A quarter-century ago, this week, NASA’s Voyager 2 spacecraft became the first machine fashioned by human hands to travel close to the giant planet Neptune. It was a stunning finale to a 12-year voyage, which had also featured encounters with three other gaseous worlds—Jupiter, Saturn, and Uranus—and the adventure continues to the present day as Voyager 2 and its twin, Voyager 1, explore the outer reaches of the Solar System and the first wisps of conditions beyond. Yet today (20 August) is a special date in Voyager 2’s history, for it was on this day, in 1977, that the spacecraft parted company with its planet of origin for the final time and set sail for the stars. Two weeks later, on 5 September, Voyager 1 followed. Their launches and early weeks of operations were filled with drama and provided a fitting prelude for the exciting missions which were to come.
Voyager 2 rose from Earth at 10:29 a.m. EDT on 20 August 1977 to begin its voyage of exploration, but only after having suffered a double computer failure as its Titan IIIE-Centaur launch vehicle sat on Launch Complex (LC)-41, which is today known as “Space Launch Complex (SLC)-41,” at Cape Canaveral, Fla. This launch complex has supported dozens of space missions, boosted by a range of Titan and Atlas rockets, since December 1965, and most recently hosted the launch of the latest Global Positioning System (GPS) satellite, atop an Atlas V, on 1 August 2014.
Even after the computer problems were rectified, Voyager 2 suffered another glitch as the Titan IIIE-Centaur roared into the clear Florida sky. They appear to have been caused by the spacecraft suffering robotic “vertigo” as it rolled and pitched in its ascent trajectory. It also separated from the final stage of the booster too quickly, which convinced its on-board computers that its primary attitude control system had failed, and it promptly switched to a backup. Fortunately, its attached Centaur upper stage remained in control, autonomously correcting the error before releasing the spacecraft. At length, matters settled down. Seventy-one minutes after leaving Cape Canaveral, Voyager 2 fired its solid-propellant motor for 45 seconds to heave it out of Earth and onto a course for its first planet, Jupiter.
Still, none of the flight controllers at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., could breathe easily, for more trouble was in store. Less than two minutes after the conclusion of the trans-Jovian “burn,” Voyager 2 sensed and reacted to another orientation error. It had been pre-programmed in such a fashion that if it encountered difficulties, it was to “safe” itself and reposition its high-gain antenna toward Earth. For another 1.5 hours, it struggled to stabilize itself. The procedures put in place by NASA to protect the spacecraft, far from Earth, had inadvertently tripped, only hours into the mission. It seemed that Voyager 2’s brain was a little too sophisticated for its own good. Other unwanted surprises on the first day of flight included disturbing telemetry, which suggested that the scan platform—laden with a battery of scientific instrumentation—had not deployed correctly. Fortunately, this proved a false alarm and was caused by a troublesome sensor.
Voyager 1, however, was not quite so lucky when it rocketed into space, also atop a Titan IIIE-Centaur from LC-41 at 8:56 a.m. EDT on 5 September 1977. Although launched 16 days later, and numerically in the wrong order, its shorter, faster trajectory allowed it to eventually overtake its sister and be first to reach both Jupiter and Saturn. The decision to launch Voyager 2 first stemmed from the option to continue out to Uranus and Neptune. It was recognized that by launching Voyager 2 “early,” it might be possible to exploit an extremely rare alignment of all four gaseous planets and enable a “Grand Tour.” This had been predicted by JPL researchers Michael Minovich and Gary Flandro in the early 1960s and required a launch window before the end of August 1977.
By lofting Voyager 2 early, NASA was thus in with a fighting chance of getting to Uranus. Voyager 1 could then be launched, two weeks later, to Jupiter and Saturn only, and its faster trajectory meant that it had overtaken its twin by December 1977. It reached Jupiter four months and Saturn nine months, respectively, ahead of Voyager 2. Consequently, it was dubbed “Voyager 1” because it was first to reach its initial planetary targets, rather than being first in the launch pecking order.
Two weeks after Voyager 1’s launch, from a distance of almost 7.5 million miles (12 million km), it took a remarkable “departing” image of Earth and our Moon. For the first occasion in human history, our home planet and its only natural satellite—both crescents at the time—were captured in a single photographic frame. It was during this early part of the mission that Voyager 1’s scan platform jammed. Although it was successfully freed, mission controllers handled it with kid gloves for some months.
As Voyager 1’s team fought to keep their vehicle on the straight and narrow, the Voyager 2 staff fared little better. One of the hydrazine maneuvering thrusters ended up pointing in the wrong direction, with the result that it sprayed propellant toward the spacecraft. This action did not cause physical damage, but created the same effect as firing a high-pressure jet washer and caused Voyager 2 to drift slightly off-course. This required more hydrazine wastage to correct the problem. At length, it was resolved. However, the sensitivity of Voyager 2’s infrared imaging spectrometer became seriously degraded by December 1977, due to crystallization of bonding material which held its mirrors in place. This caused the mirrors to become warped and misaligned, and, as a consequence, the instrument’s sensitivity diminished. Fortunately, a flash-off heater was employed to reverse the process of evaporation and the instrument returned to its pre-flight sensitivity.
These glitches were soon eclipsed by something far more threatening. In April 1978, nearly eight months into the mission, a series of problems led Voyager 2 to lose control of its main radio receiver, effectively making it deaf in one “ear.” The first problem arose when the spacecraft’s seven-day events timer ran to zero, without receiving a command from the ground, to which Voyager 2 responded—as programmed—by assuming a failure and switching to its backup receiver. Correctional instructions were transmitted from Earth to return to the primary receiver, but, at first, the spacecraft failed to respond. At this stage, another issue reared its head. Each radio receiver was fitted with circuitry to automatically detect the frequency of the incoming signal and tune itself appropriately to accept it. One of this circuitry, known as a tracking loop capacitor, had failed, presumably due to an electrical short, with the result that the backup receiver would only respond to command transmitted at precisely the right frequency. The result: Its “good” ear was tone-deaf.
Calculating this “right” frequency was no mean feat, because engineers had to cater for the motion of the transmitting station on the rotating Earth, which was itself circling the Sun, and for Voyager 2’s own motion. Moreover, the receiver frequency of the spacecraft “slipped,” due to changes in ambient deep-space temperatures. A difference of a single degree Celsius (33.8 degrees Fahrenheit) could shift the frequency by up to 96 hertz. At length, a breakthrough was achieved and contact was restored. Unfortunately, during the process to switch from the backup to the primary receiver, an electrical short blew both fuses in the primary, effectively killing it. This raised the very real possibility of an impaired or ruined mission, but extensive reprogramming on Earth and transmission over many weeks to the spacecraft calmed Voyager 2 down for its mission of exploration.
The near-calamitous circumstances which thus surrounded the launch and early operations of both Voyagers make it all the more remarkable, not only that both spacecraft are still functioning—with Voyager 1 currently more than 128 Astronomical Units (AU), or 12 billion miles (19.2 billion km), and its twin about 105 AU, or 9.7 billion miles (15.7 billion km) from Earth—but that they have endured for so long. Current estimates are that their plutonium-fed Radioisotope Thermoelectric Generators (RTGs) will continue to supply electrical power until about 2025, during which time they will continue to probe the outer reaches of our Solar System. Their success is a testament to the ingenuity of both human beings and the technologies and equipment of their far-flung emissaries.