PASADENA, Calif. The annual Leonid meteor shower can be a not-so-gentle reminder of how small cometary particles can cause trouble for Earth-orbiting spacecraft.

Each year, without fail, satellite controllers fret the speedy particles from Comet Tempel-Tuttle will do everything from damage solar panels, short-out delicate electronic components and if well enough placed send a satellite spinning. The particles, like buckshot sent flying from the blast of a shotgun, are to be ducked at all costs.

But for a small flotilla of missions already or soon under flight, there will be no shying from the peril. Instead, the spacecraft will soon head straight into the breach.

What volleys will meet the four U.S. spacecraft Deep Space 1, Stardust, Deep Impact and CONTOUR as they fly by a half dozen comets, remains unknown.

But with the first encounter merely a year away, scientists are already scrambling to model what dust environment at each target comet will greet the various probes.

"Its not like going by an asteroid or a planet, which are quite benign, because comets are actually throwing things at you," said Donald Yeomans, a cometary expert at NASAs Jet Propulsion Laboratory (JPL).

Preserved by the cold of space, comets are made up of ice and dust left over from the solar nebula, out of which our solar system formed. Scientists believe these primitive frozen remnants hold the clues to the solar system's earliest and coldest period.

But as these 4.5 billion-year-old frozen reminders approach the warming rays of the Sun, they can grow active, often unpredictably so, as they spew jets of gas and dust.

The European Space Agency (ESA) learned its lesson well during the March 1986 flyby of Comet Halley by its Giotto spacecraft.

Scientists began meeting last fall to work on modeling comet nuclei and what the various missions might see and image of them during their respective encounters. Missions members met again in recent weeks at JPL to further tinker with those models, which include complex matrices they hope will predict the dust environment at each fickle comet.

"They vary from day to day, week to week and from appearance to appearance," said Ray Newburn, a JPL co-investigator on the Stardust mission. Stardust will fly by the comet Wild 2 in January 2004, collecting dust and volatile samples for return to Earth two years later.

For example, models predict that Deep Space 1, which will make the first encounter of the bunch when it flies by the comet Borrelly in September of next year, will encounter a single particle 0.015 to 0.028 inches (0.4 to 0.7 millimeters) in size during the entire flyby. Although the expected dust flow will be light, a single, well-placed particle of sufficient size could send the spacecraft spinning.

"If you get a particle impact at an edge, it can impart considerable torque to the spacecraft," said Marc Rayman, the missions chief engineer. Particles could also knock out individual components or create plasma upon impact that can short-out portions of the spacecraft.

To lessen the danger, Deep Space 1 will fly into the dust stream with its solar panels edge-on, thus minimizing the surface area it exposes to harm.

Unlike the other three missions, however, the probe does not have the luxury of bumpers called Whipple shields. The shields would deflect the particles it might meet during the flyby, a bonus tacked on to the end of the already completed mission. Instead, Deep Space 1s multi-layer insulation is set off from the spacecraft about 2 inches (5 centimeters) to help absorb any impacts.

"We will be somewhat more bold than the other missions will be," Rayman said.

Assuming Deep Space 1 survives its flyby which in all likelihood will be the spacecrafts swan song, as its on-board supply of hydrazine is dwindling the data it can provide about Borrelly will be included in the modeling work.

"That information will be factored into the next flyby and likewise that data will be factored into the next one after that," said Yeomans, who has a hand in all four cometary missions.

Should the risks prove larger than expected, mission navigators for the most part, common to all four missions can always set a course that takes the spacecraft farther from the comets. That might lessen the odds of getting a decent look at the comets nuclei, but may mean the difference between a successful flyby and disaster.

"None of us are flying what we think of as kamikaze missions by any means," Newburn said. "All of us flying cometary spacecraft plan to survive."