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Comet 81P/Wild-2 is a fresh periodic comet. Until
September 10, 1974, when it passed within 0.006
AU of Jupiter, its orbit lay between Jupiter and
a point near Uranus. That encounter with Jupiter,
at only 10 times the distance which fragmented
P/Shoemaker-Levy 9 in 1994, brought it into the
inner solar system, where its perihelion now lies
just beyond the distance of Mars and its aphelion
near Jupiter. During its first passage relatively
near to the Earth (1.21 AU), the comet was discovered
by Paul Wild on January 6, 1978. See
the orbital evolution of Comet P/Wild 2.
Since 1978 it has faded slightly, as might be
expected, but is a very active and interesting
comet for one that comes only within 1.6 AU of
the sun. Bright ones such as Halley get much closer,
and Halley is much larger than Wild-2 (8 x 8 x
16 km vs 4 km).
Comet P/Wild 2 Observational
History
Date |
Duration of Observations* |
1978 - Visible
continuum, Common radicals |
-100 to -8
days |
1984
- Visible continuum, Common radicals |
-234 to -175
days |
· Water
from OH |
-202 to -175
days |
· Photos |
-169 days |
1991 - Visible
continuum point |
+172 days |
· CN
Point |
+172 days |
· Photos |
throughout
apparition |
· Broadband
R magnitudes |
throughout
apparition |
Light Curve
- 1978 |
good preperihelion |
Light Curve
- 1984 |
a few very
early points |
Light Curve
- 1990-91 |
good preperihelion |
* Relative to Perihelion
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In 1996-1997, the best apparition or appearance
of the comet since it has been making close approaches
to the sun will occur for Earth viewers. It will
peak around February-March, 1997 coming within
0.85 AU of the Earth with a brightness of 12th
magnitude in February, brightening to around 10.4
in March. From late 1996 until July 1997, the
comet will be within 1.6 AU from Earth. The comet
will be well placed in the sky, not at twilight
for example, and far enough North to be relatively
easy to observe throughout North America, Europe,
and Asia.
It will be observable in large telescopes from
-250 to +100 days from perihelion with the best
ever 0.85 AU closest approach occuring in mid-February.
Amateur astronomers should be able to view it
with no problem with small telescopes, as long
as the focal ratio is not too large. Fast telescopes
(small f number) work well with comets that have
low surface brightness.
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The comet should move noticeably with the star
background from night to night. Identify it with
a nearby star to detect movement over a period
of 5 days. The comet itself, not coming very close
to the sun, will not develop spectacular tails
like Halley, but will look more like a fuzzy blob.
It will be visible to amateur astronomers for
about 6 months, while it is brighter than 13th
magnitude. Amateurs who have observed comets before
and have experience in determining their apparent
brightness, will be able to assist scientists
in determining a light curve for 1997.
To determine a cometary light
curve one must use a starchart with
accurate magnitudes to determine how bright the
comet is. Experiment by identifying known stars,
then throwing the star images out of focus to
simulate a comet and determine a proper brightness
reading. Much practice is required to do this
well.
Ephemeris
data will be posted during the time
that the comet is visible. Donald K. Yeomans at
JPL does all of our ephemeris work. He provided
all of the navigation ephemeris data for Giotto
and VEGA, and ephemerides for Ida and Gaspera
which allowed Galileo to get data from those asteroids.
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With the Stardust Mission we hope to fly within
150 km of the nucleus of the comet. The nucleus
is the true comet, since it creates all of the
cometary phenomena. When it nears the sun, it
outgases, meaning that gases boil off of the nucleus,
creating a vast atmosphere which fluoresces in
sunlight. The gases also blow small dust particles
off of the nucleus which reflect sunlight. The
dust and gas together creates the fuzzy coma around
the comet which is what we see. The Stardust Mission
will look at nucleus structure. The primary goal
of the mission, however, is to return dust samples
to Earth for geochemical analysis of their isotopic,
elemental, chemical, and mineralogical composition.
Data for both dust and gas production rates will
also be collected.
If Comet P/Wild-2 were close enough to the sun,
the dust and gases blown off of the nucleus by
the outgassing would form visible dust and gas
tails. See the attached table of P/Wild 2 dust
models. This does occur for Wild-2, but the surface
brightness is such that you can't observe them
with a small telescope, or easily with a professional
one for that matter.
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With the Stardust mission we'll be able to image
the nucleus of Wild-2 far better than has been
done before. We'll be going much closer to the
nucleus of Wild-2 than Giotto or VEGA went to
Halley's Comet. Giotto only saw Halley from a
single viewpoint, meaning that the illumination
angles between the sun and spacecraft were constant
before it was damaged by dust flowing out from
comet Halley. VEGA covered a range of illumination
angles but only at a large distance.
We hope to cover Wild-2 over a wide range of illumination
angles so that we'll get a much better, 3D picture
of the entire illuminated side of the nucleus.
Giotto saw roughly 25% of the illuminated hemisphere
of Halley with a resolution, at best, of 100 m,
or 0.1 km. They came in from the back side so
they saw only a tiny part of the illuminated hemisphere.
Their equipment was damaged and mirrors destroyed
before they were able to image it on the way out.
We hope to image the entire nucleus with a resolution
of 30 m or better.
We will be able to view the entire sunside, from
48 illumination angles and with color filters
at four angles so we'll be able to see any subtleties
in color which would indicate the presence of
different minerals or structures on the surface
of the nucleus. We should be able to do a far
better job of imaging than Giotto or VEGA did
on Halley 10 years ago.
The spacecraft will have a "Whipple shield"
to protect if from the dust grains through which
it is "plowing," dust grains which impact
the shield like tiny bullets. A periscope will
be used by the camera to look around the shield
on approach. The first mirror of that periscope
will gradually become sand blasted, but by the
time it is no longer optically useful, a movable
mirror in front of the camera will be able to
look around the shield and supply clear images
to the camera.
Nucleus Imaging Comparison Between the Giotto/Halley
and Stardust/Wild2 Missions
|
Giotto |
Stardust |
Phase Angles to
Enhance Resolution
of Surface Morphology |
170º |
62
to 2 to 95 º |
Coverage |
25%
lighted area |
All
lighted area |
Best Resolution |
100
m |
<
30 m (we hope 10-20 m) |
Color Filters to
Enhance Surface
Features |
-169
days |
4,
Red, Green, Blue and Infrared
@62, 64, 95 and 97º |
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The nucleus of Comet P/Wild-2 is thought to be
of low density, with a radius of about 2 km. We
can calculate a volume, assume a density, and
get a mass, but we don't really know. The comet
could be on the order of a 15 trillion kg which
is light and small compared to most cosmic objects.
P/Wild 2 Model Assumptions at Encounter
Nucleus Radius |
2000 m, 10% of surface
is active |
Dust Density |
500
kgm-3 |
Nucleus Density |
500
kgm-3 |
Mean Dust Albedo |
0.04 |
Water Production |
8.4
x 10 27 mol/s |
Total Gas Production |
An
additional 20% of mass 44 |
Interpolated Continuum
Strength Ap/s |
17
meters |
Last updated November
26, 2003 |
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