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Voyager 1 Fires Up Thrusters After 37 Years (nasa.gov)
If you tried to start a car that's been sitting in a garage for decades, you might not expect the engine to respond. But a set of thrusters aboard the Voyager 1 spacecraft successfully fired up Wednesday after 37 years without use. NASA announces: Voyager 1, NASA's farthest and fastest spacecraft, is the only human-made object in interstellar space, the environment between the stars. The spacecraft, which has been flying for 40 years, relies on small devices called thrusters to orient itself so it can communicate with Earth. These thrusters fire in tiny pulses, or "puffs," lasting mere milliseconds, to subtly rotate the spacecraft so that its antenna points at our planet. Now, the Voyager team is able to use a set of four backup thrusters, dormant since 1980. "With these thrusters that are still functional after 37 years without use, we will be able to extend the life of the Voyager 1 spacecraft by two to three years," said Suzanne Dodd, project manager for Voyager at NASA's Jet Propulsion Laboratory, Pasadena, California.
What's harsh about space other than it being relatively cold? Besides some solar radiation (which it's probably too far for anyway) there is nothing to interact with the systems, no molecules or fluctuations of radiation or physical pressure/stress that interact with it so it won't corrode or fatigue.
Yes, it's amazing that it still works because the design was good. What's more amazing is that they didn't fuck up the data transmission to interact with the hardware/software on the system. You're talking about 50's and 60's era electronics and hand-made/hand-programmed stuff that's still working at rates like 40 bits per seconds with nibble-sized serial communications to the CPU, how many programmers still know how much a nibble is these days and how to craft a message using it on modern systems?
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For these thrusters (which I think are Aerojet 0.2-lb monoprop, MR-103 series), there's nothing to degrade from mere age or vacuum, and the environmental thermal cycling is negligible. So it is not that surprising that they still work. Using them is another story, and pulsing certainly does degrade them (eventually - hundreds of thousands of pulses).
It's not terribly unusual to switch to redundant equipment after decades and have it work. Essentially we rely on that working, and I have seen many examples of it working perfectly well. Vacuum is a very good storage medium for anything that does not outgas. Radiation degrades solid-state electronics and power supply components (particularly high-voltage components) are somewhat prone to degrading from age or outgassing. Longest I have personally been involved with is a prime flight computer that was believed to have failed on the day after the launch that was turned on 32 years later as a final test, and that worked fine for at least a few hours. But other components were switched to (of necessity) after ~20 years fired right up and worked fine, showing the same parameters as the day we turned it off.