As the United States enters the second quarter-century of the space age, scientists, planners, and officials who carry out the nation's civilian space program share two major concerns.
They say they are disheartened by what they view as a continuing lack of well defined national goals for the program. And they express alarm at the possibility of a military takeover.
These twin themes permeated a symposium held by the National Academy of Sciences to commemorate Sputnik - the Soviet satellite whose appearance on Oct. 4, 1957, opened the space age. These themes also surface in conversation with officials and researchers of the National Aeronautics and Space Administration (NASA).
Twenty-five years after Sputnik, these scientists and officials say they believe that the US civilian space program is at a critical stage. They explain that lack of a well defined program plus increasing involvement of the military, with attendant secrecy, could compromise the gains made by that program as a technically successful, open activity which fosters international cooperation. At the same time they foresee an explosion in practical uses of space as Japan, Europe, the Soviet Union, and even some third world countries expand their own civilian space activity.
Harvey Brooks of Harvard University, a physicist and veteran science policy analyst, reminded the Academy symposium that the US civilian space program had been founded by law on a set of principles which, among other things, include:
* ''The separation of military and civilian space activities to the greatest degree feasible.''
* ''A high degree of openness . . . a strong emphasis on public information, and a willingness to expose mistakes as well as successes.''
* ''A strong international commitment. . . .''
Brooks noted that space is, by nature, an international ''commons.'' Many activities, such as world communications satellite networks, demand multinational cooperation. Also, the great cost of major exploration should be shared among nations, he said.