A scientific paper by H. F. Matthews and Charles F. Hall delivered to the American Astronautical Society's June 1969 meeting in Denver, Colorado, described the first mission to the outer planets. '`An exciting era of exploration of the outer planets has been initiated by NASA in recently approving the Pioneer F/G mission for flights to Jupiter in 1972 and in 1973."
Preceding years had seen a number of proposals and scientific papers about exploration of the outer planets, including missions to visit several planets by one spacecraft using gravity assist from other planets. Several NASA centers and private industry had completed studies showing that the gravity field of Jupiter combined with the orbital motion of the planet could accelerate spacecraft to speeds at which they could complete missions to more distant planets in reasonable times with useful payloads.
In March 1967, for example, a paper presented at the Fifth Goddard Memorial Symposium in Washington, D. C., discussed several types of galactic Jupiter probes aimed at exploring the interplanetary space beyond Mars, the solar wind and its interaction with deep space, and the environment of Jupiter. The paper pointed out, too, that such a Jupiter probe would be accelerated by the large planet sufficiently for the spacecraft to escape completely from the Solar System and cruise into interstellar space.
Official approval of a mission to Jupiter came from NASA in February 1969, and it was assigned to the Planetary Program Office, Office of Space Science and Applications. NASA selected the Pioneer Project Office at Ames Research Center, Moffett Field, Mountain View, California, to manage the Jupiter project, and TRW Systems Group, Redondo Beach, California, as the contractor to design and fabricate two identical Pioneer spacecraft for this new mission.
Relative positions of Earth and Jupiter on their orbits permit a spacecraft to be launched to Jupiter every 13 months with minimum launch energy. The first opportunity that seemed feasible for the Pioneer mission, taking into consideration the time needed to build the spacecraft, select its scientific experiments and build instruments to perform them, appeared to be the 1972 opportunity extending from late February through early March. NASA scheduled the first spacecraft, Pioneer F, to meet this launch window. A second spacecraft, Pioneer G was planned for launching approximately 13 months later during the 1973 opportunity.
Planning for the Pioneer mission to Jupiter involved organizations within NASA and industry. The Pioneer Program was managed at NASA Headquarters, originally by Glenn A. Reiff and later by F. D. Kochendorfer.
At NASA-Ames Research Center, Charles F. Hall became manager of the Pioneer Project. The experiments  system of the spacecraft became the responsibility of Joseph E. Lepetich, and the spacecraft system, that of Ralph W. Holtzclaw. The flight operations manager was originally Robert R. Nunamaker and later Norman J. Martin. Dr. John H. Wolfe became project scientist; Robert U. Hofstetter, launch vehicle and trajectory analysis coordinator; Richard O. Fimmel, science chief; and Gilbert A. Schroeder, spacecraft chief.
The Jet Propulsion Laboratory of California Institute of Technology, Pasadena, California, provided tracking and data system support with originally A. J. Siegmeth and later Richard B. Miller, manager of the Deep Space Network for Pioneer, NASA-Goddard Spaceflight Center, Greenbelt, Maryland, provided the worldwide communications.
The launch vehicle system became the responsibility of NASA's Lewis Research Center, Cleveland, Ohio, under management of D. J. Shramo. Launch operations were the responsibility of NASA's John F. Kennedy Space Center, Florida, where the representative for the Pioneer Jupiter project was J. W. Johnson.
At TRW Systems Group, B. J. O'Brien managed the Pioneer Jupiter project. At the Atomic Energy Commission, B. Rock became project engineer for the SNAP-19 radioisotope thermonuclear power generators to be built by Teledyne Isotopes. Bendix Field Engineering Corporation, under the management of Walter L. Natzic, supported the mission operations system. As appropriate, these responsibilities are, of course, continued into the mission beyond Jupiter until communication is lost with the spacecraft nearly a decade after launching.
Objectives of the Pioneer Jupiter mission were early defined by NASA as:
Ames Research Center was chosen for the mission because of previous experience with earlier spin-stabilized spacecraft that are still exploring the inner Solar System on a continuing basis. The new Pioneer was required to utilize proven spacecraft modules of Pioneers 6 through 9 to produce a small, lightweight, magnetically clean, interplanetary spacecraft.
To propel the 550-pound spacecraft to the unprecedented velocity needed to enter a transfer trajectory to Jupiter, the Atlas-Centaur launch vehicle (Figure 2-1) was equipped with an additional solid-propellant third stage.
