SP-480 Far Travelers: The Exploring Machines


Music of the Spheres


[33] After Mariner 1 was destroyed, only a month remained before Venus would move out of reach. There was a lot to do to prepare for Mariner 2, and we actually had only 3 weeks, since the fourth would be consumed by the full countdown and its inevitable holds. The pad was quickly inspected to assess the damage done by the blastoff fire of Mariner 1; fortunately, damage was minor, and the necessary rewiring and other refurbishments could be accomplished in time. The causes of the failure of the first launch were soon known and the corrections found to be straightforward. Changes to the Atlas guidance antenna and the addition of a hyphen to the guidance program, once recognized, presented no significant problems It was not deemed necessary to punish anyone on the team for the failure, for in tightly knit groups the individuals at fault usually won compassion, their consciences causing more anguish than any formal reproach. At least one company official was very apologetic, however, for he had been responsible for programming before being promoted to his management position. His colleagues ceremoniously awarded him a plaque with the missing hyphen on it.

After the traumatic failure of Mariner 1, those at Cape Canaveral would have liked a few months to wind down, regroup, and carefully prepare for the second launch. However, we had to reckon with an absolutely firm deadline-set not by an overzealous program manager but by the strict geometry of our solar system. There were 4 weeks in which to complete our task and no more; a delay beyond that point amounted to certain failure. This time restriction, while especially frustrating in the summer of 1962, is a fundamental problem for any planetary mission.

Although diagrams often show the planets in neat, circular orbits around the Sun, the actual geometry of the solar system is tremendously more elaborate The Earth and its sister planets revolve about the Sun in unique ellipses, each moving in a separate plane and with varying, precisely changing velocities Considering the multitude of factors involved, the orbital rela [34] tionships among the planets are beautiful in their geometry but bewilderingly complex. Thus, in sending a spacecraft from one planet to another, proper orientation of the launch vehicle is not enough to ensure an acceptable flyby distance. We cannot simply point a rocket toward Venus, fire it, and expect the payload to neatly sideswipe its planetary target. Once in flight, a rocketlaunched spacecraft itself becomes a planet, obeying the same curvilinear laws of orbital behavior. For a successful interplanetary launch, proper timing is every bit as important as correct orientation.

The best opportunities for launching a spacecraft from Earth to Venus occur once every 19 months. During these short periods (about a month or so in duration), the energy required for a successful launch is at a minimum. If a voyage were attempted at any other time, the amount of energy needed would be prohibitive. The reason is that while Earth and Venus both circle the Sun in near-circular orbits, the spacecraft must leave Earth and travel in its own elliptical orbit about the Sun, arriving at the orbit of Venus at a time when Venus is also there. Since Venus moves faster around the Sun than does Earth, it would move from behind to ahead of Earth during the total interplanetary flight time of Mariner 2.

In addition to the approximately month-long period when Venus is accessible, there is a daily window amounting to less than an hour. This additional complication is caused by Earth's rotation about its own axis. We were able to extend the window slightly by varying the timing delay in the parking orbit before the Agena's second burn, but this nevertheless imposed a tight restriction on the last part of the countdown.

Mariner 2's countdown began on August 25 at launch time minus 205 minutes. The spacecraft had by then accumulated a total test time of 690 hours-better than 4 full weeks of operation in which its parts had been given a chance to fail and had demonstrated their likelihood of lasting through the long trip in space. Soon, however, the count had to be scrubbed because of an indication of stray voltage in the Agena destruct circuit. This was corrected and the count restarted on August 26. In all, there were four unscheduled holds, and the launch was delayed a total of 98 minutes. There were several tense interludes in those last hours, particularly after the Agena had been loaded with propellants. If the daily window had been missed, the vehicle would have had to be detanked and elaborately purged of its volatile chemicals. The Atlas battery also proved worrisome as launch time neared, for it had been replaced once and its replacement was down to a life expectancy of 3 minutes at the moment of liftoff.



Relative orbital positions of Earth, Venus, and Mariner 2.

Relative orbital positions of Earth, Venus, and Mariner 2.


