[meteorite-list] Viewer's Guide to New Comet Ikeya-Zhang

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:02:26 2004
Message-ID: <200203081612.IAA27832_at_zagami.jpl.nasa.gov>

http://www.space.com/spacewatch/spacewatch_comet_020309.html

Viewer's Guide to New Comet Ikeya-Zhang
By Joe Rao
space.com
08 March 2002

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.

Comet Ikeya-Zhang will make its closest approach to the Sun on March 18,
when it will be roughly 47 million miles away or midway between the orbits
of Mercury and Venus. Shortly after it was discovered on Feb. 1, it appeared
there was a chance that Ikeya-Zhang might evolve into the comet of the
decade, judging by an initial rapid brightening and its possible link to a
spectacular 16th Century comet.

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.

Ikeya-Zhang: What You Can Expect to See

The next few weeks will afford skywatchers an unusual opportunity.

In any year, several comets will typically come within range of good
binoculars or small telescopes. But a comet bright enough to be readily
visible to the unaided eye comes along usually only once or twice per
decade.

If Ikeya-Zhang attains third magnitude, it would be comparable to the star
Megrez, the star in the Big Dipper that joins the handle with the bowl.

Of course, a comet's appearance is much different and less well defined than
the sharp image provided by a star. In 1974, Kenneth Weaver, an assistant
editor of National Geographic magazine, provided an excellent description of
a comet possibly comparable to Ikeya-Zhang. Weaver had traveled to the dark
skies of Kitt Peak National Observatory near Tucson, Arizona, for a glimpse
of comet Kohoutek. He wrote:

"No spectacular lighting of the sky, no radiant display such as early
predictions had led us to expect. One moment it was not there, the next
moment it appeared. To the naked eye, it seemed as though a giant
paintbrush, dipped in whitewash, had been drawn swiftly across the black
wall of heaven, leaving a long, ghostly swath above a gently glowing blob."

When Weaver wrote these words nearly three decades ago, he was describing a
moderately bright comet from a location completely free of light pollution.
Today, with the glare of bright lights seemingly metastasizing everywhere,
it is far more difficult to find such dark, starry and pristine conditions.
Urban skyglow has robbed many of us of our night skies, and the vast
majority of the population of the United States now live in regions where
the stars are mostly blotted out by overly excessive lighting.

If you live in an area where light pollution is severe, the first
prerequisite in obtaining a good view of comet Ikeya-Zhang will be to travel
to a darker location where you can see many of the fainter stars. Anything
less will likely mean that, at best, the comet may only be barely visible
through the bright light haze, and more likely not at all.

Comet metamorphosis

Comet Ikeya-Zhang's overall appearance could change radically between the
time it makes its closest approach to the Sun in late March and when it
passes closest to the Earth near the end of April.

If it proves to be a particularly active comet, noticeable changes may occur
even from night-to-night. It probably won't lose much in overall brightness
during most of April, as it approaches Earth and swells in apparent size.
Although its tail will then be shrinking in physical size, it also will be
swinging toward the Earth, causing our viewing angle of the whole tail
structure to change.

It should be stressed, however, that the shape and size of a comet's tail is
very hard to predict.

What we'll ultimately see depends on several variables -- the comet's orbit,
the relative locations of the comet, Earth and Sun, and of course the size
and composition of that icy clumping of solar system detritus that forms the
nucleus of a comet.

The most important factor is the amount of dust released from the comet
nucleus as its surface ice vaporizes in the heat of the Sun.

John Bortle, a long-time comet consultant for Sky & Telescope magazine, has
previously noted that the tail of Ikeya-Zhang is likely to be composed
chiefly of gas and not dust, meaning it will likely appear rather faint with
a bluish hue. The gas tail is a fairly thin, straight line pointing away
from the Sun.

On the other hand, comets that produce prodigious amounts of dust become
eye-catching because the dust tails appear much brighter. The dust tail is a
flat sheet of material spread out in the plane of the comet's orbit with one
edge immediately adjacent to the gas tail.

Past comet comparisons

As a good example of how these factors can affect appearances, most people
were far more impressed with comet Hale-Bopp in 1997, as opposed to comet
Hyakutake in 1996. Although both comets were similar in brightness,
Hale-Bopp shed significant tails of both dust and gas, as opposed to
Hyakutake whose tail was mainly gas. Even people in brightly-lit cities
could trace some of Hale-Bopp's tail.

Yet in comparison, although Hyakutake's narrow gas tail stretched across a
much greater expanse of the sky, only those blessed with very dark skies,
far from any light pollution, actually saw it. A similar fate may await
those who attempt to view Ikeya-Zhang.

The only way you'll know for sure is to actually go out some evening soon
after sunset or, better yet, on a morning during early- or mid-April before
dawn breaks, and attempt to see the comet for yourself.

The comet's production of gas or dust could halt without notice, making the
tail vanish and causing Ikeya-Zhang to appear much like comet Halley's
disappointing apparition back in 1986. Or you might witness a sudden
increase in dust or gas production, and the tail might lengthen before your
eyes.

Footnote

Ikeya-Zhang was discovered by Kaoru Ikeya of Japan and Daqing Zhang of China
on Feb. 1, 2002. While this was Zhang's first comet discovery, it was
Ikeya's sixth.

At one time, Ikeya was considered one of the world's most prolific comet
hunters. He was the discoverer or codiscoverer of five comets, bagging them
between January 1963 and December 1967. One of these, comet Ikeya-Seki, went
on to become the brightest comet of the 20th Century during late-October
1965.

But after 1967, nothing had been heard from Mr. Ikeya for more than three
decades. Exclaimed one Japanese astrophotographer after hearing of Ikeya's
latest find: "He is the phoenix!"
Received on Fri 08 Mar 2002 11:12:32 AM PST