 Scientific experiments were selected over a series of planning meetings in the late 1960's, and by early 1970, all science experiments had been settled: measurements of magnetic fields; measurement of plasma; Zodiacal Light measurements; polarimetry and imaging of Jupiter and several Jovian satellites; determination of composition of charged particles; recording of cosmic rays; ultraviolet and infrared observations of Jupiter; and detecting asteroids and meteoroids. In addition, as with other spacecraft, the radio communications signal would provide a probe into the atmosphere of the planet as the spacecraft passed behind it, and tracking data from the signal would provide information about the mass of Jupiter and its satellites. Principal investigators were appointed for all the experiments, and contracts were awarded to build the instruments and conduct the experiments. All experiments are described more fully in Chapter 4.
The two spacecraft for this mission were identical. The first, Pioneer F, blazed the trail; had the environment of the asteroid belt or of Jupiter caused a failure, the second spacecraft, Pioneer G, would have provided a backup. Initially, Pioneer G was launched and targeted to follow the path of Pioneer F. However, the capability existed and, therefore, it was planned that Pioneer G be retargeted as necessary, based upon the results from the first spacecraft's encounter with Jupiter. And this was done.
The launch vehicle boosted the spacecraft in direct ascent, i.e., with no parking orbit, to start the flight to Jupiter at about 51,500 km (32,000 mi.) per hour. A trip of just under 600 days was the shortest time to Jupiter within the capabilities of the launch vehicle, and a trip of 748 days' the longest.
In-flight maneuvers were planned to take place several times during the mission to target the spacecraft to arrive at Jupiter at a time and position suitable for best observing the planet and several of its large satellites.
Design requirements were established for each spacecraft to mate physically with the launch vehicle, and for its communications system to be compatible with the Deep Space Network. Each Pioneer had also to provide a thermally controlled environment for scientific instruments.
The Pioneer spacecraft must operate reliably in space for many years (Figure 2-2). Each carries a data system to sample the scientific instruments and to transmit scientific and engineering information about the "health" of the spacecraft and its instruments over the vast distances to Earth. The spacecraft also have to be capable of being commanded from Earth to perform their mission and....
 ....to change the operating modes of equipment aboard them (Figure 2-3).
Each Pioneer's curved path to Jupiter is some 1000 million km (620 million mi.) long. covering about 160 degrees azimuthly around the Sun between the orbits of Earth and Jupiter. During Pioneer's flight from Earth to Jupiter Earth travels almost twice around the Sun while Jupiter moves only about one sixth of the way around its solar orbit.
There were options on the path to Jupiter. Some arrival dates were forbidden because the sensors would not have been able to perform the scientific experiments desired; others because they would have clashed with the arrival of another spacecraft, Mariner 10, at Venus or Mercury and given rise to conflicting requirements for the use of the big 64-meter (210-foot) antennas of the Deep Space Network.
Launch windows were available for Pioneer from February 25 to March 20, 1972, with arrival at Jupiter between the middle of October 1973 and late July 1974. Arrival had to be timed so that Jupiter and the spacecraft would not appear too close to the Sun as observed from Earth. Approximately 300 to 325 days and 700 to 725 days after launch' the motions of Earth and the spacecraft put them on opposite sides of the Sun. Thus arrival at Jupiter beyond 700 days after launch would have been impractical. The earlier passage of the spacecraft behind the Sun, just over 300 days after launch, interrupted communication with Pioneer 10 but not at a critical period of the mission. There were similar options for Pioneer G one year later.
There were also targeting options at Jupiter itself, such as how close the spacecraft should be allowed to approach the planet, how the trajectory should be inclined to the equatorial plane of Jupiter, and the position of the closest approach relative to the equatorial plane of Jupiter.
Early in planning the program, a decision was made that the encounter trajectory (Figure 2-4) for Pioneer F should be one that would provide maximum....
....information about the radiation environment, even if this damaged the spacecraft and ended the mission at Jupiter. Hence' imaging of Jupiter could only be ensured before closest approach. An approach trajectory was' therefore, selected to present a well-illuminated planet for the pre-encounter phase, and a partially illuminated (crescent) planet as seen from the spacecraft after encounter. Although at first it seemed desirable that occultation of the spacecraft should be avoided, an occultation was selected since it could provide useful information about the atmosphere of Jupiter unobtainable in any other way.
