Science Feature

Asteroid attack: Putting Earth's defences to the test

IT LOOKS inconsequential enough, the faint little spot moving leisurely across the sky. The mountain-top telescope that just detected it is taking it very seriously, though. It is an asteroid, one never seen before. Rapid-survey telescopes discover thousands of asteroids every year, but there's 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. Oh, and it will be here in three days.

Also known as the "eye of Quebec", Manicougan Crater in Canada is one the Earth's oldest known impact craters, and is about 200 million years old. Today it contains a 70-kilometre hydroelectric reservoir along its edge. The island in the centre of the crater was formed by post-impact uplift of the land. Also visible in the bottom left-hand corner is the fin of the space shuttle from which this image was taken. (Image: LSTS-9 Crew/NASA/GSFC)

Far-fetched it might seem, but this scenario is all too plausible. Certainly it is realistic enough that the US air force recently brought together scientists, military officers and emergency-response officials for the first time to assess the nation's ability to cope, should it come to pass.

They were asked to imagine how their respective organisations would respond to a mythical asteroid called Innoculatus striking the Earth after just three days' warning. The asteroid consisted of two parts: a pile of rubble 270 metres across which was destined to splash down in the Atlantic Ocean off the west coast of Africa, and a 50-metre-wide rock heading, in true Hollywood style, directly for Washington DC.

The exercise, which took place in December 2008, exposed the chilling 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 woefully inadequate. The meeting provided just the wake-up call organiser 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 civilisation-ending event," he says.

The latest space rock to put the frighteners on us was 2008 TC3. This car-sized object exploded in the atmosphere over Sudan in October last year. A telescope first spotted it just 20 hours before impact - at a distance of 500,000 kilometres - and astronomers say we were lucky to get any warning at all.

Thankfully, 2008 TC3 was far too small to do any damage on the ground, but 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 neighbourhood, any one of which could unleash as much destructive power as a nuclear bomb on impact.

Asteroid impacts are not as rare as you might think. It is widely accepted that an asteroid or comet 30 to 50 metres across exploded over Tunguska in Siberia in 1908, flattening trees for dozens of kilometres all around. The chance of a similar impact is about 1 in 500 each year (Nature, vol 453, p 1178). Put another way, that's a 10 per cent chance of an impact in the next 50 years (see "Should we panic?").

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

During the US 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-metre asteroid would weigh hundreds of thousands of tonnes, 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 (see "Could we nuke it?").

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.

Panic on the streets
Realistically, though, the nuclear option would not be on the table in the first place: the nuclear-tipped missiles sitting patiently in silos around the world are not designed to track and home in on an asteroid or even survive for more than a few minutes in space. Instead, we would simply have to brace ourselves for the impact.

The good news is that even a little warning makes a big difference, simply because it would allow us to predict the time and location of impact. In the case of 2008 TC3, just a few hours after the asteroid's discovery, NASA scientists completed calculations that predicted an atmospheric plunge over an unpopulated desert area of northern Sudan, with timing accurate to within a minute.

War of The World - Synopsis

A meteor like object crash-lands near the small town of Linda Rosa. Among the crowd of curious onlookers is Pacific Tech scientist Gene Barry, who strikes up a friendship with Ann Robinson, the niece of local minister Lewis Martin. Because the meteor is too hot to approach at present, Barry decides to wait a few days to investigate, leaving three townsmen to guard the strange, glowing object. Left alone, the three men decide to approach the meteorite, and are evaporated for their trouble. It turns out that this is no meteorite, but an invading spaceship from the planet Mars. As Barry and Ann seek shelter, the Martians go on a destructive rampage. Nothing-not even an atom-bomb blast-can halt the Martian death machines. The film's climax occurs in a besieged Los Angeles, where Barry fights through a crowd of refugees and looters so that he may be reunited with Ann in Earth's last moments of existence.