[230] In the preparation of this book, I was continually reminded of the amazing way the threads of technology were woven into the fabric of missions to the Moon and planets. Developments pursued independently in laboratories throughout the world evolved miraculously to make space flight possible in the 1960s. Some of these developments had been ready and waiting for years; others barely arrived ahead of the need.
Many who contributed to successful space missions were not present to see their research pay off. Because of their special contributions, I would like to illustrate briefly how the works of two such researchers came into play long after their deaths.
The first is Johannes Kepler, who, after working for 18 years, developed laws of planetary motion which are beautifully simple, but crucial to defining the behavior of planetary bodies and spacecraft in orbits. As is usually the case in science, his work built on the efforts of others; his findings would not have been possible without the observations, the positional sightings, the recording of times, and the determination of planetary periods obtained earlier by Tycho Brahe and other astronomers. But by 1618, with these data and his own efforts, Kepler was able to arrive at three basic laws that clearly define the orbital relationships of satellite systems.
His first profound determination was that the planets move in elliptical orbits, with the Sun at one focus. Before Kepler, most astronomers believed that the heavenly bodies moved in circles, and their planetary systems concepts were based on this premise. Of course, the ellipse is a conic section which becomes a circle when its eccentricity is reduced to zero; in other words, a circle is simply an ellipse having coincident foci). Kepler's determination of this feature of planetary behavior was based on years of studying the orbit of Mars, leading to his final conclusion that its path was accurately defined as an ellipse.
[231] His second law, called "the law of areas," says that the line joining the Sun and a planet sweeps out equal areas in equal times. While this simple geometrical relationship was also the result of observations, it provided a basis for relating the speed of a planet to its position in orbit.
The third law, called "the harmonic law," simply says that the square of the time of revolution (in years) of any planet is equal to the cube of its mean distance from the Sun (in astronomical units). While the law of areas enabled changes in a planet's orbital speed to be calculated, from the harmonic law we can obtain either the distance of the satellite from the parent body or the period of revolution, provided the other is known from observation.
At the time these discoveries were presented, Kepler was working in Prague, Czechoslovakia. Many miles away in England and some 47 years later, Isaac Newton studied the laws produced by Kepler's observations, trying to understand the causes. His studies led him to develop gravitational theories as to why the planets move in elliptical orbits, and he produced the mathematical relationships of attractive forces between bodies. I find it interesting that it was almost 20 years before his works were published, reportedly because he was persuaded to do so by Halley, an astronomer who saw the significance of the work. Newton's brilliant discussion in 1687, called the Principia - System of the World, showed mathematical relationships for all the known motions of the Moon, the planets, and comets-even the rise and fall of the ocean tides-allowing precise calculations in terms of his laws of motion and gravitation.
Of course these principles are fundamental to all spacecraft trajectory determinations. When Apollo 13 was disabled by an explosion on its way to the Moon, for example, the only hope for recovery required a combination of velocity and lunar flyby distance such that the gravitational effect of the Moon would return the spacecraft to Earth in the proper direction for reentry into the atmosphere. Thus, gravitational forces and their effects, as originally worked out by Kepler and Newton, became the tools by which Apollo mission controllers and astronauts were able to direct the damaged spacecraft to a safe return and recovery on Earth.
The same classical developments, while used in the conduct of every space mission, became strikingly significant in some of the planetary flyby missions. A notable case was the Mariner 10 mission, in which a single spacecraft was sent from Earth to Venus and from Venus to Mercury, making orbits around the Sun and returning to Mercury for three close [232] encounters before completing its mission. It is doubtful that Kepler and Newton ever dreamed of the Apollo 13 and Mariner 10 applications of their findings, but surely they would have respected the engineers who so deftly applied them.
Feasibility studies for using gravity-assist trajectories, as they became known, were first recorded in the 1920s and 1930s, although it was during the 1960s that a team of JPL engineers dedicated to trajectory design became aware of their potential for the Venus-Mercury mission. One of the first studies was a flyby mission to Venus and return to Earth, a trajectory which would be extremely important to a manned flight for reconnaissance. During these studies, it was determined that a near minimum energy condition would exist for a launch past Venus and on to Mercury in 1973.
