How NASA swapped Voyager 1's thrusters from 50 billion miles away

Voyager 1 is in interstellar space, and it’s taken a lot of work from brilliant engineers at NASA’s Jet Propulsion Laboratory to get it there. But a recent engine problem threatened to cut off the beloved spacecraft’s communication with Earth. Let’s dive into how engineers diagnosed and fixed Voyager 1 from 15 billion miles away.

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Voyager 1’s ongoing issues

You may recall that there was a long period of time from late 2023 to mid 2024 where Voyager 1 was having a computer issue. It was sending back basically garbage data, and we didn’t know whether it could be fixed — but engineers pulled it off.

What we’re going to talk about today is different: Voyager 1 has been having thruster problems, and they’ve built up into a serious concern. The spacecraft isn’t using thrusters to propel itself through space — that’s happening just based on momentum. But in order to communicate with Earth, Voyager 1 needs to be able to point its antenna towards our planet, and for that, it needs functional thrusters. Engineers thought they could fix the problem, but there was a question of whether implementing that fix would further damage the spacecraft. Here’s how they managed it.

The problem: Clogged fuel tubes

Voyager 1 launched on September 5, 1977, and its original mission was to fly by Jupiter, Saturn, and Saturn’s largest moon Titan. It was the first spacecraft to enter interstellar space, which occurred on August 25, 2012, and since then, it’s just been a game of prioritization to keep the spacecraft operational and in communication with Earth as long as possible.

The problem here is the thrusters. Basically, when engineers were designing Voyager 1 and 2 (the two spacecraft, both currently outside our solar system, are identical), they chose hydrazine as the propellant for a simple reason: It’s very reliable. The probe has 16 hydrazine thrusters, and a healthy Voyager 1 spacecraft would conduct about 40 thruster pulses a day to accomplish this.

But sadly, Voyager 1 is not a healthy spacecraft. The side effects of these constant thruster pulses have been building, and over the years, the fuel tubes inside the thrusters have become clogged. These tubes specifically direct liquid fuel to the catalyst beds, where it’s turned into a gas. What that means is that more fuel is required to accomplish the same amount of work, pointing the spacecraft, with each of those pulses — and Voyager 1’s fuel supply is finite. They don’t want to be using more fuel than they need to, nor do they want the tubes to get so clogged that they can no longer fire the thrusters. Right now, the twin Voyager spacecraft have about a decade of fuel left.

If it weren’t for the computer issue last year and early this year, the hydrazine thruster problem would have been easily labeled the most significant problem facing Voyager 1 over the past decade.

Engineers have been taking steps to mitigate this problem over the years. The Voyager 1 team first noticed the problem in 2002 with one branch of attitude control thrusters and switched to the other one. The spacecraft has three different branches of thrusters, the attitude control thrusters take up two of these branches and one is the trajectory correction maneuver, or TCM, thrusters. The TCM thrusters are identical to the attitude control thrusters, they’re just located on the back of the spacecraft.

But as I mentioned, Voyager 1 isn’t using its engines to correct its trajectory. That means these TCM thrusters hadn’t been used since the Saturn flyby on November 8, 1980. Well, on November 28, 2017, for the first time in 37 years, engineers powered up the TCM thrusters. They worked, and so Voyager 1 switched to exclusively using these thrusters in January of 2018.

Well. Now these thruster tubes are also clogged, and they’re even worse than the attitude control thrusters were. They were originally .25 mm in diameter, now they’re .035 mm, which according to NASA is about half the width of a human hair. So, now they have to switch the thrusters back from the TCM thrusters to a branch of the attitude control thrusters. Simple, right? Well, when it’s an almost 50-year-old spacecraft 15 billion miles away in interstellar space, NOTHING is simple.

Why the solution is complicated: Power and heat

To understand why this is a problem, we need to discuss Voyager 1’s dwindling power supply.

