PLEASE NOTE:


*

CAMBRIDGE-CONFERENCE DIGEST, 23 April 1998
------------------------------------------

(1) US DEPARTMENT OF THE DEFENSE CONFIRM RECENT METEOROID IMPACTS
    George Zay <GeoZay@aol.com>

(2) LETTER TO THE WASHINGTON POST
    Tom Gehrels <tgehrels@LPL.Arizona.EDU>

(3) A NEW THEORY OF CHONDRULES FORMATION
    H.C. Connolly & S.G. Love, CALTECH

(4) THE ORBITAL AND ABSOLUTE MAGNITUDE DISTRIBUTION OF MAIN BELT
    ASTEROIDS
    R. Jedicke & T.S. Metcalfe, University of Arizona

(5) ASTEROID TRAILS IN HUBBLE SPACE TELESCOPE IMAGES
    R.W. Evans et al., CALTECH

(6) MAIN BELT ASTEROID COLLISION PROBABILITIES AND IMPACT VELOCITIES
    J.D. Vedder, PAREXIC RESEARCH

(7) A THERMAL MODEL FOR NEAR-EARTH ASTEROIDS
    A.W. Harris, DLR, BERLIN

(8) WHAT EVER HAPPENED TO FRAGMENT D OF COMET SHOEMAKER-LEVY 9?
    J.E. Little et al., Queens University Belfast

(9) LONG-TERM EVOLUTION OF COMET SL-9 FEATURES
    A. SanchezLavega et al., University Pais Vasco, Bilbao

==========================
(1) US DEPARTMENT OF THE DEFENSE CONFIRM RECENT METEOROID IMPACTS

From  George Zay <GeoZay@aol.com>

Here's some more info about last Decembers event over Greenland and the
Denver object in January from data aboard DoD surveillance satellites
detailed by Peter Brown.

George Zay

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

from <http://phobos.astro.uwo.ca/~pbrown/usaf.html>

The following press announcement was released on 22 April by the USAir
Force Technical Applications Centre (AFTAC) Office of Public Affairs.
The release relates to satellite observations of recent impact events
detected in the atmosphere by optical and infrared sensors aboard DoD
satellites.

DOD 98-001 (April 22, 1998)

Fireball Detection

On 9 December 1997, sensors aboard DOD satellites detected the impact
of a meteoroid at 08:15:55 UTC roughly midway between Nuuk and
Qaqortoq, Greenland. The object broke into at least 4 pieces. One
piece detonated at an altitude of about 46 km at 62.9 degrees North
Latitude, 50.9 degrees West Longitude. The remaining 3 pieces
detonated in close proximity to one another at altitudes between 28
km, at 62.9 degrees North Latitude, 50.1 degrees West Longitude and
25 km at 62.9 degrees North Latitude, 50.0 degrees West Longitude.

Fireball Detection

On 11 January 1998, sensors aboard DOD satellites detected the impact
of a meteoroid at 07:11:13 UTC roughly midway between Denver and Grand
Junction, Colorado. The object was detected at 39.4 degrees North
Latitude, 106.4 degrees West Longitude. 

PLEASE NOTE THAT THIS SATELLITE BOLIDE RELEASE AND ALL PREVIOUS SUCH
RELEASES CAN BE FOUND ON THE WWW AT
http://phobos.astro.uwo.ca/~pbrown/usaf.html

=====================
(2) LETTER TO THE WASHINGTON POST

From Tom Gehrels <tgehrels@LPL.Arizona.EDU>

THE GLOBAL ASTEROID PROBLEM

Joel Achenbach wrote a perceptive article (April 19) on the asteroid
hazard. If a quote checker had called me, I would have set mine
straight, less cavalier and more appropriate. I spell out to all
reporters, "This is the most serious environmental danger facing
humanity, because it can take all of us out at once; but we are taking
care of it such that it might become an example for other global
problems."
                                          Tom Gehrels

============================
(3) A NEW THEORY OF CHONDRULES FORMATION

H.C. Connolly*) & S.G. Love: The formation of chondrules: Petrologic
tests of the shock wave model, SCIENCE, 1998, Vol.280, No.5360, pp.62-67