Looking back on those Cape launches in the early 1960s, I remember human aspects fully as much as mechanical mischances. A nagging feeling of helplessness plagued me, a former "hands-on" engineer, now a Headquarters official, because of my total dependence on others, including many persons I had never met. My only recourse was to do everything possible to ensure team spirit and singlemindedness of purpose.

One idea that became a tradition involved a visit to the blockhouse in a 30-minute, built-in hold, about an hour before launch. During the Mariner 2 countdown, I accompanied the project manager and one or two other officials from the spacecraft control station in Hanger AE to show our "colors" and to wish the launch crew well during those final critical minutes before launch. In addition to helping ease the tension of the count, I knew deep inside that sharing moments when things were going well might make it easier to work together should things go wrong. Those early blockhouse visits are remembered as warm and friendly interludes during times of considerable anxiety-a chance to share "Hellos" with the Launch Conductor, Orion Reed, and other respected friends who played vital roles in the operation. Whether or not these visits had a good effect on team spirit or were otherwise helpful is uncertain, but they at least gave us something to do during the tense time of waiting that was, for me, the hardest part of the job.

[36] I think the idea of visiting the blockhouse during the T minus 1 hour hole came from Jack James, Mariner Project Manager. Jack was extremely sensitive to the importance of each team member in the launch operation and genuinely cared about peoples' feelings. These qualities made him an excellent choice to lead operations at the Cape where Air Force, NASA, JPL, and contractor personnel were trying to do a most difficult task as a relatively unrehearsed team. The concept of a project manager and field center project management assignments was new and being used for the first time. Jack's powers were largely dependent on his ability to develop respect and on his skills in persuading people to do their tasks in a coordinated manner. The project manager was answerable to NASA for the entire operation; however, he had "hiring and firing" control over only a few staff members-the thousands of others who worked on the project did so through a complex of indirect assignments.

Jack had grown up in Texas. He confessed to a certain amount of bumming around and doing odd jobs after finishing high school; when he finally decided to enter college at Southern Methodist University, he chose electrical engineering for "default" reasons. When I asked him to explain, he said he knew that civil engineers built roads and bridges and that he didn't want to do that, mechanical engineers dealt with big machinery and he didn't want to do that either, but since he didn't know anything about what electrical engineers did, he decided it might be fun to learn and chose to enter the field. It's clear now that he made an excellent choice; solid-state physics began opening the field of electronics with semiconductor developments about the time he was in school and within a few years revolutionized our lives.

Jack's early assignments at JPL involved missiles being developed for the Army. His experiences with missile launchings were good preparation, but compared with Mariner operations involving literally thousands of people, missile launchings were games. Coupled with the usual "people" problems were the unusual schedule demands dictated by planetary orbit constraints. Getting diverse groups to work together was tough enough; getting them to mesh their efforts to meet an unyielding deadline was far more challenging. No allowances could be made for one element of the team to slip its schedule. This extra dimension, added to an effort that had never been approximated before, called for a high level of insight to plan and direct technical and human activities.

[37] Jack had an unusual blend of experience, horse sense, and humanistic qualities. We were extremely lucky to have him around, for it was apparent by the success of the early Mariners that he was a singularly gifted project manager, one of the best we ever had. He became, in a shy, winning way, almost a national resource: technically astute, uncanny with schedules, and masterful with people.

We did not always agree on everything, but my respect for Jack's abilities made it possible for me to compromise without distress. One such compromise sticks in my mind, when we disagreed on a "judgment call." It seems relatively unimportant now, but I was concerned at the time. Our disagreement arose over Jack's plan to have one of the shiny aluminum covers on a Mariner Mars spacecraft compartment embossed with the seal of the United States. He had a cover made up with the seal so we could see what we were talking about, but since it was fastened by only a few screws, the final decision on whether to use it could be made at the last minute. His view was understandable; we were competing with the Russians in the race to the planets, and Americans could be proud that our "trademark" would be exhibited for current and future generations to see. My concern was that we might be accused of exhibitionism, something distasteful to me, for I was deadly serious about doing the mission for other reasons. The Russians had bragged about landing a pendant on the Moon, and I wanted no part in that disgusting game.

