Target: Earth

Watch the TEDTalk that inspired this post.

Buying insurance is seldom gratifying. But here's a case in which plunking down cash for a policy is just ... good policy.

The threat is familiar: rocks from the sky, known as asteroids -- the subject of a 2011 TEDx talk by astronomer Phil Plait. These leftovers from the birth of the Sun and planets race around at thirty thousand miles per hour, and can wreak formidable damage if they collide with Earth. The threat is also multiple, since at least 40,000 asteroids bigger than a football field silently careen through our neck of the solar system. The larger of these -- comparable in size to downtown San Francisco -- could devastate our world's flora and fauna, and deep-six humans in the same way that the dinosaurs were snuffed 65 million years ago. The Earth is a metal duck in a very busy shooting gallery.

In 1908, there was a persuasive demonstration of the power of high-speed, low-mass asteroids in rural Siberia. The Tunguska impactor iced millions of pine trees and about a zillion mosquitoes - Seth Shostak

But here's the good news: Thanks to the work of a series of telescopic observations collectively known as Spaceguard, we've learned that none of the really big objects in this swarm of cosmic projectiles is likely to hit us soon. Approximately 90 percent of these solar system bullies have been pinpointed, and their whereabouts for the next century carefully computed. Planet Earth is not in their crosshairs.

So the odds are good that neither you, nor your children or grandchildren will be victimized by a reprise of the KT extinction -- the lethal event that took out the thunder lizards and half of all the other colorful Jurassic megafauna.

Consequently, you can go back to worrying about climate change, home-brew pandemics, or just good old-fashioned nuclear holocaust. Assuming you worry about such things at all.

Alas, the picture's not quite so rosy. You still have reason to fret about death from the skies, as Plait lovingly calls it. Not from the big rocks, but from the small. Consider that the overwhelming majority of those 40,000 near-Earth asteroids are small enough to fit on the parking lot at the mall. And while these rocky runts won't cause Armageddon, they could still flatten such popular hominid hangouts as Manhattan or downtown Des Moines.

In 1908, there was a persuasive demonstration of the power of high-speed, low-mass asteroids in rural Siberia. The Tunguska impactor iced millions of pine trees and about a zillion mosquitoes -- and was no larger than an office building. Despite its modest caliber, this rock exploded with the energy of 300 Hiroshima-type bombs (although, thankfully, with none of the radioactivity). Imagine how ruinous such an unguided missile would be if it smacked into a major metropolis. And because of the great abundance of these second-tier rocks, you can expect a Tunguska-like collision every few centuries, on average. That's enormously more frequent than impacts by the larger asteroids, which occur at intervals of 1-100 million years.

Of course, it may occur to you that only a tiny fraction of these smaller projectiles will actually hit a city. Often, they'll simply create a new tourist attraction in farmland or desert. Two out of three will cannonball into the ocean. Sure, that's unnerving for fish, but it could also be bad news for humans, because the resulting tsunamis might threaten the large number of people who live near the coast.

Small is not beautiful. So what to do? We could confront the threat directly by developing technology to dodge these bullets. And indeed, there are some clever schemes for nudging any incoming cosmic ordnance out of the way, including the gravitational tractor described by Plait in his talk.

But before we can develop -- let alone deploy -- effective countermeasures for these bantam brickbats, we need to know the territory. We need to know which rocks are coming our way.

A decade ago, NASA investigated the costs to locate and define the orbits of asteroids between 140 meters and one kilometer in size. The space agency presented their study, and the U.S. Congress signed on -- instructing NASA to track down 90 percent of these objects over the course of the next two decades. As you read this, telescopes are scanning the skies.

The price tag for this catalog will likely turn out to be $200-400 million, or roughly $500 per rock. That's really cheap, and is less than the payout for the most recent Powerball jackpot. Put another away, this work costs the same as buying a single, high-grade military helicopter once a year.

Since the property damage from an impact -- not to mention the loss of life -- could far exceed the expense of this catalog, this project should be considered a no-brainer, even in times when government budgets are tight. It's insurance with a very affordable premium.

Disasters happen. We still have no way to eliminate earthquakes, wildfires, hurricanes, floods or droughts. We cope as best we can by fortifying ourselves against danger with building codes and levees, and by setting aside money to clean up afterwards. FEMA's annual budget is running about $14 billion, and we generally accept that burden as the cost of doing business in a hazard-packed world.

But for much less than one percent of the annual outlay for FEMA, we can continue to systematically reconnoiter the rocks that might visit death and destruction upon our urban areas in the foreseeable future. And the first step in defeating an enemy is to know the enemy.

Buying insurance is no one's idea of fun. And it's especially easy to berate something as funky-sounding as writing checks to defend our neighborhoods against apartment-size rocks from space. But this is one insurance pitch that makes perfect sense. Ask the dinos.

Seth Shostak is Senior Astronomer at the SETI Institute, and host of its weekly science radio program, "Big Picture Science."

Ideas are not set in stone. When exposed to thoughtful people, they morph and adapt into their most potent form. TEDWeekends will highlight some of today's most intriguing ideas and allow them to develop in real time through your voice! Tweet #TEDWeekends to share your perspective or email to learn about future weekend's ideas to contribute as a writer.