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(meteorobs) Excerpts from "CCNet 74/2001 - 1 June 2001"
More fascinating stuff on the subject of "asteroids - or are they?" :)
Lew Gramer
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From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 74/2001 - 1 June 2001
Date: Fri, 1 Jun 2001 10:43:41 +0100
CCNet 74/2001 - 1 June 2001
----------------------------
"Eros was the subject of detailed study for over a year by the Near
Earth Asteroid Rendezvous (NEAR) spacecraft. The magnetometer on NEAR
failed to measure any sign of a global magnetic field for the
asteroid down to the limits of the instrument's sensitivity, 1-2
nanotesla. By comparison, the Earth's magnetic field is about 50,000
nanoteslas. "Eros is a remarkably unmagnetized body," said Mario
Acuna, team leader of the magnetic field experiment on NEAR. "It is
essentially a non-magnetic object." The lack of a magnetic field was
puzzling because spacecraft flybys of other asteroids, such as Gaspra
and Braille, had detected magnetic fields. In addition, most meteorites,
including those of the class most closely associated with Eros, are
also magnetic."
--Jeff Foust, Spaceflight Now, 31 May 2001
(1) WHY DOESN'T EROS HAVE A MAGNETIC FIELD?
Ron Baalke <baalke@jpl.nasadot gov>
(2) MANY ASTEROIDS TRAVEL IN PAIRS
Space.com, 31 May 2001
[...]
(5) THE ASTEROID IDENTIFICATION PROBLEM IV
Andrea Milani et al.
(6) SPECTRAL PROPERTIES OF NEAR-EARTH OBJECTS
Richard P. Binzel et al.
[...]
========================================================================
(1) WHY DOESN'T EROS HAVE A MAGNETIC FIELD?
>From Ron Baalke <baalke@jpl.nasadot gov>
>From Spaceflight Now, 31 May 2001
http://www.spaceflightnow.com/news/n0105/31eros/
Why doesn't Eros have a magnetic field?
By Jeff Foust
BOSTON, Mass. - Scientists said this week that they may know the reason =
why
the asteroid Eros appears to lack a measurable magnetic field.=20
Two researchers told attendees of the American Geophysical Union's =
spring
meeting in Boston that Eros may contain a large number of tiny,
individually-magnetized bodies whose magnetic fields are randomly =
oriented,
effectively canceling each other out.=20
Eros was the subject of detailed study for over a year by the Near =
Earth
Asteroid Rendezvous (NEAR) spacecraft. The magnetometer on NEAR failed =
to
measure any sign of a global magnetic field for the asteroid down to =
the
limits of the instrument's sensitivity, 1-2 nanotesla. By comparison, =
the
Earth's magnetic field is about 50,000 nanoteslas.=20
"Eros is a remarkably unmagnetized body," said Mario Acuna, team leader =
of
the magnetic field experiment on NEAR. "It is essentially a =
non-magnetic
object."=20
The lack of a magnetic field was puzzling because spacecraft flybys of =
other
asteroids, such as Gaspra and Braille, had detected magnetic fields. In
addition, most meteorites, including those of the class most closely
associated with Eros, are also magnetic.=20
One possible explanation for the meteorites is that they have been
"contaminated" by the Earth's magnetic field, becoming magnetized only =
after
arriving at Earth. To test this, Gunther Kletetschka, a researcher with =
the
Catholic University of America and NASA's Goddard Space Flight Center,
exposed chondrules -- tiny droplets of primitive material embedded in =
the
meteorite -- to changes in temperature and magnetic fields similar to =
what a
meteorite might experience when arriving on Earth.=20
In about half of the cases Kletetschka found that the chondrules took =
on the
magnetization they were exposed to in the experiment. However, in the =
other
half of the cases there was no change in the chondrule's magnetism,
indicating that it has a strong remnant magnetism that likely existed =
since
the chondrule was formed billions of years ago, during the formation of =
the
solar system itself.=20
Since Eros is similar to the class of chondritic meteorites studied in =
the
lab, it likely contains a large number of magnetized chondrules. =
However,
Kletetschka argues that if the chondrules are randomly oriented, they =
will
