A volcano in one sense represents the insides of a planet gushing out, a wound that eventually heals itself by cooling, only to be replaced by new stigmata. Different worlds have different insides. The discovery of liquid-sulfur vulcanism on Io was a little like finding that an old acquaintance, when cut, bleeds green. You had no idea such differences were possible. He seemed so ordinary.

We are naturally eager to find additional signs of vulcanism on other worlds. On Europa, the second of the Galilean moons of Jupiter and Io’s neighbor, there are no volcanic mountains at all; but molten ice—liquid water—seems to have gushed to the surface through an enormous number of crisscrossing dark markings before freezing. And further out, among the moons of Saturn, there are signs that liquid water has gushed up from the interior and wiped away impact craters. Still, we have never seen anything that might plausibly be an ice volcano in either the Jupiter or Saturn systems. On Triton, we may have observed nitrogen or methane vulcanism.

The volcanos of other worlds provide a stirring spectacle. They enhance our sense of wonder, our joy in the beauty and diversity of the Cosmos. But these exotic volcanos perform another service as well: They help us to know the volcanos of our own world—and perhaps will help one day even to predict their eruptions. If we cannot understand what’s happening in other circumstances, where the physical parameters are different, how deep can our understanding be of the circumstance of most concern to us? A general theory of vulcanism must cover all cases. When we stumble upon vast volcanic eminences on a geologically quiet Mars; when we discover the surface of Venus wiped clean only yesterday by floods of magma; when we find a world melted not by the heat of radioactive decay, as on Earth, but by gravitational tides exerted by nearby worlds; when we observe sulfur rather than silicate vulcanism; and when we begin to wonder, in the moons of the outer planets, whether we might be viewing water, ammonia, nitrogen, or methane vulcanism—then we are learning what else is possible.

It’s a sultry night in July. You’ve fallen asleep in the armchair. Abruptly, you startle awake, disoriented. The television set is on, but not the sound. You strain to understand what you’re seeing. Two ghostly white figures in coveralls and helmets are softly dancing under a pitch-black sky. They make strange little skipping motions, which propel them upward amid barely perceptible clouds of dust. But something is wrong. They take too long to come down. Encumbered as they are, they seem to be flying a little. You rub your eyes, but the dreamlike tableau persists.

Of all the events surrounding Apollo 11’s landing on the Moon on July 20, 1969, my most vivid recollection is its unreal quality. Neil Armstrong and Buzz Aldrin shuffled along the gray, dusty lunar surface, the Earth looming large in their sky, while Michael Collins, now the Moon’s own moon, orbited above them in lonely vigil. Yes, it was an astonishing technological achievement and a triumph for the United States. Yes, the astronauts displayed death-defying courage. Yes, as Armstrong said as he first alighted, this was a historic step for the human species. But if you turned off the by-play between Mission Control and the Sea of Tranquility, with its deliberately mundane and routine chatter, and stared into that black-and-white television monitor, you could glimpse that we humans had entered the realm of myth and legend.

We knew the Moon from our earliest days. It was there when our ancestors descended from the trees into the savannahs, when we learned to walk upright, when we first devised stone tools, when we domesticated fire, when we invented agriculture and built cities and set out to subdue the Earth. Folklore and popular songs celebrate a mysterious connection between the Moon and love. The word “month” and the second day of the week are both named after the Moon. Its waxing and waning—from crescent to full to crescent to new—was widely understood as a celestial metaphor of death and rebirth. It was connected with the ovulation cycle of women, which has nearly the same period—as the word “menstruation” (Latin mensis = month, from the word “to measure”) reminds us. Those who sleep in moonlight go mad; the connection is preserved in the English word “lunatic.” In the old Persian story, a vizier renowned for his wisdom is asked which is more useful, the Sun or the Moon. “The Moon,” he answers, “because the Sun shines in daytime when it’s light out anyway.” Especially when we lived out-of-doors, it was a major—if oddly intangible—presence in our lives.

The Moon was a metaphor for the unattainable: “You plight as well ask for the Moon,” they used to say. Or “You can no more do that than fly to the Moon.” For most of our history, eve had no idea what it was. A spirit? A god? A thing? It didn’t look like something big far away, but more like something small nearby—something the size of a plate, maybe, hanging in the sky a little above our heads. Ancient Greek philosophers debated the proposition “that the Moon is exactly as large as it looks” (betraying a hopeless confusion between linear and angular size). Walking on the Moon would have seemed a screwball idea; it made more sense to imagine somehow climbing up into the sky on a ladder or on the back of a giant bird, grabbing the Moon, and bringing it down to Earth. Nobody ever succeeded, although there were myths aplenty about heroes who had tried.

Not until a few centuries ago did the idea of the Moon as a place, a quarter-million miles away, gain wide currency. And in that brief flicker of time, we’ve gone from the earliest steps in understanding the Moon’s nature to walking and joy-riding on its surface. We calculated how objects move in space; liquefied oxygen from the air; invented big rockets, telemetry, reliable electronics, inertial guidance, and much else. Then we sailed out into the sky.

I was lucky enough to be involved in the Apollo program, but I don’t blame people who think the whole thing was faked in a Hollywood movie studio. In the late Roman Empire, pagan philosophers had attacked Christian doctrine on the ascension to Heaven of the body of Christ and on the promised bodily resurrection of the dead—because the force of gravity pulls down all “earthly bodies.” St. Augustine rejoined: “If human skill can by some contrivance fabricate vessels that float, out of metals which sink…how much more credible is it that God, by some hidden mode of operation, should even more certainly effect that these earthly masses be emancipated” from the chains that bind them to Earth? That humans should one day discover such a “mode of operation” was beyond imagining. Fifteen hundred years later, we emancipated ourselves.

The achievement elicited an amalgam of awe and concern. Some remembered the story ofthe Tower ofBabel. Some, orthodox Moslems among them, felt setting foot on the Moon’s surface to be impudence and sacrilege. Many greeted it as a turning point in history.

The Moon is no longer unattainable. A dozen humans, all Americans, have made those odd bounding motions they called “moonwalks” on the crunchy, cratered, ancient gray lava—beginning on that July day in 1969. But since 1972, no one from any nation has ventured back. Indeed, none of ushas gone anywhere since the glory days of Apollo except into low Earth orbit—like a toddler who takes a few tentative steps outward and then, breathless, retreats to the safety ofhis mother’s skirts.