PLEASE NOTE:
*
CAMBRIDGE-CONFERENCE DIGEST, 6 April 1998
-----------------------------------------
(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.
--------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK
--------------------------------
The Cambridge-Conference List is a scholarly electronic network
moderated by Benny J Peiser at Liverpool John Moores University,
United Kingdom. It is the aim of this network to disseminate
information and research findings related to i) geological and
historical neo-catastrophism, ii) NEO research and the hazards to
civilisation due to comets, asteroids and meteor streams, and
iii) the
development of a planetary civilisation capable of protecting
itself
against cosmic disasters. To subscribe, please contact Benny J
Peiser
<b.j.peiser@livjm.ac.uk>.
Information circulated on this network is
for scholarly and educational use only. The attached information
may
not be copied or reproduced for any other purposes without prior
permission of the copyright holders.