Earth-observing satellites, especially a new generation soon to be deployed, monitor the health of the global environment: greenhouse warming, topsoil erosion, ozone layer depletion, ocean currents, acid rain, the effects of floods and droughts, and new dangers we haven't yet discovered. This is straightforward planetary hygiene.
Global positioning systems are now in place so that your locale is radio-triangulated by several satellites. Holding a small instrument the size of a modern shortwave radio, you can read out to high precision your latitude and longitude. No crashed airplane, no ship in fog and shoals, no driver in an unfamiliar city need ever be lost again.
Astronomical satellites peering outward from Earth's orbit observe with unsurpassed clarity—studying questions ranging from the possible existence of planets around nearby stars to the origin and fate of the Universe. Planetary probes from close range explore the gorgeous array of other worlds in our solar system comparing their fates with ours.
All of these activities are forward-looking, hopeful, stirring and cost-effective. None of them requires "manned"1 spaceflight. A key issue facing the future of NASA and addressed in this book is whether the purported justifications for human spaceflight are coherent and sustainable. Is it worth the cost?
But first, let's consider the visions of a hopeful future vouchsafed by robot spacecraft out among the planets.
VOYAGER 1 AND VOYAGER 2 are the ships that opened the Solar System for the human species, trailblazing a path for future generations. Before their launch, in August and September 1977, we were almost wholly ignorant about most of the planetary part of the Solar System. In the next dozen years, they provided our first detailed, close-up information on many new worlds—some of them previously known only as fuzzy disks in the eyepieces of ground-based telescopes, some merely as points of light, and some whose very existence was unsuspected. They are still returning reams of data.
These spacecraft have taught us about the wonders of other worlds, about the uniqueness and fragility of our own, about beginnings and ends. They have given us access to most of the Solar System—both in extent and in mass. They are the ships that first explored what may be homelands of our remote descendants.
U.S. launch vehicles are these days too feeble to get such a spacecraft to Jupiter and beyond in only a few years by rocket propulsion alone. But if we're clever (and lucky), there's something else we can do: We can (as Galileo also did, years later) fly close to one world, and have its gravity fling us on to the next. A gravity assist, it's called. It costs us almost nothing but ingenuity. lt's something like grabbing hold of a post on a moving merry-go-round as it passes—to speed you up and fling you in some new direction. The spacecraft's acceleration is compensated for by a deceleration in the planet's orbital motion around the Sun. But because the planet is so massive compared to the spacecraft, it slows down hardly at all. Each Voyager spacecraft picked up a velocity boost of nearly 40,000 miles per hour from Jupiter's gravity. Jupiter in turn was slowed down in its motion around the Sun. By how much? Five billion years from now, when our Sun becomes a swollen red giant, Jupiter will be one millimeter short of where it would have been had Voyager not flown by it in the late twentieth century.
Voyager 2 took advantage of a rare lining-up of the planets: A close flyby of Jupiter accelerated it on to Saturn, Saturn to Uranus, Uranus to Neptune, and Neptune to the stars. But you can't do this anytime you like: The previous opportunity for such a game of celestial billiards presented itself during the presidency of Thomas Jefferson. We were then only at the horseback, canoe, and sailing ship stage of exploration. (Steamboats were the transforming new technology just around the corner.)
Since adequate funds were unavailable, NASA's Jet Propulsion Laboratory (JPL) could afford to build spacecraft that would work reliably only as far as Saturn. Beyond that, all bets were off. However, because of the brilliance of the engineering design and the fact that the JPL engineers who radioed instructions up to the spacecraft got smarter faster than the spacecraft got stupid—both spacecraft went on to explore Uranus and Neptune. These days they are broadcasting back discoveries from beyond the most distant known planet of the Sun.
