The sky's no limit for Rachel Safman, a science-minded Gaithersburg girl who was awarded a chance last week to send an experiment into orbit aboard the space shuttle Columbia.

At age 14, Rachel is the youngest of 10 finalists in a nationwide secondary school competition sponsored by NASA and the National Science Teachers Association, which supports the program as part of its effort to strengthen interest in science and engineering in the nation's schools.

"My experiment," she says, "will inject argon gas into molten gallium in a micro-gravity environment." Seeing a puzzled look on a listener, she patiently explains it again: It's a new method of metal casting that will take advantage of the near-weightless conditions aboard spacecraft.

Eventually, Rachel hopes, the method will lead to the extraterrestrial manufacture of strong, ultra-lightweight metal products.

At a recent NASA symposium, each of the 10 national finalists defended their proposals before a skeptical, demanding audience of seasoned space researchers, seeking in the name of science to poke holes in the experiments the students had devised.

"They were pretty rough at times," she said, sounding tired at the close of a long day that included a whirlwind tour of the Capitol and office visits with Maryland Rep. Michael Barnes (D) and Sens. Charles McC. Mathias (R) and Paul Sarbanes (D).

The next step for the 10 student biologists, chemists and engineers is the search for a sponsor--such as a private corporation, research hospital or university--willing to underwrite development costs necessary to make the proposals into workable experiments that will meet NASA's rigid requirements for size, safety and ease of operation.

Rachel's experiment, which could eventually have commercial application, may be underwritten by the TRW Corporation, which she says has expressed an interest in the research. Many of the past three year's student projects have been supported by companies holding major government aerospace contracts, such as General Dynamics, Thiokol, Lockheed, Martin Marietta and Rockwell International.

So far, six student projects have gone aloft and a seventh is set for Columbia's next launch, which NASA officials now expect could come as early as next month. The experiments are stowed in lockers on the flight deck and must each take less than an hour of the crew's time to execute.

Most of the students' projects are designed to test the effects of near-zero gravity on a variety of experiments in biology and materials science. They range from new methods of paper manufacture, changes in blood or bone growth in space, or the growth of metallic crystals in a weightless environment.

Rachel is a graduate of Montgomery Village Junior High School, where she participated in an accelerated program for exceptional students. She will attend Gaithersburg High School in the fall, where she plans to prepare for a premedical curriculum in college.

Her experiment, little more than theory up to now, must now be translated into hardware. Still ahead, probably a year or more, is a meeting with the shuttle crew to brief them on how to operate the metal-casting apparatus, and in the meantime, regular consultations with NASA scientists who will help with development of the design.

Whether the experiment will be airborne before Rachel graduates from high school is uncertain.

"As these shots get more routine, we expect to send up one or two student experiments on each flight," said NASA's Alan Ladwig. He acknowledged that the program had a backlog of student experiments, but said he expected more could be carried out on future flights.

"The whole purpose of the program is to stimulate the study of science and engineering," Ladwig said. "Actually getting it up on the shuttle is really just icing on the cake."

The program, now in its third year, is unrelated to the series of experiments placed in the shuttle's cargo bay, which most recently included a Camden, N.J., high school's experiment on ant behavior in space.