The fifth graders who clustered in front of Arlington's Education Center yesterday morning, squinting up at a narrow fourth-floor window, quickly learned one of the maddening realities of science:

When the planning is finished and the experiment is under way, there isn't much a scientist can do but hold his breath.

Carrie Simmons, 11, held hers for a long moment, while her creation, an eight-ounce plastic cup holding a raw egg and fastened to a parachute fashioned from garbage-bag plastic, dropped 40 feet to the education center's concrete patio. Then she raced over to peel the lid off the cup.

The egg was intact.

"Oh my God, oh my God," she gasped. "I was thinking it had already broken; when I tried it in school, the top kept coming off."

For Carrie and most of her Page Traditional School classmates, yesterday was a day of scientific success. Of 27 egg-containing contraptions, 22 made the fall without a fracture.

The egg drop, dubbed "Eggileo," was part of Arlington schools' celebration of National Science Week. It was inspired by NASA's satellite, Galileo, scheduled for launching to Jupiter in 1986, said Steven E. Smith, the schools' planetarium director.

Because no one knows what atmospheric roadblocks Galileo may meet as it descends, and because it will carry costly, sensitive instruments, the satellite must be carefully designed to protect its payload, Smith said.

Betty Jones' fifth graders, after discussing the mission and working on solutions for several weeks, came prepared with their inventions yesterday. Their goal: a special achievement award from the teacher and an unbroken egg.

As Jones, school administrators and two National Science Foundation officials stared skyward, a foil-wrapped box plummeted from the window, landing on its corner with an audible squish. Michelle Cooley, 10, clapped her hands to her forehead and ran to unpack the contents.

Her egg, in spite of the yellow terry washcloth, sponges, paper bag and toilet paper surrounding it, had not withstood the impact.

"Mine broke -- yech," Michelle said. "When I heard the loud thud, I knew it wasn't going to survive."

Howard Brown's egg, buried deep inside a rubber ball, was okay. "I recommend balloons and pillow stuffing" to cushion the egg, said Howard, who is 10. "I wanted something original and I wanted something that would be okay no matter which way it fell."

According to Chris Corkey, 10, it does matter which way the egg falls; if the point is down, he said, it is less likely to break.

"I tried it from my second-floor balcony and it worked," Chris said. "But I thought it was sort of cheating because it hit my dog first."

Medul Cordova, 10, tried hers at home, too. First she filled a small open-topped box with newspaper, fastened it with sewing thread to a plastic parachute and dropped it from a window.

The egg broke. So she took out the newspaper and added cotton balls. Yesterday the box floated perfectly, swinging a little in the warm breeze and landing gently outside the education center on North Quincy Street.

For some, successful egg drops yesterday came only after botched rehearsals at home.

"This is the seventh one I made," said Matt Brown, 11, as he rescued an unscathed egg from a plastic pantyhose container filled with cotton. "I was doing it in my front yard and a couple of them got run over by a car."

Matt's final production was launched with the leap of faith that is often the quiet companion of science.

"I made this one last night and this morning, so I didn't have time to try it," he said. "I wasn't so sure it was going to work, but it did."