Forty years ago, today, on 20 August 1977, a space explorer rose from Earth on a multi-decade adventure, which would unveil all four of the Solar System’s giant gaseous planets in unprecedented detail. Despite being originally detailed to visit Jupiter and Saturn, NASA’s Voyager 2 mission went on to reveal both Uranus and Neptune to human eyes for the first time in our species’ history. Together with its twin, Voyager 1, launched two weeks later, on 5 September, the spacecraft has revolutionized our understanding of Earth’s place in the cosmos. Yet Voyager 2’s journey into space was by no means smooth sailing.
Seemingly enigmatically, Voyager 2 launched before Voyager 1, raising the question in many minds: Why were the two spacecraft rocketing into the heavens in the wrong numerical sequence? The answer was linked to their journey times, for Voyager 1 was following a shorter, faster trajectory to reach Jupiter and Saturn, whereas Voyager 2’s route was a longer and more circuitous path, designed to potentially take in Uranus and Neptune, if the conditions were right. As such, the faster-moving Voyager 1 would overtake its mate in December 1977 and reach both Jupiter and Saturn earlier. And in the grand scheme, being first to reach its destination was the big deal.
The Voyagers were both launched atop the final two flights of the Titan IIIE-Centaur rocket, a 157-foot-tall (48-meter) behemoth previously utilized to deliver the twin Helios space probes—a collaborative U.S./West German venture to explore the Sun—and NASA’s pair of Viking orbiters and landers to Mars. All told, this booster was the most powerful vehicle in the United States’ inventory by the mid-1970s, but with the Space Shuttle on the horizon its future was limited. As circumstances transpired, the Titan IIIE-Centaur flew on only seven occasions between February 1974 and September 1977, failing on its maiden test mission, before achieving success six times to boost two Helios, two Viking and two Voyager spacecraft on their expeditions of discovery.
Three times more powerful than the earlier Atlas-Centaur rocket, it benefited from thermal insulation enhancements, which provided for longer “coasting” periods and multiple engine restarts to accomplish missions on a grand scale. It was a pity that the Space Shuttle and other factors ultimately led to no further orders for the Titan III-Centaur, which, in the words of Andy Stofan, who led the program office for NASA, represented “the largest lift capacity this country had” in the years after the retirement of the Saturn V.
Voyager 2 rose from Launch Complex (LC)-41 at Cape Canaveral Air Force Station, Fla., at 10:29 a.m. EDT on 20 August 1977. At the instant of liftoff, the Titan IIIE-Centaur’s twin solid-fueled rocket motors ignited, with a combined thrust of 2.4 million pounds (1.1 million kg), pushing the booster uphill for the first two minutes of flight. Then, at 112 seconds, the two engines of the rocket’s “core” stage roared to life, producing around 1.1 million pounds (500,000 kg) of propulsive yield to continue the journey uphill.
Shortly thereafter, the solids were exhausted and jettisoned and the Titan IIIE-Centaur continued under the impulse of its core, which itself burned out at 10:33 a.m. Next came the core’s second stage, which picked up the baton for a further 3.5 minutes, positioning the stack for a pair of burns by the broad-bodied Centaur upper stage to heave Voyager 2 out of low-Earth orbit and onto an interplanetary trajectory to reach Jupiter and beyond. First, it burned to establish a temporary “parking” orbit for the spacecraft, then ignited a second time for more than 5.5 minutes to accelerate Voyager 2 through 9 miles per second (14 km/sec), achieving an Earth-escape velocity.
Following the departure of the Centaur, the spacecraft was on its own, as the last vestige of Earthly contact was gone forever. Voyager 2’s attached Star-37E motor burned for 45 seconds to add a little more onto its velocity, setting course for a rendezvous with the Solar System’s largest planet some 23 months later, in July 1979.
As history has taught us, the Voyagers turned into a spectacular success story. Voyager 1 launched aboard an identical Titan IIIE-Centaur on 5 September 1977. Both performed close-range encounters with Jupiter and Saturn, before Voyager 1 headed for an eventual departure from the Solar System. Meanwhile, Voyager 2 was tasked with visiting Uranus and Neptune, returning the first close-range images from these two outermost “ice giants”. Today, four decades since the Voyager odyssey began, they have provided the benchmark for the exploration of the outer planets which was to follow.