Not all interplanetary travelers are like the Vikings, sophisticated caravels sailing across vast space, stabilized in three axes. Some are from a different family: spin-stabilized spacecraft that trade the complexity of three-axis attitude control systems for the simplicity of constant rotation.
Often thought of as no more than a toy for 10-year-olds, the gyroscope is actually a subtle and versatile inner ear for machines, providing attitude reference and control where nothing else can. Gyroscopic principles are used in all manner of devices, from bicycles to nuclear warheads. In a ship's wheelhouse a gyro supplements the traditional magnetic compass, using inputs from this ancient instrument given to flighty and deviate behavior, and making it useful for precise navigation. Aircraft instrumentation is rich in gyros, notably in the automatic pilot that relieves the human pilot from the constant attention needed to fly a steady course in a turbulent medium. In submerged submarines gyros are part of a marvelous machine that senses every change in heading and every variation in speed and current, integrating the multiple variables with such precision that the skipper, although functionally blinded, can know his exact position after weeks without a conventional real-world fix. In ICBM guidance systems, gyros endure a high-G launch, arc a thousand miles upward into space, survive incandescent reentry, and guide their warheads wickedly to target.
These feats, which range from the everyday to the apocalyptic, are performed by sensitive, mulishly independent mechanisms that use concepts defined by Isaac Newton to do things no mortal could manage unaided. In the delicate tasks of interplanetary navigation, gyros have earned two quite different classes of duty.
For spacecraft that are stabilized in three axes by sighting on distant objects, it is periodically necessary to give up this cruise orientation and slew to a different attitude before firing trajectory-correcting rockets. Gyros in an attitude reference package allow this to be done precisely, maintaining reference coordinates all the while. After the velocity corrections have been  made, the spacecraft may be reoriented to its original cruise attitude. For all these tasks, gyros serve nicely, keeping the control computer aware of which way is "up" in a universe without up.
The other application of gyro principles to spacecraft function is of a different order. If the entire craft is made to spin, it becomes in effect the rotor of a large gyro and is thereby stabilized in inertial space along its axis of rotation. Although it has drawbacks, this is a long-lived, low-energy way to keep a spacecraft oriented during its travels.
The principle by which a gyro works seems uncomplicated, yet its reactions to external forces are mysterious. Spin a wheel and observe that the axis on which it turns has gained an odd property. It resists deflection and "wants" to hold position against side loading. But if you overcome this resistance and compel it to point in a different direction, note that, unexpectedly, it precesses and "wants" to move in a plane 90° to the deflecting force. (This is what gives so curiously animate a feeling to a handheld toy gyro, like a little animal trying to escape.) Enclose the spinning wheel's axis in a polar hoop, and then enclose that hoop in an equatorial one, and you have the heart of a neat device capable of keeping its orientation in inertial space.
Of course, the realities of applying simple physical principles to machines can be difficult, and the gyro application invites complication. Much skilled instrument engineering has gone into gyros to make them practical, rugged, and reliable. Further effort has been devoted to attacking a constitutional sensitivity to external forces: in time the heading established by a gyro drifts into error. No matter how carefully the instrument is made, it remains susceptible to the accumulated effects of tiny forces caused by bearing friction, temperature fluctuations, or even the presence or absence of small magnetic fields. Over time, these influences add up to error. In recent years the limitations of mechanical gyros-never so great as to impair their usefulness over moderate intervals-has been moderated by an exciting development, the laser ring gyro. In effect these gyros are made by replacing the rotating mechanical parts with rings of laser light, rotating without friction. Each laser gyro consists of two rings of light traveling in opposite directions; motion causes the frequency of one beam to be upshifted and the other downshifted The sensitivities are such that changes in rotation at the rate of 10° an hour cause a detectable frequency shift. These devices are finding application as mechanical gyro replacements, and new orders of accuracy and stability can be expected when they fly on interplanetary errands.
 From the earliest days of rocketry, spin stabilization has been employed during the rocket burn. Just as the feathers on the shaft of an arrow or the rifling in the bore of a gun provide spin to stabilize a projectile, spacecraft are often mounted on final-stage solid propellant rockets that are spun to give a fixed thrust direction during burn. After rocket burnout, the spacecraft may remain attached or may be separated, in either case continuing to spin about the same axis. If the spinning is undesirable, or if the rotation rate must be changed, despinning is achieved by a simple technique of unwinding and then releasing small yo-yo weights.
