[meteorite-list] Two Naked-Eye Comets At Once! - C/2001 Q4 (NEAT) & C/2002 T7 (LINEAR)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:27 2004
Message-ID: <200402251845.KAA24063_at_zagami.jpl.nasa.gov>

http://cfa-www.harvard.edu/press/pr0409.html

Harvard-Smithsonian Center for Astrophysics
Release No.: 04-09
For Release: February 25, 2004

Note to Editors: A photograph of Comet C/2002 T7 (LINEAR) taken with a
MicroObservatory robotic telescope is online at
http://cfa-www.harvard.edu/press/pr0409image.html.

Two Naked-Eye Comets At Once!

Cambridge, MA - A naked-eye comet - one visible to the unaided eye without
telescope or binoculars - is an enjoyable sight, particularly for the
brighter comets. On average, a naked-eye comet graces our skies about once
every two years. However, most remain fairly faint or appear close to the
Sun as seen from Earth, such that even experienced observers may require
binoculars to spot them. Only rarely do two relatively bright naked-eye
comets appear simultaneously. Such an event will take place in April and
May of 2004, when skygazers will feast their eyes upon both Comets C/2001
Q4 (NEAT) and C/2002 T7 (LINEAR).

Astronomer Dan Green (Harvard-Smithsonian Center for Astrophysics),
Director of the Central Bureau for Astronomical Telegrams (CBAT), said,
"As the clearinghouse for comet discoveries, CBAT has known of these
comets for a long time. We have monitored them, collecting observations
from around the world. If they brighten as predicted, then both may be
visible to the naked eye in late April and part of May. If you haven't
seen a comet, this is a great opportunity to go out and look at one."

Dirty Snowballs

Historically, bright comets were interpreted as portents of doom, as in
1066 when the appearance of a comet, later known as Comet Halley, was
blamed for the defeat of the Saxons at the Battle of Hastings. Comets were
thought to be vaporous "exhalations" of the Earth, merely atmospheric
phenomena. Only as science advanced in the 16th and 17th centuries were
they recognized as true denizens of the solar system.

The object most people visualize when they hear the word "comet" actually
has three components - a small, irregular nucleus; a spherical, gaseous
coma surrounding it; and a broad, sweeping tail. The cometary nucleus is
the source of the gas and dust that create a comet's dramatic appearance.

In 1950, Harvard astronomer Fred Whipple coined the term "dirty snowball"
to describe a comet's nucleus. He began studying comets at a time when
little was known about them, but he said, "It turns out they were simple."
The nucleus is a chunk of ice and rock ranging in size from 100 yards or
less up to several miles in diameter. Frozen gases (ices) of water, carbon
dioxide, and methane are mixed with dust and rock into a conglomeration
much like chocolate chip ice cream. As the comet nears the Sun, the Sun's
heat vaporizes those ices, puffing off clouds of gas and dust that
surround the nucleus to form a glowing coma. Radiation pressure from the
Sun, combined with the solar wind, then sweep material from the coma
outward to form a tail that can stretch across millions of miles of space.

Scientists are interested in comets for a number of reasons. "Comets are
thought to have formed in the outer reaches of the solar system, and may
thus contain rock and ices that date back billions of years. Also, comet
tails are indicators of the solar wind and have helped us learn about the
inner solar system. And not least, comets are known to hit planets from
time to time, including Earth, so we need to keep an eye out for potential
impactors," said Green.

One intriguing possibility directly links humanity to these visitors from
the outer solar system. While Carl Sagan once said that we are star stuff,
Fred Whipple would add that we are comet stuff. "Part of the water in our
bodies comes from comets. That's because some proportion of the Earth's
water comes from comets," said Whipple.

To a layperson, the appeal of a comet may reside less in its scientific
value than in its dramatic display of cosmic splendor. Comets C/2001 Q4
(NEAT) and C/2002 T7 (LINEAR) both have the potential to provide pleasant
sights when they swing through the inner solar system this spring.

Two Bright Comets For 2004

Comet NEAT is not especially keen, nor does Comet LINEAR travel a
particularly straight line. Instead, both are named for the robotic
telescope survey programs that discovered them. The programs locate comets
so prolifically that many comets have shared the same names, including
some reasonably bright comets, hence the importance of using the comets'
full designations.

On August 28, 2001, the Central Bureau for Astronomical Telegrams at CfA
announced the discovery of Comet C/2001 Q4 by the Near Earth Asteroid
Tracking (NEAT) program, for which the comet was named.

Astronomers describe the brightness of celestial objects using a magnitude
scale: the higher the magnitude number, the fainter the object. When
found, Comet NEAT glowed at only 20th magnitude, about 400,000 times
fainter than the faintest star visible to the unaided eye. Yet predictions
indicate that Comet NEAT may brighten to 1st or 2nd magnitude in late
April and remain that bright through mid-May, making it visible to
skygazers, possibly even with light-polluted city skies. From
non-light-polluted, clear skies, Comet NEAT may be visible to the unaided
eye from early April through late June.

