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
Because the switching of the radio receivers was still controlled by the special protection sequence discussed earlier, flight engineers would have to wait for seven days—until April 13—before they could attempt communication with the spacecraft again. During that week special procedures were established and rehearsed so that commands could be sent to Voyager in the short time that the backup receiver would be on. On Thursday, April 13, 1978, the seven days were up and the spacecraft should have shifted from the dead main receiver to the sick backup system. There was just a twelve-hour “window” in which to restore communication. At about 3:30 a.m. PST the Madrid tracking station of the Deep Space Network sent its first order to the spacecraft, approximately 474 million kilometers away. Almost an hour later, word arrived from Voyager that the command had been accepted. (One-way light time for a signal to travel the distance from Earth to Voyager at that time was almost 27 minutes.) Elated flight controllers went ahead and transmitted nine hours of commands to the spacecraft.
Voyager 2 was successfully commanded again on April 18 and April 26. The April 26 commands included a course change maneuver that was executed properly on May 3. On June 23, Voyager 2 was programmed for a backup automatic mission at Saturn in the event that the secondary radio receiver should also fail. These backup mission instructions would operate all the science experiments, but only a minimum amount of data would be returned, since the scan platform would only be programmed to move through three positions rather than thousands as it would in normal operation. Instructions for a backup minimum automatic encounter at Jupiter were transmitted to Voyager 2 in two segments, the second of these on October 12, 1978.
With the backup instructions recorded on board the spacecraft, Voyager personnel felt their fears partially allayed. If Voyager 2’s secondary radio receiver failed, the spacecraft would still obtain some science data at Jupiter and Saturn. But that would mean that there would be no mission beyond Saturn; our first opportunity to explore Uranus, its satellites, its newly discovered ring system, and possibly even to get a look at Neptune, would not come in this century.
Another major concern affecting both Voyager spacecraft was the proper management of hydrazine fuel reserves. Hydrazine is used by the thrusters on the Voyagers for stabilization of the spacecraft and for trajectory correction maneuvers (TCM). Each Voyager was loaded with 105 kilograms of hydrazine budgeted for use on the long flight to Jupiter, Saturn, and beyond. Because of the excellent performance of the launch rockets, both Voyagers required less hydrazine than anticipated for their final boost into proper trajectory toward Jupiter, and at first it looked as though both spacecraft would have plenty of propellant to spare.
Charles E. Kolhase, Manager of Mission Analysis and Engineering for the Voyager Project, later explained the situation: “Voyager 1 should have been launched September 1. Had it been launched on September 1—and I’m glad it wasn’t—the maneuver to correct the trajectory for a Titan flyby would have required a change in velocity of 100-110 meters per second—an enormous maneuver—and we would have had a propellant margin for going on to Saturn of perhaps 4.5 kilograms. But, by launching on the fifth of September we increased our margin to 23 kilograms. Fortunately, for every launch date that went by, that velocity change maneuver was shrinking at a rate of 10 meters per second per day. Now, a 1 meter per second change uses about a pound of hydrazine [about 0.5 kilogram]. So when we launched on the fifth of September, now we suddenly had 40 pounds of hydrazine excess over what we would have had if we had launched on the first of September. As a result, Voyager 1 is in great shape as far as hydrazine is concerned.”
THE DEEP SPACE NETWORK
A vital component of the Voyager Mission is the communications system linking the spacecraft with controllers and scientists on Earth. The ability to communicate with spacecraft over the vast distances to the outer planets, and particularly to return the enormous amounts of data collected by sophisticated cameras and spectrometers, depends in large part on the transmitters and receivers of the Deep Space Network (DSN), operated for NASA by JPL.
The original network of these receiving stations was established in 1958 to provide round-the-world tracking of the first U.S. satellite, Explorer 1. By the late 1970s, the DSN had evolved into a system of large antennas, low-noise receivers, and high-power transmitters at sites strategically located on three continents. From these sites the data are forwarded (often using terrestrial communications satellites) to the mission operations center at JPL.
