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.


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.


>From Tuesday 9 April to Friday 12 April (am) Dr. Bailey can be contacted via
the NAM press office (see above).

Normal contact details:

Dr. Mark Bailey


Armagh Observatory

College Hill


BT61 9DG

Tel: +44 (0)28-3752-2928

Fax: +44 (0)28-3752-7174