Comets, which periodically grace our sky like celestial bottle rockets, are thought to hold many of the original ingredients of the recipe that created the planets and brought plentiful water to Earth. They are also rich in organic material, which provided our planet with many of the ready-to-mix molecules that could give rise to life. They may be the oldest, most primitive bodies in the solar system, a preserved record of the original nebula that formed the Sun and the planets.

Scientists have long sought a sample directly from a known comet because of the unique chemical and physical information these bodies contain about the earliest history of the solar system. Locked within comet molecules and atoms could be the record of the formation of the planets and the materials from which they were made.

Stardust is the first U.S. mission dedicated solely to a comet and will be the first to return extraterrestrial material from outside the orbit of the Moon. Stardust's main objective is to capture a sample from a well-preserved comet called Wild-2 (pronounced "Vilt-2").

On route to the comet Stardust has also collected interstellar dust from a recently discovered flow of particles that passes through our solar system from interstellar space.

As in the proverbial "from dust to dust," this interstellar dust represents the ultimate in recycled material; it is the stuff from which all solid objects in the universe are made, and the state to which everything eventually returns. Scientists want to discover the composition of this "stardust" to determine the history, chemistry, physics and mineralogy of nature's most fundamental building blocks.

Because it would be virtually impossible to equip a spacecraft with the most sophisticated lab instrumentation needed to analyze such material in space, the Stardust spacecraft is more of a robotic lab assistant whose job it is pick up and deliver a sample to scientists back on Earth. The spacecraft will, however, radio some on-the-spot analytical observations of the comet and interstellar dust.

The samples being collected are extremely small, less than a micron, or 1/25,000th of an inch, in size, and can only be adequately studied in laboratories with sophisticated analytical instruments.

The amount of volume of material collected is not a critical issue as even if a ton of sample were returned, the main information in the solids would still be recorded at the micron level, and the analyses would still be done a single grain at a time.

Stardust will meet up with Comet Wild-2 on January 2, 2004. A gravity assist flyby of Earth put Stardust on a trajectory that will allow it to capture cometary dust intact at a low relative speed of 6.1 kilometers per second (about 13,600 miles per hour). An onboard camera will aid in navigating the spacecraft as close as about 150 kilometers (100 miles) from the comet's nucleus, permitting the capture of the freshest samples from the heart of the comet.

Dressed for survival behind armored shields, Stardust will document its 10-hour passage through the hailstorm of comet debris with scientific instruments and the navigation camera.

On approach to the dust cloud, or "coma," the spacecraft will flip open a tennis-racket-shaped particle catcher filled with a smoke-colored glass foam called aerogel to capture the comet particles.

Aerogel, the lowest-density material in the world, has enough "give" in it to slow and stop particles without altering them too much. After the sample has been collected, the aerogel capturing device will fold down into a return capsule, which closes like a clamshell to enclose the sample for its safe delivery to Earth.

In addition, a particle impact mass spectrometer will obtain in-flight data on the composition of both cometary and interstellar dust, especially very fine particles. The optical navigation camera should provide excellent images of the dark mass of the comet's nucleus. Other equipment will reveal the distribution in both time and space of coma dust, and could give an estimate of the comet's mass.

On January 15, 2006, a parachute will set the capsule gently onto the salt flats of the Utah desert for retrieval. The scientifically precious samples can be studied for decades into the future with ever-improving techniques and analysis technologies, limited only by the number of atoms and molecules of the sample material available. Many types of analyses now performed on lunar samples, for example, were not even conceived at the time of the Apollo missions to the Moon.

Comets are small, irregularly shaped bodies composed of a mixture of grains of rock, organic molecules and frozen gases. Most comets are about 50 percent water ice. Typically ranging in size up to about 10 kilometers (6 miles) in diameter, comets have highly elliptical orbits that bring them close to the Sun and then swing them back out into deep space. They spend most of their existences in a deep freeze beyond the orbit of Pluto - far beyond the Sun's dwindling influence, which is why so much of their original material is well-preserved.

When a comet approaches within about 700 million kilometers (half billion miles) of the Sun, the surface of the nucleus begins to warm, and material on the comet's nucleus heats and begins to vaporize. This process, along with the loss of rocky debris or other particles that fly off the surface, creates the cloud around the nucleus called the coma. It is the glowing, fuzzy coma that appears as the head of a comet when one is observed from Earth. A tail of luminous debris and another, less apparent, tail of gases flow millions of miles beyond the head in the direction away from the Sun.

Comet Wild-2 was considered an ideal target for study because, until recently, it was a long-period comet that rarely ventured close to the Sun. A fateful pass near Jupiter and its enormous gravity field in 1974 pulled Comet Wild-2 off-course, diverting it onto a tighter orbit that brings it past the Sun more frequently and also closer to Earth's neighborhood. Because Wild-2 has only recently changed its orbit, it has lost little of its original material when compared with other short-period comets, so it offers some of the best-preserved comet samples that can be obtained.

Stardust was competitively selected in the fall of 1995 under NASA's Discovery Program of low-cost, highly focused science missions. As a Discovery mission, Stardust has met a fast development schedule, uses a small Delta launch vehicle, the mission is cost-capped at less than $200 million, and was the product of a partnership involving NASA, academia and industry.

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