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 July
31, 2000 -- Astronomers around the world continue to monitor
the unexpected disintegration of comet
C/1999 S4 (LINEAR). Intense solar heating apparently triggered
a massive disruption of the comet's fragile icy core when it
passed close to the Sun last week. It is still bright enough
to see through small telescopes so even amateur astronomers can
watch the comet as it dissolves. If you do plan
to look, don't wait. Experts think that comet LINEAR might
disappear completely in a few days.
Above: This R-filtered image of comet LINEAR was captured
on July 28, 2000, by M. Kidger at the Jacobus Kapteyn Telescope,
Roque de los Muchachos Observatory, La Palma, Canary Islands.
The innermost coma is elongated and rapidly fading.
The break up of a bright comet is unusual but not unprecedented.
For example, comet Shoemaker-Levy 9 (SL-9) broke up before it
struck Jupiter in 1994. SL-9 was discovered after it fragmented,
so there is no record of what happened as it came to pieces.
With comet LINEAR, astronomers have a ringside seat for the entire
show.
"We have observed a few comets in the process of breaking
up -- comet West in 1976, comet Ikeya-Seki in 1965 and others
-- but never with so much detail as we're seeing in comet LINEAR,"
says Mark Kidger, an astronomer at the
Instituto de Astrofisica
de Canarias. Comet LINEAR's demise seems to be a bit unusual.
"Cometary splittings rarely ever lead to the rapid disappearance
of a comet like this - in fact, I don't know of another case"
Kidger was the first to notice comet LINEAR disintegrating
as he monitored a cloud of gas (called the "coma")
surrounding the comet's core using the 1-meter Jacobus Kapteyn
Telescope. Comet LINEAR, which has been falling toward the Sun
since it was discovered in September 1999, made its closest approach
to our star (perihelion) on July 26, 2000. Perihelion is a critical
time for any comet. It's when solar heating of the icy core is
most intense and when the comet swings around for its long return
trip to the outer solar system.
"At perihelion there are very rapid aspect changes as regions
of the nucleus previously in shadow are suddenly subjected to
intense heating," continued Kidger. "This causes strong
thermal stresses" that may have been a primary cause of
LINEAR's breakup.
Something was already amiss the day before Comet LINEAR reached
perihelion at a distance of 114 million km (0.74 AU) from the
Sun.
"The very first images on July 25th were enough to show
me that something odd was going on," recounts
Kidger. "The comet's inner coma was no longer teardrop-shaped
(the solar wind flowing around the comet's head causes this shape).
It had a shape like a short, fat cigar. My first thought was
'Shoemaker-Levy.' It looked just like those first images of Comet
Shoemaker-Levy 9 after it was discovered."
Kidger's images on subsequent nights confirmed that something
dramatic was happening and he announced his findings in an International
Astronomical Union (IAU) Circular (IAUC
#7467) on July 27, 2000. As news of the breakup spread, astronomers
around the world trained their telescopes on the comet. In another
IAU Circular (IAUC
# 7468) published July 28th, three teams of observers reported
that they too saw evidence of a major event in the comet's nucleus.
 Left: The breakup of Comet LINEAR. Contours
represent lines of constant brightness in Jacobus Kapteyn Telescope
R-band images of the comet (credit: Mark Kidger). This 5-frame
sequence spanning the interval from July 23rd to 27th shows the
progressive elongation and disruption of the comet's core. Each
contour map is 40 arcseconds on a side centered approximately
on the core of the comet. (Mark Kidger notes that "the unusual
aspect of the innermost contours on the July 23rd frame is because
this region was so bright that it saturated completely in the
images in a 5 second exposure. On subsequent nights the comet
was nowhere near saturation."
Unlike comet Shoemaker-Levy 9, which broke into many well-defined
bright fragments, comet LINEAR seems to be dissolving into an
amorphous haze of gas and dust.
"There is some similarity of appearance to the two
comets," says Brian Marsden of Harvard's Minor Planet Center.
"An observation by Ian Griffin in New Zealand on July 29th
shows the nucleus of C/1999 S4 (LINEAR) extended into a long,
bright string. However, it does not seem to show discrete nuclei
in that string, as D/1993 F2 (SL-9) did."
The differences between comets SL-9 and LINEAR result from
their different sizes and distances from the Sun.
Comet Shoemaker-Levy 9 was larger than comet LINEAR, and it broke
apart as the result of tidal stresses it experienced when it
passed less than 100 thousand kilometers from Jupiter (within
1.4 Jupiter radii from the planet's center). SL-9 was far from
the Sun (812 million km) when it fragmented and solar heating
was not the primary cause of the break up. In fact, SL-9 wasn't
even orbiting the Sun. The comet had been captured by the gravitational
pull of Jupiter and was orbiting the giant planet instead.
Above: Comet Shoemaker-Levy 9, pictured
here in a Hubble Space telescope image, was broken into many
pieces during a close encounter with the planet Jupiter in 1992.
Two years later it came so close to the planet that the fragments
actually plunged into Jupiter's atmosphere. [more
information]
Comet LINEAR is a much smaller object that has been losing mass
rapidly during its approach to the Sun. The Hubble Space Telescope
recorded a house-sized fragment blowing away from the core on
July 5th and powerful jets of gas vaporized by solar radiation
have been pushing the comet to and fro. Solar heating is a more
important factor in its breakup than gravitational effects. [more information]
"The small size of comet LINEAR and its exposure to solar
radiation is causing a more complete and rapid dissolution than
we saw in Shoemaker-Levy 9," continued Marsden. "The
initial break-up of SL-9 was surely caused by tidal forces from
Jupiter. If they had not later collided with Jupiter, several
of those fragments would presumably still exist. C/1999 S4 (LINEAR),
on the other hand, will probably have completely dispersed in
a week or so."
 Comet
LINEAR may still be bright enough for amateur astronomers to
view in small telescopes, but it's fading fast. On July 27th,
binocular observers in South America and Europe estimated the
comet's visual magnitude to be +6.6 [ref].
That's almost bright enough to see with the unaided eye from
dark-sky observing sites. Two days later, an experienced amateur
in the Canary Islands reported a visual magnitude of +8.3, a
factor of 6 decline in brightness.
"The surface brightness of the innermost coma is fading
fast," says Kidger. "This should translate to a somewhat
slower fade of the outer coma [that binocular and small telescope
observers see] as the gas and dust in it disperses and is not
replenished. Typically a comet may take several weeks for the
coma to expand and fade down to the brightness of the sky background."
Above: On July 23rd, the Comet LINEAR's gaseous halo
was bright and centrally condensed. Since then it has steadily
faded as the core disrupts into an elongated train of debris.
Credit: Mark Kidger, Jacobus Kapteyn Telescope.
Many well-known annual meteor showers, including the Perseids,
Leonids and Geminids, are caused by dusty debris from comets
burning up in the atmosphere of Earth. Such displays are harmless
and beautiful. Unfortunately for meteor lovers, the orbit of
comet LINEAR comes no closer to our planet than 28 million kilometers
(0.18 AU). There will be no "Linearid" meteor shower.
When comet LINEAR finally disappears from view in a few days
or weeks, this memorable visitor from beyond the orbit of Neptune
will be gone forever.
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Stay tuned to Science@NASA for
news and updates about comet LINEAR.
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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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