Came across this sad news today from ARS Technica about one of NASA/JPL’s pioneering deep space probes, Voyager-1, that may now be effectively incommunicado, roughly 15 billion miles from Earth — well beyond our Sun’s heliopause that marks the true boundary with interstellar space. The current and probably fatal problem began back in November with an onboard computer glitch that is preventing any telemetry being sent back to Earth:
"It would be the biggest miracle if we get it back. We certainly haven't given up," said Suzanne Dodd, Voyager project manager at NASA's Jet Propulsion Laboratory, in an interview with Ars. "There are other things we can try. But this is, by far, the most serious since I’ve been project manager."
…
The latest problem with Voyager 1 lies in the probe's Flight Data Subsystem (FDS), one of three computers on the spacecraft working alongside a command-and-control central computer and another device overseeing attitude control and pointing.
...
When it was developed five decades ago, Voyager's Flight Data Subsystem was an innovation in computing. It was the first computer on a spacecraft to make use of volatile memory. Each Voyager spacecraft launched with two FDS computers, but Voyager 1's backup FDS failed in 1981, according to Dodd.
The only signal Voyager 1's Earthbound engineers have received since November is a carrier tone, which basically tells the team the spacecraft is still alive. There's no indication of any other major problems. Changes in the carrier signal's modulation indicate Voyager 1 is receiving commands uplinked from Earth.
…
The hope among Voyager engineers is that the transition to different data modes might reveal what part of the FDS memory needs a correction.
This is a lot more complicated than it might seem on the surface. For one thing, the data modes engineers might command Voyager 1 into haven't been used for 40 years or more. Nobody has thought about doing this with Voyager's flight data computer for decades.
…
"Not to be morose, but a lot of Voyager people are dead," Dodd said. "So the people that built the spacecraft are not alive anymore. We do have a reasonably good set of documentation, but a lot of it is in paper, so you do this archaeology dig to get documents."
At least Voyager-2 is still sailing smoothly through the void, though we almost lost it too last year due to an erroneous command sent from Earth as detailed by the BBC:
In July a wrong command was made to the spacecraft, sent to explore space in 1977, changing its position and severing contact.
A signal was picked up on Tuesday but thanks to an "interstellar shout" - a powerful instruction - its antenna is now back facing Earth.
…
Staff used the "highest-power transmitter" to send a message to the spacecraft and timed it to be sent during "the best conditions" so the antenna lined up with the command, Voyager project manager Suzanne Dodd told AFP.
Unfortunately, the plutonium power sources for both spacecraft are winding down, and both will effectively be dead probes hurtling blindly onward toward unknown fates by sometime in the next decade or so.