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Date sent:        Mon, 09 Feb 1998 09:59:15 -0500 (EST)
From:             Benny J Peiser B.J.PEISER@livjm.ac.uk
Subject:          CC DIGEST, 09/02/98
To:               cambridge-conference@livjm.ac.uk
Priority:         NORMAL

CAMBRIDGE-CONFERENCE DIGEST, 9 February 1998
--------------------------------------------

(1) NEW PUBLICATION ON LEGAL ISSUES IN NEO DEFENSE
    Michael Gerrard gerrami@aporter.com

(2) NEW MASSES AND DENSITIES ARE COMPUTED FOR THE THREE
    LARGEST ASTEROIDS
    Ron Baalke BAALKE@kelvin.jpl.nasa.gov

(3) THE TEXTURES & COMPOSITIONS OF ANTARCTIC MICROMETEORITES
    M.J. Genge et al., Natural History Museum, London

===========================
(1) NEW PUBLICATION ON LEGAL ISSUES IN NEO DEFENSE

From: Michael Gerrard gerrami@aporter.com
 

LEGAL ISSUES IN NEO DEFENSE

"Asteroids and Comets: U.S. and International Law and the
Lowest-Probability, Highest Consequence Risk" is the title of an
article that has just appeared in the New York University
Environmental Law Journal (6:1). It discusses legal issues in NEO
detection and defense, including those arising under U.S. domestic
law (particularly the environmental impact statement requirement of
the National Environmental Policy Act) and under international law
(especially the Outer Space Treaty, the Partial Test Ban Treaty, the
Anti-Ballistic Missile Treaty, and the Space Objects Liability
Convention). It also compares the risks posed by NEOs to those
regulated under U.S. environmental laws. The principal author is
Michael B. Gerrard, an environmental lawyer with Arnold & Porter in
New York and an adjunct professor of environmental law at Columbia
and Yale universities. Gerrard has offered to mail reprints of
the article to interested readers who e-mail him their
mailing address. His e-mail address is gerrami@aporter.com.

================================
(2) NEW MASSES AND DENSITIES ARE COMPUTED FOR THE THREE
    LARGEST ASTEROIDS

From: Ron Baalke BAALKE@kelvin.jpl.nasa.gov

Public Affairs Office
U.S. Naval Observatory
Washington, D.C.

Contact:
Dr. James L. Hilton
U.S. Naval Observatory
3450 Massachusetts Ave. NW
Washington, DC 20392
Phone: 202-762-1432   FAX 202-762-1612
E-mail: jhilton@usno.navy.mil

FOR RELEASE: 9:20 AM EST JANUARY 9, 1998

NEW MASSES AND DENSITIES ARE COMPUTED FOR THE THREE LARGEST ASTEROIDS

Dr. James L. Hilton, an astronomer at the U.S. Naval Observatory,
announced today at the American Astronomical Society meeting that he
has computed more accurate masses and densities for the three largest
asteroids. More accurate masses of the asteroids are important for
improving our knowledge of the motions of the planets, the
compositions of the asteroids, and the histories of the asteroids,
including those that have the potential of colliding with the Earth.

The asteroids are small rocky bodies ranging from a few hundred feet
to a few hundred miles in diameter. Most asteroids are located in the
main asteroid belt between the orbits of Mars and Jupiter. Ceres,
Pallas, and Vesta are the three largest asteroids. Even though they
are much smaller than the planets (Ceres, the largest asteroid, has
only 1% the mass of the Moon), they are still capable of causing
changes in the orbits of Mars and the Earth. These changes are seen
in data returned by interplanetary satellites such as Viking, Mars
Pathfinder, and Mars Global Surveyor. The best available planetary
positions are calculated using perturbations from 300 asteroids;
however, the masses of only a hand full of asteroids are known with
any precision. The masses of the other asteroids are estimated by
comparing them with asteroids which do have known masses.

Computing the masses of the three largest asteroids, Ceres, Pallas,
and Vesta, was part of a larger project computing the motions of
fifteen of the largest asteroids. These computations will be used in
producing future editions of The Astronomical Almanac, a yearly
publication of the U.S. Naval Observatory and Her Majesty's Nautical
Almanac Office, used by astronomers worldwide.

The masses of Ceres, Pallas, and Vesta were determined by calculating
their positions to high accuracy and then comparing those positions
with tens of thousands of positions actually observed by astronomical
observatories worldwide. The gravitational forces of the asteroids on
each other affects the observed positions of the asteroids in their
orbits. The observed positions of the asteroids are compared with
positions calculated using a mathematical model of the solar system.
The masses are determined by finding the values that make the
differences between the observed positions and calculated positions
as small as possible. Observations covered the period from 1801,
shortly after Ceres was discovered, through 1996. Using observations
over such a long period of time allows the most accurate calculation
of both the positions of the asteroids and their masses. The masses
of all three asteroids were calculated simultaneously using a special
computer program known as the Planetary Ephemeris Program, or PEP for
short.

