Astronomy Made Simple by Kevin B. Marvel Ph.D

Author:Kevin B. Marvel, Ph.D. [Ph.D., Kevin B. Marvel]
Language: eng
Format: epub
ISBN: 978-0-307-43289-6
Publisher: Crown Publishing Group
Published: 2004-06-07T16:00:00+00:00

COMETS

Comets are the dancers of the solar system. Halley’s comet, last visible from Earth in 1986, is probably the most famous, and many readers may remember seeing comet Hyakutake in 1996 as it filled up almost as much of the sky as the Big Dipper (Fig. 9.4). Spending most of their time in the outer portions of the solar system and dipping closer to the Sun only occasionally, comets, like some asteroids (discussed in the next section), visit both the inner and outer portions of the solar system, making them unique members of our planetary system.

In ancient times, comets were often thought to bring bad luck or calamity, which they certainly would if they actually hit Earth’s surface. Luckily, this has happened only rarely and is not likely to happen in the near future. Try to imagine just how dramatic the appearance of a comet in the sky must have been. There was no television, radio, or other media. There were no satellites. There was no understanding of what the objects in the sky were; even the Moon was a great mystery. The starry sky dominated the nighttime lives of everyone in creation, and the regular constellations and slow parade of the planets was a known and accepted part of everyday life. Suddenly, the sky changes—cut into by a bright object with a huge tail that rapidly moves across the sky. It is clear why these hairy wanderers (the name comet comes from a Latin phrase meaning long-haired stars, a reference to their dramatic tails) had such a dramatic impact on people.

Comets have a bright nucleus, the inner portion of the comet, sometimes called the core. The part we see is a gaseous region very close to the comet but not the comet nucleus itself. A comet’s nucleus is rarely more than a few miles in diameter, although the region we see can be much larger. This extended visible region of a comet is known as the coma.

Figure 9.4—Comet Hyakutake

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