AS American manned spaceflight planners once again set their sights on the moon, rocket pioneer and space prophet Krafft A. Ehricke has encapsuled their vision in a sentence: ''If God had wanted a species, such as this, to become a space-faring species, he would have given them a moon.''
He explains: ''Here we have a body with lower gravity . . . a body whose surface conditions are . . . far more similar to those of any other accessable surface, from Mercury to the moons of Saturn, than Earth is (Mars excepted). This is where we can cut our teeth. It will make us fit to go further out into the solar system.''
And to judge from the tenor of the recent NASA-sponsored moon-base conference , where Dr. Ehricke delivered the main lecture, the United States spaceflight community is eager to begin gnawing on the lunar challenge.
Ehricke, now with Space Global Company of La Jolla, Calif., was one of the orginal von Braun rocket team. For decades, he has been the expert to whom other Western experts have most often turned for an overarching yet technically sound vision of how the future of manned spaceflight could reasonably be expected to unfold. His concept of the moon as ''a proving ground'' where humanity prepares to colonize the solar system, meshes nicely with the American spaceflight community's own aspirations.
The National Aeronautics and Space Administration (NASA) held the conference Oct. 19-31 as part of an ongoing effort to define meaningful goals beyond the space station, which it is committed to have on orbit by the early 1990s. This follows the deliberations last April of the Lunar Base Working Group, whose report was distributed as a background document.
There was little doubt expressed at the meeting that the space station, serviced by the shuttle and with its attendant infrastructure of orbit-to-orbit spacecraft, will represent much of the basic capability needed to return to the moon. Also, there was general agreement with the working group's perception that ''the space station will change the way our society views space.''
The group's report explains that ''subsequently, major US goals will focus on occupying rather than visiting space.'' Seen in this perspective, the group said , ''A permanent base on the moon offers the most robust combination of factors that will foster steady long-term growth for our nation's space future.''
As former astronaut and US Sen. Harrison Schmitt put it, ''The frontier of space, the new ocean of exploration, commerce, and human achievement, has produced a level of excitement and motivation among young generations of the world that has not been seen for nearly a century. History clearly shows us that nothing motivates the young in spirit like a frontier.''
The reasons given by the working group for colonizing the moon reflect this new frontier spirit. They include:
* Mining lunar oxygen to help make rocket fuel, which then could be supplied to the space transportation network, including the space-station complex. This could be done more cheaply than if oxygen were carried into orbit against Earth's gravity.
* Mining other resources, including structural materials such as glass made of lunar rubble, to aid in building structures or enhancing large-scale space activities.
* Scientific research of the moon itself and using moon-based facilities such as ultrasensitive radio telescopes or particle accelerators. The accelerators would take advantage of the lunar vacuum to reach very high energies for probing the basic structure of matter.
* Aspirations of establishing the first human society on another planet - a development the working group says would mark ''a cultural watershed of sufficient magnitude to inspire broad public support and international respect.''
That is a much broader set of reasons for returning to the moon than was given for sending astronauts there in the first place. The old Apollo program merely aimed to put men on the moon and return them safely to Earth within the decade of the 1960s, doing as much scientific research as was feasible along the way.
The goals now being discussed are so far reaching that, at this stage, they can only provide the background for long-term thinking. They are not, in themselves, an agenda for specific action.
Nevertheless, the engineers, scientists, lawyers, sociologists, entrepreneurs , and NASA officials attending the moon-base conference seemed to have little doubt that, once the space-station complex is in place in the 1990s, the moon will beckon strongly. And, as a number of scientists noted, the need for extensive lunar research will likely become compelling. This proposed new home for humanity is not well known at all.
''We have sufficient knowledge to proceed with the development of a lunar base,'' says Alan B. Binder of NASA's Johnson Space Center. ''However, the surface area of the moon is that of the combined North and South American continents and the nine Apollo and (Soviet unmanned) Luna landing sites were restricted to an area about equal to that of the Midwest USA. Thus, even though we could proceed with the founding of a lunar base without a global definition of lunar resources, seismicity, tectonics, etc., it seems prudent not to do so.''
Even with this limited exploration of the moon, ''our ideas about it have undergone a revolution,'' says the University of New Mexico's G. Jeffrey Taylor. Once considered to have been cold and dead throughout its history, the moon now is known to have had a strenuous youth.
It is the same age as Earth - 4.6 billion years. Scientists can trace the moon's development throughout that time. Geological activity has destroyed Earth's records of its first 700 million years.
''The less active moon still contains that record, cryptically preserved in its ancient rocks,'' Professor Taylor explains. It is a record of extensive melting of the moon's outer several hundred kilometers, of magma eruptions, and of massive meteorite bombardment, the most intense phase of which ended around 3 .9 billion years ago. After more partial melting formed the lavas that filled the great maria basins, the moon, its internal heat exhausted, has been nearly inert for the past 3 billion years.
Between them, six Apollo moon missions and three Luna missions returned 381 kilograms of rocks and soil - more than 2,000 individual samples from nine locations.
Most of the minerals found in these materials were already known from Earth rocks, with a few exceptions that formed under the moon's waterless, low-oxygen conditions. Most highland rocks are breccias: They consist of rock fragments, which may themselves be made up of fragments. Highland rocks are rich in feldspar, a white calcium-aluminum silicate. The so-called KREEP rocks are also significantly enriched in potassium (K), rare-earth elements (REE), and phosphorus (P). The ages of highland rocks cluster around 3.8 billion to 4 billion years. A few are as old as the moon itself.
The maria - lunar ''seas'' whose dark cast forms the ''Man-in-the-Moon'' features - are covered by lavas from the moon's interior. These basaltic rocks are rich in olivine, pyroxene, and iron-titanium minerals.
