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Space Topics: VoyagerThe Story of the Mission
Together, Voyager 1 and Voyager 2 -- twin probes launched on September 5, 1977 and August 20, 1977 -- represent the most successful planetary exploration mission of all time. In their flybys of all the outer planets except Pluto, and dozens of other planetary bodies, the Voyagers set the benchmark in planetary exploration on an undertaking that has come to be deemed as one of NASA’s greatest triumphs. The two 1-ton spacecraft returned more knowledge-changing data than any mission before or since -- stunning photographs that consistently revealed our solar system to be much more diverse and complex and beautiful than anyone ever imagined and a veritable bounty of scientific information to go along with them. “Voyager revolutionized our perceptions of the solar system -- and our sense of what human beings can accomplish when they work together,” says Bruce C. Murray, then-director of the Jet Propulsion Laboratory (JPL) and co-founder, with Louis Friedman and Carl Sagan of The Planetary Society. In the late summer of 1977, Voyager 1 and Voyager 2 were launched on what was initially to be a four-year mission to Jupiter and Saturn. Voyager 2 lifted off the pad from Cape Canaveral, Florida on August 20, 1977, with Voyager 1 following on a faster trajectory September 5. Although the mission was designed specifically for encounters at Jupiter and Saturn, the team had selected launch dates that would allow Voyager 2 to fly by Saturn on a trajectory that would swing it off toward Uranus and Neptune, if Voyager 1 succeeded at both Jupiter and Saturn. JPL engineers, meanwhile, had designed the hardware for the long haul, installing a system that would allow for enhanced remote control programming to give the spacecraft even greater capability than they possessed when they left Earth if they did fly farther across the solar system. “The versatility we built in the Voyagers really paid off,” says the mission’s first Project Manager Harris ‘Bud’ Schurmeier.
As it turned out, the resilience of the robotic probes, the incredible success of their encounters, and the ingenuity and enthusiasm of engineers and scientists on the ground prompted NASA, not surprisingly, to extend the mission. Voyager 1 flew by Jupiter on March 5, 1979, and Saturn on November 12, 1980, then headed on to Saturn’s large moon, Titan. Its path, which was angled upward by the ringed planet’s gravity, eventually sent the spacecraft out of the ecliptic, a plane defined by Earth’s orbit. With mission success secured, Voyager 2 arrived at Jupiter on July 9, 1979, where it took advantage of the available energy boost and trajectory change, flying by Saturn on August 25, 1981 on its redirected course to Uranus. Despite political attempts on the ground to halt the mission at that point, Voyager 2 arrived at the seventh planet from the Sun on January 25, 1986 and reached blue Neptune, the outermost of the gas planets, on August 25, 1989. The little robot then swung downward out of the ecliptic. Depicted as “loyal,” “fearless,” “little robots that could,” “intelligent,” “curious,” “intrepid,” and “indefatigable,” the Voyagers visited four planets, 48 moons, and dozens of rings between 1979 and 1989, more planetary bodies than any other mission. By whatever description, the Voyagers have had an unparalleled journey of discovery. “That decade probably represents the greatest mission of planetary discovery in the history of humankind,” says former Mission Design Manager Charley Kohlhase. In 1990, Voyager 1 took humanity’s collective breath away when it turned around to capture one last portrait for the family album -- a sequence of pictures that revealed most of the solar system – six of the nine planets in a dazzling, orbital array. From 6 billion kilometers (3.7 billion miles) out, Earth was less than a single pixel (picture element) in the solar system -- a pale blue dot, as Sagan described it. By then, the epic tour of the outer planets was over, but the spacecraft still had places to go. Although most of the imaging instruments shut down around 1990, other instruments continued collecting data. Planetary troupers, indeed, each Voyager is still operating smoothly, decades beyond its planned lifetime. Both are headed now toward the heliopause, the never-before explored boundary between our solar system and interstellar space. And as long as they keep on going, scientists anticipate more discoveries. Surprise after surpriseDuring their travels, the Voyagers rewrote the planetary textbooks, beginning at Jupiter with an anomaly on an image of its moon Io. Although scientists had expected Io to be old, dead, and heavily cratered like our Moon, the anomaly turned out to be a volcanic plume. Before the mission was completed, the Galilean satellite eventually revealed that it had 100 times the volcanic activity of Earth.
