March 25, 1996
As I suggested last week, your best chance to look for Comet Hyakutake will be tonight and tomorrow night when it passes between the dippers. This evening it will be just below the cup in the Little Dipper (Ursa Minor) and tomorrow it will be near Polaris, the north star. It will then pass Cassiopeia on Thursday and Friday. During this entire time it will be brighter than any nearby star.
Despite that brightness, Comet Hyakutake will wander through the Solar System about 8 million miles from our own planet, much less than the 93 million miles to the sun.
But what if it came closer? Indeed, what if it smashed down through our atmosphere and struck the Earth?
My concern about those questions led me to an interesting new book by John S. Lewis, ³Rain of Iron and Ice: The Very Real Threat of Comet and Asteroid Bombardment² (Addison-Wesley). Despite Lewis¹ even-handed approach, he made me feel like Henny Penny and her entourage who worried that the sky was falling.
Lewis reviews scientific evidence of major earth impacts such as the Tungusta explosion over Siberia in 1908; the three-quarter mile wide Winslow, Arizona crater caused by a meteorite just 40 yards in diameter; the Caribbean asteroid splash down 65 million years ago that may have ended the reign of the dinosaurs; as well as hundreds of lesser strikes. He compares the violence of these episodes with other natural disasters: the Krakatau explosion of 1883 off Indonesia, the 1737 earthquake-generated tidal wave or tsunami as tall as a 21 story building that ran ashore on the Russian Kamchatka Peninsula across the Bering Sea from Alaska, and our more recent nuclear explosions.
These terrestrial incidents pale in comparison with the results of the fall of a large meteor. For example, Lewis says, even a 10 yard diameter meteor would smash down with the force of over 40 kilotons of TNT, excavate a crater 300 yards in diameter, and demolish everything within a mile of impact.
Lewis then describes computer simulations of future asteroid impacts. His calculations provide average death tolls from these collisions as high as 2450 per year, many times more than our annual loss in aircraft crashes. Even more disconcerting is the possibility of this average rising to over 6000 per year.
What should be done? Lewis offers a number of suggestions.
First, we should find and track the approximately 200,000 extremely threatening objects (greater than 160 yards in diameter) at an annual international cost of $30 million or about $22,000 each for the 1400 lives saved. He compares this with the billion dollar cost of a single communication satellite.
Second, we should employ atomic explosions or other means to nudge large asteroids headed for earth out of their collision course. Once the catalog of threatening objects is completed, there will be advance warning so that we can accomplish this economically.
Third, we must accept the risk of impact by one the 200 million smaller but uncharted bodies in near-earth orbit that still could explode with megaton force. Even though a Hiroshima-sized blast is expected only once every thousand years, it could happen tomorrow.
Finally, we should not wait until we have a catastrophe before initiating protective measures.
Lewis¹ analysis is important. I hope his message gets through to international legislators. His recommendations seem well worth the investment of our tax dollars even though they won¹t give us full protection.
Henny Penny has indeed more to worry about than that acorn that fell on her head — and so do we all.