As the two letters reproduced on this page indicate, the highly successful US space program is deeply threatened. President Reagan's additional 12 percent across-the-board cut in discretionary spending by federal agencies, which is considered for the National Aeronautics and Space Administration (NASA), among others, would emasculate the US capacity to carry out space research and apply its fruits usefully. It would also restrict, although not destroy, the country's capacity for manned spaceflight by slowing development of practical uses of the space shuttle.
In fact, the industry journal Aviation Week & Space Technology reports that the extent of the cutbacks being discussed has raised concern within NASA that they ''would end 23 years of federal support for broad civil space research areas, possibly violating the National Aeronautics and Space Act of 1958 that founded NASA in the first place.''
At this writing, it was not known how much of the cutback talk was just a trial balloon and how much was a deadly serious proposal. A definitive statement from the administration is expected sometime in November. There is no doubt at all, however, that the cutbacks under discussion would forfeit US preeminence in space.
In fiscal terms, NASA would be asked to absorb cuts of $1 billion each for fiscal years 1983 and '84, plus $367 million in 1982, the current fiscal year. These cuts are over and above what was already included in the administration's earlier financial planning, which had put NASA on a tight budget that space scientists felt dangerously restricted prospects for US space exploration.
What now is proposed goes far beyond that kind of limitation. Virtually all solar-system research would be terminated. The Voyage spacecraft now heading for Uranus and Neptune would be shut down and the deep-space tracking network scrapped. Future planetary missions would probably be unlikely for an indefinite period. The Jet Propulsion Laboratory - the preeminent space research facility in the world - might well be closed, its capacities and personnel lost as a useful national resource. Most space applications would be canceled, with the exception of the latest resources surveillance satellite, Landsat D. Much aeronautical research would be stopped.
In short, the United States would weaken itself substantially in both space and aeronautical capacity. It is hard to square this prospect with the recent statement of presidential science adviser George Keyworth that ''in setting our priorities in the support of science and technology . . . the principal criterion for the fundamental pursuit of knowledge must be excellence - excellence in the investigators and in the subject. An additional criterion for the support of areas of research directed toward technological advances is pertinence - and this means pertinence to the recognizable economic and societal needs of the nation.''
What has achieved a higher standard of excellence than the space research that has opened a new era of exploration and revolutionized earth and planetary science? What could be more pertinent than those satellite photos that augment so powerfully the daily weather forecast?