Voyager 1 uses three radioisotope thermoelectric generators, or RTGs, for power. You can think of this as, functionally, nuclear batteries. As the plutonium decays, that’s converted into electricity. But because it’s limited by the half life of the fuel source, in this case plutonium, basically these RTGs produce less power each year. Voyager 1’s available power decreases by around 4 watts per year. That’s why engineers have slowly been turning of science instruments and conserving as much as possible, because the spacecraft has to operate on less and less power each year.

One of the results of this is a loss of heat. Basically between shutting off science instruments that generated heat and shutting off heaters of thruster branches they weren’t using, the spacecraft is cold. Because of this chill, engineers couldn’t just flip a switch and turn on the attitude control thrusters due to risk of damage. They needed to turn on their heaters first, and that’s the problem: Where’s the power for that? At this point, any extra power use on Voyager 1 has to come at the expense of something else. And they didn’t want to just turn off a science instrument in order to power the heaters for awhile — because what if they couldn’t turn it back on?

And the problem is also, there’s no real way to test this on the ground. According to NASA, there are no functional models, or testbeds, of Voyager hardware or software anymore. And on top of that, even if there were, there’s no real way to simulate the conditions that the attitude control thrusters have been in these last few years, nor that the spacecraft has been in for the last 47 years.

Engineers have actually used Voyager 2 as a testbed for Voyager 1 before. Because Voyager 1 is further out into interstellar space, it’s considered more valuable basically, at 15 billion miles away. Voyager 2 is at around 12 billion miles out.

Back in 2023, engineers sent a software patch to the Voyager spacecraft — but they sent it to Voyager 2 first. That way, they would know if the software rewrite would work before sending it onto Voyager 1. In this case, though, they had to work directly with Voyager 1 and make sure they got it right on the first try.

So they had to come up with another plan, and one that wouldn’t risk damaging the spacecraft’s thrusters or its instruments.

Engineers finally find the fix

In the end, the engineers determined it would be safe to turn off one of the spacecraft’s main heaters for up to an hour, which would allow enough power to flip on thruster heaters. But keep in mind that it takes 22.5 hours for a commend to reach Voyager 1 through the Deep Space Network.

That means they had to send the command to turn off the main heater, send the command to turn on the thruster heater, then after a delay (but not more than an hour!) send the command to turn on the thrusters, then send the command to turn off the thruster heaters, then, finally, send a command to turn the main heater back on. And then…just hope that it worked, because it would take another 22.5 hours (total roundtrip communication time of 45 hours) to check and see. (To be clear, these commands probably were sent through a program Voyager executed, versus sending the individual commands manually).

Well, the sigh of relief: For the first time in six years, on August 27, that branch of attitude control thrusters pointed Voyager back at Earth. The thruster switch was successful.

How long can Voyager 1 continue operating?

Voyager 1 was designed for a five-year mission, and yet here it is 47 years later, still going. Currently, Voyager 1 has for operational instruments: the plasma wave subsystem, magnetometer instrument, cosmic ray subsystem, and low energy charged particle instrument. Six additional instruments were either turned off after the Saturn flyby or have been switched off due to dwindling power.

As I mentioned, Voyager 1 has about 10, maybe 15 years of fuel left. But really, engineers are just hoping to keep the spacecraft operational through its 50th anniversary in 2027 (current estimates are that it will continue sending back science data through at least 2025). The hope was with the switch to the TCM thrusters, they’d buy another three to five years for the spacecraft, but now we’re back to the still-clogged attitude control thrusters, and it’s not clear how long we’ll be able to point Voyager 1 towards the Earth.

If Voyager 1 can still point itself towards Earth, it will continue to send back engineering data as long as it can, through 2036. Science instruments will have to be shut off well before then, but 2036 is the point Voyager 1 will travel beyond the range of the Deep Space Network and it will no longer be able to communicate with Earth, even if it still has power and the ability to keep itself pointed at the Earth.

For now, I’m just really glad to see this spacecraft still operational. It’s hard not to get attached to various spacecraft, and Voyager 1 is one that I’m VERY attached to, so I’m glad we’re not having to say goodbye yet — even though I know it’s coming soon.

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