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Naked eye comet shines on UK National Astronomy Meeting

April 04, 2002

Naked eye comets are generally few and far between, so UK astronomers have been surprised and delighted by the appearance of comet C/2002 C1 (Ikeya-Zhang) in the night sky.

Scientists are particularly intrigued by the re-appearance of Ikeya-Zhang after a long exile in the depths of the outer Solar System. According to Mark Bailey (Armagh Observatory), who will be speaking about the rare comet at the UK National Astronomy Meeting in Bristol on Friday, this is not the first time that the icy visitor has illuminated Earth`s skies.

"Careful studies of the comet`s orbit, based on calculations by Japanese astronomers S. Nakano and I. Hasegawa, and Brian Marsden of the International Astronomical Union Minor Planet Center, prove with near certainty that Ikeya-Zhang was previously recorded in 1661," said Bailey.

Early studies of the incoming comet`s orbit indicated that it might be linked to the bright 16th-century comet C/1532 R1. Recorded as a particularly brilliant object in the sky, this comet was visible for 119 days from 1 September to 29 December 1532 (before the invention of the telescope).

The possibility that C/1532 R1 was a periodic comet, returning to the inner Solar System after a long spell in the dark regions on the edge of the Sun`s realm, has been recognised for some time. A number of leading 18th century astronomers noted a marked similarity between the orbits of comets C/1532 R1 and C/1661 C1, and in 1705 Edmund Halley suggested that they might be one and the same.

The Astronomer Royal, the Revd Nevil Maskelyne, revived the suggestion in 1786, and on the basis of the assumed identification he predicted where the comet should be during its presumed 1788/1789 return. However, extensive searches at the time proved unsuccessful.

"The failure to spot the comet at this time probably rules out Halley`s hypothesis that they are the same comet," said Bailey.

"Initially we could not be entirely sure about the precise orbit of Ikeya-Zhang, and to which if any of the 16th and 17th-century comets it is linked," he said, "but there is now strong evidence that it is the same comet as C/1661 C1, and that the 1532 comet - if it is connected at all with comet Ikeya-Zhang - must have separated from it a number of revolutions earlier."

"This means that Ikeya-Zhang has returned to the inner Solar System after a journey of 341 years that has taken it more than 100 times the distance of the Earth from the Sun," he added. "No other comet with such a long period has been witnessed on successive orbits around the Sun. Halley`s comet, by comparison, comes our way only every 76 years or so."

Studies of historical records are likely to lead to further associations between Ikeya-Zhang and comets of the past. Meanwhile, Bailey and his colleagues David Asher and Apostolos Christou are trying to calculate the comet`s long-term evolution.

Assuming the link with either C/1532 R1 or C/1661 C1, the Armagh team has considered a total of 27 clones of C/2002 C1, and integrated the resulting orbits in a model Solar System for periods ranging up to about 1 million years before and after the current apparition.

"Our investigation shows that almost 75% of the Ikeya-Zhang clones become short-period Halley-type comets at some time during their orbital evolution," said Bailey. "The majority of these return to long-period orbits (each one taking more than 200 years to come back, often much longer) after a few tens to hundreds of thousands of years."

According to Bailey, "Comets with periods less than a few thousand years often provide the most impressive visual displays and are less likely to suddenly fade in brightness."

"Observations of C/2002 C1 suggest that it fits the `bright intermediate-period comet` pattern," he added.

Royal Astronomical Society (RAS)