Neil Armstrong was right. It was indeed ''one giant leap for mankind'' when he stepped onto the moon 15 years ago tomorrow - July 20. Until then, the moon had been a metaphor for the unattainable. With the Apollo 11 mission of astronauts Edwin Aldrin, Michael Collins, and Armstrong, the unattainable became a new frontier.
Three years later the Apollo program ended. By then what had begun as a breathtaking adventure had lost much of its interest among the general public. Yet few doubted that one day there would be new expeditions to develop that frontier, perhaps even to set up permanent bases.
Meanwhile, for scientists, those years were a revelation. Six expeditions returned 843 pounds of material. At last planetologists had pieces of the moon in hand to study.
Before Apollo, some scientists had begun to question the standard notion of the moon as having been dead virtually since its formation. The moon rocks and other data gathered by the Apollo program and the Soviet unmanned explorations produced evidence of extensive early melting, vulcanism, and, most surprising of all, magnetism. Although orbiting probes had not detected it and most scientists had not expected it, the moon had a remanent magnetism. This strongly suggested that at one time it had a molten, metallic core - that its substance had at least partly melted and, like Earth, differentiated into a layered structure.
Scientists now know that Earth and moon are roughly the same age - 4.6 billion years. The lunar surface has been sculptured by heavy meteorite bombardment that ended around 3.9 billion years ago. Extensive lava flows flooded the low-lying basins - the so-called seas, or maria. This activity probably ended some 2 or 3 billion years ago.
Major questions remain - especially questions about how the moon has evolved as a planet and how it originated. Is it material hived off by Earth? Did Earth capture it after it was formed elsewhere? Did Earth and moon form together as a double planet system?
It will be a while before explorers can return to the moon to seek new data to help answer such questions or to start mining operations or other practical development. Neither the United States nor the Soviet Union now has the capacity for lunar exploration. Yet the Soviets insist that this is in their long-range planning. And, at the NASA-Johnson Space Center in Houston, a small team is considering possible lunar bases of the future.
Apollo 11 left a plaque saying, ''Here men from the Planet Earth first set foot on the Moon July 1969. We came in peace for all mankind.'' It's gathering moon dust now. But one day it will be in a museum - something for the lunar settlers to show their visitors.