Because Jupiter has radiation belts trapped by its magnetic field, the question of how close a spacecraft can approach Jupiter to take advantage of the gravity slingshot effect without damage to its electronic and optical equipment needed to be answered. This was one of several primary objectives of the first Pioneer fly-by mission.
In July 1971, scientists held a workshop at the Jet Propulsion Laboratory to define the environment of Jupiter in terms of the best information available at that date. With slight modifications, this environment was accepted as a design environment for the Pioneer Jupiter mission, for the spacecraft, and for its scientific instruments. But no one could be sure that the environment, although based on the very best observations from Earth, was the true environment of Jupiter. One Pioneer task was to determine this true environment.
 There was, of course, a trade-off to a certain extent in that although approaching closer would increase the intensity of radiation, the spacecraft would fly by Jupiter more quickly and, thus, be exposed to the radiation for a shorter time. These two factors, which determine the integrated or total radiation dosage, were carefully weighed in the light of known information about Jupiter.
In general, the mission was designed to fly by Jupiter at three times the radius of the planet (referred to as 3RJ), i.e., twice the radius of Jupiter above the cloud tops, not because the spacecraft could not be targeted to go closer, but rather because available information suggested this was the closest a spacecraft might approach without damage by radiation. At the time of mission planning, the ephemeris of Jupiter was uncertain to about 2000 km (1250 mi.), but navigationally, the spacecraft could have been sent within 3/8 Jupiter radii above the surface. Navigation to Jupiter is simplified somewhat because the massive gravity of the planet provides a focusing effect. An error in aiming by 1600 km (1000 mi.) would be narrowed to only 480 km (300 mi.) by this gravity focusing. But the error in the time of arrival is magnified by the same effect.
The choice of approach having been made, the scientific instruments were designed to survive the expected radiation intensities for the period that Pioneer F would be within the radiation belts.
Time of arrival at Jupiter could be changed by several days with the amount of propellant carried by the spacecraft, and this made it possible to fly close to a satellite for imaging, or to be occulted by a satellite.
In 1800, Johann Elert Bode called a meeting of astronomers at the observatory of Schroter in Lilienthal, Germany. He asked these astronomers to search for an undiscovered planet believed to be orbiting between Mars and Jupiter. On January 1, 1801, Giuseppe Piazzi, director of the Observatory of Palermo, Italy, discovered such a small planetary object, 1022 km (635 mi.) in diameter, which he named Ceres. But soon after its discovery, Ceres moved along its orbit into the glare of the Sun and was lost.
The great mathematician, Gauss, then developed the theory for orbit determination from a minimum number of observations and calculated the small planet's orbit and showed where it might be found again as it emerged from the solar glare. While observing Ceres again in 1802, Olbers discovered a second planetary body which he named Pallas. It was even smaller than Ceres; only 560 km (348 mi.) across. Astronomers were even more surprised when Harding discovered 226-km (141-mi.) diameter Juno in 1804, and Olbers found 504-km (313 mi.) Vesta actually the brightest of these minor planets in 1807. These diminutive planetary bodies (Figure 2-5) were termed asteroids by Herschel. They were regarded as fragments of a trans-Martian planet.
Today it is known that there are at least eight other asteroids larger than Juno but they were not found until half a century after the discovery of the first four of these bodies. Then the year 1845 saw the beginning of discoveries of many minor planets until today between 40,000 and 100,000 are postulated. Many have been discovered photographically (Figure 2-6). While most are between the orbits of Mars and Jupiter, other stray closer to and farther from the Sun in elliptical orbits. Several have approached Earth; one at least approaches the orbit of Mercury, and another that of Saturn (Figure 2-7). All are relatively small objects on the planetary scale.
While the orbits of the larger asteroids are cataloged, there are many asteroids whose orbits are unknown. The risk of Pioneer colliding with any of the charted asteroids was negligible. But there was no way of knowing how many sand-grain-sized particles might be there to impact on the spacecraft and lead to serious damage.
At the time of the program's beginning, it was not known whether Pioneer would survive its passage and reach Jupiter. But before sophisticated.....
 .....missions to the outer planets could be planned, at least one spacecraft had to penetrate and survive passage through the asteroid belt.
