Prepare for the Worst
Land or Ocean?

Even a small, 300-meter asteroid means trouble. If it struck the ocean, an epic tsunami at least 32 feet (10 meters) high would wash over coastal areas, with follow-up waves adding to the misery. The December 2004 tsunami in Southeast Asia might serve as an example, although an asteroid-induced tidal wave might behave quite unexpectedly.

If the rock struck land, it would dig out a crater 1.86 to 2.49 miles (3 to 4 kilometers) across and deeper than the Grand Canyon. Everything within a 31-mile (50-kilometer) radius of the blast would be destroyed [source: Chapman].

So, the tether on your tether-and-ballast system got tangled. The gravity tractor wasn't built Ford-tough. What do you do now about that killer asteroid barreling toward Earth? Well, if you tried one of the mitigation strategies just mentioned, the asteroid is most likely (a) big and (b) far away. That gives you some time to prepare for impact, although you won't have any historical precedent to provide best practices.

In fact, many astronomers point to fictional accounts -- "On the Beach" by Nevil Shute, for example -- as the best source material about what we might do and how we might fare in a true global cataclysm. Clearly, astronomers would try to pinpoint where the asteroid would hit so ground-zero areas could be evacuated, and governments would try to build underground bunkers, store food and water, collect animal and plant species, and shore up the global financial, electronic, social and law-enforcement infrastructures. The impact of a smaller asteroid -- say, one about 984 feet (300 meters) wide -- could devastate a region the size of small nation.But a rock bigger than 0.621 miles (1 kilometer) wide would affect the whole world. A rock larger than 1.86 miles (3 kilometers) would end civilization [source: Chapman].

Tsunamis, firestorms and earthquakes might cause additional damage. Either way -- impact in the ocean or land -- public officials might only have days or hours to evacuate heavily populated areas. Millions of lives would likely be lost.

Given these scenarios, you can see why governments around the world are so interested in keeping asteroids far from our biosphere. You can also see why dollars don't always drive decisions -- because the cost of failure far exceeds the cost of even the most elaborate deflection concept.