Since the shiny aluminum surface was important for proper thermal control, I questioned whether the embossing, however light, might negatively influence the thermal properties. Jack agreed that tests would be made with the panel in place. Since my greatest concern was that critics would misinterpret this symbol as a lack of seriousness on our part, I further insisted on a low-profile, no-publicity approach for the addition. The panel with the seal was installed, the tests were made, and even after the successful flight there was very little publicity about the seal, and none at all negative.

On our return to Hanger AE after the blockhouse visit, the final 60 minutes of the countdown began. These were the tensest moments of all, as the final checkouts began in earnest. During this period emphasis shifted from what had been predominately launch vehicle activities to include readiness reports from down-range tracking stations, from Deep Space Network stations, from weather stations, spacecraft operations, and all the [38] elements required directly after launch. At this point, anyone caught bluffing would have been a traitor, yet any group not ready to go would have provided a reason to scrub the launch. The readiness reports at T minus 5 minutes were critical; when all of these were "Go" we were fairly certain that we had a commitment to ignite the rockets and accept our fate.

On this day everything came up "Go." Liftoff appeared normal, and the distinctly frightening aspects of this launch did not occur until a few seconds before booster engine cutoff. Suddenly, control of one of the two vernier engines on the Atlas was lost for an undetermined reason, and it moved to its maximum negative mechanical stop. The main booster engines compensated and were able to hold proper roll control until they were cut off and jettisoned. Then the vehicle began to roll, slowly at first and then faster. After about 60 seconds it was turning at a rate of nearly one full turn per second in an uncontrolled, unprogrammed prelude to disaster. About 10 seconds later this aberrant behavior ceased, and the launch vehicle stopped spinning, coming to rest only a degree and a half off its proper roll position. This random restoration so close to nominal has never been plausibly explained, although Wernher von Braun may have been right when he suggested that the success during this part of the mission could only be explained by a substitution of "divine guidance" for the malfunctioning Atlas guidance system.

The launch vehicle wasn't finished with its eccentricities, however Somewhat high at booster engine cutoff, the rocket was pitched about 10° upward, and there was a slight error in azimuth. During the period of uncontrolled roll, the poor crazy Atlas had been able to respond effectively to its guidance commands-something no one would have thought possible. Separation of the Atlas and Agena occurred successfully, although the pitched-up attitude meant that the ejected shroud came perilously close to striking the Agena. In an effort to correct its attitude, Agena pitched down 2° at the start of its first burn, which prevented the horizon sensors from sensing and correcting the error until 15 seconds had passed. Further complicating matters, the excess height of the Atlas had caused the Agena start signal to be sent 8 seconds early. Then, at last, something worked precisely right. The velocity meter aboard the Agena, one of several ways to cut off the engine, sensed achievement of the proper velocity and terminated the first burn, leaving the Agena and its spacecraft in a good 115-mile parking orbit. All who knew what was going on (most of us did not ) were breathless after this series of narrowly averted catastrophes.

[39] The second Agena burn began on time and was later cut off crisply by the same perfectly operating velocity sensor. Gentle springs separated the spacecraft from the Agena, which then reversed its attitude and discharged its remaining propellant, using this residual impulse in the opposite direction to change the course of the stage and eliminate any chance that the stage itself could impact Venus. On its own at last, Mariner 2 began the long voyage to Venus.

The combination of optical and electronic tracking devices gave information suggesting that the Mariner 2 boost phase was generally satisfactory, even though the Atlas had apparently rolled 36 times during its operation. There were some anomalies in the coverage, but information was collected so that engineers could decipher what had happened at a later time. The most critical events of engine ignition and cutoff, and separations of the shroud and spacecraft, appeared to be satisfactory. When reports of all these nearEarth orbit events were complete, there were great sighs of relief from members of the project team. With all the uncertainties of the launch accomplished, it now seemed that we had a chance of success. Perhaps this feeling of relief was connected with the helplessness I felt during the launch phase, when so many strangers were involved and I had no insight as to how well they would do their jobs. Now the launch was over, and responsibility clearly rested with members of the JPL Project Team, with whom I had worked more closely.

Injection into interplanetary trajectory occurred about 26 minutes after liftoff; it was then about s more minutes until the Deep Space Instrumentation Facility (DSIF), using its big dishes, obtained contact with the spacecraft. From this time on, virtually continuous contact was maintained with the spacecraft until the end of the mission over 4 months later.