Soon thereafter, during discussions with the Space Science Board of the National Academy of Sciences, strong endorsement for such a mission was obtained. At the time, the NASA budget was beginning to decrease significantly, funds for future planetary missions were being sharply curtailed, and the Venus-Mercury Mariner concept of using interplanetary forces to obtain more science per dollar was exciting. Project evolution and mission operations provided some of the most memorable planetary experiences to date.
Mariner 10 embodied a combination of many advanced technologies that had evolved over the years. The gravity-assist concept for redirecting the orbit of a spacecraft, while requiring no additional rocket energy, demanded extremely accurate guidance and control systems to produce the precise flyby distances and velocities necessary. Earlier missions had refined our knowledge of the factors that tend to affect orbits, such as gravitational fields for the planets, and solar radiation pressure. Advances in attitude control and autopilot systems, plus improvements in tracking, allowed precise determinations of the trajectories and initial conditions required for velocity corrections. Added to this were improved vernier rocket systems used for trajectory adjustments.
To be able to achieve the close flyby of Venus with precision would require multiple trajectory corrections-at least two between Earth and Venus and two more between Venus and Mercury. You will recall that during the first Mariner mission to Venus it was debated whether a midcourse maneuver should be tried because of the hazards and uncertainties associated with the remote spacecraft orientation maneuver and rocket firing. These technologies had advanced such that we were confident the spacecraft could [233] be put within 250 miles of an aim point at Venus, so that after passage by Venus it would approach Mercury with enough precision to obtain meaningful scientific data.
Another technology that had literally soared during previous years was communications. Recalling that the first Mariner mission to Venus had a data return rate of 8 1/3 bits per second, Mariner 10 transmitted up to 118 000 bits per second-making it possible to send TV data in virtually real time while concurrently transmitting other science and engineering data at 2 450 bits per second. These phenomenal increases in data rates, plus a new command and control system for processing and programming, provided 21 data modes for television or nonimaging science, engineering data, and data storage playback.
Also significant to the Venus-Mercury spacecraft was the fact that the solar constant increased in value by four and a half times during Mariner's trip to the vicinity of Mercury, which orbits close around the Sun. Thus the thermal control system for the spacecraft could not be passive, but had to incorporate features such as solar panels that could be rotated "edge-on" toward the Sun to help keep temperatures within bounds. Mariner 10 used a combination of sunshade, louvers, and protective thermal blankets to "keep cool" during the close approach to the Sun.
One of the solar protective devices was an umbrella-like sunshade made of a Teflon-coated glass fiber fabric known as beta cloth. This simple device, suggested by Robert Kramer of NASA Headquarters, unfurled in the same manner as an umbrella, and shadow shielded the rocket system and parts of the spacecraft when pointed toward the Sun. Although Mariner 10 experienced temperatures near the Sun as high as 369° F-hot enough to melt tin, lead, even zinc-the temperature of its solar cells never exceeded 239° F. The temperatures of the television cameras dropped so low at one time during the flight that there was concern that the quality of the pictures might be degraded, but this did not happen.
In addition to the challenge of being the first mission planned for a two-planet encounter, Mariner 10 faced a number of obstacles in its approval phase that almost kept it from being. As already mentioned, when the project was presented to Congress in 1969, support for the space program had begun to wane and reductions in scientifically oriented projects were the norm rather than the exception. The Subcommittee on Science and Astronautics headed by Joseph Karth was giving a great deal of emphasis to space applications and putting pressure on NASA to use space for practical [234] purposes. This put proposals such as the Venus-Mercury mission into direct competition for funding with Earth resources, communications, and other applications missions because they were all considered by the same congressional subcommittee. It is in the record that the chairman told John Naugle, Associate Administrator for Space Science and Applications, that he didn't believe he was giving enough priority to applications as opposed to science, and that he was going to withhold funding for the Venus-Mercury mission until priorities were changed. The House of Representatives did not authorize the mission at first, and it took persuasion from the Senate plus a conference between the two houses to obtain fiscal year 1970 funding.
