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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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