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Then came the surprise. It happened when Stardust passed by the core of the comet, only 236 kilometres away, and photographed it using a navigation camera. The images were intended primarily to keep the spacecraft on course. They also revealed a world of startling beauty. At the heart of every comet lies a "dirty snowball," a compact nucleus of dust and ice that the sun vaporizes, to form the comet's spectacular tail. These nuclei are hard to see. For one thing, most are blacker than charcoal; they reflect precious little sunlight for cameras. Plus they're hidden deep inside a cloud of vaporizing gas and dust, called "the coma." Stardust's plunge into Wild 2's coma allowed it to view the nucleus at close range.
Previous flybys of Comet Halley by the European Space Agency (ESA) Giotto probe and Comet Borrelly by NASA's Deep Space 1 revealed lumpy cores without much interesting terrain. These comets have been sun-warmed for many thousands of years. Solar heating has melted away their sharpest features.
However, Comet Wild 2 looks different. "We were amazed by the feature-rich surface of the comet," says Donald Brownlee of the University of Washington, the mission's principal investigator. "It is highly complex. There are barn-sized boulders, 100-meter high cliffs, and some weird terrain unlike anything we've ever seen before. There are also some circular features," he adds, "that look like impact craters as large as 1 km across."
"The high cliffs tell us that the crust of the comet is reasonably strong," notes Brownlee. It's probably a mixture of fine-grained rocky material held together by frozen water, carbon monoxide and methanol. Certainly a lander could touch down there, or an astronaut could walk across the surface without worrying too much about the ground collapsing. An astronaut standing on Comet Wild 2 would see a truly fantastic landscape, speculates Brownlee. "I imagine them inside one of the craters, surrounded by deep cliffs." Icy spires, as tall as a person rising out of the crater floor. " Getting out of the crater would be easy. "Just jump," says Brownlee, "but not too hard." The comet's gravity is only 0.0001-g, so "you could easily leap into orbit."
Some of the photos from Stardust reveal gaseous jets. The jets come from active regions on the comet's surface, fissures or vents, where the ice is vaporizing and rushing into space," Brownlee says. This is how mass is transferred from the comet's nucleus to its tail. Viewed from the surface, the jets would be nearly transparent. But an astronaut could spot them by looking for "dust entrained with the gas. Dust grains glinting in the sunlight would look like tracer bullets shooting out of the ground."
Unlike comets Halley and Borrelly, Wild 2 is a very recent arrival to the inner solar system. For billions of years it orbited in the cold deep space beyond Jupiter, until 1974 when it was nudged by Jupiter's gravity into a sun-approaching orbit. Since then the comet has passed by the Sun only five times; solar heating has only recently stared to shape its surface.
During the dust bombardment, Stardust extended a tennis racket shaped collector to retrieves dust particle for inspection on Earth. The craft's payload will return to Earth in 2006 for analysis by scientists. If a single picture from the navigation camera can surprise researchers, just imagine what's in store when they get their hands on a pieces of the comet itself.
More info: Stardust at JPL
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