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The asteroid Itokawa.

The asteroid Itokawa, as seen by the Hayabusa spacecraft in 2005.

Sensor data courtesy JAXA, processed by National Geographic News

Traci Watson

for National Geographic News

Published October 13, 2010

In the movies, a bomb is usually the most effective way of stopping an asteroid from wiping out life on Earth. But real scientists have had their doubts about bombing the potentially hazardous objects. (See asteroid and comet pictures.)

Now, however, some researchers are finding evidence that an explosion might not, as feared, make a bad situation worse by sending a huge cloud of harmful debris raining down on the planet.

And other scientists are suggesting that, despite previous assumptions, we wouldn't need an impossibly powerful bomb to destroy a threatening asteroid.

Considering the damage a large asteroid strike could do to humanity, bombing any so-called near-Earth objects, or NEOs, headed our way might be a viable last resort "if we have the international political will," said Robert Weaver of Los Alamos National Laboratory in New Mexico.

In such a case, "my calculations show that we have the means."

(Related: "NASA to Visit Asteroid Predicted to Hit Earth?")

Asteroid Bombing Wouldn't Require Monster Nukes

Although the exact nature of asteroids is still poorly understood, scientists think the objects fall into two broad categories: solid space rocks and loose "rubble piles" held together by gravity.

(Related: "Asteroid Probe Offers New Views of Near-Earth Object.")

It had always been assumed that explosives would be more effective at blowing apart an asteroid if inserted deep in the body's interior, Weaver said.

But in his new calculations—presented last week at a meeting of the American Astronomical Society's Division of Planetary Sciences in Pasadena, California—Weaver found that the bombing option wouldn't require any drilling to get the job done.

According to his models, a nuclear explosion equal to one megaton of TNT would blow a solid asteroid to smithereens whether the bomb was placed on the asteroid's surface or deep inside the space rock. Some countries' arsenals already include nukes of that power.

The researcher also found that destroying rubble-pile asteroids would be even easier, at least in terms of the power needed to blow them apart.

His computer programs predicted that the asteroid Itokawa—a loose pile shaped like a 1,000-foot-long (300-meter-long) potato—would be broken to bits by an explosion of just 500,000 tons of TNT. Bombs big enough to create such a blast are relatively common.

The new models are the first to incorporate sophisticated shockwave physics that have been validated with laboratory experiments, Weaver said.

No Fears of "Shotgun" Asteroids

Another argument against bombing NEOs is the so-called shotgun effect, in which a bomb breaks the asteroid into a swarm of smaller chunks that can cause widespread devastation.

(See "A Hundred Years After Tunguska, Earth Not Ready for Meteors.")

To check that theory, Catherine Plesko, also of the Los Alamos lab, and her colleague Don Korycansky of the University of California, Santa Cruz, used computer programs to model the explosion of a rubble-pile asteroid a half mile (one kilometer) wide, which is large enough to cause global destruction when intact.

According to their simulations, a nuclear explosion roughly 25 times larger than the roughly 15-kiloton blast at Hiroshima would shatter the asteroid and scatter the pieces far enough that they wouldn't threaten Earth.

A weaker explosion—one slightly smaller than the Hiroshima blast—would create shards that would reconsolidate due to gravity, so that the asteroid would stitch itself back together in less than a day.

The key is knowing whether the less intense blast would be enough to knock the regrouped asteroid safely off course—something Plesko is looking in to.

But at least her work, presented Friday at the AAS meeting, means that "you don't have to worry about making a bunch of bits" that would threaten Earth, she said. "They all go away or come back together."

Space Rock Science "Still Taking Baby Steps"

Aside from total annihilation, several methods have been proposed for nudging a potentially hazardous asteroid off course, according to a report published in January by the National Research Council:

  • The gravity tractor: A nearby spacecraft uses its gravitational influence to tug the asteroid into a safer path.
  • The impactor: A large spacecraft slams into the asteroid hard enough to jolt it off its Earth-bound track.
  • The standoff burst: An explosion close to the asteroid vaporizes some of the space rock's surface, so that the asteroid recoils in the opposite direction.

But most of these options require quite a bit of advanced warning, because the technologies involved are less mature than nuclear bombs, and because some of the methods would require a longer lead time than the bomb option.

(See "'Killer Asteroid' Debate Pits Gravity Tractors Against Bombs, Projectiles.")

Blowing up an asteroid is "probably your only option" when faced with a space rock spotted less than a decade before the rock is due to cross Earth's path, said asteroid expert David Dearborn of Lawrence Livermore National Laboratory in California, who was not involved in either study.

What's more, the problem with any NEO countermeasure is that asteroid tracking is still an inexact science.

"In reality, when we have [an NEO] approaching Earth, we never know for sure, till it's way too late, whether it's actually going to hit," said Guy Consolmagno, an asteroid researcher at the Vatican Observatory, who was also not part of the two new studies.

And even once we do know that an asteroid poses a threat, properly deflecting the rock hinges on understanding its exact structure, Consolmagno said. (See "Water Discovered on an Asteroid—A First.")

The use of explosions "is an interesting issue, but I'm not sure it's a closed issue," he added. "We're still taking baby steps in this whole field."

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