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STARDUST
Comet 81P/Wild 2
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History of the Comet 81P/Wild-2 and
How to Observe it in 1997
History of Comet P/Wild 2
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 postperihelion |
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| * Relative to Perihelion |
Observational Data
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.
Methods for Observation
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.
Coma Imaging
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.
Nucleus Imaging
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
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| 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 |
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4, Red, Green, Blue and Infrared
@62, 64, 95 and 97º |
Model Assumptions
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 |
For further information about Comet
P/Wild 2 contact:
Ray L. Newburn (JPL)
Last update by Tom Meyer
Monday, 06-Nov-2000 15:49:52 PST.
