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Not With A Whimper, But With A Bang!
by Joel Marks
The U.S. space program has been in a state of vacillation for decades. Although marvelous scientific missions have been mounted, the really big-ticket items have gone begging for rationales. Gone are the days when we could set a dramatic (and expensive) goal like landing human beings on the moon for the hell of it … or to beat the Russkies … or to buck up the military-industrial complex. The International Space Station never had the allure of 2001: A Space Odyssey … not even in 2001. President George W. Bush’s goal of launching people to Mars can no longer be propelled by Ray Bradbury nostalgia. And the latest manned Moon venture has just sputtered to an indifferent, even welcomed, indefinite hold by the Obama administration.
How odd this is, though, when a glaringly important and urgent mission to outer space – indeed, a flotilla of missions – has been staring us in the face for decades. I am referring to the defense of planet Earth from asteroidal or cometary impact. Why has this been ignored or sidestepped in favor of amorphous and ad hoc justifications for building rocket ships? Partly this may be due to some fallacious reasoning: The policymakers and opinion formers have apparently bought into a complacent approach based on reassuring but misleading probabilities.
To understand this, let us begin at the beginning: How does one assess the risk of human extinction by collision with an errant extraterrestrial rock? Start with some facts. An asteroid or comet of the order of 1 to 10 kilometers in diameter is all it would take to wipe out civilization and us. There are estimated to be about a thousand such objects whose orbits currently cross near the Earth’s orbit, so-called NEOs (Near-Earth Objects). Most of these have been identified, but some remain unknown. As far as we can determine, none of those known threatens collision with our planet in the next 100 years. Other possible threats, however, are the hundreds of thousands of such-size bodies in the asteroid belt between Mars and Jupiter, the millions more in the Kuiper Belt beyond the orbit of Neptune, and the trillions more in the Oort Cloud way beyond that. At any time, one of those objects could be sent hurtling into the inner solar system where we reside due to a collision with a neighbor or the perturbation of its orbit by some passing star, or indeed may already be on its way due to such an occurrence in the past.
Counterbalancing all of the above, though, is the hugeness of space. What, then, are the chances that one of them will result in a bull’s-eye projectile? Statistically speaking, it would seem, very low indeed.
But there are two misconceptions involved in that assessment. First is that, even if unlikely, impact by a 1-kilometer-plus asteroid or comet would have practically infinite significance. For what is at stake is, essentially, everything – or everything human, at any rate. Now the standard formula for risk assessment is to multiply the probability of the occurrence by the magnitude of the projected loss. So here we have a very small probability but a humongous loss. Ergo: high risk! Those who take solace in the low likelihood of this kind of impact, therefore, are ignoring the bump-up of risk by the magnitude of what is at stake.
The second misconception is the ‘low probability’ itself. Suppose the estimates of NEOs plus the history of major impacts on our planet suggested that an object 1 km or greater in diameter hits the planet once every million years on average. What would this mean? Pretty much … nothing. There are several ways to think about this. The complacent folk would argue that, therefore, in any given year the chance of a major impact is only 1:1,000,000. And suppose even that the last major impact took place exactly 1,000,000 years ago: Still, they would argue, there is hardly any imminent threat. To think otherwise would be to commit the Gambler’s Fallacy, which is to believe that, for example, since the last ten flips of a fair coin have come up tails, the chance of the next one’s being heads is much greater than 50:50. But this is not so: the chance of a fair coin coming up heads remains 50:50 no matter how many consecutive tails have preceded the flip. Just so with a major impact: even if the annual chance were 1:1,000,000 and 1,000,000 years had elapsed since the last one, the chance of one next year would still only be 1:1,000,000. Voilà: relaxation via mathematical prestidigitation.
But there is another way of thinking about the figures that leads to anything but relaxation. Consider again the coin-flipping: (1) sooner or later there will be a head and (2) you have no idea whether it will be sooner or later. Applied to the impact scenario, this means that another extinction-type impact is guaranteed to happen sooner or later (if we don’t prevent it), and we have no idea if that will be sooner or later.
So what follows rationally? Given again the infinite magnitude of the projected loss, I would think the answer to be obviously that we should prepare for a major impact as if we knew it were going to occur just as soon as we could possibly prepare for it.
Alas, to convince the powers-that-be of this is an uphill battle. And not so much because of the technological hurdles or even the expense, but rather the absence of public understanding and political will. What would be required to deal with this sort of threat? Nothing short of continual exhaustive monitoring of asteroids and comets, a sustained research program on methods of mitigation, and the indefinite maintenance of weapons of mass destruction, specifically nukes, along with their delivery systems. (The ‘mass’ in this case is bodies of matter and not multitudes of people, fortunately.) But this flies in the face of current and foreseeable political realities, such as the push for denuclearization and the failure of global cooperation even to check global warming, which is already known to be in progress. How much less likely are the nations of the world, then, to marshal the resources and resolve needed to ward off a risk that is not known to be imminent? … and, indeed, due to the misunderstandings described above, is considered to be low-risk and improbable?
I am afraid, therefore, that Arthur C. Clarke got it right in this as in other predictions: If we are lucky, the wake-up call will be the destruction of ‘only’ a major city by a relatively small impact, as happens in his 1972 science-fiction novel Rendezvous with Rama “on the morning of 11 September.”
© Joel Marks 2010
Joel Marks is Professor Emeritus of Philosophy at the University of New Haven in West Haven, Connecticut. His thoughts about NEOs as expressed herein owe much to the National Research Council’s Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies: Final Report (2010), available online, and the website of the Gaiashield Group. More of his essays on astronomy can be found at skyskinny.blogspot.com.