effectively cancel each other's magnetic fields out, preventing a =
global
magnetic field from forming. This is seen to a limited degree in the =
lab,
where larger samples of chondritic meteorites appear to have weaker
magnetization than smaller ones as the randomizing effect begins to =
take
hold.=20
If true, this implies that Eros has not been modified since the =
formation of
the solar system in any way that would alter the magnetization of the
chondrules. "Eros is a very primitive object," said Acuna.=20
To confirm this, however, would require obtaining samples from the =
asteroid
itself, which is not likely to happen for the foreseeable future. Also
required, say scientists, are studies of other asteroids, including
follow-up studies of Gaspra and Braille to confirm that they have the
magnetic fields detected in previous flybys. =20
=A9 2001 Pole Star Publications Ltd
========================================================================
(2) MANY ASTEROIDS TRAVEL IN PAIRS=20
>From Space.com, 31 May 2001
http://www.space.com/scienceastronomy/solarsystem/asteroid_pairs_010531_=
wg.h
tml
By Andrew Bridges
AP Science Writer
LOS ANGELES (AP) -- Astronomers are discovering a bumper crop of binary
asteroids -- space rocks locked in an orbital dance with a partner.=20
The latest discovery was announced Wednesday, when radar images showed =
that
asteroid 1999 KW4 is actually two objects separated by about a mile,
something that had been suspected for the past year.=20
Radar images show a small moon just one-quarter of a mile across =
whipping
clockwise around a companion three times as large.=20
"Some day, people will go to a binary asteroid and what an interesting =
sky
they will see," said Steven Ostro of the National Aeronautics and Space
Administration's Jet Propulsion Laboratory.=20
The discovery boosts to roughly 10 the number of binary asteroids =
imaged by
radar since the spacecraft Galileo spotted the first, 243 Ida and its =
tiny
moon Dactyl. Another seven suspected pairs haven't been confirmed.=20
While the tally is still small, it is certain to grow as astronomers =
refine
the techniques used to spy the miniature planetary systems.=20
Observations of paired craters on the Earth and other bodies led =
astronomers
to suspect that binary asteroids existed.=20
On Earth, the craters -- all of equal age -- are too large and too far =
apart
to have been formed by a single asteroid breaking up in the atmosphere. =
The
odds of two asteroids hitting the Earth in the same location and at the =
same
time are slim -- unless they were paired before impact. But the first =
binary
asteroid was not seen until 1993, when Galileo spotted Ida and Dactyl =
while
en route to Jupiter.=20
Not all asteroid moons orbit asteroids. The two moons of Mars, Phobos =
and
Deimos, are probably asteroids captured in orbit by the planet's
gravitational tug.=20
Czech astronomer Petr Pravec said the study of near-Earth asteroids is
becoming more important -- especially if scientists are going to =
entertain
ways to defend the planet from potential asteroid impacts.=20
"If some of them are on a collision course with the Earth in the =
future, it
will be more difficult to divert them than if they were a single =
asteroid,"
Pravec said.=20
The asteroid pairs found so far share little more than diversity.=20
Pairs like 90 Antiope are nearly twins, each 50 miles or so across. =
Some,
like 2000 DP107, are also of about equal size, but just hundreds of =
feet in
diameter. Others are far more lopsided, like the case of 87 Sylvia, =
which at
176 miles across dwarfs its moon, just 5 percent as large.=20
Collisions may have formed many of the binary asteroids, meaning each =
little
moon is, literally, a chip off the old block. In other cases, passing =
close
to Earth may have pulled off material, dumping it into a mini-orbit.=20
In the case of 1999 KW4, the objects may be the remnants of an extinct
comet. Orbital observations will allow astronomers to determine the =
mass,
density, composition and porosity of each member of the pair.=20
"That tells us an awful lot about these things without having to go =
there,"
said Bill Merline, a senior research scientist at the Southwest =
