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(1619–1655). Cyrano, in his book
Voyages to the Moon and the Sun
, listed seven different ways in which a human being might travel from the Earth to the Moon, and one of them was by means of rockets. His hero actually performed the voyage, however, by one of the other (alas, worthless) methods.
    As the seventeenth century progressed and as observation of the Moon continued with better and better telescopes, astronomers grew aware of certain peculiarities about our satellite.
    The view of the Moon, it seemed, was always clear and unchanging. Its surface was never obscured by cloud or mist. The terminator—that is, the dividing line between the light and the dark hemispheres—was always sharp. It was never fuzzy as it would be if light were refracting through an atmosphere, thus signifying the presence on the Moon of the equivalent of an Earthly twilight.
    What’s more, when the Moon’s globe approached a star, the star remained perfectly bright until the Moon’s surface reached it and then it winked out in an instant. It did not slowly dim as it would if the Moon’s atmosphere reached it before the Moon’s surface did, and if the starlight had to penetrate thickening layers of air.
    In short, it became clear that the Moon was an airless world. And waterless, too, for closer examination showed that the dark “seas” that Galileo had seen were speckled with craters here and there. They were, if anything, seas of sand, but certainly not of water.
    Without water, there could scarcely be life on the Moon. For the first time, people had to become aware that it was possible for a dead world to exist; one that was empty of life.
    Let us not, however, hasten too quickly. Given a world without air and water, can we be sure it has no life?
    Let us begin by considering life on Earth. Certainly, it shows a profound variability and versatility. There is life in the ocean deeps and on the ocean surface, in fresh water and on land, underground, in the air, even in deserts and frozen wastes.
    There are even microscopic forms of life that do not use oxygen and to some of which oxygen is actually deadly. For them, airlessness would have no fears. (It is because of them that food sealed in a vacuum must be well heated first. Some pretty dangerous germs,including the one that produces botulism, get along fine in a vacuum.)
    Well, then, is it so difficult to imagine some forms of life getting along without water, too?
    Yes, quite difficult. No form of terrestrial life can do without water. Life developed in the sea, and the fluids within the living cells of all organisms, even those who now live in fresh water or on dry land and who would die if placed in the sea, are essentially a form of ocean water.
    Even the life forms in the driest desert have not evolved into independence of water. Some might never drink, but they then get their necessary water in other ways—from the fluids of the food they eat, for instance—and carefully conserve what they get.
    Some bacteria can survive desiccation and, in spore form, can live on for an indefinite period without water. The spore wall, however, protects the fluid within the bacterial cell. True desiccation, through and through, would kill it as quickly as it would kill us.
    Viruses can retain the potentiality of life even when crystallized and with no water present. They cannot multiply, however, until they are within a cell and can undergo changes within the milieu of the cell fluid.
    Ah, but all this refers to Earth life, which has developed in the ocean. On a waterless world, might not a fundamentally different kind of life develop that
was
independent of water?
    Let’s reason this out as follows:
    On the surface of planetary worlds (on one of which the one example of life that we know of has developed) matter can exist in any of three states: solid, liquid, or gas.
    In gases, the component molecules are separated by relatively large distances and move randomly. For that reason, gas