pale blue dot -carl sagan-第20章
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aft had to be moved during the time exposures to cancel out the motion； like panning in the direction opposite yours while taking a photograph of a street scene from a moving car。 This may sound easy； but it's not： You have to neutralize the most innocent of motions。 At zero gravity； the mere start and stop of the on…board tape recorder can jiggle the spacecraft enough to smear the picture。
This problem was solved by sending up mands to the spacecraft's little rocket engines （called thrusters）； machines of exquisite sensitivity。 With a little puff of gas at the start and stop of each data…taking sequence； the thrusters pensated for the tape…recorder jiggle by turning the entire spacecraft just a little。 To deal with the low radio power received at Earth； the engineers devised a new and more efficient way to record and transmit the data； and the radio telescopes on Earth were electronically linked together with others to increase their sensitivity。 Overall； the imaging system worked； by many criteria； better at Uranus and Neptune than it did at Saturn or even at Jupiter。
Voyager may not yet be done exploring。 There is； of course； a chance that some vital subsystem will fail tomorrow； but as far as the radioactive decay of the plutonium power source is concerned； the two Voyager spacecraft should be able to return data to Earth roughly through the year 2015。
Voyager is an intelligent being—part robot； part human。 It extends the human senses to far…off worlds。 For simple tasks and short…term problems； it relies on its own intelligence； but for more plex tasks and longer…term problems； it turns to the collective intelligence and experience of the JPL engineers。 This trend is sure to grow。 The Voyagers embody the technology of the early 1970s； if spacecraft were designed for such a mission today； they would incorporate stunning advances in artificial intelligence； in miniaturization； in data…processing speed； in the ability to self…diagnose and repair； and in the propensity to learn from experience They would also be much cheaper。
In the many environments too dangerous for people； on Earth as well as in space； the future belongs to robot…human partnerships that will recognize the two Voyagers as antecedents and pioneers。 For nuclear accidents； mine disasters； undersea exploration and archaeology； manufacturing； prowling the interiors of volcanos； and household help； to name only a few potential applications； it could make an enormous difference to have a ready corps of smart； mobile； pact； mandable robots that can diagnose and repair their own malfunctions。 There are likely to be many more of this tribe in the near future。
It is conventional wisdom now that anything built by the government will be a disaster。 But the two Voyager spacecraft were built by the government （in partnership with that other bugaboo； academia）。 They came in at cost； on time； and vastly exceeded their design specifications—as well as the fondest dreams of their makers。 Seeking not to control； threaten； wound； or destroy； these elegant machines represent the exploratory part of our nature set free to roam the Solar System and beyond。 This kind of technology； the treasures it uncovers freely available to all humans everywhere； has been； over the last few decades； one of the few activities of the United States admired as much by those who abhor many of its policies as by those who agree with it on every issue。 Voyager cost each American less than a penny a year from launch to Neptune encounter。 Missions to the planets are one of those things—and I mean this not just for the United States； but for the human species—that we do best。
CHAPTER 7 AMONG THE MOONS OF SATURN
Seat thyself sultanically among the moons of Saturn。
—HERMAN MELVILLE； MOBY DICK； CHAPTER 107 （1851）
There is a world； midway in size between the Moon and Mars； where the upper air is rippling with electricity—streaming in from the archetypical ringed planet next door； where the perpetual brown overcast is tinged with an odd burnt orange； and where the very stuff of life falls out of the skies onto the unknown surface below。 It is so far away that light takes more than an hour to get there from the Sun。 Spacecraft take years。 Much about it is still a mystery—including whether it holds great oceans。 We know just enough； though； to recognize that within reach may be a place where certain processes ate today working themselves out that aeons ago on Earth led to the origin of life。
On our own world a long…standing—and in some respects quite promising—experiment has been under way on the evolution of matter。 The oldest known fossils are about 3。6 billion years old。 Of course； the origin of life had to have happened well before that。 But 4。2 or 4。3 billion years ago the Earth was being so ravaged by the final stages of its formation that life could not yet have e into being： Massive collisions were melting the surface； turning the oceans into steam and driving any atmosphere that had accumulated since the last impact off into space。 So around 4 billion years ago； there was a fairly narrow window—perhaps only a hundred million years wide in which our most distant ancestors came to be。 Once conditions permitted； life arose fast。 Somehow。 The first living things very likely were inept； far less capable than the most humble microbe alive today—perhaps just barely able to make crude copies of themselves。 But natural selection； the key process first coherently described by Charles Darwin； is an instrument of such enormous power that from the most modest beginnings there can emerge all the richness and beauty of the biological world。
Those first living things were made of pieces； parts； building blocks which had to e into being on their own—that is； driven by the laws of physics and chemistry on a lifeless Earth。 The building blocks of all terrestrial life are called organic molecules； molecules based on carbon。 Of the stupendous ＇lumber of possible organic molecules； very few are used at the heart of life。 The two most important classes are the amino acids； the building blocks of proteins； and the nucleotide bases； the building blocks of the nucleic acids。 Mist before the origin of life； where did these molecules e from？ There are only two possibilities： from the outside or from the inside。 We know that vastly more ets and asteroids were hitting the Earth than do so today； that these small worlds are rich storehouses of plex organic molecules； and that some of these molecules escaped being fried on impact。 Here I'm describing homemade； not imported； goods： the organic molecules generated in the air and waters of the primitive Earth。
Unfortunately； we don't know very much about the position of the early air； and organic molecules are far easier to make in some atmospheres than in others。 There couldn't have been much oxygen； because oxygen is generated by green plants and there weren't any green plants yet。 There was probably more hydrogen； because hydrogen is very abundant in the Universe and escapes from the upper atmosphere of the Earth into space better than any other atom （because it's so light）。 If we can imagine various possible early atmospheres； we can duplicate them in the laboratory； supply some energy； and see which organic molecules are made and in what amounts。 Such experiments have over the years proved provocative and promising。 But our ignorance of initial conditions limits their relevance。
What we need is a real world whose atmosphere still retains some of those hydrogen…rich gases； a world in other respects something like the Earth； a world in which the organic building blocks of life are being massively generated in our own time； a world we can go to to seek our own beginnings。 There is only one such world in the Solar System。* That world is Titan； the big moon of Saturn。 It's about 5；150 kilometers （3；200 miles） in diameter； a little less than half the size of the Earth。 It takes 16 of our days to plete one orbit of Saturn。
* There could have been none。 We're very lucky that there is such a world study。 The others ill have too much hydrogen； or not enough； or no atmosphere at all。
No world is a perfect replica of any other；
