Voyager 1's Communications Issue Fixed, Back to X-Band

Back at the first of the month, we heard the story that two weeks earlier Voyager 1 had switched to its backup transmitter, down at S-Band - a transmitter that hadn't been used since 1981. 

As predictable as anything can be, the 47 year old Voyagers are doing two thing continually as they age.  First, they're going farther into interstellar space every second.  Voyager 1, for example is 15.4 billion miles from the Sun; the distance from Earth varies more as our rotation around the sun adds and subtracts miles over the course of a year - but that's in the next decimal place after the "4." 

Secondly, they're gradually losing power.  Their Radioisotope Thermal Generators are slowly degrading, losing 4 watts per year. As predictable as the power loss is, over the years more and more of the instruments on the Voyagers have been shut off to reduce power drain. Only four of Voyager 1's instruments remain powered but they're all operating at lower temperatures than they were ever designed and tested for. When some concerns about those temperature excursions started being considered, it was thought that borrowing "a few" watts for some onboard heaters would be a good thing.

So, when engineers commanded Voyager 1 to switch on one of its heaters to give the instruments a gentle thermal massage, a safety feature was tripped because of low power levels. The spacecraft's fault protection system monitors how much energy Voyager 1 has left, and if it deems there to be too little energy for the probe to continue operating, it automatically switches off non-essential systems. It seems that the heater was using too much energy, but the problem was that all the non-essential systems had been switched off long ago to conserve what little power remained, so the fault protection system took it upon itself to switch off the main X-band transmitter and activate the lower-power S-band transmitter instead. 

That description is a bit too anthropomorphic for me; this isn't AI, it's a simpler computer than anything the vast majority of us have ever seen; it was designed in the early 1970s. I'd prefer to say the conditions Voyager was seeing weren't thought of when the software was written and it merely executed code that had the same effect - shutting down the X-band  transmitter.  

The problem was understood early in November and the fix implemented. X-band communication resumed on Nov. 18, with the spacecraft once again returning data from its four remaining instruments: the Low-Energy Charged Particle Experiment, the Cosmic-Ray Telescope the Triaxial Fluxgate Magnetometer and the Plasma Waves Experiment. 

Next summer will be the 48th anniversaries of the two Voyager satellites launches: Voyager 2 launched first on August 20, 1977, followed by Voyager 1 on September 5, just over two weeks later.  Everyone is hoping to see the two make their 50th anniversary in 2027.  

Artist’s illustration of one of the Voyager spacecraft. Credit: Caltech/NASA-JPL