Zipping through the comets coma--its dust atmosphere--the probe passed 240 kilometers from the solid body, or nucleus. The close-approach record holder is the European Space Agencys Giotto spacecraft, which in 1992 came within 200 kilometers of the nucleus of Comet Grigg-Skjellerup. During the encounter, Stardust deployed a dust collector roughly the size and shape of a large-head tennis racket. The collector will soon be folded into a capsule for the return journey to Earth. In January 2006 the capsule is scheduled to separate from Stardust and land in an air force test range in Utah.
The collected dust, ranging in size from a few to a few hundred microns, is thought to be a piece of the swirling cloud from which the planets emerged. Comets, like asteroids, are planetary building blocks that were never incorporated into planets. In the intervening 4.6 billion years, they languished in the deep freeze of the outer solar system, little changed. By comparison, material on Earth and other planets has been thoroughly reworked by geological processes, erasing the record of the formative early period.
For those watching the Stardust encounter from mission control, it was a case of no news is good news. Relative to the comet, the space probe was moving at six kilometers per second. Shields--multiple layers of carbon and ceramic sheets--protected the solar panels and main body from damage by colliding dust grains. Stardust did not send images in real time, so all scientists and engineers could do was watch the radio carrier signal to see whether the spacecraft survived the bombardment. The signal from the moment of closest approach was scheduled to reach Earth at 11:44 a.m. Pacific time, and as this time came and went, the carrier held steady. Applause went up in the control room. "Weve passed the closest approach without any injury, apparently," says Donald Yeomans, a comet scientist at JPL.
Over the ensuing hours, Stardust transmitted its data back to Earth, including an image taken from about 500 kilometers away. The image shows the nucleus, about five kilometers across, and unlike other cometary nuclei, it seems to be round. Around its edge, scientists have so far counted five jets of outflowing material. The features that look like craters have the wrong proportions to be impact craters. Imaging scientist Ray Newburn of JPL says they are probably sinkholes caused by the loss of material. No such features appear on images of Comet Halley. "These pictures are really going to open up a new window to understanding how comets work," says principal investigator Don Brownlee of the University of Washington.
Stardust is one of the mid-price spacecraft in NASAs Discovery program, running $130 million plus launch rocket and operations costs. It was launched in February 1999 and, during its journey, also collected interstellar dust that had infiltrated interplanetary space.
Comet Wild-2 used to orbit beyond the orbit of Jupiter, but it made an unusually close approach to the giant planet in September 1974 and got catapulted into the inner solar system. Since then, the comet has passed near the sun five times--compared to over 100 times for Comet Halley. Consequently, the sun has hardly had a chance to boil off or otherwise alter the cometary material, making Wild-2 an especially pristine sample from the genesis of planets. --George Musser in Pasadena, Calif.