JPL engineers still recall one early Explorer that successfully progressed through a multistage launch, all going well (which was remarkable for those days), until the spacecraft and its final stage achieved the desired trajectory. Then, thanks to a certain prelaunch oversight concerning the moments of inertia, the spin axis changed from the longitudinal axis of the launch to 90° from this axis, where the small vehicle was actually more stable. The laws of physics were still perfectly obeyed, but this embarrassing bird preferred to spin sideways. It was an instructive failure, about the only salvation of the experience. A related event occurred several years later when a more expensive advanced technology satellite was tipped on separation and spun in a direction opposite to the intended one, making it impossible for its yo-yo weight system to unwind and stop its spin. In this case, the sure-fire aspects of spin stabilization will forever haunt engineers.
Spinning an interplanetary vehicle to provide orientation in space has several implications that deserve discussion. One arises from the need to manage scientific observations in some uniform fashion. A spinner with sensors looking outward radially will sweep the sky in a systematic and predictable manner. As the spacecraft orbits its parent body-the Sun in the case of most interplanetary vehicles-these swaths of coverage can be predicted and counted on to view the interplanetary medium on a regular basis. For measurements of magnetic fields, radiation background levels, and similar spatial information, this controlled scanning mode has clear merit. Of course, for a planetary flyby, where the desired look angle is much less than 360°, a spin mode offers few advantages, even though, as will be noted later, it can be employed. But for interplanetary observations, the scanning qualities of a spin-stabilized spacecraft are useful.
A second factor affected by the stabilization of an interplanetary vehicle is the generation of solar power. With three-axis stabilization it is possible to position arrays of solar cells perpendicular to the Sun, the most efficient  angle. With a spinner, the designer must settle for somewhat less, even though some arrangements are entirely practicable. If the spin axis is normal to the plane of the ecliptic (the plane occupied by the Sun and planets), then a cylindrical spacecraft having a band of solar cells that encircles the spin axis will be oriented so that the Sun serially illuminates all cells, creating a continuous ripple of power. Of course, more cells must be carried for such a cylindrical array than for a simple planar array, since the entire band of cells is never illuminated at the same time.
The third aspect of spinners to be considered involves communications to and from Earth. The earliest spinning spacecraft used low-gain, omnidirectional antennas, handy if some mischance tipped or canted the spacecraft into an unplanned attitude, but less than desirable for a large volume of error-free communication. As the two-way data link to Earth was of critical importance, higher-gain antennas that produced fan-beam, focused patterns were developed; if aligned so that the pancake-shaped beam intercepted Earth, they were not affected by the spin.
Aiming the antennas of Earth-orbiting satellites toward Earth presented small problems, but the geometry grew trickier when spacecraft were dispatched to the farther reaches of the solar system. The problem arose in the design of the second block of Pioneers, designated 6 though 9, sent into solar orbit to examine the interplanetary medium. Unlike the first block of Pioneers, which, except for Pioneer 5, were early lunar probes plagued by erratic launches and unreliability, this second block, launched from 1965 to 1968, were uncommonly successful spacecraft, reliable and richly rewarding in scientific return. The antenna-pointing problem could have been severe, as these birds were put into solar orbits not unlike the Earth's, but trailing or leading the home planet by large fractions of its annual path. They used a Franklin array antenna that transmitted and received signals in a fan-shaped pattern oriented to include both the Sun and Earth in its coverage of the ecliptic.
It may be well to examine how constantly spinning spacecraft can be adapted to the imaging of objects in space. Several ingenious methods have been used: one employed in a final block of Pioneers, the highly sophisticated Pioneers 10 and 11, made use of an instrument known as an imaging photopolarimeter. Looking radially outward from the spin axis as the spacecraft flew past a planet, it collected a narrow swath of image information as spacecraft rotation caused it to scan the target. On successive rotations an adjoining swath was viewed by slightly adjusting the field of view,  and so on until the entire planet had been imaged. The swaths of light data would be transmitted serially to Earth and reassembled into a single image. Putting this simple principle into practice involved sophistication depending on the geometry of the flyby, the prevailing angle of illumination, and the areas of particular scientific interest. However, as the beautiful Pioneer pictures of Jupiter testify, it proved entirely workable.