To judge from these findings, lunar colonists could not expect to find the full range of resources to which they are accustomed on Earth. However, of the elements most needed - oxygen, silicon, iron, aluminum, hydrogen - all except hydrogen seem to be adequately abundant. So does titanium, a structurally useful material found in some mare basalts. In fact, oxygen is quite abundant. Even though lunar materials formed under low-oxygen conditions when compared to Earth , lunar soils returned by the Apollo 11, 14, and 16 missions have 42 to 45 percent oxygen content by weight.
It is this kind of data that encouraged experts at the moon-base conference to talk repeatedly about mining lunar oxygen to make rocket fuel or to synthesize water using hydrogen brought from Earth.
Since water is 89 percent oxygen, which is 16 times heavier than hydrogen, Krafft Ehricke observed: ''If we had that kind of abundance of hydrogen on the moon, and extreme paucity of oxygen as (there) is now of hydrogen, the lunar development would be immensely more expensive. You would have to bring over oxygen, which would, of course, be (heavy and therefore) very, very expensive.'' In calculating the costs of lunar settlement, experts such as Dr. Ehricke, think primarily in terms of the energy (rocket fuel) needed to bring materials from Earth.
As for water itself, this, along with other biologically important materials such as carbon or nitrogen, is a lunar rarity, if it exists there at all. The Apollo and Luna samples are bone dry.
Some scientists speculate that water may be trapped in polar regions. However , Ehricke warned that even were water found, it might well be deposited as such a thin ice layer it would not be worth recovering.
Also, while the moon appears to have minerals worth mining, the techniques for such mining have yet to be developed.
To learn more about the prospects for lunar mining, for water in polar regions, and to gather new moon data generally, lunar scientists would like to begin a new program of unmanned exploration soon and well before any permanent base is planned.
Thus, in addition to - or in spite of - the romancing about populating the solar system and the speculation about lunar mining, lunar scientists want to gain the kind of solid knowledge required to answer the basic question with which NASA administrator James M. Beggs presented the conference.
''We now know that we can get there,'' he said. ''The question is, what should we be doing, if we establish permanent roots there, to make our presence most productive and beneficial for mankind?'' Landmarks in lunar exploration 1959
Sept. 12: Soviet Union launches Luna 2, the first man-made object to strike the moon's surface.
Oct. 4: Soviets launch Luna 3, which returns photographs of the far side of the moon.
Oct. 27: US launches unmanned Saturn I rocket, the forerunner of the launch vehicle that would loft US astronauts to the moon.
July 28: US launches Ranger 7, which returns more than 4,000 close-up photographs of the lunar surface as it plunges to the moon.
Jan. 31: Soviets launch Luna 9, the first mechanical object to land on the moon in working condition. Returns TV photos of the moon.
March 31, 1966: Soviets launch Luna 10, the moon's first artificial satellite.
May 30, 1966: US launches Surveyor I, which soft-lands on the moon and returns more than 11,000 TV pictures.
Aug. 10: US launches Lunar Orbiter I, which orbits the moon and photographs some 23 million square miles of the lunar surface, including 16,000 sq. mi. over primary Apollo landing sites.
Dec. 21: Soviets launch Luna 13, which carries soil density and gamma-ray density measuring devices, in addition to cameras.
Nov. 9: US launches first unmanned Saturn V booster, sending Apollo 4 into Earth orbit to test reentry capability.
Sept. 14: Soviets launch Zond 5, which becomes first man-made object to return from the moon. Carries tortoises, mealworms, wine flies, bacteria, plants , and seeds.
Dec. 21: US launches Apollo 8. Astronauts Frank Borman, James A. Lovell Jr., and William Anders become the first humans to fly to and orbit the moon. Astronauts, in the command service module, orbit the moon 10 times, coming within 70 miles of the lunar surface.
March 3: US launches Apollo 9, the first flight of the complete spacecraft, including the lunar module. While orbiting Earth, astronauts James A. McDivitt, David R. Scott, and Russell L. Schweickart successfully dock the lunar module with the command module and conduct first crew transfer between the two modules.
May 18: US launches Apollo 10, the first full dress rehearsal for manned lunar landing. Command module and lunar module orbit the moon, practice docking procedures. Astronauts Thomas P. Stafford and Eugene A Cernan fly lunar module to within 9.4 miles of the lunar surface, while John W. Young remains in the command module.
July 13: Soviets launch Luna 15, which orbits the moon.
July 16: US launches Apollo 11, the first manned lunar landing mission. Astronaut Neil A. Armstrong becomes first human to set foot on the moon. Edwin E. Aldrin Jr. accompanies him in the lunar module, while Michael Collins remains aboard the command module in lunar orbit. Astronauts deploy US flag, scientific instruments, and return with moon rock samples. Luna 15 plunges to the moon's surface two hours before Armstrong and Aldrin lift off from the moon.
April 11: US launches Apollo 13, but mission is aborted when fuel cells explode in the command module, some 205,000 miles from Earth. Astronauts James A. Lovell Jr., John W. Swigert Jr., and Fred W. Haise Jr. return to Earth using oxygen and power from the lunar module until just before reentry.
Sept. 12:Soviets launch Luna 12, the first successful unmanned moon craft capable of returning soil samples to Earth.
Nov. 10: Soviets launch Luna 17 with the first remote-controlled moon rover, designed to study soil samples at various locations.
Dec. 7: US launches Apollo 17, the last of the manned lunar missions.
Aug. 18: Soviets launch Luna 24, which returns core samples form the lunar surface.
Dec. 18: UN ''Moon Treaty'' is opened for signature by member nations. The treaty proclaims that the moon and its resources are the common heritage of mankind and that exploration and use of the moon ''shall be the province of all mankind and shall be carried out for the benefit and in the interests of all countries, irrespective of their degree of economic or scientific development.''