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A comet's tale

Comets are giant icebergs floating in the vastness of space. Trillions of comets surround the Sun and most are never detected, but when one or two happen to move in toward the Sun their orbits may geometrically become either elliptical (around the Sun several times) or parabolic/hyperbolic (only once around the Sun and gone forever). High-school geometry comes in handy when talking about comet orbits. While we think of space objects as hurtling through the void at thousands of miles per hour, many comets in the Opik-Oort Cloud move at speeds not much faster than a private airplane on Earth. But when they are knocked out of their deep sleep by a gravitational tug or ping from cosmic rays traveling at light speed, these lumbering space giants will take rapid flight and fall Sunward. Even the slightest of force against a comet will knock it on a vast conic loop toward the Sun. Once in motion, comets can take millions of years just to make one orbit of the Sun. Some comets loop in and get stuck in smaller orbits; they may return to the inner solar system every so often. Such a comet is famous Halleys Comet, named after astronomer Sir Edmund Halley. This comet passes near Earth every 76 years. Others may pass by us once and disappear forever on hyperbolic orbits. A typical comet can be about the size of New York City. It sometimes exudes a thin atmosphere, called a coma, and its surface, called a nucleus, is hilly and rocky with a covering of dirty snow and mixed icesand other stuffincluding water, carbon, cyanide, ammonia and ethylene (natural gas), and much more. As it warms up on a near approach to the Sun, a jet of coma dust and gas from its evaporating surface forms a long tail that can be a marvel to behold. The solar wind pushes the tail ahead of, or behind, the comet. Frenchman Jules Verne, grandfather of modern science fiction, penned one of my favorite childhood space books. Vernes two-part novel, titled Off on a Comet!, first appeared in 1878. Off on a Comet! is a fantastic adventure story about a collision of a comet with the Earth and a small band of humans who are whisked away on a chunk of Earth; the chunk is pushed through space in front of the comet. Ok, so Monsieur Verne doesnt explain how a portion of Earths breathable atmosphere remained intact with less gravitydetails, details. Later, the intrepid humans begin a fantastic journey through the solar system. Youll have to track down this out-of-print book to discover its surprise ending. While it may be more than a century old, Vernes entertaining comets tale is still worth reading to learn about how comets (and humans) behave. Whats in the Sky: Summer begins June 21 in the northern hemisphereits the first full day of the new season. The Sun begins moving southward, too; the days become shorter. On June 22, look in the west for three bright space objects: Saturn (left), the star Regulus (middle), and red Mars. Saturn will be the brightest in the trio. Lou Varricchio, M.Sc., lives in Middlebury, Vt. He was a NASA senior science writer; he is now associated with the NASA/JPL Solar System Ambassador program in Vermont. His book Inconstant Moon is available at Barnes and Noble.com

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