TWENTY years ago, our species did an extraordinary thing. Human beings stood on another celestial body for the first time in their long evolution. For those who like to be precise about historic moments, the landing craft Eagle touched down on the moon at 4 hours, 17 minutes, 40 seconds EDT on the afternoon of July 20, 1989. As Mission Commander Neil A. Armstrong put it in words that have become a space-age clich'e, when his boot hit the lunar dust, it was ``one small step for a man, one giant leap for mankind.''
Another famous sentence, inscribed on a plaque fixed to one of Eagle's legs, set an equally momentous precedent. It proclaimed, ``We came in peace for all mankind.'' For once, a major national expedition made no claims of national sovereignty over the territory it explored.
The Apollo 11 astronauts - moon-walkers Armstrong and Edwin A. (Buzz) Aldrin Jr., plus mother-ship pilot Michael Collins - were point men for a bold program. Each launch of the Saturn 5 rocket, whose 3.4-million-kilogram (7.5-million-pound) first-stage thrust boosted an Apollo mission moonward, cost about $1 billion.
It took an army of specialists and bureaucrats to put those men on the moon. A new industry grew up to support their missions. Knowledge that the United States was in a prestige race with the Soviet Union spurred the effort.
Yet, in spite of this massive investment, the United States knew it could not claim even a small piece of moonscape as its own. Its expedition was but a prelude to an exploration - and, perhaps, eventual colonization - that would require the efforts of many nations to complete. So there was explicit recognition that, when humans land on other worlds, they do so on behalf of all mankind, not of any one nation.
This may be the most enduring heritage of the Apollo lunar program.
It's the perspective in which spacefaring nations - especially the United States and Soviet Union - remember the Apollo achievements. It's the perspective in which their planners study how and when to return to the moon early in the next century. These planners talk in terms of cooperation, not of rivalry. James C. Fletcher, who recently retired as administrator of the US National Aeronautics and Space Administration (NASA), made this point succinctly last year.
``Going to the moon together,'' he said, ``would give the two leading spacefaring nations ... an opportunity to build a stable base for further cooperation.''
ANOTHER major Apollo heritage is the wealth of scientific data and lunar samples it returned. No one argues that these, in themselves, justified the program's great cost. But, given the investment, they represent a valuable dividend.
Six crews visited the moon between July 1969 and December 1972. They studied the moon from orbit. They explored both lowlands and rocky highlands on the surface. They set up a variety of instruments. And they brought home 381 kilograms (841 pounds) of rocks and soil - more than 2,000 individual samples from nine locations.
Scientists in many countries have shared - and continue to share - in the study of these data and samples. NASA receives 200 to 300 proposals a year for research on moon material.
So far, NASA has given out about 6 percent of the Apollo materials for analysis or display. The bulk of the samples rests in a protective vault at the Johnson Space Center in Houston. Michael B. Duke, who heads NASA's Solar System Exploration Division, says he considers this a reserve of virgin material held against the time when improved technology will enable investigators to learn more about it.
Scientists went into the Apollo program with some big questions. Where did the moon come from? How has it evolved? What is its internal structure? What lies on its surface? Apollo provided some answers, although the knowledge is far from complete.
Lunar experts now know the moon had a strenuous youth. The same age as Earth - 4.6 billion years - the early moon's outer several hundred kilometers underwent extensive melting. There is evidence of magma eruptions and of a massive meteorite bombardment whose most intense phase ended around 3.8 billion years ago.
After that, more partial melting formed the lavas that fill the dark maria basins that form the ``man in the moon'' features. Then, its internal heat largely exhausted, the moon appears to have been inert for the past 3 billion years.
Scientists still don't know where the moon came from. Theories that it split off from Earth continue to vie with theories that it formed separately.
A current - but unproved - favorite scheme explains how the impact of a body a little bigger than Mars could have carved the moon out of the young Earth. Horten E. Newsom of the University of New Mexico and Stuart Ross Taylor of the Australian National University in Canberra outlined this last March in the journal Nature. But lunar experts all say they must await a new round of lunar explorations to try to settle such issues.
WHILE the Apollo program was historically significant and scientifically fruitful, it had a negative aspect. Virtually all who discuss United States space policy now use it as the prime example of how not to plan a national space program. From its inception, Apollo had no future. Its goal was to put men on the moon and return them safely to Earth within the decade of the 1960s. It was not designed as a logical first step in a long-term strategy.
Indeed, Apollo turned United States space planning on end. Throughout the '50s, individual experts and study groups that looked ahead to manned spaceflight thought in terms of first learning to work in Earth orbit. They saw the building and operation of a space station as the first major step. Only then would they think of using that station as a base from which to build ships and send them to the moon.
Seen in this light, Apollo, for all its magnificent achievement, was a diversion from sound space planning - a diversion encouraged by cold-war rivalry. Every serious study of United States space policy undertaken in this decade - including those of the National Academy of Sciences, the National Commission on Space, and NASA itself - has made this point. They often make the additional point that the space station NASA now is developing also lacks a clearly defined role as a step toward a long-term goal.
Thus, in looking back to Apollo and forward to the next decade, when several countries will be actively pursuing commercial uses of space, many observers wonder if the United States has yet learned the overriding importance of having a well-defined long-range space plan.
Former astronaut Byron K. Lichtenberg, a research affiliate with the Massachusetts Institute of Technology, picked up this point last month at a space symposium at MIT. He said, ``The train has left the station and the signs aren't in English. We have the technology and we have the know-how [to play a strong role in space], but we don't have the vision - yet.''