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The Sciences

The Trouble With Rubble

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When the Near Earth Asteroid Rendezvous spacecraft passed the asteroid Mathilde last year, it sent back some unusual snapshots. The 36-mile-wide asteroid was pocked with relatively enormous craters-five were at least 12 miles across. That would be like Earth having five craters the size of North America. How could a body withstand such blows without shattering? Erik Asphaug, a planetary geologist at the University of California at Santa Cruz, thinks he knows. His computer simulations of impacts on asteroids of various compositions show that asteroids made of loose rubble held together only by gravity easily endure such impacts.

For those who stay up at night worrying about using bombs to deflect rogue asteroids from Earth, this is unsettling news. Some astronomers believe that nearly all asteroids larger than a couple of hundred feet across are essentially flying rubble piles, not solid rocks. If these asteroids have a Terminator-like ability to absorb punishment, any that threaten to hit Earth may be impossible to divert.

Asphaug modeled the asteroid Castalia, a mile-long, half-mile-wide body with a peanut shape. His simulation uses each of three proposed compositions for the asteroid: one solid rocky body, two solid bodies touching in the middle, and a pile of boulders and gravel. Asphaug then slammed a 6,000-ton, house-size asteroid into each of the three configurations and watched what happened.

The impact shattered the solid asteroid, leaving behind a flying pile of rubble. And although half of the "touching" model survived unscathed, the solid half that took the hit broke apart. But the rubble pile took the licking pretty well-its loose structure didn't transmit the energy of impact efficiently.

Asphaug's model suggests that we would need a much longer lead time than most researchers have assumed to successfully divert a threatening asteroid. To redirect an asteroid the size of Castalia, says Asphaug, a rocket would have to reach it 20 years before the predicted impact date.

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