A second problem faced by the Pioneer mission to Jupiter was how to supply electrical power to the spacecraft at the great distances it must travel from the Sun. Some of the early mission planning considered using solar cells because radioisotope power generators had not been tested over the long lifetimes required, nor were scientists sure that radiation from such generators would be acceptable to the sensitive scientific equipment carried by the spacecraft. Moreover, the hazard to solar cells by the radiation belts of Jupiter could be even more serious. It was decided not to use them.
Since the spacecraft had to fly very fast to leave Earth, the amount of payload that could be carried was restricted. Complicated on-board computing systems would be too heavy. Jupiter Pioneer had to be virtually "flown from the ground," despite long delay in communications over the distance to Jupiter and beyond.
The long mission period and weight limitations also called for unprecedented high reliability of all the spacecraft components. This was achieved by avoiding complexity in the spacecraft and by keeping the complexity, as much as possible, on the ground. Also vital items, such as transmitters and receivers, were duplicated, and only space-proven systems and components were used. Electronic components were "burned in" before assembly on the spacecraft so that components likely to fail in "infant mortality" were eliminated before the flight.
Success relied very heavily upon an advanced command, control, and communications system to link the Earth-based computers and human controllers to the spacecraft.
Command, Control and Communications
Mission Phases- Four distinct phases of command and control characterized Pioneer's mission to Jupiter and onwards into interstellar space (Figure 2-8). Each called for different approaches.....
 ....and techniques. Two phases Earth launch and Jupiter encounter were critical from a standpoint of doing things quickly, of taking necessary corrective actions as soon as possible after events called for them; while the other two phases interplanetary mode from Earth to Jupiter and beyond Jupiter permitted more leisurely actions since time was not such a critical factor while the spacecraft traveled between the planetary orbits.
Control of the spacecraft and launch vehicle during the prelaunch and launch phases at NASA's John F. Kennedy Space Center was maintained by launch teams from the Ames and Lewis Research Centers, respectively. Shortly after the spacecraft had been separated from the launch vehicle and had entered into its transfer orbit to Jupiter, spacecraft control was transferred to the Ames flight operations team at the Jet Propulsion Laboratory (JPL) (Figure 2-9). Simultaneously, control of the scientific instruments within the spacecraft was transferred to the Pioneer Mission Operations Center (PMOC) at NASA-Ames Research Center (Figure 2-10). Thus, there was a period of split control between engineering at the Jet Propulsion Laboratory and science at the Pioneer Mission Operations Center. The reason for this split control was to take advantage of the multiple consoles and backup computers at the Jet Propulsion Laboratory for the critical first few days of this epochmaking flight to the outer Solar System.
Engineer specialists were thereby able to monitor, simultaneously, all the systems, such as the spacecraft telemetry, power, thermal, attitude control, data handling, and command subsystems. Operating on three shifts around the clock, they could watch console displays of performance to make sure that each subsystem performed satisfactorily during the period of Pioneer's entry into the environment of space.
Quick reaction to unusual events was mandatory at this time when the spacecraft experienced launch stresses of high acceleration to attain the velocity needed to travel to Jupiter. As Pioneer moved away from the Earth, passing the orbit of the Moon less than 11 hours after liftoff, compared.....
 ....with three days for Apollo to reach the Moon, activities changed from checking the "health" of the spacecraft and its scientific instruments to readying Pioneer for its momentous voyage to and beyond the orbit of Jupiter.
Several days after liftoff, following midcourse maneuvers, and with all equipment and science instruments performing well, the mission crews left the Jet Propulsion Laboratory and the John F Kennedy Space Center, and returned to NASA Ames Research Center, south of San Francisco in Northern California. Now all control centered here.
Once the spacecraft settled down to the interplanetary mode, spacecraft events were expected to occur more slowly. The task changed to one of watching and waiting and becoming familiar with an increasing delay for signals to go to the spacecraft and return to Earth. In this interplanetary "cruise" phase, all monitoring of the process of "flying" the spacecraft to Jupiter was by a small group at the Pioneer Mission Operations Center, varying between five and seven mission operations people with supporting personnel.
During this interplanetary phase to and beyond Jupiter, engineering and scientific data returning from the spacecraft are continually monitored by computers and by people (Figure 2-11) to provide alerts at the earliest possible moment, should corrective action be required. This action depends upon the circumstances and the urgency of correction. A computer at NASA-Ames Research Center monitors Pioneer's telemetry signals on critical aspects of the spacecraft and its payload. Should a voltage or a temperature or some other engineering parameter rise or fall too much, or the status of an instrument change without being commanded to do so, the computer generates an audible alarm and a printed message. Whatever the hour of day or night, if the situation requires, the duty operator then immediately brings the problem to the attention of the cognizant engineer or scientist who can resolve it. Specific procedures were provided to the trained mission controllers to cover any emergency, should it occur, and to advise them whom to contact for a decision, if the unexpected occurs.
