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Space Topics: Asteroids and Comets

Comet 81P/Wild 2

Comet 81P/Wild 2
This view was composed of seven stacked frames captured by ESO's Very Large Telescope on July 31, 1998. The visible coma of the comet stretches across about 40,000 kilometers (25,000 miles). Its nucleus, which would be imaged by Stardust years later, is only 4 kilometers (2.5 miles) across. Credit: European Southern Observatory

Wild 2 is a recent arrival to the Jupiter family of comets, having been jostled into its present orbit by a close encounter with the giant planet on September 10, 1974.  That made it a very interesting target for exploration because, as a comet, it is an ancient body carrying information about the early days of the solar system; as a comet new to our neighborhood, its surface should be relatively unaffected by repeated passages close to the Sun.  Wild 2 had made only five trips around the Sun in its new, close orbit by the time Stardust arrived to examine it.

The Stardust mission flew within 236 kilometers (147 miles) of comet Wild 2 on January 2, 2004 at a speed of 6.1 kilometers per second (14,000 miles per hour).  During the flyby it exposed aerogel-covered collectors to the coma of Wild 2, collecting samples of cometary dust to return to Earth in January 2006.  Stardust captured a total of 72 images of Wild 2's nucleus, images that surprised scientists back on Earth.  At the time, the images were the highest resolution ever taken of a comet.

Wild 2's Orbit

Orbital Period: 6.39 years
Perihelion: 1.592 Astronomical Units (near the orbit of Mars)
Rotation rate: About 12 hours

What Stardust Learned About Wild 2

Unlike Borelly, Halley, and Itokawa, Wild 2's nucleus is strikingly round, a shape scientists call "oblate."  The round shape suggests that Wild 2 is probably not a fragment of something that was broken apart in a large impact -- such a fragment would be more irregularly shaped.  Instead, its present shape may represent its original shape. Its size is about 3.3 by 4.0 by 5.5 kilometers (2.1 by 2.5 by 3.4 miles), with the same volume as a sphere of diameter 4.0 kilometers (2.5 miles).

Wild 2 is also noticeably pockmarked with different kinds of topographic depressions.  Some of these, like the prominent "Left Foot" and "Right Foot" features, have flat floors and remarkably steep walls, with slopes as high as 70 degrees.  Such steep slopes suggest that the materials that form Wild 2 have enough internal strength to hold together despite gravity's tendency to flatten out such slopes.  The gravity on a body as tiny as Wild 2 is very low, but such steep slopes are surprising nonetheless.

Highest-resolution image of Wild 2Map of Wild 2's surface features
Highest-resolution image and map of Wild 2
This short-exposure image was taken only 4 seconds before Stardust's closest approach to comet Wild 2, from a distance of less than 240 kilometers (150 miles). Scientists used this image to name the features on Wild 2. The most prominent deep depressions, near the right side, are called "Left Foot" and "Right Foot" because of their resemblance to footprints in snow. They are located near Wild 2's rotational pole. The bright region near the left side is called Mayo. The bite out of the lower left side is a deep basin called Shoemaker, and the bite out of the upper right side is a basin called Walker. Source Credit: NASA / JPL

For more images of Wild 2, see the entire catalog of 72 raw images from the Stardust flyby.

Wild 2's Jets

Wild 2
A view of both the 4-kilometer (2.5-mile) nucleus and the jets of comet Wild 2 is made possible by the superimposition of a long- and a short-exposure image. The two images were taken 10 seconds apart from a distance of about 275 kilometers (170 miles) just seconds after Stardust's closest approach to Wild 2. Streams of dust can be traced back to lumpy pits on the surface of the comet's nucleus. Source Credit: NASA / JPL
Dust Flux Monitor data from Stardust's Wild 2 flyby
Stardust's Dust Flux Monitor instrument recorded repeated peaks and gaps in particle density as the spacecraft flew through Wild 2's coma at a speed of 6.1 kilometers (3.8 miles) per second. Credit: Thanasis Economou, University of Chicago

By snapping a combination of long- and short-exposure images, Stardust's navigational camera was able to image both Wild 2's nucleus and its faint jets.  Superimposing long- and short-exposure images allows scientists to trace the jets back to their likely points of origin on the surface of the comet. Many of the jets were found to emanate from "pit-halo" regions on the nucleus of the comet that the science team named "Mayo" and "Walker."  None of the jets emerged from the flat-floored "Left Foot" and "Right Foot" features close to Wild 2's sunlit pole.

The jets are the source for the material that forms Wild 2's coma.  Prior to the Stardust encounter, it was assumed that the coma had a particle density that increased uniformly toward the nucleus, but Stardust encountered quite a different pattern.  Stardust ran into three "sheets" of coma material, with its dust counter recording peaks in particle density in the middle of the sheets that were a thousand times higher than the density of particles between these peaks.

Wild 2's Particles

Embedded in two aerogel-covered dust collectors on Stardust are a few thousand dust particles from the coma of Wild 2.  After they are returned safely to Earth, the cometary dust particles will be extracted from the aerogel and studied to determine the detailed composition of Wild 2 in a study that could take years.