When the last Apollo explorers left the moon, Astronaut Eugene Cernan made a promise on behalf of his successors: ''We will return.'' Now a group of United States scientists, engineers, and space planners have been meeting here to consider fulfilling that promise.
The symposium, ''Lunar Bases and Space Activities of the 21st Century,'' is no prelude to another Apollo-like, high-pressure, single-purpose project. As the title implies, the symposium has taken a long look ahead toward feasible and worthwhile goals for the US space program in the next century. The participants seek goals that would be a natural part of the orderly development of the space frontier. But with a mandate from President Reagan to recommend such goals - and aware that the 21st century is only 15 years away - the National Aeronautics and Space Administration (NASA) has concluded that it is time to look moonward once again.
NASA administrator James M. Beggs noted in opening the meeting Monday that a lunar base has always been part of NASA's long-term vision. It was mentioned even in the earliest planning documents when the agency was founded 26 years ago.
''I certainly believe that sometime within the next 25 years we will return to the moon,'' Mr. Beggs said. Moreover, he explained, interest in lunar bases has been renewed because the US at last is preparing to build the kind of permanent infrastructure in near-Earth orbit that will provide relatively easy access to the moon.
This is the central theme of the symposium, and the essential difference between the old Project Apollo and the kind of moon program now being contemplated.
Apollo was a crash program - the US entry in a space race with the Soviets. It was designed to put men on the moon and return them safely to Earth along with whatever lunar samples and scientific data they could gather. The scientific payoff was valuable, and much was learned about space flight. Yet, as presidential science adviser George A. Keyworth II reminded the symposium, Apollo was a dead end.
''Much of the momentum of our space program was lost after Apollo, because we treated the moon landing as an end in itself,'' he said.
So the US moved on to develop a more practical spaceflight capability based on the reusable shuttle. This has given easy, routine access to low Earth orbits. Next comes the permanently manned space station, which NASA hopes to have on orbit by 1992.
Then, Beggs explained, during the decade that follows, a new capability for men and women to live and work in space will begin to build up around the station. This will include what he called the first ''true spacecraft.'' These will be units that remain at all times in space. They will enable astronauts to move from satellite to satellite or to move from low orbits to higher positions beyond the few-hundred-mile altitude range of the shuttle. They will be forerunners of craft that could carry crews to asteroids, Mars, or the moon.
The reasons for establishing lunar settlements are yet to be defined. This will require a national debate, for it is essentially a political decision - a fact that Beggs and other speakers at the symposium emphasized. A NASA sponsored workshop last April, however, identified a number of reasons for developing the moon. For example, a permanent base would allow extensive scientific exploration. It would give ready access to lunar materials, including oxygen and such metals as iron or titanium.
Also, the moon may be the best staging area from which to launch expeditions to Mars. And manned exploration of Mars is another objective that NASA expects to gain priority as the next century opens.
As for cost, this can only be determined when a specific moon program is outlined. Preliminary estimates for ''typical'' expeditions, however, suggest the costs should not be extraordinary.
To begin with, much of the basic capability will have been built and funded for commercial purposes. This includes the infrastructure of space stations and orbital transfer craft, of which Beggs spoke. Even much of the technology for living for long periods in space must be developed for the space station. Thus, to cite one example, a study commissioned by the Planetary Society estimates that it would cost $17 billion (in 1984 dollars) to mount a 30-day lunar exploration with a crew of four in order to prospect for a site for a permanent base. That compares with the $75 billion cost for Project Apollo when that cost also is expressed in 1984 dollars.