March 1981:Voyager launch and cruise

Early in the launch sequence, while the Titan stage was still firing, what appeared to be a fault was detected in the Attitude and Articulation Control System (AACS) of the spacecraft. The AACS has three gyroscopes, mounted at right angles to each other, which are used to monitor the attitude of the spacecraft and any two are adequate for normal operation. During the launch phase gyros B (roll and pitch) and C (pitch and yaw) were active and the onboard computer switched from gyro B to gyro A (pitch and yaw) when a failure was indicated. This was followed by a switch to gyros A and B when the apparent fault continued, suggesting that the problem lay with the pitch and yaw gyro, gyro C. As part of its in-built fail-safe routines the spacecraft switched over to its alternative AACS processor forcing ground controllers to check the contents of the AACS memory and determine that the commands for acquisition of the star Canopus were intact. Canopus acquisition was planned for the following day but the spacecraft was instructed to locate the Sun only, allowing it to be stabilised in two axes and this was achieved at 4.00.30 EDT. The search had taken three and a half hours, considerably longer than the planned five minutes.

Concern was also felt that the science boom had failed to deploy since a microswitch, which opens as the boom reaches within 0.05 degrees of normal deployment, appeared to have remained closed. Twelve hours after launch one of the boom instruments, the plasma science instrument,was activated and measurements relative to the direction of the solar wind and a known axis indicated that the boom was within two degrees of the correct position. Most of the remaining instruments were turned on in the next few hours and all started to return measurements indicating that they were in good condition. The remainder of the antennae and booms deployed normally.

Although most of the problems encountered during the launch now appeared to be under control there was an unexplained pitch and yaw disturbance 18 hours into the flight and mission controllers immediately set about investigating this event. There was a similar occurrence a few days later at 11.25 PDT on 25 August. The possibility that these anomalies were due to a collision between the spacecraft and the discarded propulsion module was considered and ruled out when it was confirmed that it was no longer in the vicinity of Voyager 2.

On 24 August the attitude control system acquired the star Canopus after a series of scans about the roll axis thus completing the three-axis celestial stabilisation. As Voyager 2 receded from Earth a series of photographs were taken to determine precisely the orientation of the science boom and from these it was calculated that the boom was deployed to within 0.06 degrees of the locked position. Two days later an attempt to fully deploy the boom was aborted by an error within the AACS and the spacecraft automatically returned to Canopus lock. It had been hoped that by simultaneously pitching the spacecraft and jettisoning the dust cover for the infrared interferometer-spectrometer (IRIS) with its small explosive charges, enough of a jolt would be provided to fully open the boom hinge and allow the locking pin to fall into place. The cover was jettisoned successfully a little later in the mission.

The first trajectory correction and the X-band radio transmitter calibrations were deferred because of the more immediate needs of the spacecraft and the forthcoming launch of its sister ship, but by 2 September all of the scientific instruments had been switched on. Later that day Voyager 2 was "put to bed" in its interplanetary cruise mode to allow flight controllers to concentrate their efforts on the launch of the second spacecraft, planned for 5 September.

As soon as the problem with the science boom of Voyager 2 was detected a decision was made to de-encapsulate VGR77-2 for inspection of the boom deployment mechanism and microswitch operation. This was carried out in the Spacecraft Assembly and Encapsulation Facility No 1 at Cape Canaveral. To ensure correct deployment, engineers installed five coiled springs on the boom and the spacecraft was re-encapsulated on 29 August.

After mating with the Centaur upper stage on 31 August,VGR77-2 was successfully launched at 8.56.01 a.m. EDT from complex 41 of the Air Force Eastern Test Range. Cape Canaveral, after a perfect countdown. Although the Titan second stage shut down a little early a longer burn by the Centaur stage corrected the anomaly and Voyager 1 was placed on course by a second planned Centaur burn and a brief boost from the spacecraft propulsion module. Voyager 1 did not experience any of the problems encountered by its twin and all of the booms and antennae deployed exactly as planned.

During the first 14 days of its journey Voyager 1 activated all of its scientific instruments and carried out a series of calibrations of the onboard communication systems. These were followed by two trajectory correction bums which although carried out without incident did not produce the required velocity increment. The difference of approximately 2.5 meters per sec was made up during the next manoeuvre. On completion of these two burns Voyager 1 was also commanded into its interplanetary cruise mode.