During the long voyage through interplanetary space, data from each science instrument is sampled periodically to check for scientific "health" as well as engineering "health." Controllers and scientists watch for any need to change bias voltages to adjust range or sensitivity of instruments or to switch modes of operation.
When each Pioneer reached the outskirts of the Jovian system, quick action again became the order of the day. But this was quite different action from the Earth launch phase. Since the Pioneer spacecraft was now over 800 million km (500 million mi.) away, radio signals took 92 minutes for the round trip to the spacecraft and back. All command actions had to be planned well in advance because of this delay.
The most critical item of equipment, in this respect, was the scientific instrument known as the imaging photopolarimeter or IPP for short. This instrument, which is described in detail in Appendix 1, required long sequences of commands during encounter with Jupiter to obtain best possible usage of times when the spacecraft passed by the....
.....planet and by its Galilean satellites. A sequence of contingency commands was designed to reconfigure the Pioneer spacecraft and its instruments, should spurious commands be generated by the build-up of electrical charges or by intense radiation during the close approach to Jupiter.
The fourth and final phase of command and control of Pioneer was entered as each spacecraft passed beyond Jupiter. The mode of operations is similar to that between Earth and Jupiter, but as Pioneer moves farther and farther away from Earth, its signals become increasingly fainter and also take longer and longer to return. Ultimately, contact with the tiny emissaries from Earth will be lost about 1980 for Pioneer 10 and somewhat later for Pioneer 11 if the second spacecraft survives a planned flyby of Saturn following a successful encounter with Jupiter.
Tracking and Data Acquisition Support - The NASA Communications Network (NASCOM), operated by NASA-Goddard Space Flight Center (GSFC), Maryland, provides worldwide ground communications circuits and facilities that link the Earth terminals of signals received from the spacecraft with the control centers on the West Coast.
Ranged worldwide, the Deep Space Network (DSN) operated for NASA by the Jet Propulsion Laboratory (JPL), provides deep space tracking, telemetry data acquisition, and commanding capabilities through the 26-meter (85-foot) and 64-meter (210-foot) diameter antennas (Figure 2-12) at Goldstone, California, and in Spain, South Africa (until July 1, 1974), and....
 ....Australia. Thus, as the Earth turns on its axis, the Jupiter Pioneer remains in continuous contact with its controllers; Goldstone, Australia, Spain, or South Africa, operating in turn each day. As Pioneer sets at one station, it is acquired by the next station (Figure 2-13).
Telecommunications-Communications over the vast distances to and beyond Jupiter presented new problems never before faced in the space program. To meet requirements of light weight for the spacecraft, transmitters and antennas aboard had to be designed to conserve power while at the same time being lightweight. Communications with Earth relied a great deal upon the extreme sensitivity of the Deep Space Network's 64-meter (210-foot) antennas and their advanced receiving systems, although the spacecraft can also use the smaller, 26-meter (85-foot) diameter antennas when the 64-meter (210-foot) antennas are required for other space missions.
When used in reverse as transmitters, these 64-meter (210-foot) antennas have such precision of directing a radio beam, and such high radiated power (up to 400,000 watts at Goldstone), that outgoing commands can be received by the spacecraft at greater distances (to several hundred times the distance of Earth from the Sun) than the spacecraft can send its own messages back to Earth (only 20 to 24 times the distance of Earth from the Sun).
The spacecraft carries three antennas low gain (omni), medium gain, and high gain used both to receive and to send signals. The spacecraft carries two receivers and two transmitters, which are selected for use by command from Earth.
The amount of energy received over the radio links from the spacecraft at Earth from Jupiter's distance is incredibly small. A 26-meter (85-foot) antenna collecting this energy would require 16.7 million years to gather enough to light a 7-1/2 watt nightlamp for a mere one-thousandth of a second. Only the sophisticated data coding and signal modulation techniques, coupled with the big antennas and the very advanced, ultra-cold, receiving devices attached to them, make it possible to receive and record these faint signals from Pioneer. All the pictures of Jupiter reproduced in this book, all the information from space and the environs of Jupiter, all the engineering data about the spacecraft and its battery of scientific instruments, all the tracking of the spacecraft to nearly a billion miles from the Earth, derive from this incredibly small radio signal. The communications system of Pioneer and the Deep Space Network is truly one of today's great technological achievements.
