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That the Voyagers have enough juice left to make that crossing is a tribute to their teeny, tiny nuclear power plants--otherwise known as radioisotope thermoelectric generators (RTGs)--absolutely essential for such a deep-space mission, since you can hardly rely on solar panels in a region where sunlight dwindles to a relative candle's worth of energy. The RTGs are fueled by plutonium 238, which, at the time of launch, was predicted to be good enough to keep the ships going for 50 years. So far those projections are holding, with power expected to last until 2025.
To ensure that it does, engineers have instructed both spacecraft to switch to a backup set of attitude thrusters that had not been used in flight. Voyager 1 received that command in 2004 after its primary thrusters had fired 353,000 times. Voyager 2 got the same instruction on Nov. 4 this year after 318,000 primary firings. For both ships, the switch is intended to save about 12 watts by allowing the heaters that kept the main fuel lines warm to be shut off.
It's a measure of how elegant the design of the 34-year-old spacecraft is that the 12 watts represents nearly 5% of the electricity the ships need to operate at any given time--meaning the most durable and distant machines humans have ever built do their work on less energy than it takes to run three ordinary lightbulbs. And it's a measure of how far that engineering has carried the craft that it took 13.5 hours for the thruster command--traveling from Earth at the speed of light, 186,000 miles per sec.--to reach Voyager 2 and another 13.5 for the confirmation to come back. On Nov. 13, Voyager 2 transmitted another signal, which arrived on Nov. 14, confirming that the backup thrusters were functioning as planned.
When the twin spacecraft do fall silent, they will continue to serve as earthly messengers. Attached to their flanks are their celebrated golden records, 12-in. gold-plated copper disks containing analog etchings of pictures, music and greetings from Earth in multiple languages. The disks resemble old phonograph records and work like them too--but that, of course, makes sense. In 1977, who in the world had heard of a CD?