June 8, 1998

Asteroid-impact study finds effects of collisions or explosions on small asteroids may be hard to predict

These images show the impact a house-size rock (represented by a white dot), traveling five kilometers per second, would have on the asteroid Castalia. (See larger image)

By Tim Stephens

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 UCSC research associate, 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. (Go to Asphaug's Web page)

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 theaters 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."

Contributors to this project were:

• Dr. Steven J. Ostro at the NASA Jet Propulsion Laboratory in Pasadena, CA, and Dr. R. Scott Hudson of Washington State University, who discovered the shape of the near-Earth asteroid Castalia on which the asteroid-impact model was based;

• Dr. Willy Benz of the University of Bern, Switzerland, who took part in the numerical modeling;

• Dr. Daniel M. Scheeres of Iowa State University, who worked on defining the gravitational evolution of the impact fragments.

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