PLEASE NOTE:


*

CAMBRIDGE-CONFERENCE DIGEST, 6 April 1998
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(1) NEW MITIGATION STRATEGY MINIMIZES RISK OF ASTEROID COLLISIONS
    Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>

(2) ... OR DOES IT?
    Duncan Steel <dis@a011.aone.net.au>

(3) PEEPING AT EURYDIKE & PENELOPE
    V.V. Busarev, Moskow State University

(4) MEET THE EOS FAMILY
    A. Doressoundiram et al., Paris Observatory

(5) THE FORMS OF EROS
    D.L. Mitchell et al., Caltech, JPL


=======================
(1) NEW MITIGATION STRATEGY MINIMIZES RISK OF ASTEROID COLLISIONS

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

News Bureau
University of Illinois at Urbana-Champaign
807 S. Wright St., Suite 520 East
Champaign, IL 61820-6219
(217) 333-1085   fax (217) 244-0161
e-mail: uinews@uiuc.edu

CONTACT: James E. Kloeppel, Physical Sciences Editor (217) 244-1073
E-mail: kloeppel@uiuc.edu

April 1998

New mitigation strategy minimizes risk of asteroid collisions

CHAMPAIGN, Ill. -- The spectacular plunge of Comet Shoemaker-Levy 9 into
Jupiter in July 1994 and recent concern about the projected "near miss" of
Asteroid 1997 XF11 with Earth in October 2028 brought renewed awareness that
collision events do occur within our solar system -- and the next one could
involve our planet. In fact, such a collision may be long overdue, and steps
should be taken to alleviate the risk, a University of Illinois researcher
says.

"If faced with this kind of danger, we would want to send a spacecraft to
intercept the object as far from Earth as possible," said Bruce Conway, a
professor of aeronautical and astronautical engineering. "This would allow
whatever mitigation strategy we use to have the longest time to act."

There are two practical problems that must be solved, however, Conway said.
"The first is simply getting a sizable payload to the object in the shortest
amount of time, and the second is deciding what to do when we get it there."

In a paper published in the September-October (1997) issue of the Journal of
Guidance, Control, and Dynamics, Conway described the optimal low-thrust
interception of a potential collider. The proposed mission scenario would
combine the speed of conventional chemical rockets with the increased
payload capability of continuous-thrust electric propulsion. Having arrived
at the destination, however, what should be done to prevent the impending
collision?

"For years, we assumed that the best mitigation strategy was to blow up the
object with a nuclear warhead," Conway said. "But that may not be such a
good idea. If we blow it up, instead of having just one large mass hurtling
toward the Earth, we could end up with a multitude of smaller -- but equally
lethal -- objects coming at us. A better alternative would be to deflect the
object."

One possible mechanism to accomplish this would involve detonating a nuclear
warhead above the asteroid surface, Conway said. "That would create a
crater, and a large portion of the jet of vaporized material would shoot off
in one direction -- like a rocket -- and push the object in the opposite
direction."

But which direction should the object be pushed to ensure that it will miss
the Earth? And would it make more sense to speed the object up or slow it
down?

Conway's latest research has focused on answering these questions. He
developed an analytical method that, given the orbital parameters of the
object and the interval between interception and close approach, determines
the proper direction in which to push the object to maximize the deflection
in the required time.

Such calculations may never be needed, but they're nice to have just in
case.

"While the probability of a large asteroid or comet colliding with the Earth
is low, the potential for destruction is immense," Conway said. "It's
probably not something we should lose sleep over; but, on the other hand, it
would be really silly not to do anything."

==================
(2) .... OR DOES IT?

From: Duncan Steel <dis@a011.aone.net.au>

Two comments [regarding Bruce Conway's suggestions]:

(1) One is NEVER 'overdue' for an impact. It's a poissonian process.
Even if the time since the last one is 10x the mean time between events, it
is still not correct to say that one is overdue.  The next impact is no more
likely to occur in the next year than it was to occur in the year directly
after the last impact.  This presumes, though, that impacts are random in
time, which may not be the truth, but it is the majority belief on this
question (cf. Steel et al. in the Hazards book, ed. T. Gehrels, 1994).

(2) The idea of forming a crater is daft.  It is likely/possible that such a
surface explosion would fragment the whole object.  The idea of deflection
is that one uses a nuclear explosion some distance (of order the diameter)
above the surface of an object, & the neutron flux evaporates a surface
layer which provides a push (due to reaction force from the evaporation)
over the whole surface on the side towards the explosion.
Paradoxical though this may seem, this is a 'gentle' use of a nuclear device
(in our own protection).

