Buy insurance. Tick. Health check. Tick. Drive sensibly. Tick. As a general rule, we humans like to control our lives. But let’s face i...
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 BX, which was only discovered two days before it sailed past within , miles ,km 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 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 June , when an asteroid or comet exploded . miles km above a secluded forest in Tunguska, Siberia, flattening trees over an area of sq miles 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 hours and minutes later, it would have hit St Petersburg, 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 $.tn to insurers in property damage, not to mention causing approximately . million fatalities and . 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 – 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 million miles million km – they are labelled as Near Earth Objects NEOs. ‘Near’ is clearly a relative term, though this does show how comparatively close asteroid BX 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 , over , NEOs had been found and named, with around being added to the list every year.
If an object is found that is at least ft m across, and which will pass closer than 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 , of these have been found, of which are of more than . miles km 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.
Asteroids in the - feet -m diameter range find their way to Earth around once per year, releasing energy equivalent to approximately , tons of TNT when they explode: about the same as the bomb detonated over Hiroshima. Most go unseen and unrecorded. An airburst of an ft m asteroid would release energy equivalent to around one million tons of TNT, or one megaton Mt, equivalent to around Hiroshima bombs. That same NRC report estimates an average of years between such impacts.
The Tunguska devastation in Siberia is thought to have been caused by an asteroid of around ft m diameter exploding miles km up, releasing Mt of energy. The NRC estimates an average interval of , years between events of this size, although other studies have suggested that Tunguska-like events could be caused by objects as small as ft m, which would equate to nearly a : chance of such an event occurring in a given lifetime.
Bigger asteroids start being considered as ‘continental-scale events’, although it is tricky to predict what damage such a major impact would cause. Due to the differing coverage of the Earth’s surface, there is a % chance that any such impact would hit the ocean, and models have suggested a ,ft m asteroid could cause a tsunami ft m high, although with regards to possible consequences, there is great uncertainty about whether such a wave would break on the continental shelf, whether the population could evacuate, and so on.
Mass extinctions?
Nasa estimate that there are currently around Near Earth Objects of over . miles km across, and if one hit us it would release around ,Mt of energy. Such an impact would almost certainly be globally catastrophic. Thankfully, such strikes should only be expected every , years, but even bigger collisions have occurred. Considerably more than just one cow perished when a mile km wide, ,,Mt lump hit the Yucatan peninsula in modern-day Mexico million years ago: this impact is credited with changing the climate and wiping out the dinosaurs. But such mass extinction events are only estimated to come along every million years or so. So not much need to worry there, then.
In terms of future threats, the current catalogue indicates no serious risk from asteroids that we know of. The biggest known danger Nasa can point to is a in chance of a collision with the ft m -AG asteroid sometime in the s. But most asteroids less than ,ft m across remain undiscovered, and although these would be unlikely to cause a global catastrophe, they could certainly take us by surprise. TC was -ft -m across and weighed around tons when it exploded over the Sudanese desert on October . It was the first asteroid to ever be detected before impact, but it was only picked up hours prior to its destruction, and one can imagine the crisis if its predicted path had passed over a big city. It exploded with a force of around , tons of TNT and kg of fragments were picked up afterwards – though thankfully nobody was hurt.
Is it worth getting up?
So, once again, back to that figure of deaths.
This now begins to make more sense when you realise that it is an average taken over millennia, in which almost all years show no deaths, but a few isolated events cause massive casualties. In fact, the figure of deaths is almost evenly balanced between more frequent, smaller-scale impacts, and statistically very infrequent globally catastrophic impacts.
So, I know what you are now thinking: what is the risk of being killed by an asteroid in my bed – or anywhere else for that matter – in my lifetime?
As I said in my first column, statisticians like to define such risk events in terms of micromorts: that is, a one-in-a-million chance of dying. Since there are billion people on Earth and people are expected to die every year from an asteroid impact, this works out at micromorts per person per year – about the same risk as a mile km car journey, but much less risky than riding a motorcycle for miles km in the UK around micromorts.
Assuming a lifetime of around years, this comes to the delightfully round number of one micromort per lifetime from asteroids. Which, as risks go, is hardly the end of the world.
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