Stardust will be blasted with a hail of dust particles traveling nearly four miles per second as the spacecraft approaches to within 93 miles of Comet Wild-2 (pronounced "Vilt"-2) in 2004. A special shield called the bumper shield will protect the main body of the spacecraft as it passes through the glowing gas cloud that surrounds the comet's solid nucleus. The detectors for Chicago's Dust Flux Monitor Instrument will be mounted on the front of the bumper shield.

"There, they will be exposed to the full force of the dust flux to measure the size of the dust particles the spacecraft encounters and map their distribution around the comet's nucleus," said Anthony Tuzzolino, Senior Scientist at Chicago's Laboratory for Astrophysics & Space Research.

The DFMI was not originally part of the Stardust mission. Noel Hinners, vice president of spacecraft contractor Lockheed Martin Astronautics, suggested its addition to provide rapid measurement of the dust density around the comet to help engineers and flight controllers assess the health and safety of the spacecraft as it approaches the comet. Ben Clark, also of Lockheed Martin, led the effort to find a way to integrate the experiment and the spacecraft, the design for which was already nearly complete.

"Our instrument performs an important health-hazard function," Tuzzolino said. "Conditions may be far more hazardous than we thought as we approach the comet." If so, DFMI data will warn mission controllers that it is time to take protective measures for the spacecraft.

Scientists also will correlate DFMI's data with the samples that Stardust will collect from the comet and return to Earth in 2006. Stardust will use a material called aerogel to collect the samples without damaging or altering the speeding particles. Aerogel is a silica-based solid with a porous, sponge-like structure that consists mostly of empty space. "It is so light that it has been called 'solid smoke,'" Tuzzolino said.

The other instruments aboard Stardust include a camera to take detailed photographs of the comet's surface features, and the Cometary and Interstellar Dust Analyzer, which will analyze the composition of the comet's dust particles.

The DFMI consists of an electronics box, two detectors mounted on the front of the spacecraft's bumper shield and two acoustic sensors, measurements from which will be analyzed by a team headed by Professor J.A.M. McDonnell of the University of Kent in England.

The detectors consist of a polarized plastic material. "The material is similar to Saran wrap," Tuzzolino said. The material generates an electrical pulse when hit by small, high-speed particles, even those many times smaller than a sand grain.

The two acoustic sensors are embedded between layers of the shield that protects the spacecraft from impacting dust particles. "The acoustic sensors will be triggered by a large impact particle that hits the shield anywhere," said LASR Senior Scientist Bruce McKibben.

Stardust will meet Comet Wild-2 at a distance of 242 million miles from Earth, following a flight trajectory that will loop twice around the sun. The spacecraft will loop once more around sun after its comet encounter on the way back to Earth.

The trajectory will take Stardust close to several meteor streams that the DFMI may be able to detect. The first such opportunity will occur April 20, 1999 when Stardust comes within 3.5 million miles of the Orionid meteor stream. The Orionid meteors, left in the wake of Comet Halley, can be seen from Earth each October.

The DFMI may also be able to detect particles of interstellar dust, which NASA's Ulysses spacecraft recently discovered streaming into the solar system.

"There is a chance that we can identify the trajectory of incoming particles that must have come from interstellar space," said John Simpson, Arthur Holly Compton Distinguished Service Professor Emeritus in Physics. "This is matter that is involved in the origin of the solar system itself. It's primordial material."

Stardust will be the 34th space-science mission Simpson and Tuzzolino have participated in, starting with Pioneer 2 in 1958. Last November, Tuzzolino received the NASA Public Service Medal for his role in the development of cosmic ray and dust detectors, including the first to be sent to Mercury, the moon, Mars, Jupiter and Saturn. All of their experiments have been aimed at understanding the origin of elements and matter that formed Earth's galaxy and solar system.

The DFMI is a descendant of Chicago's Dust Counter and Mass Analyzer instrument that flew on the Soviet Union's Vega 1 and Vega 2 missions to Halley's Comet in 1986. Simpson invented the instrument concept in 1983, with Tuzzolino playing a key role in its rapid development and testing.

On the Vega missions, Chicago scientists discovered to their surprise that tiny dust particles streaming from the comet had survived to the outer bounds of the comet's coma, the spherical cloud of glowing gas that develops around a comet's solid nucleus as it approaches the sun.

"We were able to show that the particles coming off the comet's nucleus had to be a conglomerate, probably bound together by an ice glue," Simpson said. "Then, as it carried far out into space, the ice glue dissolved, releasing the very small stuff that would have otherwise disappeared if it had come directly from the comet."

Two other instruments related to the DFMI are components on NASA's current Cassini mission to Saturn and on the Air Force's unclassified Advanced Research and Global Observation Satellite.

Simpson and Tuzzolino built Cassini's High Rate Detector, part of the larger Cosmic Dust Analyzer from Germany, which will collect and analyze dust particles found in interplanetary space and those that form the major components of Saturn's rings.

The ARGOS Space Dust instrument, devised by Simpson, Tuzzolino and McKibben, will measure the mass, speed and trajectory of cometary dust particles and man-made space debris found in low-Earth orbit when launched later in February.

The $350,000 DFMI was funded by NASA for Stardust, the fourth mission in the space agency's Discovery Program of smaller, faster, cheaper missions. The Stardust scientific team is led by University of Washington astronomy professor Donald Brownlee.