Asteroid collision: How to defend Earth, II

was destined to splash down in the Atlantic Ocean off the west coast of Africa, and a 50-meter-wide rock heading directly for Washington, D.C.) — would cause a giant splash that could smash coastal buildings with high-speed volleys of water. The tremendous damage and loss of life that would ensue if multiple cities around an ocean basin were flooded led NASA scientists in 2003 to rate ocean impacts by asteroids as far more dangerous than those on or over land.

Recent computer simulations offer some hope, though. They suggest that the monster waves generated by ocean impacts would typically break far from shore, dissipating most of their energy before they could reach cities - unless the impact was very close to the coast, of course. Another ray of hope is that 100-meter asteroids hit Earth only about one-tenth as often as 30-meter objects.

Lasting just one day, the 2008 U.S. air force exercise could only scratch the surface of the incoming-asteroid problem. Not surprisingly, it discovered that should this happen, there is no plan for how to coordinate the activities of NASA, emergency planners, the U.S. military and other parts of government. Further planning exercises are needed: the time saved through early preparation will be crucial if an evacuation is ever required at short notice.

Our chance of having any prior warning at all for an approaching 30-meter asteroid is no better than 25 to 35 percent with existing sky surveillance, calculates astronomer Alan Harris of the Space Science Institute in Boulder, Colorado. The sun washes out half of the sky with daylight, blinding us to 50 percent of threatening objects. Even glare from the moon can hide unwelcome incoming guests.

What is more, two of the world’s three leading asteroid surveys are based in Arizona, including the Catalina Sky Survey, which discovered 2008 TC3. The region tends to cloud over between July and September. “Shift 2008 TC3 back to July and forget it. It wouldn’t have been seen,” says Spahr.

Nuclear worries
Shiga suggests we picture this scenario, which worried some participants in the air force exercise: an asteroid flies out of nowhere and explodes over a region with political tensions and nuclear-armed adversaries — the Asian subcontinent or the Middle East. There is a reasonable chance that such an airburst could be misinterpreted as a nuclear attack. Both produce a bright flash, a blast wave, and raging winds. Now, an asteroid flying out of nowhere