Astronomy Briefly

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  Target Earth by Duncan Steel, a Readers Digest Book, 2000.

reviewed by Duane Dunkerson

In large part this book is a prophecy of our doom done with a new topic for each pair of facing pages. As with Montaigne we may not want distressful terms used rather often to make us attentive to the dangers of killer asteroids and comets. Nonetheless there are few digressions in this wakeup! book that would force Swift to satirical excellence. We are presented with questions about what is being done to save Planet Earth. It must be saved from firestorms that could consume all of North America. Highly energetic collisions of asteroids and comets with the earth would have cataclysmic events ensue unless foolhardy and irresponsible behavior on our parts is put to a stop.

The book supports a frightening comprehension of the danger we face due to instability of objects in solar orbit. Catastrophic impact is now being recognized as a controlling focus for evolution. It is a cosmic card game. You play the hand you were dealt. A mass extinction could occur. Where would that leave us? All our eggs are in one basket. These vermin of the sky could produce asteroidic Armageddon. Through a comet darkly we see our demise. We can't run for it. The shuttle only gets us a few hundred miles out. Saturn rockets are no more.

We cruise along in orbit about the Sun surrounded by plots of swarms of asteroids and comets as depicted in a photo-diagram in Chapter 1. The foreword by Andrea Carusi and
Brian Marsden, both of the Spaceguard Foundation, tell us of how every 100,000 years or so this planet is hit by a one half of a mile in diameter object.

The author changes that 100,000 interval to 1000 or tomorrow. Such a reduction in time comes about from cometary sources that could propel a comet inward toward the Sun. We might happen to be in the way. Then, too, a rogue asteroid might break from the pack between Mars and Jupiter or from the Trans-Neptune region and be incident upon us.

In the 1890's the first asteroid that could intersect earth orbit was discovered. In the 1930's, three more were found. In the past few years the number has swiftly escalated. Non-intersecting asteroids have been known since January 1, 1801 when Piazzi in Sicily saw Ceres. In 1802, along came Pallas, then 1804-1807 for Juno and Vesta. Not until 1845 was another asteroid, Astrea, discovered. The asteroid total increased in 1868 to 100, then in 1879 to 200. By 1890 there were 300 and, in 1900, the total stood at 449. Now there are 15,000. Before 1873 all were thought to be between Mars and Jupiter. Such is not the case.

In 1932 Amor came close to earth. In 1937 Hermes whizzed by, close by, twice the distance to the moon. Not enough of the orbital elements were secured for Hermes, so we don't now know where it is.

As the asteroid total increased, the occurrence of craters, presumably of asteroidal origin, on the planets began to become apparent. Spacecraft in the 60's and 70's ranged close to the planets to relay photographs or radar data to us showing craters on Mars, Venus, and Mercury. Also the Apollo program conclusively showed the lunar craters to be not of volcanic origin. Seismometers left on the moon still record objects the size of cars slam into the moon. No craters were to be seen when in 1994 Jupiter was assaulted by parts of Comet Shoemaker-Levy 9. But mighty Jupiter was punched hard by the cometary encounters.

As the author notes, comets can come from anywhere at anytime. Our main supply, however, is from the Oort cloud of objects. It is estimated that there are millions larger than about a mile across very far out there but strongly related to our Solar System.

Earth, of course, is part of the Solar System. We have found craters on other planets. Our planet is not an exception. So far 200 impact craters have been discovered on earth. The Barringer crater in Arizona, Manson in Iowa, the Chesapeake Bay, those in Canada, and Europe have been documented. They are mostly ID'd by the presence of iridium which is rarely of earthly origin. The one that caused the dinosaurs to go bye-bye is thought to be the causation of the Chicxulub basin in Mexico's Yucatan peninsula.

What did in the dinosaurs, among other things, was the chunks of soil and such blasted skyward. These fell back to earth and burned up in the plunge through the atmosphere. The temperature on the earth's surface zoomed upward. Plants burst into flames. Then the dust still in the atmosphere hung there and blotted out the Sun. It got too cool too fast. If a big blast, or parts of it occurred in the ocean, a tsunami would happen. This wall of water would go inward along the coasts for many miles.

The dreadful effects of asteroids or comets, relates the author, are not new to human thinking. Halley, of Halley's comet fame, thought a comet hit the earth and caused the Biblical flood. Whiston in 1696 believed a close passage by a comet cracked the earth open, releasing subterranean water which caused the flood Noah rode out. Then Buffon in 1745 said the planets were a rebound, the ejecta sent out from a massive comet-Sun encounter. Seas abandoned their ancient positions, in the opinion of Laplace, when a cosmic clash brought a universal earthly drowning. After the French Revolution, Cuvier found in the excavations undertaken to rebuild Paris, that fossils of sharks and deer. He put forth the notion that catastrophe regularly figured in the long-time history of the world. Lord Bryon also thought comets intersect with us and felt that in the age of steam, aspects of the Industrial Revolution could stop a comet.

The focus on cometary threats moved to asteroids in more recent times. Asteroids were getting more attention. For many, a linkage between asteroids and the death of the dinosaurs came with the publication of an article in 1980. The discovery of asteroids, as objects of scientific study had been aided by the use of the Schmidt telescopes in the 1930's to photograph larger areas of the sky. Shoemaker, decades later, started taking Schmidt photos as pairs with brief exposures. More asteroids were thus found.

The relatively recent use of CCDs (charge coupled devices) in association with telescopes has opened up optical detection of the asteroids to computer-equipped amateur astronomers as well as very sophisticated procedures put into use by professionals. The CCD allows for finding fainter objects. Computer use results in less time to process the data. Gehrels of the University of Arizona in 1989 secured CCD scans of the sky. LINEAR (Linear Near-Earth Asteroid Research) came on line in 1998. It uses a military "Star Wars" byproduct to search for asteroids. Usually one per night is discovered. Other observatories are at work in Canada, Lowell Observatory in Arizona, and Japan. The author laments that no constant surveillance is underway in the southern hemisphere. Large, automated search efforts are needed worldwide.

Asclepius in 1989 had a close enough approach to earth to cause the US Congress to authorize NASA to check into the hazards posed by asteroids. NASA produced a report in 1992 that contained the recommendation for a need to predict a collision decades ahead of time. We would need to intercept it, if such could be done, far from earth. It truly is a matter of rocket science to get to the asteroid or comet.

Most detection efforts assume an asteroid, not a comet, is to be dealt with since they are relatively closer at hand and more numerous in that range. The threat must be dealt with far from earth says the author. A blasting of the rock or comet nearer earth would produce clones not small enough so that their total effect wouldn't be as bad as one large one. The present preferred method is to alter its course away from earth. It is presumed a detonation of a nuclear device near the object would deflect it away from us.

Less deflection and more detection is the goal of the Spaceguard Foundation. Its self-proclaimed mission is to safeguard the future of the human race. The foundation take its name from Arthur C. Clarke's Rendezvous with Rama, in which a very very near earth object smacks into Italy. Thereafter characters in that book decided to begin Project Spaceguard.

Mr. Clarke has an afterword for this book. He comments that the possibility of an impact by a murderous asteroid or comet is statistically low but with hazardous consequences so dire as to be labeled tremendous and stupendous and horrendous.





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