Initially, astronomers cautioned that Comet Hale-Bopp might be experiencing an outburst -- a rapid but temporary upsurge in brightness -- and that it would soon fade to a more typical brightness level. Such outbursts have occasionally been observed in other comets. But soon after the July 1995 discovery, Rob McNaught at the Siding Spring Observatory in New South Wales, Australia went back through the observatory records and found a previously unrecognized image of Hale-Bopp. Taken in April 1993, the photo showed the comet over 13 astronomical units from the Sun, well beyond the orbit of Saturn. This meant that Hale-Bopp was inherently bright.
Since its discovery, Comet Hale-Bopp has brightened more or less on schedule. By mid-1996, sharp-eyed observers in dark sites could find it with their naked eyes, despite its being over 4 astronomical units from the Sun. As of press time in early October, it has already developed a prominent dust tail; detailed photographs have seen eruptions of dust and gas at a phenomenal rate. The signs look good for an impressive display in early 1997 (see box). A year from now, it will be gone, not to return again for 3,400 years.
Hale-Bopp is a scientific windfall, one of only two comets -- the other being Halley -- to be seen as it develops through a wide range of temperatures. Astronomical textbooks had to be rewritten after the 1986 appearance of Halley, and they may well need to be rewritten after the 1997 appearance of Hale-Bopp.
It is also an educational windfall, a chance for students of all ages to participate in gathering knowledge about this visitor. The comet will not shoot across the sky like a meteor; it will gradually shift in position and appearance from night to night. Careful observation of the comet should reveal week-to-week, even night-to-night, variations in its brightness, in the length and structure of its tails, and in the structure of its coma.
Students can record such changes photographically. An ordinary 35 mm SLR camera on a tripod, with a regular, wide-open lens (say, 50 mm at f/1.8) and fast film (ASA 400 or 1000), should be able to record detail in the tail. Comet photographers should experiment with various exposure times to see how the detail varies.
Falling apart. On March 24, 1996, three small pieces of Comet Hyakutake broke off and started to form their own tails, as hinted at in the third frame from the right. This mosaic of negative images (in which dark represents bright) shows both tails of the comet. The twisted strands toward the left are the dust tail. The gas tail is the dark spike at the back of the head of the comet; vaguely flowing from that spike is the straight, narrow continuation of the gas tail. Image and interpretation courtesy of James M. De Buizer and James T. Radomski. © 1996 James M. De Buizer, James T. Radomski, and the University of Florida.
Ironically, these claims obscure the true, subtle kinship of Hale-Bopp and us. Comets formed when our planet formed. They are made of the same water and organic materials we are. They delivered many of those materials to the early Earth and occasionally revisited to wipe the evolutionary slate clean. By turning the attention of the average layperson skyward for a while, Hale-Bopp may make people feel a little bit closer to the universe of which we are all a part.
As for me, I plan to enjoy the spectacle and to take full advantage of it for the purpose of public science education. I want to do my part to ensure that every person is aware of this comet and has the opportunity to view it. I encourage anyone who shares these goals to work with me toward making them a reality.
ALAN HALE is the founder and director of the Southwest Institute for Space Research in Cloudcroft, N.M. The institute is an independent organization devoted to astronomical research and education. Hale, as in Comet Hale-Bopp, has written a book, Everybody's Comet (available through the ASP Catalog) and coordinated an Internet project, "Hale-Bopp: Live in the American Classroom." His email address is firstname.lastname@example.org. For more information on Comet Hale-Bopp, visit http://www.halebopp.com. George Musser contributed to this article.
Anatomy of a comet. The nucleus (1) is the heart of the comet -- a fluffy, dirty snowball 1 to 50 kilometers in diameter. Generally we don't see the nucleus; it is too small. Instead, we see the dust and gas that the nucleus spews off. This material forms the coma (2), a cloud up to 1 million kilometers across which we see as the fuzzy head of the comet. Electrically charged gas from the coma forms the straight tail (4); glowing molecules give it a bluish tinge. Dust (tiny grains of rock) from the coma forms the second, curved tail; reflected sunlight colors it yellowish-white. Diagram courtesy of NASA, based on sketch by Donald. K. Yeomans.
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