Even after Mariner 10 was put in the budget, NASA Headquarters officials were concerned that it would survive only if costs were kept low. According to Bob Kramer, who was then Director of Planetary Programs, estimates based on past Mariner experience showed that the job would cost about $140 million. JPL desperately wanted to do the mission, and Bill Pickering sent a letter to Headquarters saying, "I will absolutely guarantee that JPL will do the job for 98 million dollars." This strong guarantee by the director of the project center was encouraging and was accepted by NASA Headquarters with some trepidation.
Bob Kramer told me that the budget allowed only about $6 million for the video-imaging system, including the camera, all associated mission costs data analysis, and photographic prints. The head of the imaging team was a comparatively young scientist from CalTech named Bruce Murray. Bruce knew what the budget meant, but, being an aggressive person, he also believed that JPL might be able to find a way to modify the budgeted amount. After the mission was approved, Bruce came to Washington and pointed out that the spacecraft would be going past Mercury faster than any spacecraft had ever flown by a planet before-something like three times faster than any previous planetary encounter. At that speed, he said, the cameras would not really see anything; they would produce only a blur. He proposed a film system with image motion compensation patterned after the system on Lunar Orbiter which developed film in flight and read it back slowly. A cost estimate was made for such a system, and, according to Kramer's memory, it was something like $57 million. So he said, "Bruce, that won't quite fit into your six million dollar budget."
Not giving up easily, Bruce came back with a proposal for a dielectric camera, being developed by RCA, at a price estimate of about $40 million, assuming that its development was successful. Kramer told Bruce that $6 million was still the budget and that such a camera wouldn't fit.
[235] Finally Murray started talking to the systems engineers and the imaging science team about the problems. They all recognized that the low transmitter bit rate was a major factor, along with limited tape recorder capability. As Murray had pointed out, a physical aspect of the high-speed flyby was the fact that high-resolution data had to be obtained very quickly; there seemed to be no way that pictures could be taken and stored satisfactorily for later transmission.
The imaging team and the systems engineers began to collaborate on the development of a concept for sending back a quarter of the pixels for each image in real time, while storing the others for transmittal later. This would make the best use of the limited communications and recording capabilities and ensure that some picture data were obtained, even if recording and later transmission did not pan out.
After the telecommunication, camera, and recorder tradeoffs were studied, the same vidicons that had been used successfully on the 1971 Mariner orbiter were adopted, with the basic lens or optical elements extended to 1500-millimeter focal lengths so that they became real telescopes, able to provide magnified images. The option of sending back only one-quarter of the full frame (every fourth pixel) or the full frame was retained. Either mode could be commanded from Earth. In the quarter-frame mode, thousands of images could be sent that were suitable for mosaicking the whole planet. By scanning across the planet during the fast encounter, it was possible to obtain excellent photographic coverage.
Although the system was designed to provide good coverage at high resolution, one desire was not fulfilled: full-frame imaging of the planet. For 1971 Mariners, one camera had a wide-angle lens and one had a narrow-angle lens, so that both types of information could be obtained. Since this option would not fit within the $6 million budget, ingenuity came into play again. Small mirrors were added to filter wheels used for viewing in different colors, so that images could be directed toward small wide-angle lenses mounted on top of the cameras. The mirrors and simple lens systems (just 3 or 4 inches long and 1 1/2 inches in diameter) allowed each instrument to become a wide-angle camera by simply flipping the filter wheel around to the mirror. Thus, for $6 million the imaging team got almost everything it wanted, ranging from extremely high-resolution images of Venus and Mercury to wide-angle views of the planets on approach and departure.
Such ingenuity helped ensure that the entire Venus-Mercury Mariner project was completed within the $98 million guarantee. The outstanding project management effort was led by Eugene Giberson, the first manager of [236] Surveyor who was replaced when the project got so deeply in trouble. The success of Mariner 10 provided proof that Gene really had what it took to be a good project manager, and I was very proud of his comeback. This was not to be his last success; he also managed the Seasat mission which taught us much about Earth's oceans and was the forerunner of many new activities.
With the remarkable "parentage" provided by the team of scientists and engineers who devised the Venus-Mercury Mariner mission, it is not surprising that Mariner 10 developed a very interesting personality of its own. The mission became one of the most exciting to follow on a day-by-day basis, as troubles developed and were overcome in unexpected ways.
