pale blue dot -carl sagan-第14章
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nal frog Neptune was gathered in by radio telescopes on our planet； the received power was only 10…16 watts （fifteen zeros between the decimal point and the one）。 This weak signal bears the same pro； portion to the power emitted by an ordinary reading lamp as the diameter of an atom bears to the distance from the Earth to the Moon。 It's like hearing an amoeba's footstep。
The mission was conceived during the late 1960s。 It was first funded in 1972。 But it was not approved in its final form （including the encounters with Uranus and Neptune） until after the ships had pleted their reconnaissance of Jupiter。 The two spacecraft were lifted off the Earth by a nonreusable Titan/Centaur booster configuration。 Weighing about a ton； a Voyager would fill a small house。 Each draws about 400 watts of power—considerably less than an average American home—from a generator that converts radioactive plutonium into electricity。 （If it had to rely on solar energy； the available power would diminish quickly as the ship ventured farther and farther from the Sun Were it not for nuclear power； Voyager would have returned no data at all from the outer Solar System； except perhaps a little from Jupiter。）
The flow of electricity through the innards of the spacecraft would generate enough magnetism to overwhelm the sensitive instrument that measures interplanetary magnetic fields。 So the magnetometer is placed at the end of along boom； far from the offending electrical currents。 With other projections； it gives Voyager alter a slightly porcupine appearance。 Cameras； infrared and ultraviolet spectrometers； and an instrument called a photopolarimeter are on a scan platform that swivels on mand so these device can be aimed at a target world。 The spacecraft must know where Earth is if the antenna is to be pointed properly and the data rereceived back home。 It also needs to know where the Sun is and at least one bright star； so it can orient itself in three dimensions and point properly toward any passing world。 If you can't point the cameras； it does no good to be able to return pictures over billions of miles。
Each spacecraft cost about as much as a single modern strategic bomber。 But unlike bombers； Voyager cannot； once launched； be returned to the hangar for repairs。 The ship's puters and electronics are therefore designed redundantly。 Much key machinery； including the essential radio receiver； had at least one backup—waiting to be called upon should the hour of need ever arrive。 When either Voyager finds itself in trouble； the puters use branched contingency tree logic to work out the appropriate course of action。 If that doesn't work； the ship radios home for help。
As the spacecraft journeys increasingly far from Earth； the roundtrip radio travel time also increases； approaching eleven hours by the time Voyager is at the distance of Neptune。 Thus； in case of emergency； the spacecraft needs to know how to put itself into a safe standby mode while awaiting instructions from Earth。 As it ages； more and more failures are expected； both in its mechanical parts and in its puter system； although there is no sign； even now； of a serious memory deterioration； some robotic Alzheimer's disease。
This is not to say that Voyager is perfect。 Serious mission…threatening； white…knuckle mishaps did occur。 Each time； special teams of engineers—some of whom had been with the Voyager program since its inception—were assigned to 〃work〃 the problem。 They would study the underlying science and draw upon their previous experience with the failed subsystems。 They would experiment with identical Voyager spacecraft equipment that had never been launched； or even manufacture a large number of ponents of the sort that failed in order to gain some statistical understanding of the failure mode。
In April 1978； almost eight months after launch； and while the ship was approaching the asteroid belt； an omitted ground mand—a human error—caused Voyager 2's on…board puter to switch from the prime radio receiver to its backup； During the next ground transmission to the spacecraft； the backup receiver refused to lock onto the signal from Earth。 A ponent called a tracking loop capacitor had failed。 After seven days in which Voyager 2 was entirely out of contact； its fault protection software suddenly manded the backup receiver to be switched off and the prime receiver to be switched back on。 Mysteriously—to this day； no one knows why—the prime receiver failed moments later。 It was never heard from again。 To top it off； the on…board puter now foolishly insisted on using the failed primary receiver。 Through an unlucky concatenation of human and robotic error； the spacecraft was now in real jeopardy。 No one could think of a way to get Voyager 2 to revert to the backup receiver。 Even if it did； the backup receiver couldn't receive the mands from Earth； because of that failed capacitor。 There were many project personnel who feared that all was lost。
But after a week of obdurate unresponsiveness to all mands； instructions to switch automatically between receivers were accepted and programmed into the skittish onboard puter。 During that same week the JPL engineers designed an innovative mand frequency control procedure to make sure that essential orders would be understood by the damaged backup receiver。
The engineers were now able to remunicate； at least in a rudimentary way； with the spacecraft。 Unfortunately the backup receiver now turned giddy； being extremely sensitive to stray heat dumped when various ponents of the spacecraft powered up or down。 In the following months the JPL engineers devised and conducted tests that let them thoroughly understand the thermal implications of most spacecraft operational modes： What would prevent and what would permit receipt of mands from Earth？
With this information； the backup receiver problem was entirely circumvented。 It subsequently acquired all the mands sent from Earth on how to gather data in the Jupiter； Saturn； Uranus； and Neptune systems。 The engineers had saved the mission。 （To be on the safe side； during most of Voyager 2's subsequent flight a nominal data…taking sequence for the next planet to be encountered was always sitting in the on…board puters—should the spacecraft again bee deaf to entreaties from home。）
Another heart…wrenching failure occurred just after Voyager 2 emerged from behind Saturn （as seen from the Earth） in August 1981。 The scan platform had been moving feverishly—pointing here and there among the rings； moons； and the planet itself during the all…too…brief moments of close approach。 Suddenly； the platform jammed。 A stuck scan platform is a maddening predicament： knowing that the spacecraft is flying past wonders that have never been witnessed； that we will not see again for years or decades； and the incurious spacecraft staring fixedly off into space； ignoring everything。
The scan platform is driven by actuators containing gear trains。 So first the JCL engineers ran an identical copy of a flight actuator in a simulated mission。 This actuator failed after 348 turns； the actuator on the spacecraft had failed after 352 turns。 The problem turned out to be a lubrication failure。 Good to know； but what to do about it？ Plainly； it would be impossible to overtake Voyager with ail oilcan。
The engineers wondered whether they could restart the tailed actuator by alternate heating and cooling； maybe the resulting thermal stresses would induce the ponents of the actuator to expand and contract at different rates and unjam the system。 They tested this notion with specially manufactured actuators in the laboratory； and then jubilantly found that in this way they could start the scan platform up again in space。 Project personnel also devised ways to diagnose any additional trend toward actuator failure early enough to work around the problem。 Thereafter； Voyager 2's scan platform worked perfectly。 All the pictures taken in the Uranus and Neptune systems owe their existence to this work。 The engineers had saved the day again。
Voyagers 1 and 2 were designed to explore the Jupiter and Saturn systems only。 It is true that their trajectories would carry them on past Uranus and Neptune； but officially these planets were never contemplated as targets for Voyager exploration： The spacecraft were not supposed to last that long。 Because of our wish to fly close to the mystery world Titan； Voyager 9 was flung by Saturn on a path that could never encounter any other known world； it is Voyager 2 that flew on to Uranus and Neptune with brilliant success。 At these immense distances； sunlight is getting progressively dimmer； and the radio signals transmitted to Earth are getting progressively fainter。 These were predictable but still very serious problems that the JPL engineers and scientists also had to solve。
Because of the low light levels at Uranus and Neptune； the Voyager television cameras were obliged to take long time exposures。 But the spacecraft was hurtling so fast through； say； the Uranus system （at about 35；000 miles per hour） that the image would have been smeared or blurred。 To pensate； the entire spacecraft 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