Critical spacecraft operations now began. Approximately 18 minutes after injection, the solar panels were extended. Full extension occurred within 5 minutes after the CC&S sent its command; this was considered nominal The initial telemetry data indicated that the Sun acquisition sequence was normal and was completed approximately 2 1/2 minutes after command from the CC&S. The high-gain directional antenna was extended to its preset Earth acquisition angle of 72°. The solar power output of 195 watts was slightly above the predicted amount, providing an excess of 43 watts over the spacecraft requirements for this period of flight near Earth. Although temperatures were somewhat higher than expected, they slowly [40] decreased; 6 hours later the temperature over the entire structure had stabilized at about 84° F.

With all subsystems apparently performing normally, with the battery fully charged, and with solar panels providing adequate power, a decision was made on August 29 to turn on cruise science experiments. These had been deliberately left off for about 4 days after launch to ensure that all the atmosphere in the compartments had escaped (this is referred to as outgassing), so that electrical arcing would be unlikely. Leaving the instruments off also allowed the batteries to charge and all existing power to be applied to the engineering information and commands. When finally turned on, the cruise science instruments appeared to be operating normally in all respects. Though this was promising news, only 25 percent of the total components were exercised during cruise, and there was no assurance that the other scientific instruments, those critical ones devoted solely to the planetary encounter, would ultimately function as well.

Five days after launch, temperatures had stabilized within tolerance limits, tracking had been continuously maintained with two-way lock, telemetry data were good, and all subsystems appeared to be operating as intended. For the first time the project team, now regrouping at JPL, began to feel the effects of the year of concentrated effort plus the satisfaction of successfully initiating the mission. Back in Washington I also felt good about the start toward Venus, but after so many bad experiences in the past, I could not let myself relax and enjoy the momentary success. Maybe things would continue to work, but it was a long trip to Venus and many chancy things had to happen properly before we would have a successful mission.

About 3 days later, the Earth-acquisition sequence was initiated by the CC&S. The Earth sensor and the gyros were turned off, cruise science was turned on, and roll search was initiated. At that time the spacecraft was rolling at a rate of about 720° (two revolutions) per hour. Indications were that the directional antenna and Earth sensor were pointed 72° below the Earthspacecraft plane, apparently because of a switch from the omnidirectional antenna to the directional antenna, and telemetry data were lost until Earth lock was reestablished 29 minutes later. At that time, acquisition data indicated an Earth brightness intensity measurement significantly lower than expected and comparable to that which would have resulted if the Earth sensor had been viewing the Moon. There was a possibility that the Moon had been acquired, implying a malfunction in the antenna hinge servo. As a result, execution of the midcourse maneuver sequence was postponed until [41] the following day, when it could be determined that the antenna actuator had performed properly and that the directional antenna was pointing at Earth, even though the signal seemed weak.

Tracking data indicated that the launch vehicle had provided a near nominal orbit, so that there was plenty of capability in the Mariner 2 midcourse motor to perform its correction. The midcourse maneuver was initiated on September 4 and completed early on the morning of September s, when the spacecraft was about 1 1/2 million miles from Earth.

The maneuver sequence required five commands. Two were real-time commands and three were stored. Commands sent directly from the ground ordered the changeover from the high-gain antenna to the omnidirectional antenna so that data could be received during the maneuver when the attitude of the spacecraft was not favorable for pointing the high-gain antenna. In addition, the high-gain antenna had to be moved out of the way so that it would not be affected by the rocket firing. The Earth sensor used for pointing the antenna was turned off so that the entire operation of the high-gain antenna was disabled intentionally by command. The stored commands necessary for orienting the spacecraft and for firing the midcourse motor were determined from trajectory calculations. The commands, sent to the spacecraft for storage in the CC&S until the proper clock time, contained roll and pitch turn durations and polarities, plus the velocity increment to be used for cutoff of the midcourse motor.

