FOR the United States National Aeronautics and Space Administration (NASA), bad news about its budget is set in an encouraging perspective.NASA Adminsitrator Richard H. Truly says he is "disappointed that, for the first time in many years, the total NASA appropriation [for fiscal 1992] does not keep up with inflation." Yet half a decade of serious reviews of NASA's mission have produced what looks like near consensus within the space community and the government on the space program's importance and desired goals. NASA's challenge now is to shape its plans to take advantage of that supportive consensus while keeping its ambitions within the limits of modest growth in funding. This could make the 1990s a decade of substantial space achievement. Congress is giving the agency about $14.3 billion this fiscal year - $1.4 billion less than requested. That's about 3.3 percent growth over last year. But, considering inflation, it's really no growth at all. Space Station Freedom gets the full $2 billion requested. Many other programs are curtailed, postponed, or even eliminated. A long-planned orbiting solar observatory, for example, is postponed indefinitely, leaving solar scientists who were counting on it wondering what to do with their careers. NASA and its advisory groups are rethinking their strategy to emphasize smaller, less costly space-science missions. It would be too simplistic to blame this situation simply on political maneuvering that preserves space-station funding at the expense of other programs. The United States space program is undergoing a healthy metamorphosis. It should emerge leaner and more focused on broadly accepted goals. The fact that this prospect can even be discussed represents what US Rep. George E. Brown Jr. (D) of California calls "a major step beyond the mind-set of just five years ago." That was when many concerned with the space program still considered it to be drifting without purpose. Mr. Brown - who chairs the House Science, Space, and Technology Committee - says he now sees "a limited but growing consensus on the broad objectives." Reviewing his reasoning last month in a speech to the National Space Club, he explained that he sees consensus because six major space-program reviews have all identified the same broad objectives. These include: * Knowledge, meaning largely space science. * Exploration, meaning largely manned spaceflight. * Applications of space science and technology here on Earth. Brown also cited two supporting objectives - more efficient space transportation and development of new, more- effective space technology. He also warned that NASA can't pursue all these goals at full throttle in the present budget climate. A recent review urged a 10 percent annual growth rate after inflation for the space program. But 5 percent, if that, is all that seems practical. What's needed, Brown said, is long-term planning that deals with budget stagnation and takes advantage of whatever growth in activity is possible. NASA faces a severe challenge in trying to tailor a scientifically and technically rewarding space program to those specifications. It means replacing some long-desired big projects with smaller missions and making the most of the space station to which it now is committed. Although this may seem difficult now, it could turn out to be a profitable strategy.