Within a day after launch, when instruments were being checked out, the camera heaters would not come on. Heaters were needed to keep the vidicons at reasonable temperature when the craft was far from Mercury; it was easier to shield the spacecraft when it was near the Sun and to provide heat when it was far from the Sun. There was great concern that the camera optics would cool down so much that they would not remain in focus, so the vidicons were switched on to maintain some heat within the cameras. With these precautions, the temperature of the cameras stabilized at low but viable values, and the picture data never showed any degradation as a result of the low temperatures.
About 2 weeks before encounter with Venus, the heaters for the TV cameras mysteriously came on. There had been great concern that the cameras might not operate properly during the Venus encounter, as their temperature had dropped well below freezing. It was not possible to know exactly what had happened, but engineers decided the problem might have been the result of a short circuit in another heater which had been biasing the TV heater. To avoid any risk to the camera heaters, the heaters in the related, suspect circuit were turned off. By this time, the spacecraft had warmed up enough with its closer approach to the Sun that not all of the heaters were needed.
Two months after launch, the most significant power-related problem occurred when the spacecraft automatically switched from its main to its standby power mode. This automatic switchover was irreversible. It was of great concern because of the possibility that it might have been caused by a fault common to both power circuits and might eventually cause the backup power supply to fail, ending the mission prematurely. Following the automatic switchover to the backup system, engineers were very careful when making changes in the power status of the spacecraft. Care was also [237] taken in maneuvering relative to the Sun to avoid automatic switchover from solar panel to battery power, should sunlight be lost.
Another problem occurred on Christmas Day when a part of the feed system of the high-gain antenna failed, causing a drop in signal strength. Diagnostic commands provided indications that temperature changes during flight may have caused the problem. It was of concern because should the high-gain antenna not perform properly, the real-time TV sequences would not be possible at Mercury encounter, greatly reducing the coverage and the benefits from the clever mosaic technique that had been worked out. About 4 days later, the feed system healed itself, and the high-gain antenna performed normally again. However, relief was short-lived, for within about 4 hours the fault reappeared, indicating that it was an intermittent glitch which might recur at any time. The problem with the antenna was a threat throughout the mission, but it apparently was solved by the increase in temperature and did not compromise any of the pictures.
A serious attitude control problem developed about a week before the flyby of Venus. The trouble occurred after Mariner 10 started a series of eight calibration rolls to allow the scan platform to obtain diffuse ultraviolet data over wide regions of the sky. Oscillations began suddenly in the roll channel of the attitude control system, causing the expulsion of attitude control gas at a very high rate. Watching the gas pressure drop, mission controllers knew that the spacecraft would die if this continued. In the hour it took to recognize, analyze, and respond to the problem, about 16 percent of the 6 pounds of nitrogen gas-the total supply of attitude control gas-had been lost. When the fault was determined to be the result of a gyro-induced instability, the gyros were turned off and the gas loss stopped. Later it was decided that the oscillation was caused by a long boom supporting a magnetometer at some distance from the spacecraft, which apparently entered into a resonant dynamic relationship with the attitude control system After an extensive analysis, commands were sent to place the movable solar panels and scan platform in such a position that solar pressure could help prevent the oscillation and avoid further loss of gas. Spacecraft attitude maneuvers and trajectory corrections were also modified to minimize gas usage.
It had been planned to use the gyros during the Venus encounter to ensure proper stabilization of the spacecraft. The reason was that the Canopus tracker, a light sensor, might be affected by particles near the spacecraft, by the background light from the planet, or by some other source which could [238] cause a loss in attitude stabilization at a critical period during encounter. As there was not enough time to determine the cause of the gas loss problem, a quick decision was made to take these risks and maintain attitude control during flyby using the celestial references of the Sun and Canopus. Everything worked beautifully during encounter, and all the data, including a grand total of 4165 images of the cloud-shrouded planet, were obtained as planned.