Duncan

=======================
(3) PEEPING AT EURYDIKE & PENELOPE

V.V. Busarev: Spectral features of M-asteroids: 75 Eurydike and 201
Penelope, ICARUS, 1998, Vol.131, No.1, pp.32-40

MOSCOW MV LOMONOSOV STATE UNIVERSITY, STERNBERG STATE ASTRONOMICAL
INSTITUTE, MOSCOW 119899, RUSSIA

Spectrophotometric data for the 0.338- to 0.762-mu m region on the
main-belt M-asteroids 75 Eurydike and 201 Penelope at small phase
angles have beers obtained, The spectral observations of 201 Penelope
were accompanied by photometric observations allowing an assessment
of geometric albedo variation of Penelope with rotation. The
reflectance spectra of the bodies art shown to have both similar and
differing spectral features varying with rotation. The slightly
red-sloped reflectance spectra may suggest the presence of an FeNi-
or Fe-free component in the asteroid surface material. Common
absorption bands in the spectra of Eurydike and Penelope can probably
be attributed to pyroxenes (at 0.51 mu m) and oxidized or aqueously
altered mafic silicates (at 0.62 and 0.7 mu m). A specific absorption
feature of Penelope at 0.43 mu m may be a result of electronic
transitions in crystalline lattices of several different minerals
present on the surface of the asteroid including some layer
silicates. The presumed presence of phyllosilicates on the surface of
201 Penelope is explained as a result of the aqueous alteration of
mafic silicates. Probable scenarios for the origin of the asteroids
are also discussed. The conclusion is made that in spite of the
asteroids' belonging to the M-class they have a considerable silicate
component which manifests itself in some similar absorption features,
Furthermore, a few of the main orbital elements of the bodies are
Fiery close, This permits the supposition that the bullies have a
common origin. (C) 1998
Academic Press.

=====================
(4) MEET THE EOS FAMILY

A. Doressoundiram*), M.A. Barucci, M. Fulchignoni & M. Florczak: Eos
family: A spectroscopic study, ICARUS, 1998, Vol.131, No.1, pp.15-31

*) PARIS OBSERVATORY,F-92195 MEUDON, FRANCE

The Eos family detected by Hirayama in 1918 has been always
considered to be compositionally homogeneous. To investigate
the composition and the homogeneity of the members of this family, we
started a spectroscopic survey at the European Southern Observatory
(ESO) with wavelength coverage ranging from 4800 to 9200 Angstrom. We
observed 45 Eos asteroid members, which constitutes the first large
survey of this family. Our results reveal the Eos objects have
spectral signature characterizing the whole family: a maximum at
lambda similar to 8000-8500 Angstrom and a reflectivity gradient
spanning a continuous range, Only two of the 45 investigated objects
seem to be interlopers. While the lower range of this spectral
distribution has been easily connected with CO-CV chondrites, we have
found no satisfactory meteorite counterpart to the upper range. We
have interpreted the spread out of Eos spectra to be the results of
compositional variation among the Eos members, implying that the Eos
parent body was partially differentiated. Moreover, a space
weathering effect has been proven to be present, but with a minor
role played in the diversity of Eos family, the major role being the
compositional variation. (C) 1998 Academic Press.

==================
(5) THE FORMS OF EROS

D.L. Mitchell*), R.S. Hudson, S.J. Ostro & K.D.Rosema: Shape of
asteroid 433 Eros from inversion of Goldstone radar Doppler spectra,
ICARUS, 1998, Vol.131, No.1, pp.4-14

*) CALTECH, JPL, 4800 OAK GROVE DR, PASADENA, CA, 91109

We use new analysis techniques to constrain the shape of 433 Eros
with Goldstone radar data obtained during the asteroid's close
approach in 1975. A previous analysis of these data (Ostro, Rosema,
and Jurgens, 1990, Icarus 84, 334-351) used estimates of the echo's
spectral edge frequencies as a function of asteroid rotation phase to
constrain the convex envelope of Eros' pole-on silhouette. Our
approach makes use of the echo's full Doppler-frequency distribution
(effectively similar to 15 times more echo data points) and is thus
capable of constraining shape characteristics, such as concavities,
within this convex envelope. The radar echoes are weak and
north-south ambiguous, which limits the accuracy of our models, We
present two different approaches, perturbations to an ellipsoid and
successive approximations, that help to quantify the model
uncertainties and identify features that are likely to be real.
Both approaches yield models that are tapered along their lengths,
with one or more prominent concavities on one side but not the other.
We do not have sufficient information to determine the exact nature
of the concavities, and in particular, whether they are craters,
troughs, or bends in Eros' overall shape. The pole-on silhouette of
the successive approximation model is shaped like a kidney bean,
which resembles a nearly pole-on optical image derived from speckle
interferometry (Drummond and Hege, 1989, in Asteroids II (R. P.
Binzell, T. Gehrels, and M. S. Matthews, Eds.), PD 171-191, Univ. of
Arizona Press, Tucson); however, eve (sic) cannot exclude shapes,
such as the perturbation model, with more than one large concavity.
Variations in the pyroxene/olivine ratio over Eros' surface have been
inferred from visual and infrared observations (Murchie and Pieters,
1996, J. Geophys. Res, 101, 2201-2214). Correlating these variations
with our shape information, we find that the side with concavities is
relatively pr-rich compared with the more rounded opposing side. (C)
1998 Academic Press.

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