A lump of space detritus a mile across, hitting Earth at around 15 miles per second, would release energy equivalent to a million megatons of TNT, or a hundred million Hiroshima bombs. Computations of the effect of such a blast suggest global catastrophe. No matter where ground zero was located, we’d all be in deep trouble.
You may think this is just theory, but astronomers have seen such a cataclysm, on another planet. Inmid-1994, the 20-odd pieces of a broken-up comet called Shoemaker-Levy 9 slammed into Jupiter. The black eyes that the planet received had areas ranging up to four times the surface area of Earth. And the largest of the comet fragments responsible were only a third of a mile in size.
We know also that such things have occurred on our deceptively docile planet. Geologists have so far identified more than 250 impact craters spread over the continents. On Earth, unlike on the dead, pock-marked moon, active geology and atmospheric weathering erode craters relatively quickly. In any case, 70 per cent of the globe is ocean.
But geologists have estimated how often Earth suffers a cataclysmic impact energetic enough to disrupt the climate globally. Astronomers like myself have made similar estimates from studying asteroids and comets in space.
Our answers agree. Once every 100,000 to 500,000 years, we can expect a major impact, sufficiently powerful to cause the deaths of perhaps half of humankind. That may seem like a comforting answer, but it implies, say, 2.5 billion deaths every 250,000 years, or ten thousand deaths per year, taken as a global long-term average.
Smaller events occur more often. The last really significant one was in 1908, when an asteroid 50 or 60 yards across blew up in the atmosphere above Siberia. This is what usually happens, deceiving us in several ways. For one, it leaves no crater. For another, by exploding at an altitude of a few miles (when the shock of its hypervelocity plummet into the atmosphere causes the rock to shatter into pieces) the object actually causes maximum damage across a wide area. In the Siberian case, it was over a largely uninhabited region. But the flash from the 15 megaton explosion ignited the forest instantaneously, and the following blast (taking 20 to 30 seconds to reach the ground) then blew out the fire and blasted flat the trees over an area of a thousand square miles. We expect such events about once a century; on average. If the next one were to occur over Marble Arch, the whole of London out to the M25 would be razed to the ground.
It is true that you are more likely to die in a car accident than in an asteroid impact. But you are less likely to die in an air crash, where the chances are one in 30,000 against one in 10,000 for an asteroid.
These are averages: a commercial pilot is obviously more at risk from an air crash and an inhabitant of Sydney, San Francisco, Lisbon or any other coastal region facing a large ocean is more at risk from an asteroid because of the phenomenal tsunamis that oceanic impacts produce.
At present, there is not much we can do about the smaller asteroids that hit Earth. Less than about 100 yards in size, these are too difficult to spot, without spending high sums of money. In any case, the annualised death rate they cause is low (around a hundred people per year, globally).
The very large objects (bigger than three miles in size) induce the most damage, but their rate of impact is so low that, again, the annualised death rate is low. You are unlikely to die in a mass extinction event of the sort that hit the dinosaurs.
The most dangerous asteroids are those close to the threshold for causing a global catastrophe, which we define as being an event that would kill at least a quarter to a half of humanity. The size of object that would cause such a calamity is uncertain, because it depends on the impact speed, density and where it hits. But a diameter of about a mile is in the right ballpark. If there is one of that dimension lurking around with our number on it, then we’d better find it soon.
To be conservative, we might take our size limit to be half a mile. (Later, we might set our sights on still smaller objects — the ones that could obliterate a country but not a continent.) Such half-mile asteroids strike Earth only once per 100,000 years, on average. Thus the chance of one being due within the next century is only one in a thousand. And that is the only period of interest to us: we’ll let our great-great-grandchildren look after themselves.
What we need to do, then, is to carry out a surveillance programme aimed at finding all these moderate-sized and larger asteroids, and map their orbits so as to answer one vital question: is there an impact due soon?
If there is, then the odds are that we will have years of warning time, and so we should be able to knock it off course. As they say in the movies, we have the technology — but we hope we won’t have to use it.
A parallel here is with cancer screening. It costs relatively little, and most likely you will not contract the disease in question. But if you do, then your survival depends upon an early diagnosis. None of the solutions to a cancer diagnosis is pleasant: chemotherapy, operative intervention, radiotherapy. The same is true for asteroids. Nuclear weapons provide the only known way of giving the offending object a shove (although in a quite different way from what you will see in the movies).
There is also a parallel in terms of cost. Develop cancer and the expense of tackling it is of no consequence: it’s either your bank balance and mortgage or your life. Similarly, if we found an asteroid likely to hit us in, say, 23 years’ time, then the entire global product would not be too much to spend. We would be staring down the barrel of a gun.
So the essential thing is to give ourselves enough warning time. I first wrote to the British government on this subject more than 11 years ago, when I was operating the only southern hemisphere asteroid search project, at the Anglo-Australian Observatory in New South Wales. The only thing to have changed since then is that the Australian government has closed down that programme, to the dismay of researchers around the world.
The result is that about a third of the sky is being ignored — and it would be more if the Americans could not see some way south with their telescopes in Hawaii.
Here in the UK, despite much posturing, the government has done nothing. The UK government’s task force on potentially hazardous near-Earth objects filed its report two years ago: it had a set of 14 recommendations, leading off with the need for a dedicated southern hemisphere search telescope to complement America’s efforts.
None of those recommendations has yet been implemented. Not a single Earth-approaching asteroid has been spotted by any British search team. Nor have we contributed anything to the mathematical effort to track the routes of potentially hazardous asteroids.
By comparison, the five search projects in the US are finding near-Earth asteroids at a combined rate of about one per night. Japan also has a search project up and running. Italy is a world leader in the dynamical studies.
I know this all sounds like science fiction. I know you think we have more important concerns. Indeed, most likely we have. But if, to our considerable misfortune, there is a mile-wide lump of celestial rock that the clockwork of the heavens has designated to slam into our planet any time soon, then there is no greater problem that humanity faces: not war, not famine, not disease.
To tackle that rock, if it exists, it is vital we find it soon. We have all our eggs in this one little basket we call Earth. We must safeguard it for future generations.