Once past the successful encounter with Venus, engineers had to decide how to plan the correction manuever that would allow the spacecraft to go past Mercury without using the gyros. Experiments were performed with the tilt of the solar panels to determine how to use these as "solar sails" or "rudders" and thereby save attitude control gas. About 2 weeks after encounter with Venus, the gyros were tested again. They seemed to function correctly at first, but then the oscillations began. As a result, a decision was made to plan a trajectory correction with a so-called "Sun-line maneuver." This required a wait until the spacecraft attitude relative to its trajectory was such that a simple firing of the rocket without attitude change would produce a suitable trajectory correction. Calculations showed that this would delay arrival at Mercury by 17 minutes, but would still be satisfactory for the science objectives.
Shortly after this decision, the spacecraft lost its Canopus reference and began drifting about the roll axis; the gyro mode had to be turned on and off to stop the motion and to reacquire Canopus, resulting in additional loss of the precious attitude control gas. Similar events were to occur about 10 times a week through early March, when conditions were right to make the Sunline course correction.
With the particular orientation of the spacecraft for this maneuver, it was not possible to obtain good Doppler data during the rocket firing; a considerable amount of tracking was needed after the maneuver to determine whether it had been successful. Refined trajectory calculations finally showed that the spacecraft would be passing 124 miles closer to Mercury than had been planned, but still within a satisfactory range. Like an unruly child who behaves very badly and becomes a model child just as anxious parents expect to be embarrassed, Mariner 10 began to function perfectly again just prior to its encounter with Mercury. The high-gain antenna had recovered, never to fail again, and high-resolution photographic coverage of Mercury was achieved as planned. This first return of high-resolution photographs of Mercury produced exciting new information of a Moon-like [239] planet, with many features that had never been seen from Earth. Techniques developed for the mosaic process worked as planned, and the wide-angle lens feature worked well. Scientists everywhere were ecstatic.
Shortly after encounter, in now typical Mariner 10 fashion, problems began to recur. An additional 90-watt load was registered on the power system, accompanied by a rapid rise in the temperature of the power electronics bay. This anomaly, following the still unexplained switchover from primary to standby power early in the flight, was indeed foreboding. Many tired engineers spent hours developing similies to the problem and devising work-arounds to control the temperature in the best possible way without adding stress to the power system. Other failures followed during the same week: the tape recorder power turned on and off several times without command and soon failed altogether; commands to change the transmit power level were not acted upon; and the flight data subsystem experienced a failure which caused a dropout of several engineering data channels, making it very difficult to determine what was happening and to nurse the ailing spacecraft around the Sun to reencounter Mercury. Since analyses of the loss of attitude control gas showed that gas usage would have to be reduced well below the normal cruise rate if the spacecraft were to encounter Mercury a second and third time as hoped, further multiple trajectory correction maneuvers had to be conducted, and some way had to be found to use the gyros without causing the oscillation problem. Engineers had by this time determined how the movable solar panels and the high-gain antenna worked as "solar sails," so that attitude control could be maintained, and some slight modifications in the trajectory could be effected using solar pressure.
To redirect the spacecraft for a return to Mercury, a very large maneuver was required which would have meant a long rocket burn. To prevent overheating of the rocket engine, the maneuver was programmed in two phases. This two-phase maneuver refined the aiming point of the spacecraft so that it would return to the vicinity of Mercury after making a pass around the Sun. As the spacecraft passed behind the Sun from Earth, data were obtained on the Sun's corona, adding to the planetary data collected about Venus and Mercury.
When the fifth trajectory correction maneuver was made in July 1974, the spacecraft was on the far side of the Sun from Earth. Just after the spacecraft began its attitude change for the maneuver, all the pens on the plotters dropped to zero and made straight lines, indicating that telemetry signals had ceased and no data were being returned. In spite of the fact that the mission [240] controllers were not able to see what happened, the spacecraft completed its automatic commands exactly as they had been stored; after the maneuver, the spacecraft commanded itself back to the cruise orientation, and telemetered signals were again received. With this new orbit, a passage by Mercury for the second time was assured; in addition, the trajectory change caused by the encounter with the planet and its gravitational field made possible a third encounter after another orbit around the Sun.