The spacecraft performed its maneuvers and provided the general telemetry data. All maneuvers, plus the burning of the motor, appeared to be normal. The entire midcourse correction took approximately 34 minutes. Telemetry data were lost for approximately 11 minutes because the spacecraft moved into an attitude where there was a partial null in the propagation pattern of the omnidirectional antenna. This was simply a feature of the particular orientation required for the midcourse motor burn and not a cause for concern. Initial telemetry data received after the midcourse maneuver indicated that all subsystems were still operating normally. In the Sun reacquisition sequence initiated by the CC&S at the nominal time following the maneuver, the autopilot used during the course correction was turned off and the directional antenna moved to the reacquisition position of 70°. The Earth reacquisition sequence was also initiated by the CC&S at the nominal time following the maneuver and again required approximately 30 minutes with the spacecraft rolling almost one complete revolution before Earth lock was established The transmitter was switched to the high-gain antenna at [42] the start of the sequence, just as in the initial Earth acquisition sequence, causing severe fading and a loss of signal for approximately 6 minutes while the high-gain antenna pointed in other directions than toward Earth. The spacecraft returned to the normal cruise mode of operation with all readings similar to those obtained prior to the maneuver, with the exception of the propulsion subsystem, which had expended itself in accord with its charter.

During the period between Mariner 2's launch and its encounter with Venus, I was extremely busy and glad of it. Notes I kept show that the week following the midcourse correction was filled with activities, many about to become major challenges. Hughes Aircraft officials came to see me to express concern over Surveyor and to urge more direct involvement by Headquarters; they felt the need for three-way meetings between Headquarters, JPL, and themselves, which we finally initiated after real trouble developed. I attended meetings that addressed the Centaur launch vehicle development problems and impacts on Surveyor; others with the Space Science Steering Committee were aimed at intelligently reducing the instrument complement to be carried by Surveyor because of reduced Centaur performance. A power struggle was developing between Goddard Space Flight Center in Greenbelt, Maryland, and JPL over which center would do the planned Mars '64 missions. I was seriously interested in having a healthy competition between two capable groups, but I did not like the way the battle lines were being drawn. Goddard wanted the entire spacecraft to be a spin-stabilized capsule system, and JPL preferred a three-axis stabilized bus and capsule system. I would have preferred a cooperative approach using the talents of both centers, but that was not in the cards. This was the beginning of a long and bitter struggle between the two centers over planetary assignments, with me in the middle.

It was also during that week that I initiated the first serious discussions on how to get a lunar orbiter program going. A Surveyor orbiter concept had bogged down, and I began looking at simpler spinner spacecraft concepts for achieving this desperately needed mission. On top of these concerns over future activities, Homer Newell, Associate Administrator for Space Science and Applications, was urging me to: (1) produce more creative coverage for public information on Mariner's progress, (2) initiate preparations for congressional budget hearings, and (3) draft "white papers" on the rationale for future programs and on management aspects of our Headquarters interfaces with the field centers.

[43] The first malfunction onboard Mariner occurred in the midcourse propulsion system following the completion of the correction maneuver. After the spacecraft motor had been commanded to shut off, the pressure reading in its propellant tank continued to rise. It was presumed that the normally open nitrogen shutoff valve did not close fully as the motor was shut off, letting nitrogen gas leak slowly into the propellant tank. A quick calculation showed that the equilibrium pressure, when reached, would be well below the burst pressure of the propellant tank and associated components. Accordingly, no further complications were expected or observed, since the high-pressure nitrogen was simply leaking into another tank and not escaping from the spacecraft. The leak had no effect on the attitude control or any other function of the spacecraft, and since no further rocket firing was planned, no corrective action was taken.

Post-midcourse trajectory computations indicated that Mariner 2 would miss Venus by approximately 25 000 miles and that the flight time for the entire trip would be about 109 1/2 days. A comparison of the desired and achieved encounter parameters indicated that the midcourse maneuver was accomplished with near-nominal performance. There were a number of possible explanations for a slightly out-of-tolerance correction, but telemetry data could provide no clear clues that would isolate the cause. While we would have preferred being closer-more like the planned 18 000 miles-it was believed that 25 000 miles was well within predetermined values for instrument design. As the spacecraft approached the planet and more tracking data were available, trajectory predictions showed that the actual miss distance would be 21 645 miles.