The mass for Ceres was found to be (8.7 =B1 0.1) x 10**20 kilograms
(9.5 x 10**17 tons), the mass of Pallas is (3.18 =B1 0.08) x 10**20
kilograms (3.50 x 10**17 tons), and the mass of Vesta is
(3.0 =B1 0.2) x 10**20 kilograms (3.3 x 10**17 tons). For comparison,
this means that Ceres has 1.18% of the mass of the Moon while Pallas
has 0.43% the Moon's mass and Vesta is 0.41% of the mass of the Moon.

The mass of Pallas is greater than previously thought, while the mass
of Ceres is somewhat smaller. The change in the mass calculated for
Ceres is a direct result of the change in the mass calculated for
Pallas. Confusion is caused by the fact that the two asteroids take
nearly the same time to go around the Sun and have been in the same
part of the sky ever since they were discovered. Since these two
asteroids always appear close to one another, separating the amount
of gravitational pull caused by each asteroid is difficult unless a
simultaneous solution for their masses is used as was done at the
Naval Observatory. Dr. Hilton said, "These results reduce the
uncertainty in the mass of Pallas by a factor of five, and show
that to get a good mass for Ceres you need to know the mass of
Pallas. However, calculating the mass of Pallas is tricky and it
would be nice to have some one else check this value." The mass
calculated for Vesta is in very good agreement with previous
calculations.

In addition to the masses, densities can be calculated for these
three asteroids because several groups of astronomers have determined
their radii within a few kilometers. The density of Ceres is 1.98 +/-
0.03 grams per cubic centimeter, the density of Pallas is 4.2 +/- 0.2
grams per cubic centimeter, and the density of Vesta is 3.9 +/- 0.3
grams per cubic centimeter. For comparison, water has a density of 1
gram per cubic centimeter and the average rock has a density of 3
grams per cubic centimeter. Although Pallas has long been considered
to be similar to Ceres, the new densities indicate that Pallas may,
in fact, be more like Vesta.
 

An html version of this press release can be found at

http://aa.usno.navy.mil/ephemerides/asteroid/masses/PressRelease.htm
There is also a scientific paper describing in detail the calculation
of orbits, masses, and densities of the asteroids at

http://aa.usno.navy.mil/ephemerides/asteroid/astr_alm/asteroid_ephemerides.html.

[Image: Orbits of the asteroids Ceres, Pallas, and Vesta,
http://aa.usno.navy.mil/ephemerides/asteroid/masses/Orbits.gif ]

===================
(3) THE TEXTURES & COMPOSITIONS OF ANTARCTIC MICROMETEORITES

M.J. Genge, M.M. Grady & R. Hutchison: The textures and compositions of
fine-grained Antarctic micrometeorites: Implications for comparisons with
meteorites. GEOCHIMICA ET COSMOCHIMICA ACTA, 1997, Vol.61, No.23,
pp.5149-5162

NATURAL HISTORY MUSEUM, DEPARTMENT OF MINERALOGY, LONDON SW7 5BD,
ENGLAND

Micrometeorites recovered from the Earth's surface constitute the most
abundant interplanetary dust now falling to Earth. We studied eighty-nine
fine grained Antarctic micrometeorites (fg-AMMs) to evaluate their state of
alteration and to identify the nature of their precursor materials.
Fine-grained AMMs are divided into melted and unmelted groups and subdivided
on the basis of textures related to atmospheric entry heating. The textures
of melted particles reflect crystallisation after entry heating and those of
unmelted AMMs broadly resemble CI, CM, and CR chondrite matrix. The matrix
compositions of the fg-AMMs closely resemble those of CM2 chondrites.
Matrices provide a more reliable comparison to meteorites than bulk
compositions because components such as chondrules and CAIs are not present
in representative proportions within individual AMMs. Divergences from
CM-like matrix compositions are generally minor and probably arose during
entry heating by the dissociation of volatile-bearing phases at subsolidus
temperatures and evaporation and loss of immiscible metallic liquids at
higher temperatures. Depletions in Ni and Mg relative to CM matrix are
tentatively attributed to terrestrial weathering. No conclusive evidence for
contamination during atmospheric residence was observed. Contrary to theory,
textural evidence suggests that bow shocks and high thermal gradients
existed during deceleration of micrometeoroids in the atmosphere. Copyright
(C) 1997 Elsevier Science Ltd.

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CCCMENU CCC for 1998

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