Taste of a Comet: Spacecraft samples and views Wild 2

Pummeled by dusty debris traveling six times as fast as a rifle bullet, a NASA spacecraft last week snatched up dust samples while taking the sharpest images ever of the icy core of a comet. Comets are considered to be pristine leftovers from the formation of the solar system some 4.6 billion years ago, and if all goes according to plan, the NASA craft Stardust will carry bits of that original material to Earth in January 2006.

ICY IMAGE. Comet Wild 2, as recorded by the Stardust spacecraft on Jan. 2, shows craterlike depressions. One hemisphere is in sunlight, the other in shadow. Brownlee, et al./JPL/NASA

Transporting samples of the comet, known as Wild 2 (pronounced vilt 2), remains Stardust’s primary mission, but planetary scientists this week were marveling at the images radioed by the craft from its encounter with the 5-kilometer-diameter comet on Jan. 2. Coming within 240 km of the comet’s core and discerning features as small as 30 meters across, the craft unveiled a surprisingly pocked and varied terrain.

“Everything is up and down. There are craterlike depressions, cliffs, and jets,” says Stardust investigator Donald E. Brownlee of the University of Washington in Seattle. “All of this is important in understanding how comets formed” and how they behave, he notes. Brownlee and his colleagues reported findings during a press briefing on Jan. 6 at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Born in the deep freeze of the outer solar system, comets lose material each time they visit the inner solar system. Warming from the sun causes frozen organic compounds on a comet’s surface to sublime, or vaporize. The gases, expelled as jets, drive out debris that had been embedded in the ice, endowing comets with their flamboyant tails.

In the Stardust images, which collectively show 6 to 12 jets, “we believe we’re actually seeing this [sublimation] process happen,” says Brownlee. As the comet rotates, each jet is like a fire hose that whips around and turns on and off as it moves in and out of the solar spotlight, he adds.

Three distinct, short-lived increases in the number of dust particles recorded by a Stardust detector add to the evidence that the craft flew in and out of several jets, Brownlee notes.

Moreover, the images suggest that some of the larger jets, which are about 100 m in diameter, are made up of smaller ones.

Two other features of Wild 2 are also attracting attention. The depressions are “the first direct evidence” for craters on a comet, comments Lucy-Ann A. McFadden of the University of Maryland at College Park. Also, the comet’s roughly spherical shape differs from the peanut geometry of the two other comets, Halley and Borelly, visited by spacecraft (SN: 9/29/01, p. 196: Probe’s comet encounter yields close-ups) and doesn’t conform to the similarly elongated shapes that observations from Earth have indicated several other comets have.

“It’s nice to see something different, not at all what we expected,” McFadden adds.

Wild-2’s shape and pockmarked appearance could be related, says Michael J.S. Belton of Belton Space Exploration Initiatives in Tucson. He notes that it was only in 1974 that Jupiter’s gravity put Wild 2 on its current path, which every 6.2 years places the body nearly as close to the sun as Mars’ orbit is.

Having visited the inner solar system only five times, the comet may not have yet had enough jet activity to transform into an elongated or peanutlike shape. In contrast, peanut-shaped Halley has neared the sun about 1,000 times. Moreover, the limited jet activity on Wild 2 would have left intact any craterlike depressions from previous impacts with other comets or asteroids. Some of the observed depressions that have flat bottoms and steep walls probably formed at the bases of the jets, Brownlee notes.

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