From Space.com, 19 February 2002
http://www.space.com/spacewatch/anew_comet_020219.htmlBy Joe Rao
A newly discovered comet, now approaching the Sun and Earth, could develop
into a relatively bright naked-eye object in coming weeks, researchers say.
The best views of the comet may be reserved for those under dark skies far
from bright lights, but even city dwellers should be able to spot it.
Kaoru Ikeya of Japan and Daqing Zhang from China first sighted the comet in
the constellation Cetus, the Whale, on Feb. 1. Both described it as a weak,
condensed glow in their telescopes with no mention of a tail.
The comet is called Ikeya-Zhang. The latest orbit calculation indicates it
will pass closest to the Sun, a point called perihelion, on March 18 at a
distance of 47.1 million miles (75.8 million km). After rounding the Sun,
the comet will continue moving toward Earth, making its closest approach to
our planet, called perigee, on April 28, when it will be 37.6 million miles
(60.5 million km) away.
Ikeya-Zhang's expected path across the sky in the coming weeks will greatly
favor Northern Hemisphere observers. During most of March on into early
April, the comet will be visible near to the north-northwest horizon about
an hour after sundown. Bright moonlight may hinder observations during the
last week of March.
After the first week of April, with the Moon no longer a factor, the comet
will also be visible in the morning sky, rising earlier and getting
progressively higher above the northeast horizon each night.
Seen before?
Initially, it appeared that this comet would not get brighter than fourth
magnitude, which is similar to the brightness of a relatively dim star.
Magnitude is a measure of a celestial object's apparent brightness.
But John Bortle, a longtime comet consultant for Sky & Telescope magazine,
said it could get brighter.
Soon after Ikeya-Zhang's orbit was calculated, some orbital specialists
noticed a similarity to a pair of much earlier comets that appeared in 1532
and 1661, Bortle explained in an e-mail interview last week. The 1532 comet,
in particular, was apparently a bright comet according to Oriental records.
Brian Marsden of the Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, an orbital specialist, said last Thursday that "a revolution
period of 400-500 years (for Ikeya-Zhang) is likely," keeping alive
speculations that this may be a return of the 1532 comet.
The key to figuring out if the comets are the same may lie in Ikeya-Zhang's
orbital period -- how long it takes to go around the Sun.
"In recent days, several observers have made their own independent
calculations suggesting that Ikeya-Zhang might have an orbital period of
roughly 500 years, making for a strong argument that there may indeed be a
direct connection with the comet of 1532," Bortle said.
What to expect
How comet Ikeya-Zhang ultimately performs is anyone's guess. So far it is
brightening more rapidly than originally expected. As of late last week, it
had nearly doubled in brightness in just one week and was at magnitude 6.8
as of Feb. 15.
But Terry Lovejoy, an assiduous comet watcher from Australia, says it's a
bit early to get excited.
"We've seen this situation before," Lovejoy cautions. "At first a new comet
appears to brighten at a much faster than normal rate, but then as it gets
closer to the Sun it seems to run out of puff. My best guess is that this
comet will be no different and will peak somewhere at around magnitude 3.5."
Such a brightness is just slightly fainter than Megrez, the star in the Big
Dipper that joins the handle with the bowl.
"A peak of magnitude 3.0 would not be at all surprising to me," says Bortle,
adding that he expects Ikeya-Zhang to unfurl an impressive tail perhaps up
to 15 degrees in length as it sweeps by the Earth. For comparison, your fist
held at arm's length measures roughly 10 degrees. However, because this tail
is likely to be chiefly composed of gas and not dust, it will appear faint
and bluish and likely only be visible to those with access to dark skies
free of light pollution.
So how might Ikeya-Zhang stack up against other popular comets?
The 1986 appearance of Halley's comet, considered disappointing by many,
also peaked at around magnitude 3.0. In contrast, comet Hale-Bopp, which put
on a memorable show in April 1997, attained a brightness close to magnitude
-1, or about 60 times brighter than Halley.
Binoculars or a small telescope should allow most skywatchers a view of
Ikeya-Zhang's fuzzy head, called a coma, and of the tail.
Editor's Note: Viewing tips and more information about comet Ikeya-Zhang
will be provided in our Spacewatch section in March.
Copyright 2002, Space.com
