Hunters and gatherers among the stars, migrating with the cosmic seasons, hunting adventure beyond current imagination, gathering the chance to survive in a universe not made for us.This is Carl Sagan's vision of our future in "Pale Blue Dot: A Vision of the Human Future in Space." Sagan's latest book is the sequel to "Cosmos," the 1980 best-selling account of humankind's first steps into space and the basis of the PBS television series that made Sagan perhaps the world's best-known scientist.

"Pale Blue Dot" takes readers far beyond "Cosmos" - farther along the path of the Voyager spacecraft that have made most of the discoveries presented in the books, and further in its sophistication about humankind's role in the universe.

In its middle chapters, "Pale Blue Dot" elegantly summarizes the discoveries that American space probes have made since the publication of "Cosmos." The many photographs from space and the artists' renderings of still-speculative vistas are nothing short of captivating. The accompanying text puts the wonders that quicken the hearts of astronomers into words that share the excitement with ordinary readers. Many people might want to have "Pale Blue Dot" just for these chapters.

But it is in the first and last portions of the book that the "Vision of the Human Future in Space" is explored.

"Pale Blue Dot" refers to a photograph of the Earth taken by the Voyager 2 spacecraft from beyond the orbit of Neptune. Sagan finds in this image of a tiny point of blue against a sea of black his inspiration for the next stage in humankind's life. Just as the first photograph from space of Earth as a great blue marble made us aware of the beauty of our planet, the image of the blue dot emphasizes the fragility of our home. Sagan sees in this image the ultimate reason for humankind's journey into space: the simple need to survive.

He begins by reminding his readers that humankind has ever been a restless band of wanderers. The few thousand years we have spent in relatively sedentary style in villages and cities, he says, are only a respite. The science we discovered in our intellectual wanderings has revealed how small humankind's role in the physical universe really is. Science has moved Earth itself from the center of creation to an ordinary planet circling an ordinary sun in an ordinary galaxy in an ordinary cluster of galaxies in what may even be a relatively ordinary universe within a grand complex of such universes.

We humans, he observes, now find ourselves at a crucial stage of development. We have attained the power that would let us either destroy ourselves or ensure our survival even of cosmic disasters.

His litany of possible means of self-destruction is familiar: nuclear holocaust, runaway greenhouse effect, pollution. If we are to survive as a species, we will have to conquer these problems. But that will certainly not be enough. If we humans do not move into space, Sagan argues, we are doomed to extinction, just as the dinosaurs vanished 65 million years ago.

In fact, the very catastrophe that ended the dinosaur reign over what is now our pale blue dot threatens us. One day a comet will crash into Earth, turning our pale blue dot first fiery red, then black with soot, erasing life as we know it. Just this year we saw the awesome power of a minor comet scar the face of mighty Jupiter. Earth's turn will come again.

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Science can help us learn to deflect incoming bodies so that they miss the Earth. Even that effort will force some humans to move into space, to guard the rest.

But someday will come a comet so huge as to be beyond our means to deflect. When that happens, or on that distant day when our familiar sun burns out, our only means of survival will be to be out of the way. We will need to resume the lives of our ancestors, to wander again. This time, we need not just to move south when the glaciers drift toward us. This time we need to enter a new environment entirely: We must migrate to other planets, eventually to other suns.

Sagan sees these events "as momentous as the colonization of the land by our amphibian ancestors and the descent from the trees by our primate ancestors."

In "Pale Blue Dot," beyond the grand photographic images, beyond the history of our first wandering among the planets, Carl Sagan offers a glimpse of the adventure we can create for our descendants, the wanderers among the stars.

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