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Book 4

Chicken Little Was Right 
Norm Sleep, Science Writer

Yucatan, early June, sixty-five million years ago

The summer day begins as usual on the shallow reef. Fish and ammonites forage among the vegetation as they try to avoid becoming shark bait. Pterodactyls soar in the trade winds, waiting for tidbits. Suddenly, the sky to the south brightens like a second sunrise. Within seconds, the
How do we know when this impact took place? We know the time of year from clues frozen in ponds and preserved in the geological record. The time of day is artistic license, because more goes on during the day than at night.
entire sky glows at white heat. Every mobile organism instinctively dives for cover. But there is no hope for them. One second later, coming in fast and at a low angle, an asteroid fifteen kilometers in diameter crashes to earth at twenty kilometers per second, vaporizing itself and the upper few kilometers of its center of impact. The shock produces a crater as wide as the Mediterranean Sea. A thick blanket of hot rock fragments fries every living thing within a few hundred kilometers of the crater rim.

The pent-up rock vapor expands, carried rapidly northward by the momentum of the projectile. Within minutes, the vapor cloud sears the surface of western North America, igniting any exposed plant or animal. This is not an ordinary forest fire; the massive heat wave destroys even seeds buried underground.

Several minutes later, across the ocean in Europe and Australia, dawn comes early as ejected sand-sized fragments return to Earth at cosmic velocities. They glow like meteors, filling the sky for hours. Their heat ignites exposed vegetation, leaving surviving animals with nothing to eat. Soot from the fires fills the lower atmosphere, quickly bringing darkness.

The calamity is just beginning. The meteor fragments vaporize into a fine dust that circulates in the upper atmosphere, blotting out sunlight. Sulfate, vaporized from anhydrite beds beneath the Yucatan, contributes to the opaqueness of the stratosphere. It remains in the air longer than the dust. Darkness brings cold after the heat. Lilies freeze in Wyoming ponds. Photosynthesis stops in the open ocean for more than a year. Plankton species perish, along with the creatures in the food chain above them. Animals not dependent on photosynthesis eke out a living. Survivors include crocodiles in ponds, our insect-eating ancestors in logs, and a species of shore bird-the only remaining dinosaur.

Long before the great asteroid forever changed life on Earth, even worse disasters occurred that make this more recent apocalypse pale, a mere pebble in a pond. Four billion years ago, objects hundreds of kilometers in diameter hurtled from the sky, reshaping the planets. These gigantic impacts produced the enormous basins still visible on the Moon and Mars. The heat from the kinetic energy of the projectiles partly vaporized the whole terrestrial ocean. Amazingly, life was able to weather these storms in the "Goldilocks Zone," which is located in rocks over a kilometer deep, the only safe place for thousands of years after an impact. The surface and shallow subsurface alternately teemed with life, and became a death trap when a large asteroid hit every few ten million years. Both the surface and the deep subsurface were too hot to sustain life. Only heat-loving (thermophilic) organisms in the Goldilocks Zone, living at 100 degrees Centigrade, survived these tribulations to root the tree of life.

Evolution does not directly prepare organisms for conditions they do not regularly experience, like a year of darkness in the tropical ocean. So most organisms are unable to cope with the disastrous results of such freak events. Some adaptations that arose to cope with other environmental conditions, however, provided salvation for a species. For example, in pre-human times the pike evolved sharp teeth for catching and eating its usual diet of smaller fish. Today, these teeth are advantageous for biting through fishing line. Other coincidental adaptations help during rare events. For example, sixty-five million years ago, when most creatures boiled to death during the great cataclysm, animals and plants that were low on the food chain and nestled deep in swamps managed to survive.

Chicken Little was Right: The Risk Is Real!

Your chance of being killed by an asteroid is about the same as in a passenger plane crash: one in a million per year. But with an asteroid, billions of humans will be killed at the same time. How do we avoid the indignity of a mass extinction, with no one left to bury us? Our species possesses one helpful adaptation: our intelligence, which has evolved for hunting, gathering, and social interaction. Unlike dinosaurs, we can observe, predict, and act decisively. Can our smarts save us from an otherwise inevitable tragedy?

Asteroid orbits are predictable in the short run, but over a geological length of time the orbits change. The Earth is a tiny target in the vastness of the solar system. Moreover, earth-crossing orbits are chaotic-a series of minute changes in an asteroid's actual position and velocity build up to huge changes over time. Thus, in the long term, asteroid orbits are effectively random-more so than even a fair roulette table. We know an asteroid will hit us, sooner or later, but we can't say which one, or when.

Even a fair roulette wheel gives nonrandom results due to mechanical variations. By collecting statistics and betting appropriately, Joseph Jaggers broke the bank at Monte Carlo in 1873, and mathematician Claude Shannon built a wearable computer to outwit the roulette wheel in 1961. Nowadays, casinos regularly rebalance their wheels to keep the spin results as random as possible.

NASA monitors near-Earth asteroids. Before this program began a few years ago, we did not know whether an impact was more likely next year or a million years from now. Now, visual tracking and heavy mathematics predict the orbits of large objects five hundred years in the future. Early results are in: no ten-kilometer-diameter object has Earth's name on it. And five centuries from now, society should be better equipped to deal with the hazard of an impending collision. However, the more numerous one-kilometer objects also present a danger of global catastrophe. We do not yet have a complete manifest of these vermin of the sky.

What do we do if we find an asteroid in our path? Civilization will face this calamity sooner or later, certainly within a million years. If we have been diligent, we will have hundreds of years to prepare for an impact. We will be able to soft-land a probe to track the object's course and confirm the danger of collision. We will probably not choose to blow it up, as that would turn one dangerous object into several. More likely we will change its orbit by detonating a nuclear explosive nearby, to spall off some material. Newton's law of the conservation of energy predicts that the equal and opposite force would change the asteroid's orbital velocity by a few centimeters per second. At that time, there may even be advanced rocket motors with enough power to do the job.

The take-home message is that, thanks to the development of human intelligence and our continually increasing knowledge base, the next time an asteroid threatens to destroy the planet, there's a good chance that life on Earth will be saved. And even if most life is destroyed, take heart. After another sixty-five million years, the cycle of evolution may lead to another civilization with the ability to protect Earth against these behemoths from outer space.

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