On Jan. 2nd, 2004, NASA's Stardust spacecraft approached Comet Wild 2 and flew into a storm. Flurries of comet dust pelted the craft. At least half a dozen grains moving faster than bullets penetrated Stardust's outermost defenses. The craft's 16 rocket engines struggled to maintain course while a collector, about the size of a tennis racquet, caught some of the dust for return to Earth two years hence. All that was expected. Then came the surprise. It happened when Stardust passed by the core of the comet, only 236 km distant, and photographed it using a navigation camera. The images were intended primarily to keep the spacecraft on course. They also revealed a worldlet of startling beauty.At the heart of every comet lies a "dirty snowball," a compact nucleus of dust and ice that the sun vaporizes, little by little, 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 Giotto probe and Comet Borrelly by NASA’s Deep Space 1 revealed lumpy cores without much interesting terrain--as expected. These comets have been sun-warmed for many thousands of years. Solar heating has melted away their sharpest features.

View: Full Article | Source: Nasa.gov