Interplanetary Cruise

As the two Voyagers pursued their 18 month journey to Jupiter mission operations settled into a routine punctuated by minor difficulties and an occasional crisis. Many of these events are chronicled below, with occasional details of distance from Earth,heliocentric velocity and one-way communication time.

On 23 September 1977 Voyager 2 suffered a failure of its Flight Data Subsystem (FDS) circuitry which caused the loss of 15 of the 243 engineering measurements, but this did not prevent a successful course correction being carried out a few weeks later on 11 October. Voyager 1 also made a small correction in October, on the 29th. which removed the residual error from the first burn.

On 31 October controllers instructed Voyager 2 to rotate and acquire the star Deneb as a celestial reference point. Deneb lies on the opposite side of the sky from the usual reference star Canopus and this manoeuvre effectively placed the spacecraft "upside down". The reason for this unusual step was an attempt to reduce the effect of the solar wind which had been continually altering the attitude of the spacecraft and causing an increase in the rate of propellant used as the AACS strove to correct for the drift. Studies by the Voyager engineering team indicated that this fuel depletion would not prejudice the mission and that a 9 kg margin existed even for the possible Uranus option.

Apart from minor problems including a stuck filter wheel on the photo polarimeter Voyager 1 continued in good health and on 15 December it overtook the slower Voyager 2 spacecraft. It would continue to pull ahead and its lead would have increased to four months at the time of Jupiter encounter. On 5 January 1978 Voyager 1 was 177 million kilometers from Earth travelling at 27 km per sec; the one-way communication time was nine minutes and 49 seconds. The figures for Voyager 2 were 174 million km. 26 km per sec and 9 minutes 40 seconds respectively.

Throughout January and February the two spacecraft continued to take scientific measurements during the cruise phase including regular 360 degree scans of the whole sky and occasional camera calibrations. Work was continued to determine the reason for a failure of one such scan which terminated early on 17 February. A problem with the gyros of Voyager 1 was believed to be the cause. On 21 February Voyager 1 was 322 million km from Earth and had slowed to 23 km per sec. Voyager 2 was 9 million km nearer to home,traveling at 22 km per sec, and the one-way communication time had increased to over 17 minutes. It was during a camera calibration scan on 21 February that the scan platform on Voyager 1 ground to a halt. It took three weeks to free the platform fully and on 23 March the scan platform was positioned in such a way that should it fail again it would be in the most favourable position for the encounter with Jupiter.

In April a series of communications problems developed with Voyager 2 which seriously endangered the mission. At the beginning of the month the main radio receiver apparently failed and after a period of seven days had passed without any commands being received the back up receiver was selected by the on-board computer. Attempts to contact the spacecraft on the backup receiver failed, probably due to a failed tracking loop capacitor. After twelve hours the spacecraft, following its protection routine, switched back to the main receiver. This switch over appeared to reactivate the receiver and for 30 minutes several commands were accepted by the spacecraft,reseting the seven-day timer in the main radio system. Then the main receiver suffered an excessive current surge from an unknown source and the receiver fuses blew. This left the spacecraft still switched to the main radio and out of contact with Earth for a further period of seven days. An additional problem was that due to the failed tracking loop in the reserve receiver the spacecraft would be unable to follow the changes in frequency in up-linked messages caused by the Earth’s rotation. On 13 April the back-up receiver switched back into service and contact with Voyager 2 was re-established. In an impressive feat of improvisation engineers had determined the frequency on which the spacecraft was listening and computed the changing frequency on which to transmit commands. Various steps were then taken to prevent changes in the receiver's temperature which could alter its frequency characteristics and to boost the transmitters on the spacecraft to full power in an attempt to ease the tracking requirements.

After a critical phase hopes rose at mission control that all the objectives of the flight could still be achieved. On 26 April the onboard computer of Voyager 2 was successfully updated using new techniques which had been developed to communicate with the spacecraft through its remaining,crippled receiver. The transmitted signal was being programmed to match the receiver frequency to within 50 Hz; it was indeed fortunate that the 64 metre antennae of the Deep Space Network already had this capability.

