As the United States begins to define its future in space, there's no secret as to who is the program's cheerleader. He sits in the Oval Office. Special presidential assistant Craig Fuller notes that ''the space program has occasionally had a hard time finding a home in the White House. But this President has an enthusiasm for it.''

This enthusiasm - reflected by the National Aeronautics and Space Administration (NASA) and other relevant agencies - permeates the new US National Space Strategy. Signed by the President Aug. 15 and outlined for the press at a White House briefing Sept. 7, this formal plan aims to give the US a leading role in the dawning age of profitable, commercial space activities. (See accompanying box.)

In a sense, it's easy for Mr. Reagan to be enthusiastic. He's the first president to hold office when, with the exception of communications satellites, long prophesied fruits of this country's space enterprise are beginning to ripen.

For example, in spite of an equipment malfunction on the recent shuttle flight, McDonnell Douglas Corporation's payload specialist Charles D. Walker obtained 80 to 90 percent of the expected yield of a biological material best produced in space. That's enough to begin Federal Drug Administration certification tests. Soon the drug, whose nature is a commercial secret, should be available with a market value of many millions of dollars. Yet McDonnell Douglas and its partner Johnson & Johnson have estimated that the cost of such items to consumers should be substantially less than that of comparable products produced on Earth, if such products exist at all.

Furthermore, highly precise latex microspheres - also best made under weightless conditions - are already being sold. NASA Administrator James M. Beggs says this shuttle-produced item commands a price that more than justifies the production cost and has a market that ''is quite large.'' He notes that one small vial containing 15 million of the identical-sized spheres suspended in water will cost about $400. Six hundred of these units are available for sale by the National Bureau of Standards. The precisely shaped spheres - 10 micrometers in diameter - have been certified as a standard reference material for calibrating scientific instruments. Such tools are used to monitor air pollution particles, finely ground products such as paint pigments, ink, or explosives, or in such medical applications as counting red blood cells.

With such returns from the nation's space investment being realized and with more on the horizon, Reagan says he has little patience with those who fail to see the long-term potential of space. He told reporters at the recent briefing that, like astronaut footprints on the moon, ''the maiden voyage of the space shuttle Discovery tells us that our future will be determined by our dreams and our visions.''

He added: ''Now there are some, however, who may believe that space is a luxury that we can't afford. They may work to protect the past. And I believe it's up to us to invent the future. Pushing back the frontiers of space is a critical investment that will lead to better times to come.''

Foreseeing benefits that will ''literally dazzle the imagination,'' he explained: ''... we can produce rare medicines with the potential of saving thousands of lives and hundreds of millions of dollars. We'll be able to open the door to new opportunities for important breakthroughs in cancer research and diabetes and other diseases. We'll be able to manufacture superchips that improve our competitive position in the world computer market. And we'll be able to develop new metals that are lighter and stronger than any we've ever known. And in partnership with industry and academia, the opportunity to extend basic research will grow and grow. And there will be new discoveries and breakthroughs - new progress.''

It's quite a vision. But it's now shared by the Japanese, the West Europeans, and, of course, the Soviets. They, too, see the beginnings of the new space industry. And they will be tough competitors in the marketplace for space products and services: the European Space Agency's Ariane rocket already is the shuttle's competitor for the burgeoning business of launching commercial satellites.

Thus, the main thrust of the administration's new strategy is to help ensure US commercial preeminence in space.

This is a stiff challenge. To begin with, the commercial viability of the shuttle itself is still uncertain. It is now able to compete with Ariane only because the fees NASA charges customers are heavily subsidized. NASA charges $71 million per mission (prorated by customer) against a true cost of about $170 million.

The new strategy mandates that NASA charge full costs beginning Oct. 1, 1988. Mr. Beggs estimates that by then true costs should be about $100 million a mission. However, this depends partly on spreading costs over 20 missions a year. Beggs says that with the recent successful Discovery mission, the shuttle program has demonstrated that it is healthy and back on schedule. But whether it attracts enough business to fly 20 missions a year by 1988 remains to be seen.

Besides the West Europeans, NASA will be competing with the United States' own fledgling commercial launch businesses. Using unmanned rockets, several companies are now preparing to offer such service. Indeed, one reason the new strategy mandates full costs for shuttle services is to prevent a subsidized shuttle from undercutting these new enterprises, which the government wants to foster.

This is just one of many problems to be solved as the US space endeavor evolves from a largely government effort to the more normal status of an activity where government and private business work together, as in air transport or shipping. There is a tangle of regulations and laws written in a prespace age which needs to be revised. New ways of regulating launch ranges, which up to now have known only government users, need to be devised.

For example, Tony Calio, deputy administrator of the National Oceanic and Atmospheric Administration, points out that it can take three years and the involvement of several agencies to assign radio frequencies for a private rocket launch site. A working group, he said, hopes to find ways to cut this time to six months. Also, he noted that tariff codes need to be revised to avoid such absurd situations as treating space-made goods as imports that pay customs duties.

The Department of Transportation, which is the lead agency in encouraging private launch-vehicle operators, has created working groups and a joint government, industry, university advisory panel to help unravel needlessly tangled regulations. It now has a mission to help guide this new business as it develops into what one day will probably be a major space-transport industry.

Detailing this challenge, Transportation Secretary Elizabeth Hanford Dole said her department's job now ''is to ensure the (new) industry every opportunity to establish itself in a free, competitive marketplace.''

This does not mean her department will take the risk out of the enterprise, but, she said, ''to reduce economic risk to a reasonable level - not to pave the way, but to point the way.''

And that would seem to be a motto that reflects the commercial goals of the new space strategy.

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