Astronomers are completely confident that the 45m wide asteroid 2012 DA14 is not going to hit us, passing "only" at 28,000km from Earth — the closest encounter with an asteroid ever predicted. It's a close call, but we will be safe. Even so — what would really happen if their calculations were wrong?
To answer this, the first question is how much energy can 2012 DA14 release if it crashed against our planet. According to Denton Ebel — Chair of the Division of Physical Sciences and Curator of the Department of Earth and Planetary Sciences at the American Museum of National History — three other people asked themselves this question in 2010: Robert Marcus, H. Jay Melosh and Gareth Collins.
Ebel says that they calculated the kinetic energy of a 50m wide asteroid with a density of 2.6 grams per cubic centimetre and a speed of 12.7km per second hitting Earth at a 45-degree angle. This would be a very similar sceneario to 2012 DA14, which is only 5m smaller and in the opinion of Ebel could have a density of about 2.6 grams per cubic centimetre since it is a stony asteroid. He says that the density may be less, since "most asteroids are underdense (lots of void space)", but let's assume it's 2.6 grams/cc for the sake of this argument.
Marcus, Melosh and Collins's calculations resulted in 3.3 megatons of kinetic energy at entry, with an airburst energy of 2.9 megatons at about 8.5km from the surface, "about the cruise altitude of passenger jets". This means that an asteroid like this would likely explode in the air, releasing the energy equivalent of about 138 atomic bombs like the one that the Boeing B-29 Superfortress Enola Gay dropped over Hiroshima on August 9, 1945. Or, if you would like a more modern equivalent, 2012 DA14 could have exploded with the energy of nine W87 nuclear warheads like the ones carried by the American Minuteman III Iintercontinental ballistic missiles.
If the magnitude of such an explosion is hard to imagine, take a look at this Chinese 3.3-megaton atomic bomb test:
Not too shabby.
Needless to say, the effects of such an explosion over New York, San Francisco, London, Madrid, Moscow, Rio de Janeiro, Sydney, Beijing or Tokyo, would be quite significant.
But — BUT! thankfully there is a but — fear not: Ebel says that, while "direct hits are bad, these cities are very small targets". So even if 2012 DA14 were to impact Earth, the probabilities of a direct hit like the one described above would be very, very thin.
But what if we were really really unlucky?
OK, let's assume things get hairy. We know that if this rock were in a collision trajectory with a major city, it would likely not directly impact its surface. Scientists believe that most asteroids enter Earth's atmosphere at 45 degrees. If this were the case for 2012 DA14, the enormous heat generated by the compression of air in front of the asteroid will make it explode way above the city, not on the ground. The destructive power would be diminished considerably. According to Ebel, it would require a much bigger asteroid to impact Earth:
Asteroid 2012 DA14 is probably not tightly bound, so its density is low. That makes an airburst at any impact angle, according to impact calculations. The Meteor Crater impactor was very tough, dense iron. At a shallow angle any incoming object sees a lot more air, so loses more kinetic energy. A vertical impactor sees the least amount of air. According to impact calculations, a low density object like DA14 would have to be about 90 meters diameter (twice its inferred diameter), impacting at 90 degrees, at 11 km/sec, to make a crater. For the same case, an impact at 45 degrees does not make a crater.
Sadly, none of this means cities are safe against these beasts. Airbursts can be dangerous too, depending on the size of the object. On June 30, 1908, a stony asteroid just a bit bigger than the one about to pass Earth exploded over the Podkamennaya Tunguska River in Siberia, Russia. There was no city near the explosion site — which happened at about 5-10km above the surface — but the airburst obliterated an estimated 80 million trees over an area of 2150sqkm. In 2011, the estimated population of New York City were about 8.2 million people living over an area of about 789sqkm. You do the maths.
On top of that, according to the Imperial College, London, and Purdue, "large fragments may strike the surface" in the case of 2012 AD14.
The angle of entry
All this is assuming a 45-degree angle of entry. If 2012 DA14 entered Earth perpendicular to the surface, it will still not hit the ground, but it will result in a more powerful explosion closer to the city. The destructive power of asteroids increases as the angle increases because there's less air resistance and they lose less kinetic energy.
Now, if 2012 DA14 entered at something like 15 degrees, it will encounter much more air resistance, so it would explode at a much higher altitude with a lot less power. That would be good for us.
However, it appears that a lower angle of impact can actually cause more damage than a perpendicular strike if the asteroid is big enough to survive the heat and pressure stress, like the Río Cuarto event. That impact occurred 10,000 years ago in Córdoba province, in north-central Argentina, hitting Earth at 15 degrees from the horizontal and resulting in 30 times more destructive power than Tunguska. But that was a much bigger asteroid than 2012 DA14. Definitely not the case here.
The good news
An impact over a major city would be terrible news, but at least it wouldn't bring a global extinction event.
According to Ebel, the effect would be "very local for an impactor about 45 meters in diameter. As an airburst, Tunguska was a more intense event. As a crater-forming impact, Meteor Crater in Arizona was formed about 50,000 years ago by an estimated 30m diameter iron meteorite." Civilisation would not be destroyed. No global nuclear winter, no collapse of the ecosystem, nothing. Destruction would be entirely localised to wherever the asteroid hit.
But what would happen if we got really, really, REALLY unlucky, and 2012 DA14 hit over the Yellowstone supervolcano caldera? Would that trigger a supervolcano? Ebel says that he doesn't think that "either an airburst or a crater forming impact would trigger a supervolcano. Nor do I think the energy would be sufficient to trigger earthquakes." Some theories say that something big enough, like a few powerful nuclear warheads detonated on a fault, could trigger an earthquake. These theories, however, have never been tested.
Let's hope we never have to find out.