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 July 5, 2000 -- Later this
month a fuzzy blob will glide across the sky near the Big Dipper.
Called "C/1999 LINEAR S4", or LINEAR-S4 for short,
it's the brightest comet to come along in more than 3 years.
Comet LINEAR-S4 was discovered on September 27, 1999, by the
Lincoln Near-Earth Asteroid Research program in New Mexico. It
appears to be a first-time visitor to the inner solar system
traveling in an
orbit that will return it beyond distant Pluto after it passes
114 million km from the Sun on July 26, 2000.
Right: This time-lapse sequence of Comet LINEAR-S4 was
taken on July 2, 2000, from Arizona and shows the comet's movement
over only 19 minutes. Credit & Copyright: Wil
Milan
"Estimating how bright this comet will become is
tricky," says an astronomer at the NASA Marshall Space Flight
Center. "Comet LINEAR hasn't been here before so we can't
use past experience as a guide."
Astronomers have been monitoring
LINEAR-S4 since its discovery last year. The comet has brightened
slowly but steadily and it is now a 9th magnitude
object visible through binoculars. Experienced comet-watchers
expect LINEAR-S4 to continue brightening until it peaks at 4th
to 6th magnitude on July 23, 2000. That's when the comet will
make its closest approach to Earth at a distance of 56 million
km. (Note: When the brightness of an object equals 6th magnitude,
it is about as bright as the faintest stars visible to the unaided
eye on a dark night. A 4th magnitude star is approximately 6
times that bright.)
If these projections are correct, LINEAR-S4 will put
on a modest show compared to the great comets of 1996 and 1997,
Hyakutake
and Hale-Bopp.
At its brightest in March 1996, Hyakutake was a 1st magnitude
object sporting a tail that stretched 60 degrees across the sky.
Comet LINEAR-S4 will probably be 20 to 100 times dimmer than
Hyakutake and its tail will look more like an elongated smudge
than a sky-rending blaze of light.
Nevertheless, comet LINEAR-S4 could be a beautiful sight as it
moves through the relatively star-poor area around the Big Dipper
in late July. In fact, if the comet reaches 4th magnitude it
will probably resemble another popular fuzzy blob, our nearest
neighboring galaxy, the Andromeda
Nebula. Experts stress that predicting what LINEAR-S4 will
do is more of an art than hard science. The comet might fail
to develop or it could be far more striking than expected.
Comet Linear Finder Chart -- July
23, 2000
Above: Comet LINEAR-S4 is expected to reach peak brightness
around July 23, 2000. Skywatchers in the northern hemisphere
should look for the comet near the Big Dipper around 9 p.m. local
time approximately 30 degrees above the northern horizon. Although
the comet will appear to be stationary when viewed for a short
time, it is actually moving 8 degrees per day with respect to
background stars. (The bowl of the Big Dipper is 10 degrees across.)
The comet will move past the Big Dipper in just a few days. To
view finder charts for other dates, visit Sky
& Telescope and CometLinear.com.
Note for Southern Hemisphere observers: when comet LINEAR
is brightest on July 23 it will be impossible to view from most
southern latitudes as it rises just a few degrees above the horizon
during the day. In August the fading comet will be a good target
for southern telescopes for a few hours after sunset.
LINEAR-S4 is expected to become a naked-eye object around
July 17, 2000, when it brightens to 6th magnitude. Even if the
comet eventually reaches 4th magnitude, it will be invisible
in light polluted urban areas. Dark skies are essential. For
comet watchers, this is a good time for a vacation in the country!
The comet will fade after its closest approach to our planet
on July 23rd and by the end of July, LINEAR-S4 it will be too
dim for naked-eye observations. It will remain a good target
for binoculars and small telescopes well into September. On August
20th, telescopic observers in the southern hemisphere will be
treated to a rare sight when the 7th magnitude comet passes almost
directly in front of the Sombrero galaxy M104. [See Sky
& Telescope for more information.]
 By
the end of 2000, LINEAR-S4 is expected to be a faint 13th magnitude
object, fading as it races back to the outer solar system. Comets
like LINEAR-S4 may originate in the Oort
Cloud, a giant reservoir of comets 50,000 AU from the Sun.
(One AU is an Astronomical Unit, 149.6 million km, equal to the
average distance between the Earth and Sun.) Although the Oort
Cloud is speculative, many astronomers believe that it exists
and is populated with icy relics from the formation of the solar
system more than 4 billion years ago. Occasionally a galactic
gravitational perturbation tweaks the orbits of the Oort
comets and sends some of them plunging toward the Sun. Comet
LINEAR-S4, with its highly elliptical long-period orbit, is probably
one of these. After it passes by the Sun later this month it
may not return for millions of years, if ever.
Right: This 10-minute exposure of comet LINEAR S4 was
captured in Austria on June 28, 2000. Copyright © 2000 by
Michael Jäger and Gerald Rhemann.
Although we probably won't see comet LINEAR-S4 again after this
summer, it's almost certainly not the last "Comet LINEAR."
The LINEAR project
has discovered more than 40 new comets since 1998 and shows no
signs of slowing.
Will this Comet LINEAR be worth watching? Only time will tell.
The best way to find out is to go outside and look!
Science@NASA readers are invited to send their descriptions
and photos of Comet LINEAR to the
editor for possible inclusion in future articles about this
comet.
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Anatomy of a Comet:
 The nucleus of a comet is an irregular
ball of ice and dust typically 1 to 10 km in diameter. When the
nucleus approaches the Sun, sunlight warms the surface and the
solid ice sublimes (turns to vapor). The resulting cloud of water
vapor and carbon dioxide surrounding the nucleus is called the
coma. Most comets come from the most distant reaches of
the solar system, far beyond the orbit of Pluto. By the time
they are as close to the Sun as the Earth, the coma can be larger
than Jupiter. Together the coma and the nucleus form the head
of the comet.
Right: This image of Comet Halley's nucleus
was taken by the European Space Agency Giotto
spacecraft during a flyby on March 13, 1986. Scientists estimate
that about 10% of the surface was boiling off into space. The
stuff that boiled off Halley in 1986 may one day be seen again
during an eta Aquarid meteor shower.
 When a comet is far from the Sun, only
the head is visible as a smudge in photographs. As the comet
nears perihelion (closest approach to the Sun) it sprouts two
tails. The dust tail is composed of small (smoke-sized)
dust particles carried off the nucleus by escaping gases. The
dust tail shines by means of reflected sunlight and is the part
of a comet that is usually easiest to see. A longer, blue-colored
ion tail is made of charged gas that glows as electrons
re-combine with ions to make uncharged molecules. The gaseous
ion tail is pushed straight away from the Sun by the solar wind,
while the brighter dust tail traces the comet's curved orbit.
Usually the two tails point in slightly different directions.
[more]
Above: This
photograph of Comet Hyakutake highlights different components
of the tail. The gold and red tail features are dust, made predominately
of little bits of rock and carbon. The dust tail shines by reflecting
sunlight. Extending past the dust tail is the comet's ion tail,
shown here glowing in blue. The ion tail is composed mostly of
ions of water, carbon monoxide, and cyanogen. The ion tail glows
by emitting light when elections re-combine with electrically
charged ions to make uncharged molecules.
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