Of course, the probability of such a catastrophic asteroid strike is very small. Researchers at NASA's Spaceguard Survey estimate that a hit similar in size to the 1908 Tunguska event in Siberia occurs once every 300 years and would take out an area of about 5,000 square kilometers (1/100,000th of the Earth's surface). Huge portions of the planet are uninhabited (ocean) or sparsely inhabited, so Spaceguard calculates that another Tunguska is apt to hit a large urban area about once every 100,000 years. Ultimately the researchers calculate that the annual probability of an individual's death from a Tunguska-type impact is 1 in 30 million.
But what about the even more fearsome Earth Killing strikes? The Spaceguard Survey estimates, "The threshold for an impact that causes widespread global mortality and threatens civilization almost certainly lies between about 0.5 and 5 km diameter, perhaps near 2 km. Impacts of objects this large occur from one to several times per million years." The hit that is thought to have given the kibosh to the dinosaurs—and 70 percent of all other plant and animal life—65 million years ago was probably caused by an asteroid 10 kilometers in diameter.
Just last month, a 300-meter asteroid was detected which, initial calculations suggested, had a 1 in 300 chance of hitting our planet on April 13, 2029. (Fortunately, later calculations showed that it should miss us by several thousand miles.) A 30-meter asteroid zipped by at only 26,500 miles from the Earth on March 18, 2004, and another 70-meter asteroid whooshed pass at 288,000 miles on March 8, 2002. Several objects of this size probably speed by between the Earth and the Moon every year. So far NASA's Spaceguard Survey has identified some 700 of the estimated 1,100 possible "Earth Killer" asteroids measuring 1 kilometer or more in diameter.
The consequences of a strike by an object 1 to 2 kilometers in diameter would be so widespread—killing perhaps a quarter of the world's population—that the annual probability of an individual's death from such a Civilization Killer is actually higher than that from a more probable, but far less devastating, Tunguska-style hit—it's 1 in 2 million. An American's annual risk of dying from lightning is 1 in 6.5 million; from cataclysmic storms (say, hurricanes), 1 in 5 million; and from an earthquake, 1 in 10 million. We do protect ourselves from such low risks by spending on lightning rods and on flood- and earthquake-resistant structures. So protecting against devastating risks posed by asteroid strikes is not completely out of the question.
For example, Posner makes a quick and dirty calculation that the average American is willing to pay $70 per year to protect herself against a 1 in 100,000 risk. Scaling that to the risk of being obliterated by an asteroid, she should be willing to spend about $3.50 per year on planetary defense. Now multiplying that by 280 million Americans, it would seem that Americans as a whole would be willing to spend about $1 billion per year on planetary defense. Posner notes that NASA currently spends only about $4 million per year finding and tracking Near Earth Objects (NEOs) that could strike our planet and end civilization.
Assuming that Spaceguard did identify an asteroid that was going to strike the Earth several years in advance, what could be done to deflect it? Not much, right now. In November 2004, the American Institute of Aeronautics and Astronautics called for stronger measures to protect our planet from impacts, including the creation of an agency directly responsible for planetary defense, the development of techniques to deflect menacing NEOs, and eventually, test missions on non-threatening NEOs to make sure those techniques work.
One proposal for diverting NEOs, devised by SpaceWorks Inc., is called "The League of Extraordinary Machines." This plan involves an armada of small spacecraft attaching themselves to the NEOs surface. Once attached, the landers would heat up and eject material from the asteroid's surface as a way to change its path away from the Earth.
However as Russell Schweikert, a former Apollo astronaut and chairman of the B612 Foundation, observes, deflecting an asteroid would also raise novel international problems. As the asteroid's trajectory was being diverted, the spot it would hit on the surface of the Earth would move across political boundaries until its trajectory becomes tangential to the planet and misses it. But China might well wonder why it should risk being hit by a 200-meter asteroid that was originally going to hit Europe. Schweikert is right to suggest that it would be a good idea to work out international agreements beforehand on these matters.
Defending against NEOs is a classic example of a public goods problem. A public good is one, such as national defense, with low costs for an extra individual to enjoy, and high costs to exclude anyone from enjoying. This means that it's unlikely that any particular individual (namely you or me) would pay out of his own pocket to see that the good is provided.
In such situations, we turn not to markets, which excel at providing private goods, but to mechanisms of collective action, generally the government. Naturally, any new Planetary Defense Agency will suffer all the problems that afflict government bureaucracies, especially the innate drive to seek more money and power by exaggerating risks. But given the devastating consequences of an Earth Killer strike, it makes sense to take some strong collective action to protect ourselves and future generations against this risk.