A different approach to compensating for the inconvenience of spinning instruments was used on the Orbiting Solar Observatory satellites in the 1960s. A separate, free-turning portion of the spacecraft was made to spin while an instrument-carrying portion was oriented relative to the Sun (the object being viewed). The gyroscopic forces on the spinning portion thus maintained orientation, and, in the weightlessness of orbit, the forces on the connecting bearing were minimal, so that friction was not a significant factor in maintaining the spin rate of the rotating section.
 The engineering problem of carrying multiple electrical connections across the spinning interface was solved by using slip rings made with exceptional quality and precision However Rube Goldbergian they may have seemed, the Orbiting Solar Observatories worked nicely in orbit, which was what counted. The concept of the two-part spin-stabilized spacecraft is destined to fly again when the Galileo spacecraft, scheduled to study Jupiter in the late 1980s, will be spin stabilized, with a nonrotating instrument platform.
Although they never won much public attention and respect, the early Pioneers were interesting spacecraft. The first one, launched in August 1958, suffered the misfortune of a flawed first stage that failed; it became known as Pioneer Zero. It nevertheless lingers in the memory of Charles P. "Chuck" Sonett, then a scientist nursemaiding a magnetometer aboard the craft. Just before launch, he climbed up the gantry for a last look at his instrument. Horrified to find that a vital shield had come loose, he hurried down, borrowed a soldering iron, and was starting back up again when he was stopped by an imperious safety officer. "You can't plug that thing in," he was ordered. "We've got live rockets stacked here." Expostulation was useless. A technician found an electrical outlet away from the rocket, heated the soldering iron, unplugged it, raced up the gantry, made a few dabs at the loosened shield until the iron cooled, scurried back down to reheat the iron, and repeated the process until the shield was secure. The valiant effort was futile, of course; the rocket failure launched the spacecraft to disaster.
Three months later Pioneer 1 was launched. It failed to reach the Moon, its nominal destination, but it did return 43 hours of data about the then mysterious interplanetary medium. It is not easy to recapture the extent of our ignorance a quarter-century ago; everything we learned was new. The first four Pioneers had been planned as lunar reconnaissance spacecraft, at which they failed; Pioneer 4 achieved the highest orbit, approaching within 37 300 miles of the Moon and sending back significant quantities of interplanetary data. It was this series of spacecraft that greatly advanced the definition of the Van Allen and other radiation belts in the vicinity of Earth, following their initial discovery by Explorer 1.
Pioneer 5 had originally been planned for a possible flyby of Venus but was not ready in time for launch at the planetary opportunity in late 1959. It did achieve a solar orbit and became the first spacecraft to send data back over a distance of 22.5 million miles, the longest radio transmission distance achieved at the time. The information that it transmitted from March  through June 1960 fascinated interplanetary scientists by revealing temporal and spatial variations of particles and fields in the region between Earth and the orbit of Venus.
This series of spinners-Pioneer 0 through 5-was begun prior to the creation of NASA and was the continuation of a program started in the earliest days of U. S. space development. With NASA attention turned toward Rangers, Surveyors, Mercurys, and a full complement of physics and astronomy satellites, the appetites of a small but increasingly interested cadre of interplanetary scientists were whetted just when the outlook for future interplanetary launches disappeared.
Having been heavily involved in the early Explorers and Pioneers at Space Technology Laboratories in California, Chuck Sonett was a leader in the interplanetary field. He came to work at NASA Headquarters in November 1960, bringing not only a strong scientific background and understanding about fields and particles in interplanetary space, but also a significant amount of engineering experience in the design of instruments and interplanetary spacecraft. His early attempts to satisfy the increased interests of interplanetary scientists with instruments riding on Ranger and Mariner spacecraft resulted in frustration, because of the priority conflicts in the selection of scientific objectives. Experiments aimed at gathering new information about the Moon or a planet at arrival always seemed to receive priority over those examining the interplanetary environment. This resulted in compromises that prevented orderly planning and acquisition of interplanetary facts.
