Although it’s been quite some time since I’ve written about Voyager, our two interstellar craft (and this is indeed what they are at present, the first to return data from beyond the heliosphere) are never far from my mind. That has been the case since 1989, when I stayed up all night for the Neptune encounter and was haunted by the idea that we were saying goodbye to these doughty travelers. Talk about naivete! Now that I know as many people in this business as I do, I should have realized just how resilient they were, and how focused on keeping good science going from deep space.

Not to mention how resilient and well-built the craft they control are. Thirty five years have passed since the night of that encounter (I still have VCR tape from it on my shelf), and the Voyagers are still ticking. This despite the recent issues with data return from Voyager 1 that for a time seemed to threaten an earlier than expected end to the mission. We all know that it won’t be all that long before both craft succumb to power loss anyway. Decay of the onboard plutonium-238 enabling their radioisotope thermal generators (RTGs) means they will be unable to summon up the needed heat to allow continued operation. We may see this regrettable point reached as soon as next year.

But it’s been fascinating to watch over the years how the Voyager interstellar team manages the issue, shutting down specific instruments to conserve power. The glitch that recently occurred got everyone’s attention in November of 2023, when Voyager 1 stopped sending its normal science and engineering back to Earth. Both craft were still receiving commands, but it took considerable investigation to figure out that the flight data subsystem (FDS) aboard Voyager 1, which packages and relays scientific and engineering data from the craft for transmission, was causing the problem.

What a complex and fascinating realm long-distance repair is. I naturally think back to Galileo, the Jupiter-bound mission whose high-gain antenna could not be properly deployed, and whose data return was saved by the canny use of the low-gain antenna and a revised set of parameters for sending and acquiring the information. Thus we got the Europa imagery, among much else, that is still current, and will be complemented by Europa Clipper by the start of the next decade. The farther into space we go, the more complicated repair becomes, an issue that will force a high level of autonomy on our probes as we push well past the Kuiper Belt and one day to the Oort Cloud.

Image: I suppose we all have heroes, and these are some of mine. After receiving data about the health and status of Voyager 1 for the first time in five months, members of the Voyager flight team celebrate in a conference room at NASA’s Jet Propulsion Laboratory on April 20. Credit: NASA/JPL-Caltech.

In the case of Voyager 1, the problem was traced to the aforesaid flight data subsystem, which essentially hands the data off to the telemetry modulation unit (TMU) and radio transmitter. Bear in mind that all of this is 1970s era technology still operational and fixable, which not only reminds us of the quality of the original workmanship, but also the capability we are developing to ensure missions lasting decades or even centuries can continue to operate. The Voyager engineers gave a command to prompt Voyager 1 to return a readout of FDS memory, and that allowed them to confirm that about 3 percent of that memory had been corrupted.

Culprit found. There may be an errant chip involved in the storage of memory within Voyager 1’s FDS, possibly a result of an energetic particle hit, or more likely, simple attrition after the whopping 46 years of Voyager operation. All this was figured out in March, and the fix was determined to be avoiding the now defunct memory segment by storing different portions of the needed code in different addresses in the FDS, adjusting them so that they still functioned, and updating the rest of the system’s memory to reflect the changes. This with radio travel times of 22 ½ hours one way.

The changes were implemented on April 18, ending the five month hiatus in normal communications. I hadn’t written about any of the Voyager 1 travails, more or less holding my breath in hopes that the problem would somehow be resolved. Because the day the Voyagers go silent is something I don’t want to see. Hence my obsession with the remaining possibilities for the craft, laid out in Voyager to a Star.

Engineering data is now being returned in usable form, with the next target, apparently achievable, being the return of science data. So a fix to a flight computer some 163 AU from the Sun has us back in the interstellar business. The incident casts credit on everyone involved, but also forces the question of how far human intervention will be capable of dealing with problems as the distance from home steadily increases. JHU/APL’s Interstellar Probe, for example, has a ‘blue sky’ target of 1000 AU. Are we still functional with one-way travel times of almost six days? Where do we reach the point where onboard autonomy completely supersedes any human intervention?