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Cambridge Conference Digest - February 9, 1998

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

(1) NEW PUBLICATION ON LEGAL ISSUES IN NEO DEFENSE
    Michael Gerrard                             

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

(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                                                                               

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 

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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