About 3 days after the midcourse maneuver, telemetry information showed that the autopilot gyros had automatically turned on and that the cruise science experiments had automatically turned off, possibly because of an Earth sensor malfunction or an impact with an unidentified object which temporarily caused the spacecraft to lose Sun lock. All attitude sensors were back to normal before the telemetry measurements could be sampled to determine whether an axis had lost lock. A similar occurrence was experienced 3 weeks later when the gyros were again turned on automatically and the cruise science experiments were automatically turned off. Here again, all sensors were back to normal before it could be determined which axes had lost lock. By this date, the Earth sensor brightness indication had become essentially zero. The significant difference between the two events was that [44] in the second case, telemetry data indicated that the Earth brightness measurement had increased to the nominal value for that location along the trajectory. This problem remained a mystery and was somewhat worrisome because of the possible implication that the attitude control system sensors were marginally effective in maintaining the desired attitude references.

On October 31, there was an indication that Mariner's power production had decreased, a malfunction later diagnosed as a partial short circuit in a solar panel. As a precaution against the possibility of the spacecraft rapidly sapping its battery stores, a real-time command was transmitted from the Goldstone station in California, turning off the cruise science experiments and thereby reducing power consumption. The lower power condition existed for some 8 days, when suddenly the telemetry data again indicated that the panel was operating normally. After this was confirmed, another command was transmitted from Goldstone to reactivate the cruise science experiments. The science telemetry data remained essentially the same as before the experiments had been turned off; however, engineering telemetry data indicated that most temperatures had increased shortly after the science experiments were reactivated, probably due to the increased power requirements. A recurrence of the panel short was experienced on November 15; by this time, however, the spacecraft had proceeded nearer the Sun and the power supplied by the one operative panel was enough to meet the spacecraft's needs; thus, the cruise science experiments were permitted to remain active. Along with this anomaly, the magnetometer experienced a high offset, probably caused by a current redistribution when the power failure occurred. This made readings more difficult to interpret, but the recorded data indicated reasonably steady magnetic fields.

The radiometer calibration performed during the cruise phase indicated that the instrument would malfunction when activated for the flyby of Venus. It was considered possible that when the cruise science mode was changed to the encounter sequence the radiometer would remain in a permanent slow-scan mode, and no high-speed scan or automatic scan reversal would occur. In addition, the telemetry data indicated that only one of the two microwave radiometer channels would have the desired sensitivity. It turned out, however, that both the microwave radiometer and the infrared radiometer channels had acceptable sensitivities at encounter and that one scan rate change occurred, allowing three successful scans of the planet.

[45] Another worrisome problem in the scientific instrumentation was soon detected On November 27, the calibration data for the cosmic dust experiment indicated that either the instrument sensitivity or the amplitude of the calibration pulse had decreased by 10 percent. By December 14, a further decrease by a factor of 10 had occurred. These figures suggested that the instrument's operation would be severely impaired at the time of flyby.

Mariner's technical troubles were not limited to malfunctions onboard the spacecraft. On one occasion, a commercial power failure at one of the tracking sites caused the loss of 1 1/2 hours of data. In mid-November, an occasional out-of-sync condition in the telemetry data was determined to be the fault of a telemetry demodulator at the tracking stations and not of the spacecraft's instrumentation. No real-time telemetry was transmitted from Goldstone and Johannesburg during the November 26 view period. The information was not lost, however, since all data were recorded on magnetic tape at each station and could later be sent to the Space Flight Operations Facility for full processing.

Except for problems of this nature, the DSIF stations covered the Mariner 2 operations continuously and successfully. In taking two-way Doppler data for orbit determination, one Goldstone antenna transmitted to the spacecraft and the other received signals from the spacecraft. On one occasion, the spacecraft antenna reference hinge angle changed slightly, an event which should have occurred only at cyclic update times. This phenomenon had appeared several times during preflight system tests and was not considered serious. With the exception of this anomaly and the Earth sensor anomalies noted earlier, the attitude control system performed without fault through the mission.

