It looks inconsequential enough, the faint little spot moving leisurely across the sky. The mountaintop 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 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.
As far-fetched as it might seem, this scenario is all too plausible. 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 off the west coast of Africa, and a 50-metre-wide rock heading, in true Hollywood style, directly for Washington.
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 the organiser Peter Garreston had hoped to create. 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 500,000km - and astronomers have said 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.
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. Put another way, that is a 10 per cent chance of an impact in the next 50 years. "Fifty-metre asteroids scare me to death," said Timothy Spahr, the 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 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 nuclear weapon near it in space would not provide a sufficient impulse so late in the game to cause a miss. To sufficiently deflect an asteroid, 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. Realistically, though, the nuclear option would not be on the table in the first place: 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. But participants in the planning exercise worried that if the situation were not handled properly, panic and lack of co-ordination could lead to chaos.
Mr Spahr was not involved in the exercise, but shares those concerns. "With a three-day warning, you can walk away and be safe. But it scares me, given how poorly we've handled things of this nature in the past," he said, citing the failure to fully evacuate New Orleans ahead of hurricane Katrina in 2005 Well-thought evacuation plans should ensure the streets would be very quiet as an object such as Innoculatus plunges into the atmosphere and makes its final approach to Washington. The compression of the atmosphere in front of the asteroid and friction with the air would cause rapid heating. At lower altitudes, where the air is denser, the heating becomes so intense that the asteroid vaporises and explodes. For the Tunguska event, this happened at about 8km above ground.
If you were unfortunate enough to be looking up from directly below, the explosion would be brighter than the Sun. The visible and infrared radiation would be strong enough to make anything flammable ignite, said Mark Boslough of Sandia National Laboratory in Livermore, California. "It's like being in a broiler oven. Anyone directly exposed would quickly be very badly burned." Even before the sound of the blast reaches you, your body would be smashed by a devastating supersonic shock wave as the explosion creates a bubble of high-pressure air that expands faster than the speed of sound. At ground zero below an exploding asteroid, the shock wave would be powerful enough to knock down buildings. It would arrive about 30 seconds after the blazing hot flash of light, and could also knock any nearby planes out of the sky, Mr Boslough said. Any surviving buildings would be pummelled by raging winds blowing faster than any hurricane can muster.
While asteroid impacts are much rarer than hurricanes and earthquakes, they have the potential to do much greater damage, warned Lindley Johnson at Nasa headquarters, who oversees the agency's work on near-Earth objects. "It's not something I think there needs to be billions of dollars per year spent on, but it does warrant some priority in the list of things that we ought to be worried about." The cash would at least give us a better idea of when the next asteroid might strike. "From what we know today," he said, "it could be next week."