Buy insurance. Tick. Health check. Tick. Drive sensibly. Tick. As a general rule, we humans like to control our lives. But let’s face it, all of this caution is a complete waste of time if a huge rock from space has your name on it.
Take the recent ‘near-miss’ by the poetically-named asteroid 2012 BX34, which was only discovered two days before it sailed past within 40,000 miles (60,000km) of Earth. What if it had been heading straight for us?
A wonderful report from the US National Research Council (NRC) says that on average there should be 91 deaths per year from asteroid strikes - a remarkably precise figure and one that deserves some digging.
Try to think of when you last heard about an asteroid striking the earth. There really aren’t that many of them, or at least that many that are noticed or reported in newspapers.
One of the last significant impacts occurred on 30 June 1908, when an asteroid or comet exploded 6.2 miles (10km) above a secluded forest in Tunguska, Siberia, flattening trees over an area of 625 sq miles (1600 sq km), which surprisingly few people cared about at the time due to the remoteness of the region and the fact that there seem to have been no casualties.
Calculations suggest that if it had landed 4 hours and 47 minutes later, it would have hit St Petersburg(1), in which case people might have cared a lot, particularly as it was rather a delicate time in Russian history. According to estimates, such an airburst occurring over New York would cost $1.19tn to insurers in property damage, not to mention causing approximately 3.2 million fatalities and 3.76 million injuries.
But that has not occurred. In fact, there are surprisingly few reports of fatalities involving asteroids. A few cars in the United States have been damaged and there was a case of a cow being killed in Valera, Venezuela in 1972 – the unfortunate animal was duly eaten and bits of the meteorite were later sold to collectors. A home outside Paris was also recently hit by an egg-shaped meteorite, but the appropriately-named Comette family were away at the time.
So how can the NRC be so precise? Well, to understand we need to understand how astronomers and statisticians think about these risks.
As these threats come in all shapes and sizes astronomers have established a classification system to help gauge the potential level of risk. If asteroids or comets come within one-third of the distance from the Earth to the Sun – just over 30 million miles (48 million km) – they are labelled as Near Earth Objects (NEOs). ‘Near’ is clearly a relative term, though this does show how comparatively close asteroid 2012 BX34 came. Fortunately, there are observatories watching over us and Nasa’s Near Object Program is keeping count of what is passing through our local area. By December 2011, over 8,500 NEOs had been found and named, with around 500 being added to the list every year.
If an object is found that is at least 480ft (150m) across, and which will pass closer than 20 times the distance between the moon and the Earth (around five million miles, or eight million km), it earns itself the status of Potentially Hazardous Asteroid (PHA). If anything this big were to hit the Earth, the consequences would be serious. So far 1,271 of these have been found, of which 151 are of more than 0.6 miles (1km) in diameter, a size that could be globally catastrophic.
Working out the chances of an Earth-asteroid collision and the damage it would cause is not like an insurer dealing with a collision between two cars: there is almost no direct historical data, so astronomers create equations relating to the size of an asteroid, how many there are around, how often they might hit the Earth and what the explosive force of any impact would be. These estimates are continually being revised and are subject to some esoteric disputes.