The second bright comet now approaching the Sun, Comet C/2002 T7 (LINEAR),
was found by the Lincoln Laboratory Near Earth Asteroid Research (LINEAR)
program and announced on October 29, 2002. To an experienced observer,
Comet LINEAR may become visible to the naked eye in mid-March, when it is
expected to brighten to 6th magnitude. (Stars as bright as 7th magnitude
are visible to the unaided eye in dark skies, if they are directly
overhead. A comet is harder to spot since its light is spread out rather
than concentrated in a star-like point, and it may be located close to the
horizon where extinction dims it further.) However, the position of Comet
LINEAR in the constellation Pisces places it very close to the Sun in our
sky, so observers will have to wait several weeks for their first good
viewing opportunity.

Both comets are likely on their first trip through the inner solar system
after having been nudged out of the Oort Cloud, the spherical reservoir of
comets that surrounds our Sun far beyond the orbit of Pluto. As a result,
peak brightness estimates are uncertain. "Comets do a lot of things that
are unpredictable," said Green. If a comet should break apart, as happened
with Comet C/1999 S4 (also called LINEAR) in the year 2000, it would never
become bright in our skies. On the other hand, if a comet should undergo a
sudden outburst, it could brighten substantially above predicted levels.
Both professional and amateur astronomers currently are monitoring and
will continue to monitor the comets.

Viewing Tips

Both comets will present viewing challenges, since at their brightest they
will also be relatively close to the Sun as seen from the Earth. As a
result, they will appear either in the western sky shortly after sunset,
or in the eastern sky shortly before sunrise. The glow of twilight may
interfere with viewing.

Spotting either comet will be easier if observers first locate their
target through binoculars, searching the relevant area of the sky for the
fuzzy round glow of the coma, with a tail pointing up from the horizon.
Once you know where to look, try lowering your binoculars and looking with
the eyes alone.

Green recommended, "You'll need a good observing site with a low horizon,
few city lights, and clear skies. Neither comet will be particularly easy
to pick out, especially in the light-polluted skies that most people face.
Your best bet is to attend a public open night at your local observatory
or astronomy club, where experienced observers can help you."

The Southern Hemisphere will enjoy the best views of Comet C/2001 Q4
(NEAT), which moves from the constellation Tucana through Hydrus and into
Dorado during April, passing by both the Small and Large Magellanic
Clouds. It will be visible in the evening sky between sunset and about
11:00 PM local time. During May, Comet NEAT moves northward, becoming
visible to observers at mid-northern latitudes early in the month. Comet
NEAT appears in the evening sky after sunset as it slides from Canis Major
through Cancer and into Ursa Major by month's end. Comet NEAT passes
closest to Earth on May 7th at a distance of around 30 million miles (48
million kilometers). (For comparison, the Earth is at a distance of 93
million miles, or 150 million km, from the Sun.)

The most eye-catching views of Comet NEAT in the Northern Hemisphere are
likely to occur during May 12-16, when the western horizon after sunset
shows the comet and four bright planets (Venus, Mars, Saturn, and Jupiter)
all in a line. The dramatic lineup not only offers wonderful photographic
opportunities, but also graphically demonstrates the contrast between the
clockwork regularity of orbiting planets and the irregular serendipity of
visiting comets.

Comet C/2002 T7 (LINEAR) may be visible in the morning twilight just
before sunrise in late April and early May to Northern-Hemisphere
observers with a flat eastern horizon. The comet then disappears into the
solar glare again as it moves from Pisces through Cetus, Eridanus, and
Lepus to Canis Major. Comet LINEAR finally reappears in the twilit evening
sky in late May, but will be fading, so observers should use binoculars to
locate it before attempting to view it with the naked eye.

Southern Hemisphere observers will enjoy better views of Comet LINEAR,
which will be visible in the early morning sky to the east from mid-April
through early May. In mid-May, Comet LINEAR begins to swing into the
evening sky, and for a few days may be visible both immediately after
sunset in the west and immediately before sunrise in the east. Around May
20th, the comet becomes an evening object visible only in the western sky
after sunset. Comet LINEAR passes closest to Earth on May 19th, at a
distance of about 25 million miles (40 million km).

As June opens, both comets will fade as they speed ever farther from both
the Sun and the Earth. Yet if current predictions hold, the brief but
enjoyable appearances of Comet NEAT and Comet LINEAR will be remembered
for years to come!

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

For more information, contact:

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468
daguilar_at_cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
cpulliam_at_cfa.harvard.edu
Received on Wed 25 Feb 2004 01:45:01 PM PST