As Voyagers 1 and 2 turn 46, a software patch

Voyager 2 launched on August 20, with Voyager 1 following 16 days later on September 5, 1977, which means the two probes turned 46 just over a month ago.  Put another way, they entered the 46th year of their four year missions.  

Forty-six years in deep space have taken their toll on NASA's twin Voyager spacecraft. Their antiquated computers sometimes do puzzling things, their thrusters are wearing out, and their fuel lines are becoming clogged. Around half of their science instruments no longer return data, and their power levels are declining.

All that said, they are the farthest any objects from Earth mankind has gotten.  The backwards order of their launch with Voyager 2 starting out 16 days ahead of Voyager 1, and so being farther from Earth, was reversed early in the mission when Voyager 1 was given a shorter, more direct path to Jupiter and then Saturn while Voyager 2 was sent on path that would allow it to visit all four "gas giant" outer planets and the first human visits to Uranus and Neptune.  

Today, Voyager 1 is farthest from Earth, not quite 22-1/2 light hours from Earth, over 15 billion miles.  Voyager 2 is "just" 18.7 light hours.  This is only far compared to our day to day lives; the nearest stars are 4.3 light years away, closer to 37,700 light hours.  It will take the Voyager 1 over 77,000 years to get that far.  

Still, the lean team of engineers and scientists working on the Voyager program at NASA's Jet Propulsion Laboratory are taking steps to eke out every bit of life from the only two spacecraft flying in interstellar space, the vast volume of dilute gas outside the influence of the Sun's solar wind.

"These are measures that we're trying to take to extend the life of the mission," said Suzanne Dodd, Voyager project manager at JPL, in an interview with Ars.

Voyager's instruments are studying cosmic rays, the magnetic field, and the plasma environment in interstellar space. They're not taking pictures anymore. Both probes have traveled beyond the heliopause, where the flow of particles emanating from the Sun runs into the interstellar medium.

"These two spacecraft are still operating, still returning uniquely valuable science data, and every extra day we get data back is a blessing," Dodd said.

We know that spacecraft designers love redundancy - backup systems for everything.  Both Voyagers are out of backups and running "single-string."  Power is decreasing at about 4 Watts per year as their Radioisotope Thermal Generators' fuel is decaying with age.  Much of the two satellites has been turned off already.  Will they make it to 50 years?  For 46 years they've been one little piece of space rock or meteor away from death.  Same as today.  It's the slow decay of various parts that's the problem.  

Over the weekend, ground controllers at JPL planned to uplink a software patch to Voyager 2. It's a test before the ground team sends the same patch to Voyager 1 to resolve a problem with one of its onboard computers. This problem first cropped up in 2022, when engineers noticed the computer responsible for orienting the Voyager 1 spacecraft was sending down garbled status reports despite otherwise operating normally. It turns out the computer somehow entered an incorrect mode, according to NASA.

Managers wanted to try the patch on Voyager 2 before transmitting it to Voyager 1, which is flying farther from Earth, deeper into interstellar space. That makes observations of the environment around Voyager 1 more valuable to scientists.

During the SW development, the engineers developed a different way to use the spacecrafts' thrusters.  The thrusters fire occasionally to keep the high gain antenna pointed back at Earth and the buildup of hydrazine residue in the fuel lines is becoming problematic.  Engineers beamed up fresh commands to the spacecraft in the last few weeks to allow the probes to rotate slightly further in each direction before firing the thrusters.  This means the engines will make fewer but longer burns.  The issue there is the longer burns might make the residue problem worse,.

With these steps, engineers expect the propellant inlet tubes won't become completely blocked for at least five more years, and "possibly much longer," NASA said. There are other things engineers could try to further extend the lifetime of the thrusters.

“This far into the mission, the engineering team is being faced with a lot of challenges for which we just don’t have a playbook,” said Linda Spilker, Voyager project scientist at JPL, in a statement. “But they continue to come up with creative solutions.”

The major subsystems of the Voyagers.  NASA/JPL-Caltech image.

The power decrease with aging has been planned for all along since it's a very predictable aging curve.  That aspect of aging is stable.  Earlier this year, engineers bypassed a voltage regulator on Voyager 2 to allow the spacecraft to draw on more power (there's always power loss in a voltage regulator). This means they expect that ground controllers won't have to shut off one of Voyager 2's five remaining science instruments until 2026, after previously expecting to deactivate one of the instruments this year. Dodd said ground teams will do the same with Voyager 1, which only has four active instruments, and therefore uses less power.

If you only look at the power situation, the Voyagers should make it until 2030, and maybe slightly longer, before the decay of the plutonium power source forces NASA to switch off all their science instruments.

"The transmitter takes about 200 watts of power, so once we get down to that level of power, that will be the end of the mission," Dodd said.

Even when they stop working, NASA's Voyagers will continue on to the stars.

"A lot of things could break before we run out of power," she told Ars. "Just like this thruster issue sort of popped up, there are a lot of other issues that could pop up and cause a mission to fail."

I've said before that I think of the Voyagers as the crowning accomplishment of NASA and among the coolest things ever done in human history.  I hope they make it to 50 years in 2027.  

Closing words to Suzanne Dodd, Voyager project manager. 

"They've overcome lots of issues, and the engineers have been very clever in overcoming those issues," Dodd said. "I think the focus now is let's get to 50 and have the biggest party we can."