*) CALTECH, DIV GEOLOGY & PLANETARY SCIENCE, MAIL CODE 100-23,
   PASADENA, CA, 91125

Chondrules are millimeter-sized rounded igneous rocks within chondritic
meteorites. Their textures and fractionated mineral chemistries suggest
that they formed by repeated, localized, brief (minutes to hours)
melting of cold aggregates of mineral dust in the protoplanetary
nebula. Astrophysical models of chondrule formation have been unable to
explain the petrologically diverse nature of chondrites. However, a
nebular shock wave model for chondrule formation agrees with many of
the observed petrologic and geochemical properties of chondrules and
shows how particles within the nebula are sorted by size and how rims
around chondrules are formed. It also explains the volatile-rich nature
of chondrule rims and the chondrite matrix. Copyright 1998, Institute
for Scientific Information Inc.

==================
(4) THE ORBITAL AND ABSOLUTE MAGNITUDE DISTRIBUTION OF MAIN BELT
    ASTEROIDS

R. Jedicke & T.S. Metcalfe: The orbital and absolute magnitude
distributions of main belt asteroids. ICARUS, 1998, Vol.131, No.2,
pp.245-260

UNIVERSITY OF ARIZONA, LUNAR & PLANETARY LAB, TUCSON, AZ, 85721

We have developed a model-independent analytical method for debiasing
the four-dimensional (a, e, i, H) distribution obtained in any asteroid
observation program and have applied the technique to results obtained
with the 0.9 m Spacewatch telescope. From 1992 to 1995 Spacewatch
observed similar to 3740 deg(2) near the ecliptic and made observations
of more than 60,000 asteroids to a limiting magnitude of V similar to
21. The debiased semimajor axis and inclination distributions of main
belt asteroids in this sample with 11.5 less than or equal to H < 16
match the distributions of the known asteroids with H < 11.5. The
absolute magnitude distribution was studied in the range 8 < H < 17.5.
We have found that the set of known asteroids is complete to about
absolute magnitudes 12.75, 12.25, and 11.25 in the inner, middle, and
outer regions of the belt, respectively. The number distribution as a
function of absolute magnitude cannot be represented by a single power
law (10(alpha H)) in any region. We were able to define broad ranges in
H in each part of the belt where alpha was nearly constant. Within
these ranges of H the slope does not correspond to the value of 0.5
expected for an equilibrium cascade in self-similar collisions
(Dohnanyi 1971). The value of alpha varies with absolute magnitude and
shows a 'kink' in all regions of the belt for H similar to 13. This
absolute magnitude corresponds to a diameter ranging from about 8.5 to
12.5 km depending on the albedo or region of the belt. (C) 1998
Academic Press.

==========================
(5) ASTEROID TRAILS IN HUBBLE SPACE TELESCOPE IMAGES

R.W. Evans*), K.R. Stapelfeldt, D.P. Peters, J.T. Trauger, D.L.
Padgett, G.E. Ballester, C.J. Burrows, J.T. Clarke, D. Crisp, J.S.
Gallagher, R.E. Griffiths, C. Grillmair, J.J. Hester, J.G. Hoessel,
J. Holtzmann, J. Krist, M. McMaster, V. Meadows, J.R. Mould, E.
Ostrander, R. Sahai, P.A. Scowen, A.M. Watson, J. Westphal: Asteroid
trails in Hubble Space Telescope WFPC2 images: First results. ICARUS,
1998, Vol.131, No.2, pp.261-282

