Washington — As United States space planners set their sights on the moon, Mars, and the asteroids, they also see new opportunity for international cooperation. They discuss the development of what they like to call the ''last frontier'' in terms of a number of nations working together to benefit mankind.
But the new thrust into space has its darker, nationalistic, military side. The water, metals, and other ''strategic'' materials that civilian planners hope to get from the moon - and especially from asteroids - also would be ideal materials to harden orbiting weapons systems against enemy attack. Hence, they are being eyed by military analysts as well. Indeed, the possibility of getting relatively cheap shielding from asteroids is already cited in support of the administration's strategic defense initiative, or ''star wars'' space weapons program.
This prompts Louis Friedman, director of the Planetary Society, to ask whether these parallel approaches to space - civilian outreach and military defensiveness - are compatible. Speaking at the end of a three-day conference on new space initiatives, sponsored by the National Aeronautics and Space Administration (NASA), he said there is a fundamental ''conflict in motivation'' between the star-wars approach and the vision of global cooperation in solar-system exploration. He considers it to be one of the biggest challenges to developing the space frontier.
The meeting was held Oct. 29-31 on the assumption that a new expansion of space activity is imminent. Dozens of technical papers backed up the statement of NASA administrator James M. Beggs that the shuttle-supported space station and its attendant infrastructure of space ferries, workshops, and communications links will open the way to the inner solar system over the next 15 years.
The use of space-derived materials to harden weapons systems will also become practical. In fact, this is the same time scale that star-wars planners have discussed for establishing the space-based missile defense - that is, toward the end of the century.
Arthur Kantrowitz, retired director of the Avco-Everett Research Laboratory and now a professor of engineering at Dartmouth College, stressed this point in one of the meeting's keynote speeches. He said that criticism of the star-wars concept has turned partly on the impracticality of carrying shielding material into orbit from Earth. The weapons satellites would be vulnerable to attack because it would cost too much to harden them. Such criticism, he said, assumes ''the fragility of today's spacecraft without including the possibility of ETM (extraterrestrial materials)'' as shielding.
John S. Lewis of the University of Arizona and Carolyn P. Meinel of Analytic Decisions Inc. have studied this possibility. Their work details how materials such as water, iron, and high-strength nickel can be obtained from asteroids. These and other materials could provide effective shielding. Large amounts of water, for example, could absorb the energy of attacking laser beams and dissipate it as the water evaporated.
The central point is that it takes much less work to recover such materials from an asteroid and bring them to low Earth orbits than it does to lift the materials into orbit against Earth's gravity and through the resistance of the atmosphere. In papers distributed at the meeting, they estimated at least a tenfold reduction in cost - say, $100 per kilogram or less as opposed to $1,000 per kilogram to orbit materials from Earth.
Whether or not such savings can be realized in practice cannot be judged until asteroid mining is actually tested. But the fact that materials from the moon or asteroids are seriously considered for military, as well as civilian, uses will put a strain on efforts to internationalize US space development, Dr. Friedman pointed out.
The desirability of international participation was stressed repeatedly during the conference. NASA administrator Beggs summed it up this way in a keynote address: ''We know that any enterprise of the magnitude and scope of a permanently manned lunar base would be an enormous challenge.... This implies even greater international cooperation and international sharing of risks and benefits in the future.
''In this connection, we expect our friends and allies to join us in developing the space station. Such cooperation could lay the groundwork for even greater international collaboration in space for the future. Indeed, an internationally developed lunar base might even prove an irresistible lure to the Soviets. And if they were to join with us, I believe that would certainly enhance the prospects for peace in the world and in space.''
President Reagan's endorsement, Tuesday, of the congressional resolution pledging the US to seek cooperation with the Soviets in space activities reinforced this view. But in citing this resolution, Dr. Friedman said that the inconsistency between the desire to internationalize space development and the drive to build a national space weapons system prompts him to ask: ''Can they co-exist?''