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(meteorobs) Excerpts from "CCNet DIGEST 14 Oct 98"


A very interesting issue, with several items of interest to
us amateur theoretical (and observational) meteoricists!

Lew

------- Forwarded Message

From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST 14/10/98
Date: Wed, 14 Oct 1998 11:25:07 -0400 (EDT)

CCNet DIGEST, 14 October 1998
-----------------------------
...
(3) CLOSE TO RENDEZVOUS: SATELLITE NEARING ASTEROID ORBIT
    ABCNews Online
  http://www.abcnews.com/sections/science/DailyNews/asteroid981012.html
...
(5) COLORADO RESEARCHERS DISCOVER MISSING LINK IN PLANET FORMATION
    Andrew Yee <ayee@nova.astro.utorontodot ca>=20
...
(7) NONGRAVITATIONAL MOTION OF COMET 32P/COMAS SOLA
    M. Krolikowska et al., POLISH ACADEMY OF SCIENCE
...
(9) SATELLITE ORBITS AROUND A COMETARY NUCLEUS
    D.J. Scheeres et al., UNIVERSITY OF PADUA

(10) LUNAR ICE: COMETARY ORIGIN?
     A.A. Berezhnoi & B.A. Klumov, SHTERNBERG STATER ASTRON INST

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(3) CLOSE TO RENDEZVOUS: SATELLITE NEARING ASTEROID ORBIT

>From ABCNews Online
http://www.abcnews.com/sections/science/DailyNews/asteroid981012.html
By Paul Recer
The Associated Press

M A D I S O N,   Wis.,   Oct. 12 =97 A small satellite launched more than
two years ago is just weeks from becoming the first manmade object to
be placed into an orbit of a distant asteroid.

The Near Earth Asteroid Rendezvous, or NEAR, spacecrafts will fire
rockets on Dec. 20 to settle into an orbit of an asteroid called Eros
that is streaking across space some 163 million miles from Earth.

Andy Cheng, a Johns Hopkins University researcher and the project
scientist for NEAR, said today that all instruments on the spacecraft
are working well and that the small satellite is on target for its deep
space meeting with Eros.

The rocket firing comes while NEAR is still distant from Eros, and the
satellite will not start orbiting the asteroid until Jan. 10, 1999.

Up-Close With an Asteroid=20
=20
During months of circling the asteroid, the craft will be slowly=20
lowered until its orbit is just 21 miles above the space rock, said=20
Cheng. The craft will give the first prolonged, up-close look at an=20
asteroid, which are minor planets, in space.=20
=20
Six instruments aboard NEAR will analyze the composition, magnetic=20
field and mass of the asteroid, sending the data back to Earth by=20
radio.=20
=20
Eros is something of a mystery. The rock is about the size of a=20
mountain, 24 miles by 8 miles. It is a near-Earth asteroid, one whose=20
orbit routinely carries it close to the orbit of Earth.
=20
Scientists don=92t know if the rock is solid or if it is a highly =
porous=20
body with empty cavities or chunks of ice.=20
=20
But it similar to the asteroids that pounded Earth early in the=20
planet=92s history. One such asteroid smashed into Earth 65 million =
years=20
ago and is thought to have caused the extinction of the dinosaurs.=20
=20
Eros=92 Mysterious Origin=20
=20
Like most asteroids, said Cheng, Eros probably formed early in the=20
history of the solar system. What isn=92t known is whether Eros was =
once=20
part of a bigger planet or if it formed independently.=20
=20
=93There were many collisions in the early solar system,=94 said Cheng. =

=20
Sometimes huge chunks of a planet are knocked into space by such=20
celestial smashups and became independent bodies. The Earth=92s moon is =

thought to come such a collision.
=20
Asteroids could also form from the leftover debris of planet-building.=20
Bits of stray rock and dust slowly clump together until they form one=20
object.=20
=20
Concern Over Space Rocks=20
=20
The process may not be finished, said Cheng. He said there is some=20
concern that wandering space rocks or gravel that imperil the NEAR.=20
=20
As NEAR approaches Eros, researchers will use cameras and instruments=20
on board to search for any stray bits of rock or gravel that are still=20
orbiting the asteroid.=20
=20
After months in orbit of Eros, researchers may attempt to put the craft =

onto the surface of the asteroid. Cheng said the spacecraft was not=20
designed to land, but that is one option the researchers are=20
considering. The density of Eros is unknown, but the asteroid is so=20
small that its gravity force will be only a fraction of Earth=92s, =
making=20
landing there less violent.=20
=20
But it would still be tricky steering the craft to a touchdown because=20
Eros is so far away that there is a long delay between sending a signal =

and getting a response. Cheng said it takes about 45 minutes for a=20
radio signal to make a round trip between Earth and NEAR.=20
=20
Copyright 1998 Associated Press

------------------------------------------------------
(5) COLORADO RESEARCHERS DISCOVER MISSING LINK IN PLANET FORMATION