*) CALTECH, JPL, MAIL STOP 183-900, 4800 OAK GROVE DR, PASADENA, CA,
   91109

Careful examination of 28,460 selected Wide Field and Planetary Camera
2 (WFPC2) long exposures from 1994, 1995, and early 1996 has revealed
trails of 96 distinct moving objects. They have been reported to the
International Astronomical Union's (IAU) Minor Planet Center for their
asteroid database and a few have been identified with known asteroids
and used to update their orbits. Most of the objects are new, as they
are too faint to show up on ground-based surveys. The trails often show
a characteristic curvature due to the parallax induced by HST's orbital
motion during the exposures. Using ephemerides for HST, the distance to
each object can be directly determined from the parallax contribution
to the trail shapes. Based on these distances, constraints on the
orbits, and photometry of the trails (16 < V < 24), most of the moving
objects appear to be small, main-belt asteroids a few km in diameter. A
few are known objects-three are potential Mars crossers. Modern
wide-field CCD surveys detect asteroids nearly as faint as these(V <
21), but the corresponding absolute magnitudes are uncertain unless
their orbits have been established. The detected objects span the
absolute magnitude range 13.6 < H < 19.3 (H is the symbol for absolute
magnitude, not H-band). Statistics of the detections imply a reservoir
of (3.1 +/- 0.6) x 10(5) such asteroids within 25 degrees of the
ecliptic. We find that the slope of the cumulative distribution of
absolute magnitudes follows a power law N proportional to H-0.2 to N
proportional to: H-0.3 over this absolute magnitude range in the three
distance ranges defined by the Palomar-Leiden survey. These are
significantly shallower slopes than those inferred by the
Palomar-Leiden survey or extrapolated from population studies of larger
asteroids. (C) 1998 Academic Press.

====================
(6) MAIN BELT ASTEROID COLLISION PROBABILITIES AND IMPACT VELOCITIES

J.D. Vedder: Main belt asteroid collision probabilities and impact
velocities. ICARUS, 1998, Vol.131, No.2, pp.283-290

PAREXIC RESEARCH, 15827 STONEHAVEN DR, HOUSTON, TX, 77059

A new method for estimating asteroid collision probabilities and impact
velocities (Vedder 1996, Icarus, 123, 436-449) is extended from single
to multiple targets. It is then applied to estimating collision rates
and velocities of all asteroids in the main belt with each other. The
results obtained are compared with those obtained by previous
investigators using other methods. The new method yields main belt
collision probabilities about 10-15% higher than earlier results, and
the impact velocity distribution is shifted downward by about 20%.
Analysis of the velocity components shows that the most significant
difference is in the component normal to the plane of the ecliptic,
with the new method yielding a higher likelihood of this Z component
having a low magnitude. The nonnormal distribution of the Z component
in previous results may be due to underestimating the collision
probability for nearly coplanar or nearly tangent orbits. For these
frequently occurring cases, the new method may well have advantages,
because it is singularity-free and does not depend on an elaborate and
very sensitive analytical model of the geometry of close encounters.
(C) 1998 Academic Press.

========================
(7) A THERMAL MODEL FOR NEAR-EARTH ASTEROIDS

A.W. Harris: A thermal model for near-Earth asteroids. ICARUS, 1998,
Vol.131, No.2, pp.291-301

DLR, INSTITUTE OF PLANETARY EXPLORATION, RUDOWER CHAUSSEE 5, D-12489
BERLIN,GERMANY

It has become common practice in studies of the thermal emission from
near-Earth asteroids to employ thermal models developed on the basis of
infrared observations of large, main-belt asteroids. However,
near-Earth asteroids are relatively small and are expected to have less
dusty, 'rockier' surfaces with higher thermal inertias than main-belt
asteroids. Furthermore, they tend to have irregular shapes and are
often observed at large solar phase angles, compared to main-belt
objects. Therefore, the applicability of the commonly used asteroid
thermal models to these objects is not self-evident. Radiometric data
in the 4-20-mu m range of several near-Earth asteroids are compared
with model predictions of the thermal emission. For all the objects
considered both the 'standard' and 'fast rotating' thermal models
produce poor fits to the observational data. Simple modifications to
the standard thermal model are described that lead to significantly
improved fits to the data. Revised estimates of the albedos and sizes
of a number of near-Earth asteroids are derived using the resulting new
thermal model with an empirically derived calibration parameter and
compared with existing data from other sources. The new model is
proposed as a default simple thermal model for estimating albedos and
diameters of near-Earth asteroids. (C) 1998 Academic Press.

========================
(8) WHAT EVER HAPPENED TO FRAGMENT D OF COMET SHOEMAKER-LEVY 9?

