Asteroid collision: How to defend Earth, I

Published 12 October 2009

There are thousands of Near Earth Objects (NEOs) orbiting Earth; some of them are of a civilization-ending size, others are smaller — they will take out “only” a country or a city were they to collide with Earth; scientists say we should focus our minds on this danger

Here is a scenario we should think about: A mountain-top telescope has just detected an asteroids never seen before. Rapid-survey telescopes discover thousands of asteroids every year, but there is something very particular about this one. The telescope’s software decides to wake several human astronomers with a text message they hoped they would never receive. The asteroid is on a collision course with Earth. It is the size of a skyscraper and it’s big enough to raze a city to the ground. It will be here in three days.

David Shiga writes that far-fetched though it might seem, but this scenario is all too plausible. It is realistic enough that the U.S. Air Force recently brought together scientists, military officers, and emergency-response officials for the first time to assess the U.S. ability to cope with an asteroid collision, should it come to pass.

The participants in the meeting were asked to imagine how their respective organizations would respond to a hypothetical asteroid called Innoculatus striking the Earth after just three days’ warning. The asteroid consisted of two parts: a pile of rubble 270 meters across which 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.

The exercise, which took place in December 2008, exposed the dangers asteroids pose. Not only is there no plan for what to do when an asteroid hits, but our early-warning systems — which could make the difference between life and death — are inadequate. The meeting provided just the wake-up call organizer Peter Garreston had hoped to create. He has long been concerned about the threat of an impact. “As a taxpayer, I would appreciate my air force taking a look at something that would be certainly as bad as nuclear terrorism in a city, and potentially a civilization-ending event,” he says.

Shiga notes that the latest space rock to put fear in those in the know was 2008 TC3. This car-sized object exploded in the atmosphere over Sudan in October last year (see 10 October 2008 HSNW). A telescope first spotted it just twenty hours before impact — at a distance of 500,000 kilometers — and astronomers say we were lucky to get any warning at all.

The 2008 TC3 was too small to do any damage on the ground, but Shiga writes that we are nearly as blind to objects big enough to do serious harm. We have barely begun to track down the millions of skyscraper-sized asteroids zipping around Earth’s neighborhood, any one of which could unleash as much destructive power as a nuclear bomb on impact.

Asteroid impacts are not as rare as one might think. It is widely accepted that an asteroid or comet 30 to 50 meters across exploded over Tunguska in Siberia in 1908, flattening trees for dozens of kilometers all around (see 16 July 2008 HSNW and 2 July 2008 HSNW). The chance of a similar impact is about 1 in 500 each year (Nature, vol 453, p 1178). In other words, this is a 10 percent chance of an impact in the next fifty years.

Fifty-meter asteroids scare me to death,” says Timothy Spahr, director of the Minor Planet Center in Cambridge, Massachusetts. “I could easily see a 50-meter object hitting in three days causing absolute pandemonium.”

During the U.S. Air Force planning exercise, the participating scientists explained that with so little warning there would be no hope of preventing an impact. Even Innoculatus’s smaller 50-meter asteroid would weigh hundreds of thousands of tons, requiring an enormous push to change its trajectory appreciably — so much so that detonating a nuke near it in space would not provide a sufficient impulse so late in the game to cause a miss. To deflect an asteroid sufficiently, force would need to be applied years in advance.

In fact, it could make things worse by breaking the asteroid into pieces, some of which could be large enough to do damage, and even create a blizzard of meteors that would destroy satellites in Earth orbit.

Tomorrow: What can we do to avert an asteroid catastrophe