The Norm Chronicles (31 page)

Subject: Apocalypse

Norm, dearest,

The asteroid was on the news, as you said. Too soon to tell, they said, but could be a chance. Wondered if you knew any more goss among the star-gazing fraternity, such as whether the devastation would be global, as we were thinking of that holiday home in Portugal.

‘I was just wondering, love, about the asteroid,’ said Mrs N a few days afterwards, ‘whether it was worth re-mortgaging – I mean, if we never had to repay …?’

NOT LONG AGO
Norm discovered he was
l’homme moyen
and hit the peak of hope and self-belief. Now he faces an existential crisis. Why? Because with the danger from asteroids he confronts one of the most absurd averages ever in the ultimate life-or-death calculation. That makes it not just a distant threat to survival but an immediate threat to everything Norm stands for. We’ll come to this strange average and the calculation behind it in a minute.

First, who has the more reasonable fear: Norm or Prudence? Heavenly bodies falling from space or human bodies from planes? And why?

Prudence has one advantage: familiarity. She can imagine bodies, planes and pigeons easier than apocalypse – for obvious reasons. Easier too, perhaps, to get your head around cosmic doom by thinking in terms of house prices.

According to news reports, the Heathrow flight path has seen a few falling bodies over the years, the tragic results of desperation and the freezing, oxygen- starved atmosphere above 30,000 feet. In 2001 the body of Mohammed Ayaz, a 21-year-old Pakistani, was found in the car park of a branch of Homebase in Richmond. Four years earlier, a stowaway fell from the undercarriage of a plane onto a gasworks near by. No one was hit.

Nobody was at home at the house of the perfectly named Comette family in Paris, either, in the summer of 2011, when an egg-sized meteorite seared into the roof.
³
The rock, blackened as it passed through the Earth’s atmosphere, smashed a roof-tile and buried itself in the insulation. It wasn’t until Martine Comette noticed the rain coming in and called someone to fix it that she discovered the cause. The rock was thought to be about 4 billion years old, from a belt of asteroids between Mars and Jupiter. When Martine’s son Hugo took it to school in a piece of kitchen roll, his friend said it looked like a lump of concrete.

A few months later, in September 2011, a NASA satellite fell somewhere off the west coast of America – prompting concern about how likely it was to land on a human head. At the same time Lars von Trier released his film
Melancholia
, in which ‘two sisters find their already strained relationship challenged as a mysterious new planet threatens to collide with the Earth’, all to music from Wagner’s
Tristan and Isolde
.

All of which gives the impression that there is a lot of heavenly debris about. So what are the risks that a solid object will appear from space and land on your head?

For heavenly bodies the calculation is tricky, partly because of the chance of a truly big hit. And this is what Norm finds so disturbing. An insurer dealing with a car–car collision has abundant direct historical data to help calculate the risks. Astronomers have little. Instead, they devise equations relating the size of an asteroid, how many of them are out there, how often they might hit the earth and what the explosive force would be. These estimates are continually revised and subject to esoteric dispute. The average bottom-line risk that they produce turns out to be truly absurd. We’ll come to it in a moment.

There are two main considerations in calculating potential damage. First, the size of the object; second, where it strikes. If a tree falls in a forest and there’s nobody there to hear it, does it make a noise? And if an asteroid strikes the atmosphere at a speed of 15 km a second and explodes 10 km above a Siberian forest, where it flattens an area of trees measuring 25 by 25 miles but scarcely anyone is there to see, does it matter? When this happened on 30 June 1908 in Tunguska, the few eyewitnesses willing to talk spoke of a heat so intense it felt as though their
clothes were burning, even 40 miles away, of a sky split in half with fire, of being knocked from their feet and running in panic, thinking the end was nigh. It was dramatic, leaving 80 million scorched and flattened trees; otherwise there was some confusion, not many hurt and no direct evidence that anyone perished.

