Astronomers are completely confident that the 45-metre-wide asteroid 2012 DA14 is not going to hit us, passing “only” at 17,200 miles from Earth — the closest encounter with an asteroid ever predicted. It’s a close call, but we will be safe. Given what’s happened in Russia today — what would really happen if their calculations were wrong?

To answer this, first we should find out how much energy could 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 scientists asked themselves this question in 2010: Robert Marcus, H. Jay Melosh, and Gareth Collins.

Ebel says that Marcus et al calculated the kinetic energy of a 50-metre-wide asteroid with a density of 2.6 grams per cubic centimetre and a speed of 12.7 kilometres per second hitting Earth at a 45 degree angle. This would be a very similar scenario to 2012 DA14, which is only five metres 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’ calculations resulted in 3.3 Megatons of kinetic energy at entry, with an airburst energy of 2.9 Megatons at about 8.5 km 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 explode 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 London, New York, San Francisco, 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 much less.

Sadly, this doesn’t mean cities are safe against these beasts. 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 to 10km over the surface — but the airburst obliterated an estimated 80 million trees over an area of 830 square miles. In 2011, the estimated population of New York City were about 8.2 million people living over an area of about 302 square miles. You do the math. Hell, just today we’ve seen the horrifying destruction a meteorite exploding the the atmosphere can cause.

All this is assuming a 45 angle of entry. Here’s another really bad luck scenario: the Río Cuarto event — an impact that occurred 10,000 years ago in Córdoba province, in north-central Argentina — hit Earth at 15 degrees from the horizontal, resulting in 30 times more destructive power than Tunguska.

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 45m 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.