No Shelter From the Rain of Death

No Shelter From the Rain of Death

by Adam Frank for McGraw-Hill

It comes in steep and fast. More than a billion tons of rock dives into the atmosphere at 20,000 miles per hour. Years ago, the gentle tug of a passing outer planet set this asteroid on a collision course with Earth, a course now coming to its climax. The flying mountain screams through the sky and strikes the surface with a world-shuddering impact. The blast wave that explodes away from the impact site sears all air, water and life in its path. Billions of tons of superheated rock blows high into the atmosphere only to rain back down across the rest of the planet. Fires are lit everywhere. A cloud of dust, smoke and debris spreads across the planet. The long, cold night for life on Earth has begun.

Asteroid collisions with the Earth are the stuff nightmares and Hollywood science fiction movies are made of. In the last ten years at least two films, "Deep Impact" and "Armageddon," have sprung from the collective shudder we get considering our planetary vulnerability. While it's clear that Hollywood knows a good scare when it sees one, how scared are scientists who study the subject? Is this really something any of us should be losing sleep over? Even more to the point, if the worst-case scenario did come true and we found ourselves in the path of a cosmic rock heap, is there really anything we could do about it?

From a long-term, global, "life on Earth" perspective, the danger posed by asteroids and comets is real. There is now a substantial body of evidence that shows Earth has been struck a number of times by objects with enough punch to do serious damage to the biosphere. Some scientists call these disasters Mass Extinction Events and they occur when a large fraction of species on the planet are wiped out by the impact and subsequent changes to the ecosystem. Darkening skies from smoke and debris block sunlight wiping out plants and animals at the bottom of the food chain. Everything above simply collapses. The most famous mass extinction event came some 65 million years ago when a comet or asteroid brought the rein of the Dinosaurs to an abrupt and fiery end. Scientists believe the object that killed the dinosaurs was at least 10 miles across and even believe they have found the crater from the impact in the Gulf of Mexico (it is called the Chicxulub Crater, closer to home there is also a large crater from an impact called Meteor Crater in Arizona).

So, the danger is real. Is it, however, immanent? For all the big press the subject gets, it is unlikely that we are going to be hit by any 10-mile wide flying mountain any time soon. After all, the last mass extinction event was 65 million years ago. That should tell you something right away. Formally, the odds of being hit by say a 2 km size body with the energy of 1 million megatons in a human lifetime are about 1 in 20,000. You can take one perspective, we are tampering with the environment in so many other ways why worry about the remote possibility of an asteroid impact. On the other hand, while they may be rare, asteroid impacts have a lot of …well, impact. One big collision can ruin your day for quite a while. Maybe it is worth at least some of our attention.

Is there anything we could do if we found we were in the cross hairs of an oncoming asteroid? Some people have argued that with enough warning we could fire a flotilla of nuclear weapons at the asteroid and blow it into dust. This was the premise behind "Deep Impact," "Armageddon" and "Asteroid." Unfortunately, computer simulations of this process do not show much promise for this approach. In 1998, researchers used powerful NASA supercomputers to track the results of "explosions" on a 1-mile long asteroid. This was a difficult calculation to make and was only possible because of the speed of modern advanced computers. Instead of blowing the asteroid apart, the simulations showed the space rock being fragmented but holding together via its own gravity. Instead of "asteroid death," the explosions only created a rubble-pile that would be just as dangerous to Earth. It is still too early to say for sure that space-bombing would not work since results may depend on what the asteroid is made of.

Still, it may not be too early to think, "score one for the asteroids." If nature wants to wipe us out, there may not be much we could do about it. If the time comes, let's hope we have more tricks up our sleeve.

Points to Ponder

If nuclear bombs cannot destroy an asteroid is there any other way we might avoid an impact?
Should resources be spent to prepare for the possibility of finding an asteroid on a collision course with Earth?

Check Out These Websites

Chicxulub Crater
http://www.space.com/scienceastronomy/planetearth/asteroid_jello_001122.html

Near-Earth Asteroid Tracking
http://neat.jpl.nasa.gov/