Following a brilliant performance in the vicinity of Mercury, Mariner once again acted up. This time it lost Canopus lock and began an uncontrollable roll. The automatic reaquisition sequence had been inhibited to save gas, and repeated reacquisition attempts using commands on the basis of the star tracker roll-error signal telemetry were unsuccessful. Each of these attempts required a momentary turn on of the gyros and the attendant use of the almost depleted gas supply. Roll axis stabilization had to be abandoned for this portion of the trajectory in which the attitude, other than solar orientation, was not critical. A roll-drift mode, allowing the spacecraft to roll slowly, was used. The rate was controlled by differentially tilting the solar panels; in a sense these became "propeller blades," with pitch changes commanded from Earth to moderate the roll rates.
This complicated operational technique was made more difficult by the loss of the engineering telemetry channel that had occurred earlier. But, after some study, engineers found that they were able to measure the roll rate by analyzing the signal from the low-gain antenna. This signal varied with roll position due to the nonuniformity of the antenna radiation pattern. Of course, signal strengths had been measured during testing before the mission began; after a few hours this technique became a suitable means of determining the roll attitude and drift rate of the spacecraft. By using this "roll stabilized" mode, only 25 percent of the normal cruise usage of attitude gas was required, allowing Mariner to reach Mercury for the third time with a slim margin-just enough to cover the encounter and a few days after. Three important trajectory correction maneuvers were completed, and the spacecraft was placed on a very close planetary encounter, determined to be only 2035 miles above the surface.
A few days after the encounter, trouble again developed, and the final significant drama for Mariner 10 engineering operations occurred. During an attempt to reacquire Canopus, the spacecraft rolled into a position such that the low-gain antenna was in a deep null and communications with Earth using the 85-foot dishes were completely interrupted. To compound the [241] problem, the large dishes of the Deep Space Network were tied up with a very important Helios mission that was approaching the Sun. In order to save Mariner, the controllers of Helios were asked to allow some use of the 210-foot antennas, and they acceded to this request. Using the more powerful transmitter and antenna at Madrid, it was possible to arouse the Mariner spacecraft and command it to its proper orientation just in time for the third flyby of Mercury. The third encounter produced some of the most remarkably detailed pictures of the planet and additional information on the magnetic field because of the very close 620-mile flyby trajectory
About a week later, Mariner 10's nitrogen supply was depleted, and the spacecraft began an unprogrammed pitch turn which told engineers it had finally exhausted its capabilities. Commands were immediately set to turn off its transmitter, and radio signals to Earth ceased. It then became a silent partner to Mercury, forever in orbit about the Sun. But it had performed brilliantly, and all associated with it had learned to respect its personality. However obstreperous, Mariner 10 always came through in the crises.
The interplanetary billiards successfully initiated by Mariner 10 and used by Pioneer 11 to swing by Jupiter and on to Saturn were followed by the spectacular flights of Voyagers 1 and 2. Both spacecraft have visited Jupiter and Saturn, with close encounters of several moons in orbit about those gas giants. Voyager 1, having completed its planetary exploration, is now sailing into the far reaches of the solar system. Voyager 2 is on a course to Uranus and is expected to continue to Neptune for close encounters in 1986 and 1989, respectively.
It is appropriate to class the Voyagers as planetary systems explorers, for, by judicious use of sophisticated navigation and guidance techniques, they examined 20 known satellites and more than a dozen new ones discovered during Pioneer and Voyager missions. The four planet-like Galilean satellites of Jupiter were of special interest, as was Titan, the almost Earth-like moon of Saturn. The Voyagers also examined Saturn's six icy satellites, of interest because water-ice is the dominant material on their surfaces. Among the most exciting findings about the moons of Jupiter and Saturn is the fact that several of them are still active volcanically; some have active atmospheres, and Titan at least may have oceans of liquid nitrogen or methane.
These extraordinary achievements resulted from a fall-back position taken after a program called "The Grand Tour" failed to win approval. At the time gravity-assisted trajectories were being studied for the Venus-Mercury mission, engineers discovered that in the late 1970s the outer [242] planets would be roughly aligned in a manner to make all of them observable by a single flyby spacecraft. After passing by Jupiter, the craft could be redirected toward Saturn; from there it could go by Uranus, then Neptune, and finally, after about a decade, past Pluto.
