It will be a moment tinged with history when the Stardust spacecraft makes an encounter with Asteroid 5535 Annefrank this weekend. The flyby will test many of the systems and procedures to be used when Stardust makes its encounter with comet Wild 2 in little more than a year.

"It turns out to be a tremendous plus because you end up having a full dress rehearsal more than a year ahead of the encounter," said Donald Brownlee, a University of Washington astronomy professor who is the mission's chief scientist. "It's a little like a dress rehearsal for a wedding - you expect things to be fine, but you practice just to make sure. If the unexpected does happen at the rehearsal, it's not a problem at the real ceremony."

Stardust, launched in February 1999, is designed to capture particles from Wild 2 and return them to Earth for analysis. The spacecraft already has collected grains of interstellar dust. It is the first U.S. sample-return mission since the last moon landing in 1972.

Brownlee described Annefrank as typical for asteroids found in the inner asteroid belt, just beyond the orbit of Mars. Stardust's main camera will capture images, but the asteroid's relatively small size (2½ miles across) and the spacecraft's distance (about 1,900 miles) mean the images won't be very detailed, he said. The closest approach to the asteroid will be at 8:50 p.m. PST (11:50 p.m. EST) on Friday.

"We're just fortunate to have a target there that we can approach at this time," he said.

Asteroid 5535 was discovered by prolific German asteroid hunter Karl Reinmuth in March 1942 but was not named Annefrank until long after World War II.

The discovery came barely three months before Frank, a Jewish teenager, joined her parents, her sister and four others hiding from the Nazis in Amsterdam, Holland. For two years the group remained in their hideaway, subsisting with help from a small circle of outsiders. Anne recorded their life and her thoughts in a diary that was to become one of the world's most famous books. The group was discovered in 1944 and sent to Nazi concentration camps. All except Anne's father perished. Otto Frank survived the war and returned to Amsterdam, where he published his daughter's diary.

Now Annefrank happens to be the asteroid that lies on the right course to help Stardust and its controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., prepare for the tasks they face come Jan. 2, 2004.

On that day, Stardust will fly within 75 miles of Wild 2's main body, close enough to trap small particles from the coma, the gas-and-dust envelope surrounding the comet's nucleus. Stardust will be traveling at about 13,400 miles per hour and will capture comet particles traveling at the speed of a bullet fired from a rifle. The main camera, built for NASA's Voyager program, will transmit the closest-ever comet pictures back to Earth.

There are differences, however, between how the spacecraft will function during the Annefrank flyby and the comet encounter. For one thing, if it runs into serious problems during the asteroid encounter it will be able to go into "safe mode," where the spacecraft turns its solar power collectors toward the sun and essentially protects itself. But when it approaches Wild 2 (pronounced Vilt two), Stardust will be working without a net - the "safe mode" function will be turned off.

Brownlee said the Annefrank flyby is "a very good test," the kind that ideally every mission should have. Such tests are particularly important, he said, for low-cost missions such as those in the National Aeronautics and Space Administration's Discovery program, of which Stardust is a part.

"When we have the comet encounter, we want as few first-time events as possible," Brownlee said. "This fortunate opportunity at the asteroid increases our probability of success next year at the comet."