POSTED 6 SEPTEMBER 2007
At the outset, the Voyager spaceships were designed to visit all of the outer planets, and they had the grand title of "Grand Tour." Then a funding shortage forced designers to focus on Jupiter and Saturn, and after a name change, Voyager 2 was launched on Aug. 20, 1977. Voyager 1 left home on Sept. 5.
Both Voyagers explored Jupiter and Saturn. Then Voyager 1 headed north, above the plane of the solar system, while Voyager 2 visited Neptune and Uranus, and then headed south. (To tell directions in space, figure that the skies above Earth's North Pole are on the north side of the solar system.)
Voyagers were a shot in the dark in more ways than one. Although space technology was rapidly evolving, it was tremendously primitive by today's standards. "The space age was 20 years old, and there was no basis for knowing how long they would last," says Ed Stone of Caltech, recalling the early design discussions. "We hoped, but we had no long-term data."
Stone credits Voyager's long life to several factors:
Extra parts: "Redundancy in all the space subsystems" came in handy, for example, when part of the radio transmitter broke and a spare was pressed into service.
Experience: Voyagers' predecessor, Pioneer 10, found the radiation environment of Jupiter "so horrendous that we spent nine months redesigning the circuits to make them more robust." The redesigned circuits were more durable, "so we have Jupiter to thank" in part for Voyager's longevity.
Everlasting (almost) electricity: The electric generator is powered by heat from the decay of plutonium 238. With only a few instruments still turned on, the spacecrafts should have enough power to transmit signals until 2025. When Voyager 1 goes silent, it should be about 170 times as far from the sun as Earth is, "almost surely in interstellar space," Stone adds.
Caution was not the only the design criterion, however. Stamatios Krimigis, emeritus head of the space department at Johns Hopkins University Applied Physics Laboratory, and principal investigator of the low energy charged particle instrument, wrote us to say, "I took a risk in my instrument and included a small motor that turns the detectors so as to view all directions in space. My engineering colleagues said it would fail soon after launch, and we tested it for 500,000 steps, just enough to get to Saturn. Well, it's still stepping along every 192 seconds, some 30 years later, a total of about 5 million steps!"
Bye-bye planets. Hello Milky Way.
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive