A new tail for Comet Hale-Bopp.

Even as it begins to fade, Comet Hale-Bopp continues to beguile skywatchers. Two week ago, astronomers spied a huge tail, of a type never seen before, emanating from the comet's dusty shroud, or coma. Composed of sodium atoms, the tail stretches some 50 million kilometers in length and 60,000 km in width and is distinct from the familiar dust and ion tails that Hale-Bopp and other comets flaunt when they near the sun.

Researchers detected the tail on April 16, using a wide-field camera at La Palma Observatory in the Canary Islands, Spain. Sodium has been detected before in comets, but never in the form of a tail and never this far from the coma. Gabrielle Cremonese of the Astronomical Observatory in Padua, Italy, reported the finding in an April 18 circular of the International Astronomical Union. An image taken by the Polar satellite on March 31, but only recently analyzed, confirms the presence of the tail.

Michael F. A'Hearn of the University of Maryland at College Park says he was surprised by the discovery but suspects "it is a common phenomenon that nobody looked for before." Sodium atoms absorb radiation at wavelengths that coincide with the peak intensity emitted by the sun. As a result, the atoms interact strongly with sunlight and are easily pushed great lengths from the coma by radiation pressure, he suggests.

Michael R. Combi of the University of Michigan in Ann Arbor argues that radiation pressure isn't strong enough. He says that the solar wind, the sun's stream of charged particles, pushes on sodium-containing material in the inner coma. After leaving the coma, the sodium separates from its parent molecule.