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Why Comet Wild 2?
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+ Observational History

+ Observational Data
+ Methods for Observation
+ Coma Imaging
+ Nucleus Imaging
+ Model Assumptions
+ Ephemeris Data



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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

170 62 to 2 to 95

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 97

 

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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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