatiles relative to water in the coma is 
a function of the heliocentric distance and provides important 
information about the chemical composition and structure of the 
nucleus. Our goal is to determine the abundance ratios of various 
mother molecules relative to water from many remote-sensing 
observations of the coma as a function of heliocentric distance. Comet 
Hale-Bopp is ideal for this purpose since it has been observed using 
instruments in many different wavelength regions over large ranges of 
heliocentric distances. The ratios of release rates of species into the 
coma are than modeled assuming various chemical compositions of the 
spinning nucleus as it moves from large heliocentric distance through 
perihelion. Since the heat flow into the nucleus will be different 
after perihelion from that before perihelion, we can also expect 
different gas release rates after perihelion compared to those observed 
before perihelion. Since not all the data are available yet, we report 
on progress of these calculations. Copyright 1999, Institute for 
Scientific Information Inc.

===============
(8) GAS AND DUST RELEASE FROM COMET HALE-BOPP

D. Prialnik: Modelling gas and dust release from comet Hale-Bopp. EARTH 
MOON AND PLANETS, 1999, Vol.77, No.3, pp.223-230

TEL AVIV UNIV,DEPT GEOPHYS & PLANETARY SCI,IL-69978 RAMAT AVIV,ISRAEL

Numerical simulations of the evolving activity of comet Hale-Bopp are 
presented, assuming a porous, spherical nucleus, 20 km in radius, made 
of dust and gas-laden amorphous ice. The main effects included are: 
crystallization of amorphous ice and release of occluded gas, 
condensation, sublimation and flow of gases through the pores, changing 
pore sizes, and flow of dust grains. The model parameters, such as 
initial pore size and porosity, emissivity, dust grain size, are varied 
in order to match the observed activity. In all cases, a sharp rise in 
the activity of the nucleus occurs at a large heliocentric distance 
pre-perihelion, marked by a few orders of magnitude increase in the CO 
and the CO2 fluxes and in the rate of dust emission. This is due to the 
onset of crystallization, advancing down to a few meters below the 
surface, accompanied by release of the trapped gases. A period of 
sustained, but variable, activity ensues. The emission of water 
molecules is found to surpass that of CO at a heliocentric distance of 
3 AU. Thereafter the activity is largely determined by the behaviour of 
the dust. If a dust mantle is allowed to build up, the water production 
rate does not increase dramatically towards perihelion; if most of the 
dust is ejected, the surface activity increases rapidly, producing a 
very bright comet. Copyright 1999, Institute for Scientific Information 
Inc.

============
(9) IS COMET HALE-BOPP A JUVENILE COMET?

J. Benkhoff*) & H. Rauer: Is Hale-Bopp a 'juvenile' comet? Study of the 
energy balance to explain the vapor flux of volatiles from the surface. 
EARTH MOON AND PLANETS, 1999, Vol.77, No.3, pp.231-236

*) DLR,INST PLANETENERKUNDUNG,RUDOWER CHAUSSEE 5,D-12484 BERLIN,GERMANY

Sublimation of minor gases from ices inside of a porous comet nucleus 
strongly depends on the effective energy input. Our model meant to 
describe the gas flux inside and out of the porous nucleus has been 
used to study the influence of physical and structural parameters on 
the effective energy input. We solve the conservation equations for H2O 
and CO as the most abundant minor component of higher volatility under 
appropriate boundary conditions. From the calculations we obtain the 
gas flux from volatile, icy components inside the porous nucleus, 
temperature profiles, changes in relative chemical abundances, and the 
gas flux into the coma for each of the volatiles. We will show results 
from our calculations for a model comet in the orbit of Hale-Bopp 
(C/1995 O1). In this paper we focus on the energy balance at the 
surface. We will also relate measurements of molecule fluxes to 
available energies and try to provide hints about the evolutionary 
status of the comet. Copyright 1999, Institute for Scientific 
Information Inc.

============
(10) MONTE CARLO SIMULATIONS OF OORT CLOUD COMETS

P. Nurmi*), M.J. Valtonen, J.Q. Zheng, S. Mikkola, H. Rickman: Monte 
Carlo simulations of oort cloud comets: The influence of mantle 
blow-off on the inclination distribution of Jupiter family. EARTH MOON 
AND PLANETS, 1999, Vol.77, No.3, pp.239-244

*) UNIV TURKU,TUORLA OBSERV,PIIKKIO 21500,FINLAND

We have developed an efficient Monte Carlo method by which we can 
evaluate the evolution of comets. There are many poorly known 
evolutional parameters, and we have investigated the influence of these 
parameters on the final populations and the inclination distributions 
of short-period comets. We compare the observed and calculated 
inclination distributions of different comet populations and obtain a 
good fit for the inclinations of the Jupiter family comets by assuming 
a mantle blow-off and a sudden brightening of the comet when its 
perihelion distance is lowered in a major jump. Copyright 1999, 
Institute for Scientific Information Inc.

- ----------------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK (CCNet)
- ----------------------------------------
The CCNet is a scholarly electronic network. To subscribe/unsubscribe, 
please contact the moderator Benny J Peiser <b.j.peiser@livjm.acdot uk>. 
Information circulated on this network is for scholarly and 
educational use only. The attached information may not be copied or 
reproduced for any other purposes without prior permission of the 
copyright holders. The fully indexed archive of the CCNet, from 
February 1997 on, can be found at http://abob.libs.ugadot edu/bobk/cccmenu.html 




------- End of Forwarded Message



To UNSUBSCRIBE from the 'meteorobs' email list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html