Research
Institute in Boulder, Colo., who has discovered three binary asteroids. =
Copyright 2001, AP
========================================================================
* ABSTACTS *
========================================================================
(5) THE ASTEROID IDENTIFICATION PROBLEM IV
Andrea Milani*, Maria Eugenia Sansaturio, Steven R. Chesley: The =
Asteroid
Identification Problem IV: Attributions. Icarus, Vol. 151, No. 2, June =
1,
2001, pp. 150-159 =20
Existing archives of asteroid observations contain many objects with very
short observed arcs. In this paper we present a method that we have used
with considerable success to attribute these short arc "discoveries" to
other objects with better defined orbits. The method consists of a
three-stage filtering process whereby several billion possible
attribution/orbit pairs are systematically analyzed with more and more exact
algorithms, at each stage rejecting improbable cases. The first stage
compares an attributable, by definition a synthetic observation
representative of all the observations over a short arc, with the predicted
observation for each available orbit. The second stage compares the proposed
attributable observations with predicted positions from the known orbit
using conventional linear covariance techniques, considering both the
position and motion on the celestial sphere. In the final filter we =
attempt to compute a best-fitting orbit by differential corrections using the
combined dataset. With this algorithm we have found 1675 attributions =
in approximately one year of operations, in addition to 902 identifications
found with another algorithm. We discuss the lessons learned from this
one-year experiment and the possibilities of further improvement and
automation of the procedure. Copyright 2001 Academic Press.=20
*Dipartimento di Matematica, Universit=E0 di Pisa, Via Buonarroti 2, =
Pisa,
56127, Italy, milani@dm.unipidot it=20
=A9 Harcourt, Inc.
========================================================================
(6) SPECTRAL PROPERTIES OF NEAR-EARTH OBJECTS
Richard P. Binzel*, Alan W. Harris, Schelte J. Bus, Thomas H. =
Burbine:
Spectral Properties of Near-Earth Objects: Palomar and IRTF Results for =
48
Objects Including Spacecraft Targets (9969) Braille and (10302) 1989 =
ML.
Icarus, Vol. 151, No. 2, June 1, 2001, pp. 139-149
We present results of visible wavelength spectroscopic measurements for =
48
near-Earth objects (NEOs) obtained with the 5-m telescope at Palomar
Mountain Observatory during 1998, 1999, and early 2000. The =
compositional
interpretations for 15 of these objects have been enhanced by the =
addition
of near-infrared spectra obtained with the NASA Infrared Telescope =
Facility.
One-third of our sampled objects fall in the Sq and Q classes and =
resemble
ordinary chondrite meteorites. Overall our sample shows a clear =
transition
between S-type and Q-type compositional classes over visible and
near-infrared wavelengths. Taken together these results point toward an
abundance of near-Earth asteroids capable of providing sources for =
ordinary
chondrite meteorites. Our sampling strategy favors targeting the =
smallest
observable objects and we report results for the 15-m diameter object =
1998
BT13, the smallest spectroscopically measured NEO to date. NEOs show a
greater spectral diversity than main-belt asteroids, and our small =
sample
includes objects falling in the rare categories of K, L, O, and V =
classes.
The K-class object 1999 JD6 is found to match CV chondrite meteorites.
Potential spacecraft targets received top priority for observation, =
with the
ordinary chondrite-like composition of (9969) Braille being reported =
prior
to the Deep Space-1 encounter. The relatively accessible asteroid =
(10302)
1989 ML displays a neutral spectrum that may be interpreted as a
shock-darkened ordinary chondrite. Copyright 2001 Academic Press.=20
*Department of Earth, Atmospheric, and Planetary Sciences, =
Massachusetts
Institute of Technology, Cambridge, Massachusetts, 02139, rpb@mitdot edu=20
=A9 Harcourt, Inc.
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