The “big surprises,” as Project Scientist Ed Stone calls them, just kept on coming. The Great Red Spot on Jupiter that had been seen for years was found to be a raging storm . . . Saturn’s rings were discovered to be oddly intertwining, and kinked . . . Saturn’s moon Titan turned out to be so heavily shrouded in organic smog that it for years had deceived us into thinking it was much larger than it really is . . . Uranus was found to be orbiting oddly, sideways, and its moon Miranda bared a surface as bizarre and complex and puzzling as the surface of a moon could get . . . Neptune boasted the fastest winds in the solar system while its moon Triton flaunted icy volcanoes. From one discovery to the next, our knowledge and thinking about the outer planets changed. The Voyager mission has been, and will remain well into the future, no doubt, NASA's biggest planetary expedition. The original price tag of nearly one billion dollars made it the second most expensive planetary voyage, exceeded only by Viking, which sent orbiters and landers to Mars in 1976. Nevertheless, the mission, by all accounts in the community, has been worth every single penny. On February 17, 1998, Voyager 1 cruised beyond Pioneer 10 to become the most distant human-created object in space, 6.5 billion miles (10.4 billion kilometers) from Earth, setting a new milestone in space exploration. Although it is very, very cold out in the far reaches of our solar system, where the Sun is only 1/5,000th as bright as on Earth, the radioisotope thermal electric generators (RTGs) on both spacecraft have kept them going. “The Voyagers owe their ability to operate at such great distances from the Sun to these nuclear electric power sources, which provide the electrical power they need to function,” says Stone, who has retired as director of JPL, but continues teaching and researching from his quarters at Caltech and as Voyager Project Scientist.
The Voyagers are still relaying their data via 23-watt transmitters through the 34-meter Deep Space Network (DSN) antennas located in California, Australia, and Spain. For a few hours each day, these antennas tune in to the weak signals emanating from the probes. Every three months or so, new commands are radioed up to them from the ground crew. It’s a slow conversation – with one two-way communiqué with Voyager 1 taking an estimated 23 hours, and about 18.5 hours with Voyager 2. Sail on silver dreamsWhile the available electrical power will no longer support science instrument operation after about 2020 and our contact with them will be lost forever, the two intrepid robots will journey on through the emptiness of space where the interstellar winds blow and cosmic rays abound. Even without power, the Voyagers, theoretically, can drift on and on. In fact, they could survive for millions of years, speeding along diligently as ever – unless, perchance, they are intercepted by beings from distant galaxies far, far away. It will take them another 40,000 years, scientists estimate, to pass through the Oort Cloud, the sphere of cometary nuclei that are the last of the known objects held by a faint gravitational pull of our Sun. Then, Voyager 1 will be within 1.6 light years (15 trillion kilometers, 9.3 million miles) of a star in the constellation Camelopardalis, the first time Voyager will be nearer to another star than they are to the Sun, according to Stone. Then 250,000 years after that, Voyager 2 will fly by Sirius, the brightest star in our night sky, 4.3 light years (40 trillion kilometers, 25 trillion miles) away. We may never know if they make it, but if they do -- or even if they don’t -- the Voyagers are humankind’s first emissaries to the stars. “Voyager has been the journey of a lifetime,” sums up Stone. He pauses a moment, then quickly adds: “And it’s not over yet.” Indeed, long after those of us on Earth now are all gone, Voyager, in all probability, will be sailing on, silver dreams of humanity’s quest to reach the stars. -- A.J.S. Rayl |
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