The success of the early Pioneers, although modest, was enough to convince Sonett that special interplanetary spacecraft were a much-needed element in a total program, rewarding not just for their return of interplanetary data but also to support the engineering modeling and design of spacecraft that were to journey through space to other planets. Many questions remained about the radiation environment and its effects, especially transient energetic events like solar flares, and about such ill-defined factors as micrometeorites and magnetic fields. At the time, data did not exist to properly model the solar constant at distances related to the nearer planets.
This special interest in interplanetary study eventually became a major factor involving the Ames Research Center, a NASA laboratory that previously had played a large role in developing reentry aerodynamic concepts, but which had not become a participant in space project activities.
 When NASA was created and former NACA laboratories became heavily involved in space projects, there was a great deal of change and, some thought, erosion in existing research activities. This was a concern to NASA's Deputy Administrator Hugh Dryden and to Ira Abbott, who headed the office responsible for advanced research. As a result, Headquarters established guidelines that encouraged research and development work at Ames, Langley, and Lewis, with minimum dilution from space project activities. Langley had already undergone a significant transformation to manned space activities, with the assignment of a Space Task Group, resulting eventually in the transfer of some 250 researchers to Houston. Several key personnel from the Lewis Research Center had come to Washington to help staff the space flight organization under Abe Silverstein. Only Ames had failed to undertake any major space project after 2 1/2 years as part of NASA.
By this time, the Goddard Space Flight Center had been assigned a principal role in Earth satellite projects for physics, astronomy, and applications areas, JPL was up to its ears in lunar and planetary programs, and the options for new efforts were limited. Furthermore, senior management officials at Ames and at Headquarters did not seem impelled to strain against the "avoid diluting research" guidelines.
This view was not shared by a small group of engineers and scientists at Ames. They were specifically interested in the Sun and its effects on Earth, and they conceived a solar probe that would travel inward toward the Sun and be ideal for making interplanetary measurements. The technical requirements for a spacecraft that was to operate in an extremely hot environment could be studied with facilities existing at Ames and appeared to be a good match for their scientific talents. Like the other NACA laboratories, Ames had an unusual array of talented people who had been working in high-technology areas on the fringes of space for years and were ready to contribute more than research support to the rest of NASA. Names like Harvey Allen, Alfred Eggers, and Al Seiff were synonymous with high-temperature, high-speed flight. Harvey Chapman had made planetary entry trajectories and other analytical determinations easier, and many engineers at Ames understood the physics and chemistry of aerodynamic heating better than most.
Charles Hall came to Headquarters to make a presentation in December 1961; Ames engineers had done their homework toward defining a good solar mission, and it was evident that the group very much wanted to  become involved in the project management of a space mission. At the same time, plans were underway to define experiments in support of an International Quiet Sun Year, and there was interest in a meaningful mission.
At Headquarters we were interested but wary. While the project could fulfill a basic scientific need, and the Ames engineers had distinguished themselves in research activities, none of them had obviously relevant project management experience. The proposed project effort would clearly not be simple; one wondered how Ames, starting from scratch, would deal with the launch vehicle interface problems, the scientific community, and the challenging data acquisition problems that would have to be solved. Although it was not squarely in my province of lunar and planetary programs, I could see the problems and possibilities. I was also aware that Chuck Sonett, an outstandingly good man, was becoming saturated with the papermill aspects of Headquarters and yearned to return to the world of hardware and experiments. Sonett and I paid a visit to Ames, talking with members of the enthusiastic group there, and I also discussed the matter with Ed Cortright and Homer Newell.
The pieces began to come together in May 1962 when Homer Newell, Chuck Sonett, and I met with Smith DeFrance, Director, and John Parsons his deputy, at Ames. A general approach was outlined, subject, of course, to approval by higher authority. Ames would consider a role in space exploration with a three-part plan consisting of (1) advanced studies and analytical efforts pointed toward a solar probe, (2) project management of an interplanetary program based on the Pioneer series, and (3) establishment of a space science division headed by Sonett, who would be transferred to Ames. The logic for a Pioneer-based flight program included several factors thought to be favorable: the spacecraft concept seemed developed to the point where it was understood; the Delta launch vehicle to be used was proven, and tracking and data acquisition services could be obtained either through the Deep Space Network at JPL or from the Goddard Satellite Net. For starting up a new project and developing the skills of project management, this plan seemed well suited.