As mission operations again settled into a routine the engineers at Pasadena continued to refine their control techniques and made some minor changes to the AACS. One of these was to modify the computer software to allow the spacecraft to be turned at a very slow rate without the automatic drift compensation attempting to stop the motion. A second was to compensate for attitude changes or changes in rate caused by the movement of the digital tape recorder. The changes, although small, were enough to cause smearing in the imaging and the AACS was instructed to sense when the recorder started, stopped or changed direction and to pulse the control jets to offset the torque.

During September of 1978 tests were conducted to determine if the radio astronomy experiment of Voyager 2 could be used "in reverse" to receive ground commands should the remaining receiver fail.The radio telescope at Stanford University in California transmitted signals on a frequency of 46.72 MHz and it was concluded that the spacecraft could indeed receive this frequency and that the signal-to-noise ratio was acceptable. Since major changes in the spacecraft’s onboard computer programs would be required if such a technique was invoked further studies of this possibility were put under way.

On October 25 Voyager 1 was about 675 million km from Earth and travelling with a heliocentric velocity of 14.9 km-sec. One way radio communication time was now 37 minutes and 36 seconds, but of more concern to mission controllers was that the spacecraft had safely traversed the asteroid belt and encounter operations with Jupiter were only 78 days away. For this reason a series of tests to ensure that both spacecraft and ground systems were ready for encounter were in progress and proceeding satisfactorily. Sister ship Voyager 2 was just leaving the asteroid belt 637 million km from Earth and still 6 months from the beginning of its encounter operations.

The final few weeks of Voyager 1’s cruise to Jupiter was a busy period for the engineers at Pasadena with a series of crucial calibrations and tests to review the project's readiness for the Jupiter encounter. On 10 and 11 December a series of photographs covering two rotations of the giant planet were recorded so that identification of features of special interest could be made. Also on the 11th the Sun sensors, high gain antenna and scan platform pointing and imaging optics were calibrated. This was followed by a 39 hour Near Encounter test, essentially a dry run of the operations planned to occur around the time of closest approach; then as Christmas drew near a two week period of relative quiet followed to allow personnel a chance to rest before Voyager 1 entered its observatory phase on 4 January of the new year.

Voyager 1 Observatory Phase

On 4 January 1979 Voyager 1 was sixty million kilometres and sixty days away from Jupiter and detailed long range observations of Jupiter were just beginning. The observatory phase of the mission was designed to provide a time history of scientifically important phenomena on the giant planet. Most of the material recorded during this phase was of a repetitive nature to provide a data base for all the ensuing data.

After a series of calibrations made over two days, the long range imaging experiments began. Every two hours, a period representing one fifth of a rotation, a series of four narrow-angle photographs were taken, each in a different colour. The series of pictures were part of a study of the large scale dynamic properties of the turbulent atmosphere and from them JPL scientists were able to produce a motion picture covering many revolutions of Jupiter. Individual images were then examined to identify features of particular interest to allow them to be targeted and examined in more detail during the closest approach. Meanwhile the remaining optical instruments were far from quiet. The ultraviolet spectrometer observed the Jovian system eight times a day,mapping the distribution of UV emissions,and about one hundred infrared spectra were recorded every day. The photopolarimeter was also in use as it scanned for the edge of Io’s sodium cloud. Other instruments probed the electromagnetic and particular environment searching for the interaction between the solar wind and the planet's magnetosphere and listening out for bursts of radio emissions. As the spacecraft closed in with its target the detail evident in the pictures began to increase and by the end of January the circulation patterns, especially around the Great Red Spot,were beginning to appear.

Daily system scans, infrared mapping and ultaviolet searches continued as the observatory phase drew to a close during the beginning of February. For a four day period beginning on 30 January a 100 hour intensive imaging experiment took place with photographs being recorded every 96 seconds and transmitted to Earth in realtime via the high data rate (115,200 bits per second) X-band. The use of the X-band transmitter required continuous coverage from the three 64 metre antennae of the DSN and so contact with Voyager 2 and other NASA deep space probes was maintained using other smaller antennae. The termination of this period of intensive imaging on 3 February effectively marked the end of the inbound observatory phase of Voyager 1 since at its end the spacecraft was so near to its target that the planet filled most of the image from the narrow angle camera and mosaics would soon be necessary to ensure full coverage of the planet.