|
 July
28, 2000 -- Dashing the hopes of many sky watchers around
the world, comet C/1999 S4 (LINEAR)
was too faint to see with the unaided eye as it made its closest
approach to Earth on July 22nd. Its visual magnitude appears
to have peaked between magnitude +6.0 and +6.5, just below the
threshold for naked-eye observations. Nevertheless, the comet
is still a good target for binoculars or small telescopes. [See
Sky & Telescope for finder charts].
LINEAR may have disappointed casual stargazers, but it is intriguing
professional astronomers with unpredictable behavior including
jets that are perturbing the comet's orbit and an outburst in
July that may have sent a fragment hurtling away from the comet's
core.
Above: This 3-frame sequence of Hubble Space Telescope
images spanning July 5th through 7th shows the brightness of
comet LINEAR increasing by 50% and then subsiding again as it
blows off a piece of its crust, like a cork popping off a champagne
bottle. [more
information]
Comet LINEAR surprised astronomers
using the Hubble Space Telescope (HST) with a brief, violent
outburst on July 5, 2000.
The comet's brightness soared by a factor of 1.5 during
a four hour period. Two days later astronomers spotted at least one house-sized fragment trailing
the nucleus by more than 450 km.
"We lucked out completely," said Hubble comet-watcher
Harold Weaver of the Johns Hopkins University, Baltimore, MD
in a
press release. "In one surge of brilliance this under-performing
comet showed us what it could have been. Comet LINEAR generally
has not been as bright as we had hoped, but occasionally does
something exciting."
"The July 5th flare was probably associated with
the separation of the fragment," says Brian Marsden of the
Minor Planet Center at Harvard University. "Fragmentation
releases fresh icy material and exposes it to solar radiation,
causing the comet momentarily to brighten."
If such a flare had occurred this week as LINEAR was approaching
the Sun (it was closest to the Sun on July 26th) the comet might
have become visible without a telescope.
Something similar happened to the already-brilliant comet C/1996 B2 (Hyakutake) in 1996. Ground based telescopes recorded
transitory
flares while the Hubble Space Telescope and others
captured pictures of fragments flying away from the nucleus (see
below).
 Like
comet Hyakutake, comet LINEAR is a "dirty snowball"
from the outer reaches of the solar system. Its nucleus is laced
with volatile gasses that vaporize furiously as the comet falls
toward the Sun. Marsden explains that this is probably LINEAR's
first visit to the inner solar system, and it has a greater proportion
of vaporizable material than comets that have passed by the Sun
many times before (e.g., Halley's Comet). C/1999 S4 is losing
so much of its mass to solar vaporization that it's being pushed
and shoved by the reaction force of its own gaseous jets. Just
as a jet airplane under its own power does not follow a ballistic
trajectory, LINEAR's orbit is not a perfect gravitational ellipse.
Right: This 1996 Hubble Space Telescope picture shows
three small pieces that broke away from the core of comet Hyakutake.
The fragments were forming their own tails when Hubble captured
this image. [more
information]
"Basically, I think C/1999 S4 is simply a rather
small comet," says Marsden. "If comets of similar composition
lose material by means of a constant shrinking of the radius,
the relative mass loss will be largest for the smallest comets.
So that's why the non-gravitational terms in the equations describing
LINEAR's orbit are so large for this first-pass comet, as opposed
to C/1956 R1 (Arend-Roland) in 1957, for example."
LINEAR S4's erratic jets make predicting the long term fate
of the comet tricky.
 "The
large nongravitational effect complicates the calculation of
long-term motion," continued Marsden. "Without the
nongravitational effect, comet LINEAR would be back in some 30,000
years. With it, I don't know."
Left: Is comet LINEAR breaking up? Mark
Kidger, an astronomer at the Instituto de Astrofisica de
Canarias, captured this picture of comet LINEAR's coma (the cloud
of gas surrounding the icy nucleus) on July 25, 2000, using the
one meter Jacobus Kapteyn Telescope in the Canary Islands. The
contours show lines of constant brightness in the inset image.
Kidger reports that the coma has developed a peculiar elongated
shape unlike its appearance on previous nights. He speculates
that the rapidly-changing comet could be fragmenting into smaller
pieces following its recent close pass by the Sun. Click
for a side-by-side comparison of contour plots from July 23rd
and July 25th.
Comet LINEAR reached its maximum northern declination
on July 18, 2000, and now it's heading for the southern sky.
Amateur astronomers in the northern hemisphere can track the
comet through early August as it moves from Ursa Major, through
Leo and into Virgo. After the beginning of August the fading
comet can be seen best from south of the equator. Southern observers
with access to a telescope are in for a treat on August 20th
when the comet passes close to the Sombrero
galaxy (M104). From southern Africa LINEAR S4 will pass right
in front of M104, affording an opportunity for truly unique astrophotos.
[more
information from Sky & Telescope]
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Above: Science@NASA reader Larry Koehn contributed this
illustration of comet LINEAR's motion through the Solar System.
The comet passed 56 million km from Earth on July 22nd and 114
million km from the Sun on July 26, 2000. It appears to be a
first-time visitor to the inner solar system traveling in an
orbit that will eventually return it beyond Neptune in 2013.
The Space Telescope Science
Institute is operated by the Association of Universities for
Research in Astronomy, Inc., for NASA, under contract with Goddard
Space Flight Center. The Hubble Space Telescope is a project
of international cooperation between NASA and the European Space
Agency. |
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