NASA's Stardust spacecraft, on a mission to collect and return the first samples from a comet, began Monday to collect tiny specks of solid matter, called interstellar dust grains, that permeate the galaxy.

An artist's concept of Stardust collecting samples from Comet Wild 2. Credit: NASA

"If you look at the Milky Way on a dark night you may see a black band stretching along the center. The band is interstellar dust blocking the light from distant stars. These are the particles that Stardust will be collecting," said Dr. Don Brownlee, an astronomy professor at the University of Washington, Seattle, and the principal investigator of the Stardust mission.

This dust, passing through the solar system like a wind, is made of particles smaller than one-hundredth the width of a human hair. The particles are made of varying amounts of most of the elements in the periodic table. The Stardust mission will use its special formulation of aerogel, the world's lightest solid, to try to capture these small solid particles as the spacecraft travels in the same direction as the dust stream until December 9, 2002.

"Stardust's tennis-racket-shaped particle collector has shoulder and wrist joints that will point one side of the aerogel collector material into the dust stream to collect interstellar dust," said Tom Duxbury, the project's manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "When Stardust encounters comet Wild 2 in early 2004, the reverse side of the collector will trap particles from the gas and dust escaping from the inside of the comet. When the dust samples return to Earth in 2006, we will extract and analyze the particles,"

The Stardust mission collects both ancient and young dust. Comets are made of interstellar particles that clumped together with ices more than 4.5 billion years ago. When the spacecraft flies past comet Wild 2, it will attempt to collect ancient dust samples stored for billions of years in, effectively, a deep freeze.

The mission began yesterday collecting a younger type of stardust: the free-flowing interstellar dust that was produced by the current generation of stars. Comparing the ancient and newer types of dust may provide clues to the evolutionary changes in the galaxy and the composition of the early galaxy. This is the second and final time Stardust will collect these dust particles. It previously collected samples during a six-week period in 2000.

Comet Wild 2 is a particularly good example of preserved interstellar dust because its path through space brings it no closer to the Sun than Mars' orbit, about 228 million kilometers (about 141 million miles) from the Sun. Before 1974, the closest Wild 2 came to the Sun was Jupiter, Brownlee said.

NASA's Galileo and Ulysses spacecraft both detected a stream of dust particles flowing between stars and into the solar system. The particles did not come from the Sun, but from another direction that showed their origin was outside the solar system.

Interstellar dust may have played a role in bringing the building blocks of life -- carbon and other organic materials -- to the young Earth. Similarly, comet impacts may have also brought these elements to Earth. Brownlee expects to find a lot of carbon in the interstellar dust particles. "When Earth-like planets form, comets and interstellar grains may bring carbon and organic material," he said.

The interstellar dust stream differs from the solar wind in that the solar wind is made of individual atoms, while the interstellar dust is made of small particles of rocks with complex compositions.

Stardust, a part of NASA's Discovery Program of low-cost, highly focused science missions, was built by Lockheed Martin Astronautics and Operations, Denver, Colo. and is managed by JPL for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.