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Heavenly Skies: A comet cometh

By: By Forrest Lockhart, Special to the Telegraph
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Since recorded history, the passage of a comet through the night sky has inspired awe and fear. Comets that pass close to the Sun along their orbit frequently sport long diaphanous tails of gas and dust. Seen in centuries past, comets were interpreted variously as harbingers of plague, impending disaster, or the ascension of a new king. The origin of comets was not fully understood until 1704, when Edmund Halley explained how comets travel about the Sun. Noting the recorded passage of a comet in 1456, 1531, 1607, and 1682, Halley theorized that the objects were in fact one object with an orbital period of 76 years. He predicted that, according to his theory, the comet would reappear in 1758. When it did appear according to his prediction, it was named for him. Unfortunately, Halley, who died in 1742, did not live to see the return of his comet.

Comets are now known to be balls of dust and ice sized from about 300 feet in diameter to more than 25 miles across.  Popularly described as dirty snowballs, comets were formed billions of years ago when the solar system was formed. Scientific observations reveal that in addition to very dark, dusty surfaces, the interiors contain ice, a variety of organic compounds and even amino acids, thought to be the precursors to life here on Earth.

Comets that reappear on a regular schedule are classed as periodic. Short-term periodic comets may have orbits that bring them back into view anywhere from a few years to two centuries. These objects originate in the Kuiper Belt, the region occupied by our former ninth planet, Pluto. Long-term comets possess orbital periods of over 200 years, and originate far out in the Oort Cloud. Infrequent gravitational perturbations in the spherical cloud may send cometary objects plunging towards the Sun.

Another class of comets is the non-recurring type; objects from the Oort Cloud that have been kicked into hyperbolic orbits inward toward the Sun and then propel them completely out of the solar system. This March we may be witnesses to the passage of one of these unique comets. Beginning about March 8th, comet Pan-STARRS C/2011 L4 will whip around the Sun and be visible to keen eyed observers in the evening sky on its outward passage. Astronomers forecast that comet Pan-STARRS will have a total brightness of third magnitude, and may display a tail of gas and dust in the twilight sky. Each evening after March 8, the comet will rise higher in the western sky, but is expected to dim considerably as it retreats from the Sun and returns into the deeps of space.

Observers, preferably equipped with binoculars, who travel to high ground with an unobstructed view of the western horizon may catch a glimpse of comet Pan-STARRS. If you do spy the comet, remember that no human has ever seen it before, nor will any have your experience again.

Forrest Lockhart is a docent with the Cameron Park Rotary Club Community Observatory in Placerville.