J.E. Little*), A. Fitzsimmons, P.J. Andrews, R. Catchpole, N. Walton,
I.P. Williams: The evolution of debris from Comet D Shoemaker-Levy 9 on
Jupiter. ICARUS, 1998, Vol.131, No.2, pp.334-340

*) QUEENS UNIVERSITY BELFAST, DEPARTMENT OF PURE & APPLIED PHYSICS,
   BELFAST BT7 1NN, ANTRIM, NORTHERN IRELAND

We present low-resolution photometric spectra in the wavelength range
407-907 mu m of impact sites G, H, E, C, K, L, Q1, and W plus K
together, arising from the collision of Comet D/Shoemaker-Levy 9 with
Jupiter. The spectra were obtained from narrow band CCD images taken
between July 15th and July 22nd using the 1-m Jacobus Kapteyn Telescope
on La Palma. Our results indicate the impact debris to have low overall
optical depths (tau < 1) which decrease with increasing wavelength for
all individual sites observed. Our results also imply that mass
absorption of sunlight is the dominant process occurring in the impact
debris and that a decrease in optical depth with time is due to the
material dissipating in the jovian atmosphere. (C) 1998 Academic Press.

======================
(9) LONG-TERM EVOLUTION OF COMET SL-9 FEATURES

A. SanchezLavega, J.M. Gomez, J.F. Rojas, J.R. Acarreta, J.
Lecacheux, F. Colas, R. Hueso, J. Arregui: Long-term evolution of Comet
SL-9 impact features: July 1994 September 1996. ICARUS, 1998, Vol.131,
No.2, pp.341-357

*) UNIVERSITY PAIS VASCO, ETS INGN, DEPT FIS APLICADA 1, ALDA URQUIJO
   S-N, BILBAO 48013, SPAIN

We present a two year study of the evolution of SL9 impact aerosol
debris we observed between 0.4 and 0.9 micrometers with continuous high
temporal coverage from July 1994 through September 1996 and at 1.7 and
2.3 micrometers during three observing runs in July 1994 and March and
August 1995. Temporal cylindrical map projections at red continuum
wavelengths in the region covered by the impact debris show the
contributions of different mechanisms in producing the complicated
morphological evolution of the sites during the first month. Long-term
horizontal aerosol transport was mainly due to the zonal jets in the
upper troposphere with extreme measured velocities of - 10 and 20 m
s(-1). A comparison of the zonal drift of the core sites in the red
continuum and in the 890-nm methane band (sensitive to higher levels)
during the first month do not show significant velocity differences
between these filters, indicating a low vertical wind shear in the
upper troposphere. The spread of the aerosols resulted from the
meridional and vertical shears of the zonal winds. Rapid initial
outward expansions (speeds of similar to 30 to 60 m s(-1)) and
interactions with nearby vortices (speeds of similar to 10 to 25 m
s(-1)) also contributed to the dispersion of particulates. Using
methane band images we have measured a steady poleward and equatorward
meridional transport of the particulates with velocities of similar
to-6 and 40 cm s(-1), respectively. Particulates were detected up to
similar to-20 degrees by August 1995. Limb brightening in the 890-nm
methane band was observed up to similar to-30 degrees during the last
observation (September 1996) reported here, indicating that a small
population of aerosols was still present two years after impact,
Photometric observations in the 890-nm band, together with a radiative
transfer model, allowed us to calculate the evolution of the aerosol
optical depth in the main impact core areas and in the subsequent SL9
band. We found a rapid decrease in optical depth in the largest impacts
during July and August 1994 (from approximate to 3.2 to 2.1), followed
by a gradual decrease during the next two years to approximate to 0.3
(June 1996). This behavior can be explained by simple models of
debris horizontal dispersion by the wind shear and by sedimentation.
Calculations of the characteristic times related to the microphysical
processes in the aerosols (sedimentation, coagulation, and coalescence)
together with their observed residence times (greater than or equal to
2 years) indicates that this persistent population of particles had
sizes less than or equal to 0.1 micrometers during 1995 and 1996. (C)
1998 Academic Press.

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