Had the meteorite landed 4 hours and 47 minutes later, it would have hit St Petersburg, or so it has been calculated.
⁴
One estimate is that such an airburst over New York today would cost $1.19 trillion to insurers in property damage, not to mention roughly 3.2 million fatalities and 3.76 million injuries.
⁵

So where an object lands is of at least equal significance to what it is. There is a lot of stuff flying around up there: asteroids made of rock, comets made of ice and frozen gases. At the smallest scale – according to the riveting US National Research Council (NRC) document
Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies
⁶
– around 50 to 150 tonnes of ‘very small objects’, mainly dust, drop to Earth every day. Simply looking into a clear night sky can reveal regular trails of rock or dust burning up in the atmosphere.

Bigger – but only a little more serious – asteroids of 5–10 metres diameter call by around once a year, releasing energy equivalent to around 15,000 tonnes of TNT when they explode in the upper atmosphere, about the same as the Hiroshima bomb. Most go unseen and unrecorded.

Occasionally something gets through and leaves a visible crater or disappears harmlessly into the sea. There is no recent record of human fatality from a meteorite strike, although a few cars in the US have been damaged over the past century and one, the famous Peekskill meteorite car, a Chevrolet Malibu with a boot that looks as if someone took a sledgehammer to it, has toured the world. A cow was also killed on 15 October 1972 in Valera, Venezuela, and duly eaten. Bits of the meteorite were sold to collectors.

A little bigger again, an airburst of a 25-metre asteroid – about the volume of 60 or 70 double-decker buses – would release energy of around 1 million tonnes (1 MT) of TNT, equivalent to around 70 Hiroshima bombs. The Tunguska meteorite is thought to have been about
50 metres across, although some astronomers suggest that Tunguska-like events could be caused by objects as small as 30 metres.
⁷
Up to this point, even if such an asteroid does strike the earth, there is about a 70 per cent chance that only water will be in the way. So we might still be lucky. Unlucky, and it could kill millions.

Still bigger asteroids begin to fall into the range described as ‘continental-scale events’, although again it is tricky to say what damage such an impact would cause. On land, it could be devastating. As before, there is a 70 per cent chance it would strike ocean, but this time with more consequence. Models have suggested a 400-metre asteroid could cause a tsunami 200 metres high – almost twice the height of St Paul’s Cathedral
⁸
– although, of course, there is great uncertainty about whether such a wave would break on the continental shelf, whether a population could evacuate and so on.

In the seriously big league, an asteroid more than 1 km across would release around 100,000 megatonnes of energy: equal to about 700,000 Hiroshima bombs and potentially globally catastrophic. Even bigger collisions can and have occurred. More than a cow perished when a 10 km, 100 million MT lump hit the Yucatán peninsula in Mexico 65 million years ago. It left a crater more than 180 km in diameter and probably wiped out the dinosaurs.

That gives some sense of the potential range of damage. The next step is to discover how many of these things are out there and from this derive an estimate of the likelihood that Earth will be in their way.

Fortunately, NASA’s Near Earth Object Programme is watching over us, and reports, for example, that object 2009TM8, about 10 metres across, was due to pass closer than the moon on Monday 17 October 2011.

Near Earth Objects (NEOs) are those which could come within one-third of the distance to the sun – around 30 million miles – showing that ‘near’ is a relative term. When Perry Como sang ‘Catch a Falling Star and Put it in Your Pocket’ in the 1960s, there were only 60 NEOs known, but by December 2011 over 8,500 had been found and named by the Minor Planet Center. Each year about another 500 are added to the list. There are enough of these for the NRC to estimate an impact
equivalent to the devastation from the 50-metre Tunguska meteorite in Siberia about every 2,000 years on average, although if it’s true that Tunguska-like events could be caused by objects as small as 30 metres,
⁹
they would occur ten times more frequently, giving odds of a hit of this size of nearly 50:50 in anyone’s lifetime. An even chance that a baby born today will see a Tunguska equivalent, capable of wiping out New York, is about as scary as asteroid statistics can be made to appear. But it is contentious.