After reaching a gentleman's agreement with DeFrance on how the three activities would be viewed by Headquarters and what controls and interfaces would be logical, we also discussed the importance of getting Hugh Dryden's approval, the final prerequisite. On my return to Washington, I outlined for Dryden the general plan we had worked out, and he explained in some detail his concern that in the rush toward space, NASA might inadvertently injure  the continuance of the research for which it had become known. But he was sympathetic to the idea, and agreed to consider the proposal on its merits in a face-to-face discussion with DeFrance, if that could be arranged.
In the 1920s, a near-fatal plane crash had caused Smitty DeFrance to pledge to his wife that he would never fly again, a pledge that he honored into the jet age and throughout his directorship of an outstanding aeronautical laboratory. His trips across the country were limited to about one train ride each year. DeFrance made his annual pilgrimage to Washington the following week, endorsed the plan, satisfied Dryden that Ames would continue to excel in research, and Dryden approved. It then took only a few months of countless meetings and memoranda to establish a project office, define the mission, obtain billets for the necessary manpower, arrange funding for the three parts of the plan, and see to Chuck Sonett's transfer and replacement.
As mentioned earlier, this second block of Pioneers was to use the Delta as a launch vehicle. The Delta dictated a modest spacecraft weight of something less than 150 pounds, including instruments. However, since it had been used on many missions, it was thought to be a mature launch vehicle suitable for interface with a new project team. As it happened, the launch vehicle status soon became fuzzy: improvements being made on the Delta for other projects became options for Pioneer, and the new project team became entangled in resolving these choices. With the scientific payload restricted to 20 to 40 pounds, various specific objectives shaped the spacecraft's design. Among these were the desirability of pointing instruments in all directions along the plane of the ecliptic; continuous data sampling from instruments, as opposed to recording and transfer part-time; high data rate transmission from spacecraft to Earth; several commendable modes of operation, allowing experiments to modify their use of the instruments over a period of time; a favorable spacecraft environment, particularly a low residual magnetic field (spurious fields had plagued many prior experiments); and a long useful life in orbit of 6 months to a year. Added to these tough engineering requirements was the fact that the spacecraft was to be a spinner. The net effect of these constraints and desirable qualities was to drive the available technology to the limits, placing unexpected demands on the skills of the Ames team.
A spin-stabilized spacecraft had to be sensitively balanced. Every part had to be designed and placed in such a way that it matched something of equal weight and moment on the other side, and all subsystem components had to be chosen with balancing the spacecraft in mind. It was impossible to  do this perfectly on the drawing board; only after actual flight hardware was delivered and installed and the craft experimentally spin tested could the last few pounds held back for balance weights be added and adjusted. Allowances had to be made for everything aboard that moved or that had any weight change during flight.
Magnetic cleanliness was especially important if magnetometers-instruments of particular interest in the interplanetary medium-were not to be affected by the spacecraft's own field. This was a difficulty because almost everything dealing with electrical power and metallic structures could affect the spacecraft field. To measure the very small levels of interplanetary fields, the spacecraft's own field had to be as small as possible, and furthermore, it had to remain the same throughout the mission. Twisted wire pairs, the sedulous avoidance of any cabling that created a magnetic loop, and extensive use of nonmagnetic materials in components all helped. The onboard transmitters used traveling-wave tubes that seemed at first an uncorrectable source of magnetic contamination; the remedy was to spot nearby small permanent magnets oriented to cancel out the tubes' magnetic influences. Considerable effort went into the design of a facility to test the magnetic cleanliness of the spacecraft, not merely at one instant, but under all conditions. This attention to magnetic cleanliness and ways to achieve it were major contributions of Ames and its contractors.
The Franklin array antenna was another concept that had not been extended as far in a technological sense as Pioneer required. This involved not only orienting the antenna on the spin axis but also a design to produce as high a gain as feasible in the toroidal (doughnut-shaped) pattern it produced. As the gain increased, the sensitivity to exact alignment increased; thus the pointing of the antenna had to be corrected as the spacecraft traveled farther away from Earth. For Pioneers it was decided that the spin axis of the spacecraft should be changed as needed by the commanded firing of a small thruster on a boom at right angles to the axis, changing the spin axis and the swath swept by the instruments to the precise plane desired. (It also set up a modest wobble in the spin, like the wobble in a slowing top, but a wobble damper took care of that.) Two different spin-correcting maneuvers were called for: an automatic one during the launch sequence, occurring right after injection, to ensure that the spacecraft's spin axis was as intended; and a commendable one to be initiated from Earth as needed after weeks or months of cruise had altered the geometry of the antenna and instruments. Persons responsive to the aesthetics of mechanism will find pleasure in studying the  axis-torquing systems aboard these Pioneers; they were simple, clever, imaginative, and they worked!
