Voyager 2 was the first of the spacecraft to be launched, on August 20, 1977, propelled into space in a Titan/Centaur rocket. [P-19450AC]
The First Year Is the Roughest
During the autumn of 1977 Voyager 2, and to a lesser extent Voyager 1, continued to plague controllers with erratic actions. Thrusters fired at inappropriate times, data modes shifted, instrument filter and analyzer wheels became stuck, and the various computer control systems occasionally overrode ground commands. Apparently, the spacecraft hardware was working properly, but the computers on board displayed certain traits that seemed almost humanly perverse—and perhaps a little psychotic. In general, these reactions were the result of programming too much sensitivity into the spacecraft systems, resulting in panic over-reaction by the onboard computers to minor fluctuations in the environment. Ultimately, part of the programming had to be rewritten on Earth and then transmitted to the Voyagers, to calm them down so that they would ignore minor perturbations, yet still be ready to perform automatic sequences required to protect the spacecraft from major threats. Meanwhile, however, more serious problems were developing.
On February 23, 1978, during a series of movements or slews, Voyager 1’s scan platform slowed and stopped before completing the maneuver. This failure caused a great deal of concern, since the scan platform houses the optical instruments that are crucial to the observation of the Jovian system—the ultraviolet spectrometer, the IRIS, the photopolarimeter, and the two TV cameras. At JPL, tests were run on a proof-test model—an exact copy of the Voyager spacecraft—to try to find out why Voyager 1’s scan platform had become stuck. On March 17, Voyager 1’s scan platform was tested—JPL engineers instructed the platform to move slowly for a short distance, and Voyager responded as ordered. Further tests were conducted on March 23. This time the scan platform was ordered to execute a sequence of four slews, moving away from the part of the sky where the original failure had occurred and ending with the position that it would be most useful to leave the platform in—just in case the platform should become stuck again. On April 4 the scan platform was commanded to perform a sequence of 38 slews, and fifty more slews were performed on April 5. All were successful. Yet engineers were still hesitant to force the platform to move through the region where it had originally stuck, and extensive discussions were held to determine if the Jupiter observations could be carried out without risking a return to the danger area. It was argued, however, that full mobility of the scan platform really was required, and on May 31 commands were sent to maneuver the scan platform through the danger region. It moved normally: The scan platform was operating properly again. After additional slewing tests were run in mid-June, the scan platform was pronounced fit for operation. Engineers suspected that the material caught in the platform gears must have been crushed or moved out of the way by the continued slewing, allowing the platform to move once more.
Voyager 1 was launched on September 5, 1977. The launch was delayed 5 days to make last-minute adjustments to avoid the postlaunch difficulties experienced by Voyager 2. [P-19480AC]
An even more serious crisis soon endangered the Voyager 2 spacecraft. In late November 1977, the S-band radio receiver began losing amplifier power in its high-gain mode, so the solid-state amplifier was switched to its low-power position. No further problems were noted until April 5, 1978, when Voyager 2’s primary radio receiver suddenly failed, and shocked engineers discovered that the backup receiver was also faulty. The trouble was detected after Voyager’s computer command subsystem directed the spacecraft to switch from the primary radio receiver to the backup receiver. This command was issued as part of a special protection sequence: If the primary radio receiver receives no commands from Earth for seven days, the backup receiver is switched on instead; if the secondary receiver in turn receives no instructions over a twelve-hour period, the system reverts to the main receiver. When, on April 5, Voyager 2’s radio reception was switched from the primary to the secondary receiver, flight engineers found that they were unable to communicate with the spacecraft—the secondary receiver’s tracking loop capacitor was malfunctioning. That meant that the secondary receiver could not follow a changing signal frequency sent out from Earth. The frequencies of signals transmitted from Earth are affected by the Doppler effect—just as the siren on a fire engine seems first to rise in pitch as the truck approaches, then falls as the truck speeds away, so the frequency of signals transmitted from Earth fluctuates with the Earth’s rotation as the Deep Space Network’s radio antennas move toward or away from the spacecraft. The engineers had to wait until the primary radio receiver was switched back on before they could communicate with the spacecraft. Once the primary receiver was on, Voyager 2 began receiving instructions from Earth, but approximately thirty minutes later, there was an apparent power surge in the receiver. The fuses blew. There was no recourse. The main receiver had failed; its loss was permanent. It remained for the engineers to devise a way to communicate with the slightly deaf spacecraft.
Each Voyager spacecraft follows a billion-kilometer path to Jupiter. Except for minor thruster firings to achieve small trajectory corrections, each Voyager coasts from Earth to Jupiter, guided by the gravitational pull of the Sun. At Jupiter, the powerful tug of the giant planet deflects the spacecraft and speeds them up, imparting an extra kick to send them on their way toward Saturn.
Voyager 1 Voyager 2 Jupiter-Saturn-Uranus Sun Earth 8/20/77 Earth 9/1/77 Mars 8/20/77 Jupiter 8/20/77 Jupiter 3/5/79 Jupiter 7/9/79 Saturn 8/20/77 Saturn 11/13/80 Saturn 8/27/81 Uranus 8/20/77 Uranus 1/30/86