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Comet Hale-Bopp Update



Sky & Telescope News Bulletin
April 25, 1997


Comet Hale-Bopp (C/1995 O1) has a tail of sodium atoms that is distinct from its dust and gas tails. This third tail was first seen on April 16th in images filtered to isolate the yellow light (D line) emitted by neutral sodium atoms. The sodium tail is nearly 7 degrees long but only 10 arcminutes wide. Too faint to be seen in normal photographs, it lies a few degrees east of the anti-Sun direction. Spectra show that the sodium atoms are accelerating along the tail and reach 95 km/second at a point 11 million km from the nucleus. Apparently this acceleration is linked to the process that gives rise to sodium's fluorescent glow. Its atoms absorb photons from the Sun but reradiate them in random directions, an imbalance that provides the accelerating force.


Space scientists from NASA and the European Space Agency predict that the ion tail of Comet Hale-Bopp will be particularly susceptible to the solar wind's influence between April 28th and May 15th. They say the blue-colored tail could show brightness changes, kinks, and perhaps moving structures. Also possible are "disconnection events," when the ion tail breaks off from the comet's coma and reforms a few hours later. Scientists have issued this "tail watch" because on the 28th the comet will be within 10 degrees of the plane of the solar equator. In this region the outflowing solar wind is gusty and the magnetic field irregular. When the plane is crossed on May 3rd, the direction of the interplanetary magnetic field will change. Amateur astronomers are encouraged to monitor the tail's appearance carefully during this period. Interested observers can access the comet watch home page at


Comet Hale-Bopp has faded to about magnitude 0, but it is still obvious and impressive in the evening sky. Observers in the Southern Hemisphere are getting their first glimpses of it too, spotting it low on the horizon during twilight. Strong moonlight ceases to be a problem this week. So look for the comet after sunset; it should be obvious about 20 degrees above the west-northwestern horizon.


Comet Hale-Bopp is still in view this week, though it has faded and shrunk considerably now that it has been traveling away from both the Earth and Sun for about a month. It's getting lower each evening too. Look low in the west-northwest right at the end of twilight. Bring binoculars!

Copyright 1997 Sky Publishing Corporation. S&T's Weekly News Bulletin is provided as a service to the astronomical community by the editors of SKY & TELESCOPE magazine. Widespread electronic distribution is encouraged as long as this paragraph is included. But the text of the bulletin and calendar may not be published in any other form without permission from Sky Publishing (contact S&T's Weekly News Bulletin and "Sky at a Glance" are available via SKY Online on the World Wide Web ( At present they are not available via electronic mailing list. comethome.gif Comet Hale-Bopp Home Page