How Voyager engineers found two more years in a 50-year-old power budget

SPACE EXTENDERS Adding a few extra years to Voyager 2's prodigiously long life was the result of an effort that began more than a year ago, as engineers and managers pondered how to eke out a few more watts from the spacecraft's dwindling power supply.

"We came up with the 'Big Bang' idea probably more than a year ago now," recalls Suzanne Dodd, Project Manager for the Voyager project.

"We were looking for ways to extend the lifetime of the mission, which means really extending how long we can operate the instruments."

Power and thermal requirements are key. "We either run out of power, or we get too cold so that our propellant lines freeze and we can't point the spacecraft at the antenna anymore."

The original plan was to turn off the spacecraft systems sequentially to save power. "The problem with turning things off in serial is that you get the power, but you don't get the extra thermal heat when you do that… and all we really have left to turn off is instruments," Dodd explains.

A few years earlier, the team had established that the instruments could survive without their heaters. It had now reached the point where keeping the remaining instruments powered would require shutting one down.

That prompted the approach that became the Big Bang. "The idea behind the Big Bang… If we were to switch systems and do a lot of power switching all at once within the bus of the spacecraft, we would gain a lot of power and it would still keep the electronics warm, as opposed to when we do it serially (in a series), it doesn't provide enough of the heat input to keep the propellant lines warm."

The approach required careful power and thermal modeling. The draw of each component had been documented, but the figures were nearly 50 years old and might contain some hidden margins. "We're talking about a half a watt, or two tenths of a watt, and that makes all the difference on the spacecraft," Dodd explains.

"So if something in the documentation written 50 years ago that says it's four watts and it's really 3.8 watts, that makes a difference to us."

After extensive testing on Earth, the only way to validate the model was to see how the spacecraft reacted. The first orbital test focused on the power commands: Voyager 2 entered the new configuration for a few minutes before switching back. Enough telemetry would be generated in that time to show that "the power commands acted as we expected and that they drew the amount of power that we expected and they did."

The team then had to test the thermal model. "Thermal is much more difficult to predict," says Dodd. "It's not instantaneous… it kind of cools slowly or the heat rises slowly over like a 48-hour period so it's harder to model and it's harder to predict."

After confirming that the power commands behaved as expected, the team sent them again at the beginning of June. This time, Voyager 2 remained in the new configuration for six hours – long enough to reveal the thermal trend while limiting the danger if the model proved wrong.

Every test was slowed by distance. Commands took up to 20 hours to reach Voyager 2, with the response requiring another 20 hours to return.

The results matched expectations, and occasionally exceeded them. There are often surprises when commanding spacecraft, especially the Voyagers. "Generally they're positive surprises!" Dodd says. "Like 'Oh look! [The temperature] didn't drop as much as we thought!'"

"And sometimes we actually gain more power than we think too, which is also a nice surprise."

With the thermal test complete, the team was ready to make the configuration permanent. "July 9 was when it executed on the spacecraft," explains Dodd.

"The nail-biter was that we had executed the commands twice before – we were pretty certain that those would work – but we had only done a limited thermal test. So we weren't really sure after 24 or 48 hours where we would settle out thermally.

"Turns out when we executed it permanently, and then we watched the next 24 or 48 hours, 36 hours kind of thing, that it really came down to a very nice spot temperature-wise."

There wasn't the drop predicted by the model, so the engineers now have to update it accordingly.

The update has postponed the next instrument shutdown – the Cosmic Ray System – to after the 2027 50th anniversary of Voyager 2's launch, and well into 2028. "Solely based on power," says Dodd, "it's moved out two years."

Voyager 1 is farther away and operating with even narrower power margins. The team now plans a similar update and may even attempt to revive some of the instruments previously shut down to conserve power.

"There's a concern we're working on now," explains Dodd, "on what would the danger be to the spacecraft if you turned on an instrument? There could be a short, or something that would affect the bus that could take out the whole spacecraft."

Back from the dead?

And then there's the worry that, for an instrument that has been off and cold for years, would it even work?

Still, it's remarkable that, after almost 50 years, the agency is even having a conversation about turning instruments back on.

Dodd laughs. "There are definitely scientists that want those instruments back on!"

Other upgrades are under discussion. With funding, a flight software update could restore high-rate data from Voyager's Plasma Wave Subsystem (PWS). However, ultimately, time is running out for the veteran spacecraft, despite the rabbits being pulled from the engineering hat.

"I'm not sure we have many more rabbits though," says Dodd. "But when I came on the project in 2010 it was the same thing… we'll have to turn off instruments. And I think we've done a lot of clever engineering things: the Big Bang is certainly one of them… running on what we call voltage management versus a power margin is another thing that's given us a couple extra years.

"What really is the margin? What really is zero? If you have one subsystem engineer and he puts his margins in, and then there's a system engineer and they put their margins in. So there's kind of, over the course of building the spacecraft, there may be margins on margins on margins, and we don't know."

Despite the publicity surrounding the upcoming 50th anniversary of the spacecraft's launch, the Voyager mission will eventually end, and the signal from the spacecraft will fade before it passes out of range of NASA's Deep Space Network. Dodd notes that another famous probe, Cassini, "did everything right in their final end of mission… they dove through the rings and they had a great phrase: 'Grand Finale'. They had a great ending; it was a definitive mark when you knew the signal was not going to come down.

"Unfortunately, Voyager's not going to be that way… we're just going to one day not hear the signal from the DSN, and we're going to rush around and make sure that it's not something on the ground.

"We'll try commanding… see if we can get a carrier signal or something and, honestly, that could probably take a couple of months before we declare the spacecraft dead."

And then? Perhaps point a Deep Space Network antenna toward Voyager's last known position and transmit farewells, memories, or a song.

"That's how I want Voyager to end," says Dodd, "because I don't want Voyager to end because somebody says the mission should be over. I want Voyager to end on its own terms." ®