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Us travel destinations for Generally speaking, with respect to shape, comet comas fall into two major categories. Some are more circular globular, and others are oval elliptical ig, such as the comets Hale-Bopp and Ikeya-Zhang, or fan-shaped parabolic, like Tebbutt’s Comet of FIG A comet with a globular coma compared to a comet with an elliptical coma. Image credit Sirscha Nicholl. Globular comas often have a blue or green hue, because they are more gassy than dusty. Elliptical and parabolic comas often appear more yellowish, because they are dustier. The most spectacular comets in history, heavy dust-producing comets that have made particularly close passes by the Sun coming within the orbit of Mercury, tend to have elliptical or parabolic comas. Because of the Sun’s intense radiation pressure, the dust expelled sunward by the nucleus is immediately forced back behind it, with the result that the nucleus is very close to the sunward side of the coma. Us travel destinations 2016.