In mid-November, spacecraft temperatures became a cause for concern as they began to exceed predicted values. On November 16, the temperature of the lower thermal shield reached its telemetry limit and pegged-this corresponded roughly to 95°F. Seven of the eighteen temperature measurements were pegged for the encounter phase, and the actual temperatures had to be estimated by extrapolation. It seems that spacecraft, like people, suffer at times from extremely high temperatures; there was considerable concern that electronic components would be adversely affected by this condition. On December 9, a failure in the data encoder circuitry disabled four telemetry measurements: antenna hinge angle, propellant tank [46] pressure, midcourse motor pressure, and attitude control nitrogen pressure Though the loss of these particular measurements did not affect the outcome of the mission, failures of this sort were deeply troubling to the project team. Not knowing exactly what caused these malfunctions, we worried that more critical systems might suddenly (and inexplicably) begin to deteriorate.

The CC&S was designed to perform various functions, one of which was to provide the attitude control subsystem with a timing or cyclic update to change the Earth-pointing antenna reference hinge angle. Each cyclic update pulse was indicated by telemetry. Until December 12, the pulses occurred with predictable regularity. On that day, however, only 2 days before the encounter phase, the CC&S failed to issue the 155th or subsequent cyclic pulse. As a result of this malfunction, the spacecraft was switched on December 14 to the encounter mode of operation by a prearranged backup command transmitted from Goldstone. Just prior to this transmission, seven spacecraft temperature sensors had reached their upper limits. The Earth sensor brightness data number had dropped, and approximately 149 watts of power were being consumed by the spacecraft. About this much power was available from the one good solar panel, and a small excess of about 16 watts was actually being dissipated. All science experiments were operating, and coverage by the DSIF remained continuous and appeared normal. Signals were clear, and data quality was good. Of course, there was considerable concern over the fact that several minor failures had occurred in telemetry measurements, the failure of the CC&S update of the antenna, and the associated possibility that the scientific scan platform might not operate as designed. With the spacecraft running a high fever, the preencounter hours were extremely tense.

We will probably never know for certain what went wrong inside the CC&S, although higher than expected temperatures surely played a part. It was suspected that a single component was the culprit. Within the region where the failure was isolated there were 160 resistors, 51 transistors, 50 cores, 40 diodes, 25 glass capacitors, and 4 tantalum capacitors. Any single one of these could have been the cause.

The operation of all science experiments during encounter was essentially as planned, except for the sensitivity decrease in the cosmic dust experiment. The encounter mode lasted approximately 7 hours, being terminated by a ground command from Goldstone at 20:40:00 GMT on December 14, 1962. Engineering telemetry data transmitted after the encounter phase indicated that all systems appeared to be performing essentially as before. However, [47] temperatures continued to rise and were not expected to decrease as the spacecraft was approaching the Sun, scheduled to arrive at its perihelion or closest approach on December 28.

As a result of the CC&S malfunction for orienting the Earth-pointing antenna, the antenna reference hinge angle had not been updated since December 12. Since the change of angle was slight during the period of encounter, no correction had been necessary. Afterwards, however, it became apparent that some adjustment was needed to ensure continued communications between the spacecraft and Earth. Two series of commands were transmitted from Goldstone, on December 15 and December 20, updating the reference hinge angle. Five of these commands were accepted and acknowledged by the spacecraft, and an effective reference angle change of 8° occurred as desired.

On December 17, after an extremely busy 3-month period, the continuous coverage of the DSIF was reduced to approximately 10 hours per day to provide relief to overworked personnel. As expected, peribelion occurred on December 28. On this date an attempt was again made to command the reference hinge angle to change, but Goldstone was unable to lock up the command loop, indicating that command thresholds had been passed. On December 30, a reference frequency circuit failure in the CC&S countdown chain resulted in a temporary loss in telemetry; however, radio frequency lock, that is, the closed-loop coupling of the spacecraft transmitter and the ground station receiver, was maintained. When the telemetry signal was again acquired 1 1/2 hours later, the telemetry bit rate had dropped from the nominal 8.33 bits per second to approximately 7.59 bits per second. Simultaneously, internal temperature readings increased due to the inefficiencies of the power system at lower frequencies.

The spacecraft was tracked for the last time at 07:00:00 GMT on January 3, 1963, by the Johannesburg station. During this pass, about 30 minutes of real-time telemetry data were received. Although the demodulator went out of lock and remained out during the later part of the tracking period, good tracking occurred for most of the interval. Examination of the recorded data showed that the spacecraft was still performing normally, with a power consumption of 151 watts and available power of 163 watts from the single operating solar panel.

