Central Bureau for Astronomical Telegrams

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Two Bright(ish) Comets in 2000?

Produced at the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, Massachusetts, U.S.A.

Although it is not anticipated that those contending with Y2K problems this coming New Year's Day will have a spectacular comet to grace their darkened skies, there may be one bright enough to be glimpsed with the naked eye next July, as well as one that could be modestly visible with binoculars six months after that, when the millennium really does turn.

Comet C/1999 S4, discovered by LINEAR (the Lincoln Laboratory Near Earth Asteroid Research project) on September 27, is currently little closer than the planet Jupiter and requires at least a moderately large telescope in order to be seen. Steadily approaching during the next few months, it ought to brighten somewhat before disappearing into the sun's rays in March 2000.

Reaching Mars' distance next May, the comet should be a binocular object in the northeastern sky before sunrise in June, before moving under the pole and into the northwest by late July. A little outside Venus' distance when at its closest to the sun on July 26, C/1999 S4 will then also be near its minimum distance of 35 million miles from the earth and should be at its peak brightness. What that peak will be is currently anybody's guess, but it is quite unlikely that this comet will become anything like as bright as comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp). The nominal prediction for C/1999 S4 currently gives a peak of fourth magnitude. One of the most important factors in predicting a comet's brightness is whether or not the object has passed near the sun before. A comet that is on its first approach from the Oort Cloud cometary reservoir on the outskirts of the solar system is less likely to please than one that has survived previous passages. Whether C/1999 S4 may be on its first or is on a subsequent pass is not yet known. The latest available information is contained on Minor Planet Electronic Circular 1999-U14, issued today.

Comet C/1999 T1, discovered by Scots-born astronomer Rob McNaught on a photograph taken by Malc Hartley with the U.K. Schmidt telescope in New South Wales on October 7, is currently even a little beyond Jupiter's orbit and will be conveniently observable for the next year or so only from the southern hemisphere. Passing about 108 million miles from the sun on December 13 next year, this comet will then be moving quickly northwards to be best observable in the northern hemisphere when expected at its brightest, during the week the twentieth century comes to an end. Again, whether C/1999 T1 could be on its first pass is still not known, so how bright it will become is anybody's guess, the nominal prediction currently being at best sixth magnitude. As 2001 dawns, C/1999 T1 may be a noticeable binocular object in the dawn sky. The latest orbit calculation is contained on Minor Planet Electronic Circular 1999-U15, issued today.

Brian G. Marsden

1999 October 19

E-mail: bmarsden@cfa.harvard.edu

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