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Asteroid Impact Study Finds Effects Of Collisions Or Explosions On Small Asteroids May Be Hard To Predict

Contact:
Tim Stephens, science writer
University of California
Public Information Office
1156 High Street
Santa Cruz, CA  95064
Phone: (408) 459-4352   Fax: (408) 459-5795
stephens@cats.ucsc.edu

May 29, 1998

This release is EMBARGOED until 4 p.m. EST Wednesday, June 3, 1998. The
research will be published in the June 4 issue of the journal Nature.

ASTEROID IMPACT STUDY FINDS EFFECTS OF COLLISIONS OR EXPLOSIONS ON SMALL
ASTEROIDS MAY BE HARD TO PREDICT

SANTA CRUZ, CA -- An analysis of collisions between asteroids may help
explain the structure and evolution of these small planetary bodies and
also raises concerns about the feasibility of disrupting or deflecting an
asteroid in the event that one is discovered hurtling through space towards
Earth.

	Astronomer Erik Asphaug, a research associate at the University of
California, Santa Cruz, used computer simulations to study the effects of
powerful impacts on asteroids with different internal structures. He and
his colleagues found that the outcome of such impacts depends on the degree
to which the asteroid has been fractured and made porous by earlier
collisions.

	Asphaug said he is primarily interested in understanding the
geophysics of asteroids and the evolution of small bodies in the solar
system. But the implications of his findings for deterrence of an asteroid
collision with Earth are compelling. Nuclear explosions have been proposed
as one way to break up or alter the course of an asteroid headed towards
Earth. But Asphaug found that some types of asteroids could absorb a
powerful explosion with little or no effect.

	"It's a lot more difficult to nudge these asteroids around than we
had thought," said Asphaug, who completed the study while working at NASA
Ames Research Center and the SETI Institute in Mountain View, CA. "More
work needs to be done before we can decide whether nuclear warheads provide
a viable deterrent," he added.

	Previous studies by Asphaug and others suggested that many of the
asteroids in our solar system are aggregates of debris left over from
previous collisions -- either a few large fragments held together by
self-gravity or "rubble piles" consisting of numerous smaller pieces. The
new study shows that the porous nature of such asteroids damps the
propagation of shock waves, thereby limiting the effects of an impact or
explosion to a localized area. Asphaug and his collaborators at several
other institutions published their results in the June 4 issue of the
journal Nature.

	For their simulations, the researchers started with a computer
model of an asteroid 1.6 kilometers (1 mile) across, based on radar images
of a near-Earth asteroid named Castalia. They gave this peanut-shaped
target asteroid three different internal structures: solid rock, a pair of
solid rocks in close contact, and a rubble pile with pore space accounting
for 50 percent of its volume. The researchers subjected each of these to
impact by a house-sized rock traveling 5 kilometers per second, a typical
speed for collisions in the asteroid belt. This is equivalent in energy to
the 17-kiloton Hiroshima bomb, although impacts are more devastating than
explosions of equal energy, Asphaug said.

	The results may explain some of the bizarre shapes and structures
scientists have observed in recent years as they have begun to get detailed
images of near-Earth asteroids. For example, in June 1997 the Near Earth
Asteroid Rendezvous (NEAR) spacecraft sent back remarkable images of the
asteroid Mathilde, showing five huge craters, some larger in diameter than
the radius of the asteroid itself. The impacts that created these enormous
craters did not break the asteroid apart, did not erase or disturb
preexisting craters, and left no sign of fractures on Mathilde's surface.

	"A rubble-pile model provides a good explanation for a low-density
body like Mathilde, because an impact can blast the heck out of a local
area and have little effect on the rest of the asteroid; the shock wave
dies out quickly, so a large crater can be excavated without the rest of
the asteroid noticing what happened," Asphaug said.

	At the opposite extreme, he noted, an asteroid consisting of solid
rock throughout may shatter into many smaller pieces when hit by another
object. Depending on the energy of the impact, those pieces might disperse
to form a family of smaller asteroids or remain aggregated, forming a
rubble pile. A solid asteroid might also break into several large pieces
plus debris.

	Many asteroids (including Castalia) have a twin-lobed structure
that suggests they consist of two separate pieces held together by gravity.
Such "contact binaries" proved somewhat resistant to impacts in Asphaug's
simulations, because the shock wave reflects off the boundary between the
two components. As a result, one side of a binary asteroid could be blown
apart by an impact, while the other side remains relatively unaffected,
Asphaug said.

	These results suggest that to a large extent the fracture pattern
resulting from one impact determines the outcomes of future impacts. "Once
an asteroid has been broken, it becomes more resistant to subsequent events
because the impact-generated shock waves can't propagate across the
fractures," Asphaug said.

	The same would hold true of explosions. Therefore, to predict the
effect of a nuclear explosion on any particular asteroid, scientists would
need to understand its internal structure, Asphaug said. The internal
structures of asteroids, however, are probably just as diverse as their
external shapes, he added.

	Asphaug's ongoing research may shed light on the evolution of the
solar system from a disk of gas and dust to the familiar set of planets
circling the Sun. The asteroids are presumably representative of the small
bodies that eventually grew into planets by accumulating additional matter.
This accretion process must have involved numerous collisions between
smaller bodies, and Asphaug is trying to find out what factors in a
collision favor the accumulation of mass (allowing for the growth of
planets) rather than disruption and loss of mass.

	"Asteroids may be like a snapshot of the first stages in planetary
evolution," Asphaug said. "We're in the midst of an epoch of discovery in
which we are just beginning to see what asteroids look like and to
understand how they got to be the way they are."

	Additional studies may explain many of the key processes involved
in the evolution of the planets. Of course, one process involving asteroids
will always be of special interest to the inhabitants of planet Earth, and
that one is currently playing in theatres nationwide. Movies such as Deep
Impact dramatize the scenario in which an asteroid collision threatens life
on Earth.

	According to Asphaug, there are hundreds of thousands of asteroids
in near-Earth space whose impact on Earth would be equivalent to the
largest thermonuclear device ever exploded.  Thousands, like Castalia, are
larger still. While the probability of such an impact occurring in the near
future is extremely small, the consequences would be disastrous.

	"Asteroids are not an imminent threat, and I am far more concerned
about what humans are doing to the planet," Asphaug said. "But in case we
ever identify an asteroid or comet on a collision course, it would be best
to know our enemy so that we can get it before it gets us."

				#####

IMAGES AND VIDEO AVAILABLE -- Contact Tim Stephens at (408) 459-2495 or
stephens@cats.ucsc.edu.

Editor's note: You may reach Erik Asphaug at (408) 459-2260 or
asphaug@earthsci.ucsc.edu, or through his web page at
http://www.es.ucsc.edu/~asphaug/.