Of the big, wipe-out-sized NEOs more than 1 km across, 834 asteroids and 90 comets have been identified, and NASA’s Near Object Program estimates that there are only around 70 left lurking out there that we don’t know about.
¹⁰

A little closer to home than the 30 million miles that qualifies as ‘near Earth’ is 20 times the distance to the moon (a total of about 5 million miles), within which range any lump of rock that is found to be at least 150 metres across will earn Potentially Hazardous Asteroid (PHA) status. So far 1,271 PHAs have been found, 151 of which are in the potentially end-of-the-Earth-as-we-know-it bracket of more than 1 km diameter.

Still, these quantities are low enough to suggest that a clash with debris of that calibre is only expected every few million years. The NRC stoically points out that ‘while this apocalyptic possibility is extraordinarily unlikely to happen in the lifetime of anyone living now, traditional approaches to preparing for disaster would become irrelevant.’ Have they not heard of Bruce Willis? Mass extinctioners of the 10 km size that did for the dinosaurs are estimated to come along every 100 million years or so, so not much need to worry there.

These are all averages, of course, seen from a perspective of thousands or millions of years, but the NASA catalogue of NEOs allows us to escape just a little from largely theoretical calculations and talk about what might be in store for those of us living now from specific rocks, all with their own personalised name or number. Each NEO is classified according to its size and probability of impact: these probabilities do not reflect any randomness, since it is assumed that the asteroid is either going to hit us or not, but is simply due to ignorance of its precise trajectory.

The TORINO scale
¹¹
– named after the conference venue in Italy at which it was adopted – expresses the appropriate level of concern:

Level 0 (white) says no problem;

Level 1 (green) is a routine safe fly-by;

Level 2 (yellow) merits attention by astronomers but not the public;

Level 3 means the public should be told;

And so on up to level 10 – a certain collision threatening global civilisation.

At any time there are usually a few level-1s, but Apophis, which is around 200–300 metres across, was given level-2 status in 2004, when it was discovered, and temporarily upgraded to level 4, when a 2.7 per cent chance of impact in 2029 was estimated.
¹²
New information has shown we’re safe after all, although it should be visible to the naked eye on 13 April 2029, when it passes only 18,300 miles from earth. Note that 13 April that year falls on a Friday.

The current catalogue indicates no serious risk from asteroids that we know about. In November 2012 the greatest danger that NASA could point to was a 1-in-500 chance of a collision with the 140-metre 2011-AG5 some time in the 2040s. But most asteroids less than 500 m across remain undiscovered, and although these are unlikely to bring the end of the world, they can take us by surprise: 2008 TC3 was around 2 to 5 metres across and weighed 80 tonnes when it burst above the Sudanese desert on 7 October 2008. It was the first such asteroid to be detected before impact, but there was only 19 hours’ warning and one can imagine the crisis if the predicted path had contained a big city. It exploded with a force of around 2,000 tonnes of TNT, and 10 kg of bits were picked up afterwards. Nobody was hurt.

So what is the risk of being killed by one of these rocks from space in your lifetime? This is impossible to assess accurately, and that’s where Norm comes unstuck. For although there is rapidly increasing understanding of the chance of an impact, predicting the consequences requires many assumptions (or guesses, depending on your point of
view). Nevertheless the NRC report quotes the wonderfully precise figure of 91 deaths expected per year. Of course, this is an average between almost all years with zero deaths and a few events, averaged over millennia, with massive casualties, which once again just goes to show the problem with averages. In fact, that 91 is roughly evenly balanced between the more common small-scale impacts and the very unlikely globally catastrophic impacts. Since there are 7 billion people on earth, this works out at 1/77th of a MicroMort per person per year – the equivalent of about a 3-mile car journey – which comes to the delightfully round number of 1 MicroMort per lifetime from asteroids. Not a lot. And a faintly ridiculous number with no practical purpose. It’s not surprising Norm is troubled.

What could be done about an imminent threat? The NRC report identifies four main strategies for mitigation, emphasising the massive uncertainty about the hazards, the technology, and how society would respond. The first,
civil defence
, involves standard disaster management and is suitable for small events, or anything without much warning. If, for example, it was found that Apophis did have a high probability of impact in 2029, the ‘risk corridor’ could be identified and people warned. The NRC – just like Prudence, Kelvin and Mrs N – identifies possible ‘concerns about property values’.