A communications development highly important to the success of these pioneers, though not first used on them, was phase-lock operation, a method that allowed the matching of signals from Earth and from the spacecraft to increase the sensitivity of reception over immense distances. In simplified form it worked this way. Let us suppose that a Pioneer is sending its Doppler tracking signals Earthward as it cruises along 100 million miles away. The spacecraft is operating on its own, with its transmitter frequency governed by its own crystal-controlled oscillator. This is a "noncoherent" mode of operation. Simply by listening to it, Earth can manage one-way Doppler tracking of limited accuracy. When the Deep Space Network picks up the weak signal and "locks" onto it, matters take a turn for the better.
Locking consists of directing the signal through a feedback loop and a voltage control oscillator and retransmitting it back at precisely the frequency received from the spacecraft but with a 90° change in phase. In effect, the feedback circuit forces the ground transmitter to match the spacecraft carrier frequency exactly. Once downlink lock is established, the ground transmitter sends its own carrier toward the spacecraft. When this is received, the spacecraft oscillator is automatically disconnected and switched to a voltage control oscillator that generates a signal having a precise ratio to the frequency received from the Earth station. This creates uplink lock, and the two have now formed a coherent roundtrip relationship between spacecraft and Earth that supplies Doppler tracking of exceptional precision. When tracking of this high accuracy is no longer needed, the coherent mode is simply broken at the ground transmitter, and the spacecraft automatically returns to the frequency established by the onboard crystal-controlled oscillator. Twoway phase lock has the particular merit of eliminating the effect of slight frequency drift that may have occurred onboard the spacecraft as the result of temperature changes, radiation, and aging. Another advantage is its ability to supplement the distant, relatively weak and unattended spacecraft equipment with powerful and fresh electronic gear on the ground. It makes possible those astonishingly precise calculations of spacecraft speed and position that surprise nontechnical onlookers.
There were four Pioneers in the block launched from 1965 through 1968, all productive, hardworking spacecraft, informative about the interplanetary medium away from the disturbing influence of Earth. They told us much about the solar wind and the fluctuating bursts of cosmic radiation....
... of both solar and galactic origin. They traveled in orbits approximating Earth's-two were slightly inside Earth's track and two were outside-and were spaced around the Sun to allow differential timing of the arrival of specific solar events. These four lonely sentinels in space were also an important part of a warning system designed to protect Apollo astronauts against potentially dangerous radiation resulting from solar eruptions.
The original target lifetime of a year in orbit was easily achieved. Nineteen years after the first of the four was launched, all are still working to some degree. Pioneers 6 and 9 still possess all their faculties and still speak when spoken to; Pioneers 7 and 8 have lost their Sun sensors and can respond only when the geometry of their orbits points their antennas Earthward. Such dogged longevity continues to surprise the engineers who worked on them.
Heartened by these quiet successes, Ames began developing a pair of newer, larger, more capable Pioneers designed to attempt more difficult feats. Essentially all previous interplanetary exploration had been directed toward Venus and Mars, Earth's nearest neighbors; now it was time to try to send probes through the unknown barrier of the asteroid belt to scout the distant gas giant, Jupiter. If that could be managed, it might even be possible to make a close pass through Jupiter's unknown radiation belts and gain enough swing-by energy to travel even further, to the ringed planet Saturn.
To suit the requirements of so ambitious a voyage, the spacecraft would have to be drastically modified. At Jupiter and beyond, the Sun would be too distant to create enough solar cell power; the spacecraft would have to carry a radioactive thermoelectric generator, which uses plutonium isotopes to heat an array of thermocouples. The Franklin antenna with its pancake pattern could not produce a signal strength that could cope with such a distance. It would be replaced with a parabolic antenna mounted on the spin axis and aimed back at Earth with rifle-like precision. In place of the earlier Pioneers' simple little thruster systems for initial orientation and another for nudges to precess the spin axis, there would now be no less than four pairs of thrusters arranged so that they could increase or decrease the spin rate, torque the spin axis around in different directions, or even accelerate or decelerate the whole spacecraft. Only one change was not in the direction of bigger and more; the earlier Pioneers had spun at the rate of 60 rpm; the new, larger ones had moments of inertia to hold orientation at a stately 4.8 rpm.