We tend to hear much more about the splendors returned than the ships that brought them, or the shipwrights. It has al, ways been that way. Even those history books enamored of the voyages of Christopher Columbus do not tell us much about the builders of the Nina, the Pinta, and the Santa Maria, or about the principle of the caravel. These spacecraft, their designers, builders, navigators, and controllers are examples of what science and engineering, set free for well-defined peaceful purposes, can accomplish. Those scientists and engineers should be role models for an America seeking excellence and international competitiveness. They should be on our stamps.
At each of the four giant planets—Jupiter, Saturn, Uranus, and Neptune—one or both spacecraft studied the planet itself, its rings, and its moons. At Jupiter, in 1979, they braved a dose of trapped charged particles a thousand times more intense than what it takes to kill a human; enveloped in all that radiation, they discovered the rings of the largest planet, the first active volcanos outside Earth, and a possible underground ocean on an airless world—among a host of surprising discoveries. At Saturn, in 1980 and 1981, they survived a blizzard of ice and found not a few new rings, but thousands. They examined frozen moons mysteriously melted in the comparatively recent past, and a large world with a putative ocean of liquid hydrocarbons surmounted by clouds of organic matter.
On January 25, 1986, Voyager 2 entered the Uranus system and reported a procession of wonders. The encounter lasted only a few hours, but the data faithfully relayed back to Earth have revolutionized our knowledge of the aquamarine planet, its 15 moons. its pitch-black rings, and its belt of trapped high-energy charged particles. On August 25, 1989, Voyager 2 swept through the Neptune system and observed, dimly illuminated by the distant Sun, kaleidoscopic cloud patterns and a bizarre moon on which plumes of fine organic particles were being blown about by the astonishingly thin air. And in 1992, having flown beyond the outermost known planet, both Voyagers picked up radio emission thought to emanate from the still remote heliopause—the place where the wind from the Sun gives way to the wind from the stars.
Because we're stuck on Earth, we're forced to peer at distant worlds through an ocean of distorting air. Much of the ultraviolet, infrared, and radio waves they emit do not penetrate our atmosphere. It's easy to see why our spacecraft have revolutionized the study of the Solar System: We ascend to stark clarity in the vacuum of space, and there approach our objectives, flying past them, as did Voyager, or orbiting them, or landing on their surfaces.
These spacecraft have returned four trillion bits of information to Earth, the equivalent of about 100,000 encyclopedia volumes. I described the Voyagers 1 and 2 encounters with the Jupiter system in Cosmos. In the following pages, I'll say something about the Saturn, Uranus, and Neptune encounters.
JUST BEFORE VOYAGER 2 was to encounter the Uranus system, the mission design had specified a final maneuver, a brief firing of the on-board propulsion system to position the spacecraft correctly so it could thread its way on a preset path among the hurtling moons. But the course correction proved unnecessary. The spacecraft was already within 200 kilometers of its designed trajectory-after a journey along an arcing path 5 billion kilometers long. This is roughly the equivalent of throwing a pin through the eye of a needle 50 kilometers away, or firing your rifle in Washington and hitting the bull's-eye in Dallas.
Mother lodes of planetary treasure were radioed back to Earth. But Earth is so far away that by the time the signal 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 miss
ion 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 completed 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 command 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 computers 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 computers 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 computer 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 components 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 command—a human error—caused Voyager 2's on-board computer 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 component called a tracking loop capacitor had failed. After seven days in which Voyager 2 was entirely out of contact, its fault protection software suddenly commanded 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 computer 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 commands 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 commands, instructions to switch automatically between receivers were accepted and programmed into the skittish onboard computer. During that same week the JPL engineers designed an innovative command frequency control procedure to make sure that essential orders would be understood by the damaged backup receiver.
The engineers were now able to recommunicate, at least in a rudimentary way, with the spacecraft. Unfortunately the backup receiver now turned giddy, becoming extremely sensitive to stray heat dumped when various components 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 commands from Earth?
With this information, the backup receiver problem was entirely circumvented. It subsequently acquired all the commands 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 computers—should the spacecraft again become 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 components 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 sy
stem (at about 35,000 miles per hour) that the image would have been smeared or blurred. To compensate, 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 commands 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 compensated 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 complex 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.