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Posted on 03/09/2002 10:15:59 PM PST by green team 1999
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A big question for skywatchers during the next couple of months is how bright the newly discovered comet, Ikeya-Zhang, will become. The answer can't be accurately predicted, but this much is nearly certain: The comet will provide an opportunity that comes along just once or twice per decade.
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| Comet Ikeya-Zhang: Week-by-Week Viewing Details Sky maps and viewing tips for finding comet Ikeya-Zhang during March and April 2002.  |
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| Spacewatch 101: Tips & Terms Basic terms and information to help you get started in backyard astronomy. |
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Observations of the comet in recent days however, have tempered those initial high expectations.
Currently, Ikeya-Zhang appears in binoculars and small telescopes with a faint and somewhat distorted bluish gas tail about 5 degrees long accompanying a sharp, well-condensed head of about fifth magnitude.
[Magnitude is the brightness of an object in the sky. The lower the figure, the brighter the object. The brightest stars are zero or first magnitude. The faintest stars visible to the eye on dark, clear nights are sixth magnitude. First magnitude stars are 100 times brighter than those of sixth magnitude.]
Dimly visible
Ikeya-Zhang might eventually get as bright as third magnitude, meaning that it should be at least dimly visible to the naked eye in dark skies, though better seen in binoculars or telescopes. That kind of brightness would still make Ikeya-Zhang a very fine comet from the viewpoint of an amateur astronomer, especially in April, when it will be approaching the Earth and become well placed high in a dark sky.
But at the time of this article's publication, it doesn’t appear that this comet will become the kind of spectacle that comet Hale-Bopp was in grabbing the public’s attention in 1997. However, regardless of what script we write here for Ikeya-Zhang’s performance, be advised that comets are notoriously bad actors. Few celestial events have greater false-alarm potential than the interplanetary vagabonds we call comets. Earlier this winter, for example, comet LINEAR WM1 briefly and unexpectedly flared-up, becoming as bright as third magnitude, though visible only from the Southern Hemisphere. Comet Ikeya-Zhang could brighten similarly and provide a real surprise. Ancient visitor returns Soon after a preliminary orbit was calculated for Ikeya-Zhang, some orbital experts, lead by Syuichi Nakano of Japan, noticed a similarity to a pair of much earlier comets that appeared in 1532 and 1661. The 1532 comet, in particular, was a strikingly bright comet, according to Oriental records. Curiously, during the first week or two that Ikeya-Zhang was under careful scrutiny by observers worldwide it appeared to be brightening at an unusually rapid pace. Perhaps, some thought, this was going to be the return of the great comet of 1532. Excitement began to build with the prospects of a potentially spectacular comet gracing the late winter and early spring skies. But then, during late February, Ikeya-Zhang’s brightening noticeably slowed. A more recent orbital computation by Brian Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, now suggests Ikeya-Zhang may be a return of the 1661 comet, not the one from 1532. This is a come-down of sorts for skywatchers, since historical records suggest the 1661 was a middle-of-the-road performer. Interestingly, this is not the first time that these same two comets were embroiled in an identity crisis. In the late 17th Century, Sir Edmond Halley -- the same man for whom the most famous comet is named -- compared the apparent similarities of the orbits of the comets of 1532 and 1661 as part of his own comet studies. He became convinced that they were one in the same, even implying that there would be a return of the comet in 1790. What Ikeya-Zhang might look like Joannes Hevelius of Gdansk, Poland, observed and wrote extensively about the 1661 comet in his 1668 tome, "Cometographia." He went on to report that the nucleus, or head, of the 1661 comet displayed "multiple structure," as seen in his crude telescope. Rather than seeing the break-up of the comet nucleus, which can cause a comet to brighten suddenly, Hevelius might have been observing a series of bright jets of material being expelled from the comet head. The 1661 comet also displayed a tail that measured 6 degrees in length (for comparison, 10 degrees is roughly equal to the width of your fist held at arm’s length). These descriptions may help to provide clues as to how comet Ikeya-Zhang may appear to us in the coming weeks. One important difference, however, is that the 1661 comet headed directly away from the Earth after sweeping closest to the Sun (a point called "perihelion") and quickly faded away. But Ikeya-Zhang will be approaching the Earth for a number of weeks following its perihelion and thus should remain visible for a much longer stretch of time. If the 1661 comet and Ikeya-Zhang are indeed the same, it would set a record of sorts: the longest amount of time that has elapsed between the discovery of a comet and a definitive sighting upon its return to the inner solar system. The current record is held by comet Herschel-Rigollet, discovered by Caroline Herschel in 1788 and rediscovered 151 years later by Roger Rigollet, in 1939. Comets with orbital periods of 200 years or less are considered "short period" comets. If Ikeya Zhang is the 1661 comet, this would be the very first time that the return of a "long period" comet, with an orbital period greater than 200 years, has ever been observed and noted as such. The 1661 comet might have reached the far end of its cigar-shaped elliptical orbit around the year 1830, when it was probably more than 9 billion miles from the Sun -- more than twice as far away as Pluto. If so, then ever since it has been on a slow, steady course taking it back toward the Sun, finally to reach its closest point again on March 18. Next Page: What You Can Expect to See
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FUTURE SPACE Next week: SPACE.com looks at interstellar spaceflight and the language of the long-term haul: English. |
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