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Comets Stay Hot: Hyakutake and Hale-Bopp Keep Comet Interest Coo

© Wesley Colley

Comets have dazzled Earthlings for as long as people could look up. Regarded them as signs from the gods, omens or curses, comets mystified the ancients, and the fascination has never dwindled. This month and next, we have the rare fortune to see one of the most intrinsically bright comets in recroded history. Comet Hale-Bopp is now visible and will remain so in the dawn or dusk skies. Viewers should consult Sky and Telescope's charts for exact locations, but the comet is so bright, that I doubt anyone will have trouble recognizing it if he or she looks in the right general direction during the right general time. In short, go outside after dusk and look west after March 23.

Comets have also remained a hot topic in research circles. As carriers of pristine material from the solar formation, they have much to tell us about our past. Often call "dirty snowballs" they posess a large amount of frozen volatiles, such as methane, water and cyanogen, and a fair amount of "dirt" (mainly amalgamated carbon and silicon dust grains). As they travel toward the sun, these volatiles begin to evaporate, producing a long gas tail; meanwhile the evaporation liberates dust particles, creating a second tail of dust. The tails often appear separate, because of differing affects of gravity, radiation and charge on the two types of matter in the tails as the comet flies through space.

The treasure trove whence comets originate is called the Oort cloud, after Jan Oort, who is generally regarded as the first to motivate convincingly the idea of a comet cloud in the outer reaches of the solar system. Recent research has suggested that the cloud extends as far in as Pluto's orbit, and as far out as 10,000 Earth-Sun distances. This is so far that other stars' (i.e. not the Sun's) gravitational tugs can be enough to send a comet coasting in toward the Sun, or pull it away from the Sun's grasp entirely. Closer comets seem to be affected significantly by Neptune. A few pulls from Neptune may be enough to start an inner comet on its journey toward its mother star.

Once in the planetary system, Jupiter's mammouth gravitational tug dominates the dyanmics of comets (as we saw particularly with Shoemaker-Levy). In fact, many of Jupiter's moons are buried tens of kilometers below comet ice from doomed comets Jupiter has reaped into its system. But, if things work out just right, a comet may slip into the inner solar system where Earth is the boss, and we get a free show in the sky.

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