It's hard to believe that seven years have passed since this book was published, and there's still disagreement about where it belongs. The conventional wisdom has it that it's the sequel to Cosmos--probably because it's the only the second book (along with the unfairly neglected Comet) Sagan wrote after Cosmos to have much to do with astronomy.
But Pale Blue Dot is only partly about astronomy. In the 15 or so years that separated the two books, Sagan seems to have acquired a much more political perspective on science and exploration, and it finds its way repeatedly into the later book. Time and again, we find ourselves confronted not only with what's out there, but what *should* be out there--and who.
The central motivation behind this book is the observation that manned space exploration has foundered since the end of the Apollo project in the early 1970s, in large part because of the lack of any coherent direction. As Sagan describes throughout the book, robotic exploration can be so successful, with no risk to human life, that we're left wondering what reasons could possibly justify sending people back out into space.
Sagan's proposed justifications might surprise some people who haven't yet read this book. They have little to do with the spirit of exploration (although he surely views that as an ancillary feature), or the need to have on-demand human intelligence at the site of new discoveries.
Rather, he takes a global view of the human species. Provided that we can put our social affairs in proper order, he poses, what are the dangers to humans and civilization? The short-term danger is provided by humans themselves, through their aggressiveness and short-sightedness. Voyager 2's photo of the Earth as a single cerulean pixel, taken from 5 billion kilometers away, is a kind of metaphorical plea for perspective, and the inspiration for the book's title.
Sagan's view of the long-term danger places substantial weight on the asteroid/comet impact risk, the same sort that is understood to have brought about the end of the dinosaurs. It is essential, he asserts, to have humans spread out from Earth--both to perform reconnaissance of possible impactors, and as a way to ensure the continuance of the species, should our home planet be rendered inhospitable (either through external or internal dangers).
Sagan naturally hopes it doesn't come to potential extinction, so the bet is on reconnaissance and defense. And what should we do if an asteroid is discovered to be on a potential collision path? The proposed response, nowadays, is to deflect the asteroid into some other, harmless orbit. But Sagan has a warning for us. He's concerned that this same technology could be used for harm by some madman, by turning it upside-down: deflecting some harmless asteroid into another, cataclysmic orbit.
This concern doesn't ring true for me. Sagan claims that if you can do one, you can do the other, but that's patently false: There are so many more harmless orbits than harmful ones that if your aim is only so-so, deflection is *much* easier than direction. On the other hand, raising the issue for discussion at all is a healthy idea, even if the risk is eventually judged to be minor.
The focus in Pale Blue Dot, then, is not on the science, but the engineering and the politics: How shall we revive the manned space program, what form should it take, how shall we manage it so as not to place too great a strain on human maturity? That's not to say there's no straight science in Pale Blue Dot. Sagan gives an update on the Voyager explorations of the outer solar system, from Saturn to Neptune, as well as the dramatically better picture we have of Venus, due to Magellan. But the emphasis isn't on what we know and how we know it, as it was in Cosmos, but on the behind-the-scenes work on making the science possible. It's notable that Sagan spends quite some time describing the successful efforts of NASA engineers in rescuing the Voyager explorers from mission-endangering faults. It's an inspiring narrative of heroic action--but it's also lobbying.
And perhaps that's the best way to look at it. In this book, Sagan is fighting as hard as he can for the space program, in which he believed, passionately. He is ready to recognize its certain flaws, but he is unabashed in his prejudices. In the end, perhaps not everything he tries works, maybe certain arguments are a reach, but as always, he makes us think, and it's a worthy and valiant addition to the Sagan canon.