>From Andrew Yee <ayee@nova.astro.utorontodot ca>=20

Office of Public Relations
University of Colorado-Boulder
354 Willard Administrative Center
Campus Box 9
Boulder, Colorado 80309-0009
(303) 492-6431
=20
Contact:
Henry Throop, 303-492-1628
Larry Esposito, 303-492-7325
Jim Scott, 303-492-3114
=20
Oct. 12, 1998
=20
COLORADO RESEARCHERS DISCOVER MISSING LINK IN PLANET FORMATION
=20
Three University of Colorado at Boulder researchers have observed the=20
growth of dust particles in discs around three nearby stars, a=20
phenomenon that appears to be a missing link between space dust and=20
planet formation.
=20
Doctoral candidate Henry Throop of the department of atmospheric and=20
planetary sciences said some of the dust particles surrounding one=20
"birthing star" some 1,500 light years away in the Orion Nebula are=20
about 100 times larger than particles generally found in the=20
interstellar medium. The particles apparently are growing by accreting=20
together, somewhat like a snowball that picks up flakes and grows as it
is rolled along the ground.
=20
"This is the first time scientists have been directly able to see the=20
accretion process in the formation of planets," said Throop, who is=20
affiliated with CU-Boulder's Laboratory for Atmospheric and Space=20
Physics. "These particles are much, much bigger than they started out=20
to be."
=20
Images of one of the discs were taken by John Bally, director of=20
CU-Boulder's Center for Astrophysics and Space Astronomy, with the=20
Hubble Space Telescope in 1995.
=20
The researchers used Hubble to look at the discs by using different=20
wavelengths of light, which bounce off the dust particles in different=20
ways, said Throop. The way the light scatters allows the researchers to =

estimate the particle sizes.
=20
"It's simlar to how a rainbow works," sad Throop. "The rain drops=20
themselves aren't dazzling colors, but we can tell some things about=20
the drops based on what the rainbow looks like. Likewise, by looking at =

the colors scattered off these discs, we can tell something about the=20
dust orbiting there."
=20
A paper on the subject primarily authored by Throop was presented at=20
the 30th annual Division for Planetary Sciences meeting in Madison,=20
Wis., Oct. 11 to Oct 16. Co-authors include Bally, CU-Boulder Professor =

Larry Esposito of LASP and Mark McCaughrean of the Astrophysical=20
Institute in Potsdam, Germany.
=20
When a star forms, swarms of gas and dust contract into a titanic ball, =

causing the star to "turn on," said Throop. The dust ring left circling =

the birthing star resembles a large doughnut.
=20
The interstellar dust particles drawn into the ring surrounding the=20
newborn star initially are only about one-tenth of a micron across,=20
Throop said. The Hubble images showed some of the particles had grown=20
100 times larger -- to 10 microns, or about one-tenth the width of a=20
human hair -- as the accretion process occurred.
=20
Located in the Milky Way, the Orion Nebula is one of the stellar=20
nurseries closest to Earth. The three stars under observation are each=20
about one million years old, Throop said.
=20
"This is the first direct confirmation that dust particles surrounding=20
young stars are beginning to grow into planets," said Esposito.
=20
"Another nifty thing going on here is that we see discs that appear to=20
be similar to the three we are studying around about one-third of the=20
stars in the Orion Nebula," said Throop. "If these similar discs are=20
forming larger particles, it suggests planet formation -- and thus=20
solar systems like ours -- may be relatively common."
=20
The Division for Planetary Science is part of the American Astronomical =

Society. Images of two of the discs are available on the website:
http://bogart.coloradodot edu/~throop/images.html

--------------------------------------------------------
(7) NONGRAVITATIONAL MOTION OF COMET 32P/COMAS SOLA

M. Krolikowska*), G. Sitarski, S. Szutowicz: Model of the=20
nongravitational motion for comet 32P/Comas Sola. ASTRONOMY AND=20
ASTROPHYSICS, 1998, Vol.335, No.2, pp.757-764

*) POLISH ACADEMY OF SCIENCE, SPACE RES CTR, BARTYCKA 18A, PL-00716=20
   WARSAW, POLAND

The nongravitational motion of the periodic comet Comas Sola is studied =

on the basis of positional observations made during nine consecutive=20
revolutions around the Sun. Nongravitational effects in the comet=20
motion have been examined for Sekanina's forced precession model of the =

rotating nucleus. We present three models which successfully link all=20
the observed apparitions of the comet during 1926-1996. Two solutions=20
(Models II and III) represent oblate spheroids and the third one (Model =

I)- a prolate spheroid (nucleus rotation around its longer axis). We=20
have determined values of eight parameters: A, eta , I, phi connected=20
with the rotating comet nucleus, f(p) and s describing the precession=20
of spin-axis of the nucleus, and two constant time shifts tau(1) and=20
tau(2) The last two parameters describe displacements of the maximum=20
value of the known function g(r) with respect to the perihelion time.=20
The best solution was obtained assuming that between the apparitions of =