The three DSN stations are located in the Mohave desert at Goldstone, California; near Madrid, Spain; and near Canberra, Australia. Each location is equipped with two 26-meter steerable antennas and a single giant steerable dish 64 meters in diameter, with approximately the collecting area of a football field. In addition, each is equipped with transmitting, receiving, and data handling equipment. The transmitters in Spain and Australia have 100-kilowatt power, while the 64-meter antenna at Goldstone has a 400-kilowatt transmitter. Most commands to Voyager are sent from Goldstone, but all three stations require the highest quality receivers to permit continuous recording of the data streams pouring in from the spacecraft.
Since the mid-1960s, the DSN’s standard frequency has been S-band (2295 megahertz). Voyager introduces a new, higher frequency telemetry link at X-band (8418 megahertz). The X-band signal can carry more information than S-band with similar power transmitters, but it requires more exact antenna performance. In addition, the X-band signal is absorbed by terrestrial clouds and, especially, rain. Fortunately, all three DSN stations are in dry climates, but during encounters the weather forecasts on Earth become items of crucial concern if precious data are not to be lost by storm interference.
As a result of the development of larger antennas and improved electronics, the DSN command capabilities and telemetry data rates have increased dramatically over the years. For example, in 1965 Mariner 4 transmitted from Mars at a rate of only 8⅓ bits of information per second. In 1969, Mariners 6 and 7 transmitted picture data from Mars at 16 200 bits per second. Mariner 10, in 1973, achieved 117 200 bits per second from Mercury. Voyager operates at a similar rate from Jupiter, about six times farther away. Many of these improvements in data transmission result from changes in the DSN rather than in the spacecraft transmitters.
Problems with hydrazine management developed, however. Voyager 1’s first trajectory correction maneuver achieved only 80 percent of the required speed change. Exhaust plumes from the thrusters apparently struck part of the spacecraft, causing a 20 percent loss in velocity. That being the case, Voyager might require more fuel than had been expected to complete the mission. The extra fuel requirements did not threaten Voyager 1 itself, since it held ample fuel to reach Saturn; the concern was for Voyager 2, where the effective loss of fuel might be enough to jeopardize the Uranus mission.
Because of the plume impingement problem on Voyager 1, Voyager 2’s first trajectory correction maneuver was adjusted to allow for the possibility of a 20 percent loss in thrust. The Voyager 2 maneuver was successful, but controllers felt that additional action was required to conserve fuel. One way to save was by reducing requirements on control of the spacecraft orientation. Less control fuel would be needed if the already miniscule pressure exerted on the spacecraft by the solar wind could be reduced. Flight engineers at JPL calculated that the pressure would be reduced if the spacecraft were tipped upside down; however, to accomplish this, the spacecraft would have to be steered by a new set of guide stars. By reprogramming the attitude control system it was found possible to substitute the northern star, Deneb, in the constellation of Cygnus, for the original reference star, Canopus, in the southern constellation of Carina. With this change, as well as readjustment of Voyager 2’s trajectory near Jupiter, inflight consumption of hydrazine was reduced significantly.
In late August 1978 both Voyagers were reprogrammed to ensure better science results at Jupiter encounter; for example, the reprogramming would prevent imaging (TV) photographs from blurring when the tape recorder was operating. By early November, flight crews had begun training exercises to rehearse for the Voyager 1 flyby of Jupiter on March 5, 1979. A near encounter test was performed on December 12-14, 1978: a complete runthrough of Voyager 1’s 39-hour near encounter period, which would take place March 3-5, 1979. Participants included the flight team, the Deep Space Network tracking stations, the scientists, and the spacecraft itself. Results: Voyager and the Voyager team were all ready for the encounter.
Meanwhile, the spacecraft were busy returning scientific data to Earth. Technically, the Voyagers were in the cruise phase of the mission—a period that, for Voyager 2, would last until April 24, 1979, and for Voyager 1, until January 4, 1979, when each spacecraft would enter its respective observatory phase.