The teetering space agency needs another diagnostic exam like it needs more leaky shuttles or blurred telescopes. But when presidents seek to duck the risks of assertiveness, they traditionally order a study. And that's what George Bush has prescribed for NASA rather than acknowledge that the space program is weighed down by a burdensome, dispensable cargo: the monumental costs of humans in space, easily replaced by robots and instruments.

Nonetheless, astronauts are foolishly revered in the NASA ethos as crucial for maintaining public interest in space. The result is a NASA that runs threadbare, even as Congress piles on additional billions to maintain the romance of manned shuttles and a manned space station.

The newly ordered presidential study arises from NASA's recent run of extravagant flops after a protracted though seemingly successful recovery from the Challenger disaster. First came the debacle of the $1.5 billion Hubble Telescope. It was sent aloft with a disabling optical flaw that went undetected because hardpressed NASA spared itself the testing costs of several million dollars. An encore was provided by the grounding of the shuttle fleet because of mysteriously leaking fuel valves that raised the specter of another Challenger catastrophe.

These embarrassments occurred while Congress was once again hearing that the centerpiece of NASA's efforts for the 1990s, the manned space station, would cost many more billions than the most recent estimate -- which was many more billions than the previous estimate. Initiated by Ronald Reagan in 1984, the space station has soared on hallucinatory cost estimates that started at $8 billion and are now well over $30 billion. In fact, no one can reliably say what it will cost or whether it can actually be built, since its construction depends on the lift capacity of the trouble-plagued Shuttle. Despite this uncertainty, the Bush administration has gone beyond the space station and declared its commitment to a moon-to-Mars manned voyage, for which the cost estimates are in the hundreds of billions -- but how many hundreds, no one knows.

Though over $4 billion has so far been spent on the space station, it exists only as a paper design, and with virtually no purpose beyond serving as a platform for the glamour of man in space. Nonetheless, the venture has soured relations with the European, Canadian and Japanese space agencies that NASA signed up as partners. After the foreign participants had spent several billion dollars on the design of facilities that could link up with the space station, NASA unilaterally scaled down the venture to save money. Among the items cut back was the electric-power supply for the foreign hardware.

Legitimate howls of betrayal led Bush's science adviser, Dr. D. Allan Bromley, to declare publicly that the United States must strive to be a ''more reliable partner'' in international high-tech collaboration. But not long after that, NASA, citing financial pressures, pulled out of a collaborative program to put a Franco-American research instrument on a Soviet satellite.

The villain in this tortuous succession of space follies is NASA's commitment to a manned space program, for which the rationale is keyed to public relations rather than technological necessity. Men and women are delicate and costly cargo in space, requiring huge expenditures to ensure their safety. They possess the unique capacity to ''Ooh'' and ''Ah'' about the wonders of space travel, but when it comes to getting work done, robots and instruments are cheaper and more reliable. If they fail to return, the nation does not mourn.

The curse of the space program is that when it is capable of technological maturity, it remains dominated by the circus spirit of the moon-landing project of the 1960s. It was concern about drooping public attention that inspired NASA to put a school teacher, Christa McAuliffe, aboard the ill-fated Challenger. Prior to the explosion, NASA drumbeaters ebulliently characterized the shuttle as a routinely operating space transportation system, which it never was and never will be.

The new presidential study is to be conducted by outside experts selected by NASA -- which leaves little hope for cutting through to the source of NASA's woes. But if the committee wants to take the rogue route, make history and save NASA, it need only point out that humans in space are a costly nuisance that the space program can no longer afford.

The writer is editor and publisher of Science & Government Report, a Washington-based newsletter.