In the final review of the orbits, the spacecraft was last heard from when it was 53.8 million miles from Earth and had passed Venus by about 5.6 million miles. It was traveling at 13.7 miles per second with respect to Earth [48] and disappeared at this time, never to be heard from again. Further searches for the spacecraft at later periods were unsuccessful. On January 8, 1963, the Goldstone antenna was positioned according to the projected trajectory data, and a frequency search was conducted during the calculated view period, with negative results. A similar attempt in August 1963 was also unsuccessful.

Thus ended the saga of Mariner 2-a robot, designed and directed by men, given a mission to extend the search for knowledge beyond the limited reach of Homo sapiens. Though it accomplished a voyage that was clearly "superhuman," Mariner was a simple exploring machine, with only a very modest capability to perform on its own. A total of 11 real-time commands and a spare were possible, along with a stored set of 3 onboard commands which could be modified. Other functions, such as updating antenna position and adjusting thermal control louvers, were provided, but in every sense it was a simple robot with the capability for only a small amount of human interaction.

From the meager information returned by telemetry, we know that Mariner 2 endured significant stress, but how many meteorite impacts it received and why it developed an ultimately fatal fever will forever remain a mystery. Perhaps in its passage from Earth to Venus and its transfer from orbit to orbit, it had other experiences which we will better understand when man repeats the voyage in person, with his own sensors and the additional capabilities that will exist at the time.

However modestly equipped to observe the environment and features of Venus, Mariner 2 did provide to those on Earth a firsthand, close-up impression of Earth's nearest neighbor-a brilliant object long revered as the star of the morning and evening. Indelibly imprinted in my memory is the beautiful sound of the data stream returning from the encounter science experiments during flyby. The radio telemetry signals were transmitted at L-band frequencies of about 940 megahertz and reproduced as whole tones well within the audible range. These tones were broadcast throughout the operations facility and relayed to NASA Headquarters for all to hear during an encounter press briefing. The pure tones at the low bit rate of 8 1/3 bits per second produced heavenly angelic sounds, truly music of the spheres. Words could not describe my feelings as the successful return of data from Venus at last provided evidence of a successful mission.

During the time the Mariner 2 spacecraft was on its way, a fifth Ranger mission had failed and the entire Ranger program had been interrupted for [49] review and possible cancellation. Thus the Mariner 2 flyby was a victory of far greater significance than its single-purpose objectives, for it gave some confidence to the process and to the team efforts involved in developing the capability for such exploration. Because of the "Review Board" environment and the large amount of work that resulted from the Ranger failure, there was little time to revel in the Mariner success; however, I remember that Christmas of 1962 as one of the better ones during my early years with NASA.

Jack James' penchant for patriotic display came to light as Mariner 2 was well on its way to Venus, when he disclosed that he had personally placed a small American flag between some layers of thermal material on top of the spacecraft. Had I known about this when it occurred, I would have reacted as I did when Jack later had a seal added to the Mariner 4 compartment cover.

Some day future Americans may recover Mariner 2 and rejoice in exposing its national symbol, proving that Jack was right in doing what I considered to be sensitive at the time. As things turned out, I am proud that our flag and great seal are out there in orbit about the Sun along with the planets.

Looking back on the entire experience, my warmest feeling comes from the association with the crew that produced the Mariner mission. Starting with the handful of us involved in directing the program at NASA Headquarters, the project team numbered about 250 at JPL, spreading to 34 subcontractors and over 1000 suppliers of parts for the Mariner systems. Altogether the project involved an estimated 2360 man-years of effort and cost a total of $47 million. At the time, so much effort and so many dollars expended in a year seemed large. Today, we have learned that such a price is relatively small for the results returned. Not only did the thousands of people who participated in this "once-in-history experience" gain from it, but Americans and all mankind received a boost in spirit from the adventure.

The first successful Mariner mission will surely become legend, remembered as a triumph for creative man. As someone aptly put it, "There will be other missions to Venus, but there will never be another first mission to Venus."