The greater diameter-limited by the fact that the antenna had to fit within the 10-foot shroud of the Centaur second stage-did not ease the lot  of spacecraft and instrument designers. At first it was hoped that enough weight could be spared to make these Pioneers partly autonomous, with onboard computers and memory to permit stored sequences of commands. However, as the inevitable weight crunch grew, it became necessary to leave the sophisticated brains on Earth. The long communications time imposed extra stresses on terrestrial controllers. Even though radio commands travel at 186 000 miles a second, the distances were such that it took 92 minutes between command and acknowledgment at Jupiter and more than 170 minutes at Saturn. One of the mind-stretchers of interplanetary exploration is to try to visualize long trains of commands racing at almost unimaginable speed in one direction, and long trains of data and imagery racing back to Earth, both trains, for all their velocity, requiring long periods of time to make the trip.
Fortune smiled on Pioneers 10 and 11, for both proved to be singularly effective spacecraft that accomplished historic missions. Launched on March 2, 1972, Pioneer 10 accelerated for 17 minutes atop its hydrogen-fueled Centaur to a speed of 32 114 miles per hour-at that time, the highest velocity ever achieved by a manmade object. In 11 hours it crossed the Moon's orbit, a distance that had taken Apollo astronauts some 3 1/2 days to traverse. Five and a half months later, past the orbit of Mars, it entered the asteroid belt, an utterly unknown band of scattered subplanetary debris, and in February 1973 it emerged unscathed.
Choosing the best flyby trajectory of Jupiter was agonizing, requiring not just thought about lighting, satellite position, and command sequencing, but also prudent estimation about how close the spacecraft should pass to the intense and potentially disabling radiation known to encircle the giant planet. Complexities arose from the fact that the radiation could generate false commands, and the communications delay could prevent their timely correction. The remedy was to prepare and transmit a series of redundant corrective commands against the chance that false commands would be set off by the intense radiation. Bathed in this steadying electronic reassurance from Earth, Pioneer 10 flew close to Jupiter on December 3,1973. It was accelerated to a velocity of 82 000 miles per hour by the mass of the huge planet and flung on a course that has taken it out of the solar system. In June 1983 it passed the orbits of Neptune and Pluto, still turning in its stately fashion and responding to questions at a range beyond 2.8 billion miles from the Sun. It is headed in the direction of the constellation Taurus and should reach the distance of the star Ross 248 in about 32 000 years.
 Pioneer 10's list of firsts is too long to cover in detail, but it should be credited as the first to fly beyond Mars, the first through the asteroid belt, the first to fly by Jupiter, and the first to leave our solar system. Engineers hope it will be possible to keep in touch until 1994, when Pioneer's radioisotope thermoelectric generators should expire.
Although this was a tough act to follow, Pioneer 11 succeeded and in one important aspect did even better. When it arrived at Jupiter in late 1974, its controllers were better informed about the lethal radiation and were able to manage a closer pass. In addition, the prevailing planetary configuration allowed Pioneer 11 to be guided on a course that flung it off to pass, almost 5 years later, the ringed planet Saturn, never before observed from space. It is a commentary on the pace of planetary exploration in those giddy years that, though the Pioneers added immeasurably to our scant store of knowledge about the outer solar system, the data and images they returned were soon to be overshadowed by more sophisticated exploring machines.
Like its brother, Pioneer 11 is destined to leave the solar system forever, but in an approximately opposite direction. At this writing it is perking along at a range of about 12 astronomical units (over a billion miles) from the Sun, healthy and mannerly. It bears a plate engraved with symbols and mathematical notation telling where it came from and when. This Earth's signature, or builder's mark, is situated in a place that should be shielded for incalculable ages from erosion by interstellar dust. Perhaps somewhere a hundred thousand years from now Pioneer's strange message from Earth will become a haunting reminder of beings reaching out.