1935 and of 1944 the time shift changed its value, thus tau(1) and=20
tau(2) refer to apparitions before and after 1940 Jan. 1, respectively. =

Variations of angles I and phi with time, describing the nucleus=20
spin-axis orientation, are presented. It appears that forced precession =

causes the moderate changes of the position of the rotation axis in=20
space. The ratio of rotational period to radius of the nucleus was=20
found for each model. The present precession models are in agreement=20
with sizes and periods of rotation of other cometary nuclei deduced=20
from observations. The obtained models give some strong constraints on=20
the physical parameters of the nucleus of comet P/Comas Sola. Assuming=20
a prolate spheroid for the nucleus of the comet, the expected=20
rotational period is 14 +/- 4 hours for an equatorial radius of 2 km.=20
For the same radius, the oblate Model II gives the much smaller=20
rotational period of 2.4 +/- 0.4 hours. The polar radii are 2.2 km and=20
1.3 km for the prolate and oblate model, respectively.
Copyright 1998, Institute for Scientific Information Inc.

--------------------------------------------------------
(9) SATELLITE ORBITS AROUND A COMETARY NUCLEUS

D.J. Scheeres*), F. Marzari, L. Tomasella, V. Vanzani: ROSETTA mission: =

satellite orbits around a cometary nucleus. PLANETARY AND SPACE=20
SCIENCE, 1998, Vol.46, No.6-7, pp.649-671

*) UNIVERSITY OF PADUA,DIPARTIMENTO FIS,VIA MARZOLO 8,I-35131=20
   PADUA,ITALY

This paper discusses the problem of orbiting a comet nucleus  from a=20
perspective of orbital stability. The main forces perturbing the motion =

of the spacecraft around the comet : shape and rotation rate of the=20
nucleus, comet outgassing, solar radiation pressure; are derived and=20
quantified for the nominal case of the ROSETTA spacecraft at the comet=20
Wirtanen. Their effects on the stability of the spacecraft orbit are=20
analyzed in detail and orbital stability criteria are developed=20
analytically. These criteria have been tested numerically, for select=20
cases of interest, integrating the spacecraft orbit about a Wirtanen=20
model constructed from mascons (mass concentrations at a point). This=20
numerical model allows very irregular nucleus shapes to be modelled=20
accurately if a large number of point masses is used. The stability=20
criteria derived in this paper denote stability of the spacecraft=20
against crashing onto the comet surface or escaping from the comet on a =

hyperbolic orbit. They are developed and applied only over the=20
relatively short time scales that are of interest to a spacecraft=20
mission. The stability criteria are expressed in terms of minimum=20
periapsis radii for stability against the non-spherical gravitational=20
field, in terms of maximum semi-major axis for stability against escape =

due to the solar radiation pressure, and in terms of preferred planes=20
and orbit elements of a spacecraft orbit for stability against the=20
combined non-gravitational forces of comet outgassing and solar=20
radiation pressure. For orbits dose to an irregular body the gravity=20
perturbations are minimized, and orbital stability achieved, if the=20
periapsis radius is above five mean comet radii if the inclination is=20
close to 0 degrees and above three mean comet radii if inclinations are =

between 90 and 180 degrees with respect to the comet rotation pole.=20
When considering the combined solar radiation pressure and comet=20
outgassing forces, stable or bits can be found which ''freeze'' the=20
orbit geometry with respect to the rotating reference frame defined=20
along the comet-sun line. The dependence of these stability criteria on =

the comet model parameters is discussed. The analysis in this paper is=20
general enough to be applicable to a wide range of orbital cases,=20
including spacecraft orbits about asteroids and natural satellites=20
about comets and asteroids. (C) 1998 Published by Elsevier Science Ltd. =

All rights reserved.

-------------------------------------------------------
(10) LUNAR ICE: COMETARY ORIGIN?

A.A. Berezhnoi*) & B.A. Klumov: Lunar ice: Can its origin be=20
determined? JETP LETTERS, 1998, Vol.68, No.2, pp.163-167

*) P K SHTERNBERG STATER ASTRON INST,MOSCOW 119890,RUSSIA

The comet hypothesis of the origin of lunar ice, which was recently=20
discovered in the polar regions of the moon by Lunar Prospector, is=20
examined. It is shown that a comet impact produces a temporary=20
atmosphere whose volatile component accumulates essentially completely=20
in cold traps - the permanently shadowed regions of the Moon. The=20
condensation of volatile compounds leads to the formation of ice with a =

definite chemical and isotopic composition, which contains important=20
information about the composition of the comet. This hypothesis can be=20
checked during the next lunar missions, and if confirmed, definite=20
progress will have been made in understanding the nature of comets. (C) =

1998 American Institute of Physics.

-------------------------------------------------------

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