The rate at which information is passed over a radio link is expressed in bits per second, where a bit is defined as a unit of information, analogous to the dots and dashes of the morse code. Onboard the spacecraft, a data handling system converts science and engineering information into an organized stream of data bits for radio transmission to Earth.
Just as light at night appears fainter and fainter with increasing distance, radio signals from a spacecraft also become fainter with distance. There are also natural, background radio signals, and even the components of electronic apparatus generate radio noise by the movement of electrons within them. So as signals become fainter, they tend to be drowned by the background of noise. Sophisticated techniques have to be employed to receive information over the noise at extreme distances
As the Pioneer spacecraft move out into the Solar System, their signals, too, become weaker and weaker at Earth. The telemetry system is adapted to the lesser received power by commanding a change in the rate at which information is transmitted to Earth. Power per unit of information depends upon the rate at which the information is sent; the bit rate. To extract information from a radio signal, there must be a certain energy in the signal in excess of the energy of the background noise. As the range to the spacecraft increases, the bit rate is reduced so that less information is sent per second. But each bit of information lasts longer and possesses more energy so that it can be detected above the radio noise.
 By reducing the bit rate, controllers compensated for the reduced received signal strength from Pioneer. On the way to Jupiter, the Pioneer communications system could pass a minimum of 2,048 bits of information to Earth every second, using the 26-meter (85-foot) antennas. But at Jupiter, the maximum rate could only be 1 024 bits per second, using the 64-meter (210-foot) antennas, because of the increased range.
A digital telemetry unit on the spacecraft prepares the data for transmission in one of 13 data formats at one of 8 bit rates of from 16 to 2,048 bits per second. An onboard data storage unit is able to store 49,152 data bits for later transmission to Earth. This permits data to be gathered by Pioneer during important parts of the mission faster than the spacecraft can send the data to Earth or when no data could be transmitted at all for example, when the spacecraft was being occulted by Jupiter. The data is then later transmitted in response to ground command.
Command and Control- Controllers at Pioneer Mission Operations Control use 222 different commands to operate the Pioneer spacecraft. The command system consists of two command decoders and a command distribution unit within the spacecraft. Commands are transmitted from Earth at a rate of one bit per second. Since each command message consists of 22 bits, a command requires 22 seconds for its transmission.
The spacecraft also carries a small command memory to provide storage onboard of five commands. When a series of up to five commands must be executed in less time than that needed to transmit them from Earth, i.e. 22 seconds for each, this storage is used. The command memory with time delay was also used to command the spacecraft when behind Jupiter and out of touch with Earth.
A command distribution unit routes the commands to destinations within the spacecraft: 73 to operate experiments and 149 to control spacecraft subsystems. Science commands, for example, include those to calibrate instruments, change modes. move the photopolarimeter telescope, and change instrument data types. Spacecraft commands include firing the rocket thrusters and changing from one component to another redundant component. selecting different antennas, and changing the modes of the data handling subsystem.
Any command not properly verified by the decoder in the spacecraft is not acted upon by the command distribution unit. Thus precautions are taken against the spacecraft accepting wrong commands. Commands are also verified on the ground by the computer and by controllers before transmission. A Pioneer Encounter Planning Team looked at many possible contingencies that might arise during the weeks that each Pioneer would spend passing through the Jovian system, and developed command sequences to meet them.
The decision, made early during planning, that the spacecraft would be "flown" by command demanded constant thinking planning, and acting well in advance of events occurring on the spacecraft. Indeed, two years of planning preceded the first encounter with Jupiter. All commands (over 16,000 in all for Pioneer 10) were meticulously sequenced and each was timed to be executed within one tenth of a second of a scheduled time. These were checked and stored in a computer in eight-hour long files suitable for transmission during the time that a ground station would be in contact with the distant spacecraft. The majority of these commands was transmitted to the spacecraft in a four-week period. This was an outstanding achievement and performance on the part of all personnel concerned at the Pioneer Mission Operations Center, the Ground Data System, and the Deep Space Network in sending the commands on time and with the high reliability needed for the Jupiter encounter.