They 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 of Armagh Observatory, speaking about the rare comet at the UK National Astronomy Meeting in Bristol today, 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 recognized 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 Rev. 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.

Comet C/2002C1 (Ikeya-Zhang) passed within 76 million km (47 million miles) of the Sun near midnight on 18 March. As it heads back towards the outer Solar System, it will pass within 60 million km (38 million miles) of the Earth on 30 April.

The comet is visible to observers in the Northern Hemisphere throughout this period, though it remains near the Sun in the evening sky until mid-April, when it will be close to the W-shaped constellation of Cassiopeia. By late April it will probably be fading in brightness, but it will remain above the horizon all night.

The comet was independently discovered by amateur astronomers Kaoru Ikeya of Shizuoka prefecture, Japan, and Daqing Zhang of Henan province, China, on 1 February 2002.

Ikeya-Zhang is the third bright comet to be visible around the time of a UK National Astronomy Meeting (NAM) in the past six years, the others being C/1996 B2 (Hyakutake), which passed exceptionally close to the Earth in March 1996, and the famous C/1995 O1 (Hale-Bopp), which reached its brightest in March 1997.

Although comets are notoriously unpredictable, the next prominent naked-eye comet is expected to be C/2001 Q4 (NEAT). This may become at least an easy binocular object between April and June 2004.

A more detailed analysis of the history of Comet C/2002 C1 (Ikeya-Zhang) can be found in an article by David Asher, Mark Bailey and others in the April 2002 issue of the R.A.S. journal, Astronomy & Geophysics.

Related websites:

Comet C/2002 C1 Ikeya-Zhang. Details and images

Royal Astronomy Society

UK National Astronomy Meeting

[Contact: Dr. Jacqueline Mitton, Peter Bond]

12-Apr-2002