This exciting opportunity had last occurred when Thomas Jefferson was president and would not recur until some 175 years later. To do the opportunity justice, a set of sophisticated and expensive spacecraft would have to be developed, for the requirements of the long-lived, complex operation would be demanding. Those of us supporting the plan believed the potential returns from a single program seemed too good to pass up, but waning interest in space activities and troubles with Viet Nam and other matters made the proposition less attractive to Congress. It was not very long before lesser, more affordable goals were set for a mission to Saturn by way of Jupiter. The Voyager program, approved in 1972, preserved the basic concepts of the multiplanet flyby, using advanced Mariner-class spacecraft that were the most complex ever designed and built by JPL.
These latest operational planetary spacecraft and their marvelous systems can be compared with Mariners or Vikings from the configurational viewpoint; however, a principal difference is the large central antenna 3.7 meters in diameter, outsized because of the communication requirements for the far travelers as they journey to the outer reaches of the solar system. The other obvious configurational differences are the extensible booms. One provides for a steerable platform containing TV cameras and other science instruments; another serves to locate sensitive magnetometers away from the magnetic fields produced by the spacecraft. The same Mariner-like, multisided bus structure was used, but a bank of three radioisotope thermoelectric generators replaced the solar panels which could not provide enough power so far from the Sun.
For such long distance operations, redundant radio systems were employed; even though they were expected to operate up to a billion miles from Earth, the transmitter power for each is only 23 watts. This does not seem like a large gain over the 4 1/2 watts used by Mariner 2 to transmit at 8 1/3 bits per second, but the larger antenna and several other advances in technology resulted in a bit rate at Saturn of a whopping 44 800 bits per second. Since the communication system is as critical to an automated exploring machine as a reentry rocket is to a manned vehicle, the tremendous strides in telecommunications technologies deserve great applause.
[243] At the great distances being traversed by Voyagers, accurate position determinations are aided by the use of simultaneous, two-station ranging to increase the viewing baseline. Two Earth stations many miles apart work as a team to obtain angle and Doppler data. Uplink transmissions at S-band frequencies and two downlink frequencies at both S-band and X-band that are coherent with the uplink provide discrimination for the dispersive effects of charged particles along the signal paths.
Maneuvering among the moons of Jupiter and Saturn and flying through the rings of Saturn have been facilitated by optical guidance techniques first experimentally used by Mariners 6 and 7. In principle, a camera mounted on an accurately positioned scan platform can center an object in its field of view and indicate pointing direction relative to spacecraft coordinates. The information from the optical system can be used to adjust the platform toward other objects or to reorient the spacecraft for retromaneuvers, if desired. Changing from inertial coordinates to target object coordinates can improve the approach and flyby accuracies. Optical techniques combined with Doppler systems used for baseline cruise have been very effective in obtaining close-up images of the satellites of Jupiter and Saturn.
From the navigation standpoint, the Voyager 1 encounter with Saturn was probably the most complex ever experienced. Saturn's moon Titan, the largest moon in our solar system, was of special interest for a close flyby. This was a difficult requirement to meet, partly because precise information about Titan's mass and orbit was not available in advance. Several very small rocket burns were used along with optical data to refine the trajectory, and during the Titan encounter Doppler data were processed quickly to allow accurate instrument-pointing adjustments for the outbound imaging of the satellites Mimas, Enceladus, Dione, and Rhea.
Charles E. Kohlhase, Voyager Mission Design Manager, might also be labeled "Chief Navigator." It was Charlie's job to plan the trajectories so that proper flyby times and distances would result in desired velocity changes and viewing geometries for the scientific instruments. Also his was the challenge of determining course correction rocket firings. Because of the relevance of attitude orientation and celestial mechanics, his team was also able to figure out how to rotate the spacecraft for pointing when the scan platform azimuth system malfunctioned.
It is impossible to outline the strides that have been taken during the years since Charlie first began calculating trajectories for guidance and [244] control of Mariners. I can express my respect and admiration for his achievements and for those of his colleagues, but I am sure that even greater acclaim would come from Kepler and Newton, were they here to see their principles being applied.
