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The Nation : After Jupiter: How Likely Are We to Be Hit by a Rock?

August 21, 1994|Gregory Benford | Gregory Benford is a professor of physics at UC Irvine and co-author, with William Rostler, of "Shiva Descending," a novel about an asteroid hitting Earth

LAGUNA BEACH — Last month's Jupiter fireworks struck a clear, vibrant chord in the public. Here was disaster, drama, danger. Chunks of Comet Shoemaker-Levy 9 vaporized in fiery moments. Could that happen to us? Sure--in fact, it has happened to Earth before.

About every century, somewhere on Earth a meteor strikes with impact comparable to a nuclear warhead. The most recent, in 1908, fell near the Siberian Tunguska River. It vaporized with an immense bang audible a thousand miles away, killing no one.

In 1972, a rock came breathtakingly close. It skipped across the upper atmosphere above the United States, a bright trail picked up on radar and witnessed by hundreds. It had to be at least 80 feet across to make such a trail, skimming like a flat rock spun across a pond by a cosmic child--then gone. Since then, there have been two other recorded near-misses.

These were small compared with the mountain-sized monsters that strike Earth roughly every hundred million years. One such probably ended the reign of the dinosaurs. At least 28 craters greater than 10 miles across pock our planet, records of previous hits.

Are there many of these asteroids still gliding silently in the darkness? Plenty--at least 2,000, of which we have spotted fewer than 200. A mile-wide stony asteroid could destroy civilization. Its debris would cloak the globe in dust, shutting off sunlight for years, starving a large fraction of humanity. Are such fantasies worth worrying about?

Here statistics collide with our emotions. After all, such colossal impacts are very rare; why bother?

Surveys show that people regard risks rarer than the proverbial one-in-a-million as too small to bother with. Asteroid danger is at this level. Still, statisticians forget that we do not feel through numbers. Studies show that people thinking about risk shrug off dry data, heavily favoring two emotional factors. They ask: Is it dreadful? Is it unfamiliar?

Asteroid impact is both. Dreadful means huge, uncontrollable, with high risk to future generations. The unknown nature--we haven't thought about this before, and one could be heading for us now--heightens our sense of threat.

How can we compare rare catastrophe with everyday danger? One way is to note that, averaged over long times and all humanity, you're as likely to die by asteroid impact as in an airplane crash.

Each year, about 100 airline passengers perish. We have spent hundreds of millions getting that number so low. Contrast the asteroid danger. A mile-wide stone could kill a billion. Divide that loss by the average time between such collisions, a hundred million years, and we get an average rate of 100 deaths a year.

Does this rough equality with the airline-death rate mean anything? Some are outraged at such statistics, as if to quantify human matters robs them of meaning. Alas, situations far beyond our experience, such as risks with low probability but immense consequences, require numbers outside our intuitive grasp. To get an idea of what risks are worth worrying about, consider our present policies. How much money do we spend to prevent 100 deaths?

That depends on both geography and culture. In health programs funded in the Third World, saving a single life costs about $200 (usually due to malnutrition). Cancer screening in advanced nations takes $75,000 to find an early cancer and stop its growth, extending a life for at least five years. Think about the implications of these last two numbers: society's concern rises as dangers approach our personal phobias.

Suppose we take that first number-- $200 a life saved from malnutrition. Then to avoid the average 100 deaths a year from asteroid impact, we can reasonably spend $20,000 a year. This is a fraction of what astronomers devote each year to studying asteroids.

Closer to home, U.S. highway-safety agencies spend about $120,000 to save a life, using better highway dividers, easier on-ramps and the like. Eliminating natural radioactivity in drinking water would cost $5 million a life saved--which is why we don't do it. For nuclear-plant safety, though, we spend $2.5 billion a life. The estimated death rate, nationally, from breathing the fumes of oil- and coal-fired power plants, though, is about 10,000 a year. We try to prevent this with air-pollution controls, which costs roughly a million dollars to avoid one case of deadly lung disease.

Suppose we accept the value placed on life by industrial nations. Taking the cancer-screening level of spending, multiplied by 100 deaths a year, we get $7.5 million a year--enough to find all asteroids near us within a decade and assess our danger.

Like any rational calculus, outcome strongly depends on assumptions. Ours is a rather rarefied argument, spreading the kill rate from one big hit over 100 million years--mathematically interesting, perhaps, but is it convincing? Here those two elements--dread and strangeness--may well settle the issue. People care about so immense a disaster.

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