[meteorite-list] Bombing Earth-bound Asteroids a Viable Option, Experts Say

[MEM]

Interesting photo of the Asteroid Itokawa and interview with Brother Guy.
Elton
<http://news.nationalgeographic.com/news/2010/10/101013-science-space-asteroids-near-earth-nuclear-bombs/>


Article:

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."