NASA’s Project Scientist Faces Painful Choices as the Iconic Mission Nears Its End

In 1977, two probes launched less than a month apart on a mission to the great beyond. The twin Voyager spacecraft were to travel where no other mission had gone before, exploring what lies outside the vast bubble that surrounds our solar system, beyond the influence of our host star.

Voyager 1 reached the beginning of interstellar space in 2012, while Voyager 2 reached the boundary in 2018, traveling beyond the protective bubble surrounding the solar system known as the heliosphere. The Voyager probes were the first spacecraft to cross into interstellar space and have been exploring the unfamiliar region for nearly 48 years. But all good things must come to an end, and the iconic mission is gradually losing steam as it approaches oblivion.

The Voyagers are powered by heat from decaying plutonium, which is converted into electricity. Each year, the aging spacecraft lose about 4 watts of power. In an effort to conserve power, the mission team has turned off any systems that were deemed unnecessary, including a few science instruments. Each Voyager spacecraft began with 10 instruments, but now have just three each. The two spacecraft now have enough power to operate for another year or so before engineers are forced to turn off two more instruments.

The Voyager team, some of whom have worked on the mission since it first began, are forced to make these tough decisions to keep the mission going, in addition to coming up with creative solutions to resolve menacing glitches that affect the spacecraft as they weather the harsh space environment.

Linda Spilker, the Voyager mission’s project scientist, spoke to Gizmodo about the challenges that come with operating the outdated spacecraft, and passing on the knowledge of the Voyagers to the newer generations of scientists and engineers who have joined the mission.

This interview has been lightly edited for clarity and length.

Passant Rabie, Gizmodo: How long have you worked on the Voyager mission?

Spilker: I started working on Voyager in 1977, it was my first job out of college, and I had a choice between the Viking extended mission or the Voyager mission. I, of course, hadn’t heard of Voyager. So I said, where’s Voyager going? And they said, well, Jupiter and Saturn and onto Uranus and Neptune with Voyager 2 if all goes well. And I thought, oh my goodness—I remember in third grade, I got a little telescope I used to use to look at the Moon and look at Jupiter and Saturn, and look for little moons around Jupiter and see if I could spot the rings around Saturn. So the thought of a chance to go visit these worlds that were really only tiny dots in my little telescope, I said, “sign me up.”

Gizmodo: How has the mission evolved over the years?

Spilker: The number of people that are working on and flying Voyager is a whole lot smaller than it was in the planetary days. We’ve turned off a lot of instruments on Voyager. We had some pretty big teams with the remote sensing instruments, the cameras, the spectrometers, etc, that are out on a boom on the end of the spacecraft. As the instruments turned off, the mission got smaller.

There really was the thinking after Neptune, that Voyager would probably only last a few more years and so they had a very small team, and they kind of were, in a sense, basically neatening up everything and putting Voyager in a mode that could operate longterm. A lot of the engineers, as well as the scientists, were rolling off the mission, leaving just a very small operations team for what we call the Voyager interstellar mission.

The challenge was, can we reach the heliopause? We didn’t know where it was, we had no idea how far away it was. We got to Neptune, and then we thought, “well, maybe it’s just another 10 [astronomical units] or so, a little bit further, a little bit further.” And so every time we got a little bit further, the modelers would go back, scratch their heads and say, “ah, it could be a little bit more, a little bit farther away,” and so on and on that continued, until finally, Voyager 1 crossed the heliopause in 2012. If you think about that, that’s like 21 years after the start of the mission. And then, six years after that, Voyager 2 crossed the heliopause, and ever since then, they’ve been flying in interstellar space, making unique measurements about the particles in interstellar space, the cosmic ray abundance, the magnetic field. Basically, it’s a chance to explore—once you cross that boundary, there’s a whole new region, a whole new realm out there in interstellar space.

Gizmodo: Is it an emotional decision to turn off Voyager’s instruments?

Spilker:  I was talking to the cosmic ray instrument lead, and I said, “Wow, this must really be tough for you to see your instrument turned off.” He helped build the instrument in the early 1970s. This instrument that’s been sending you data, and that’s been part of your life for over 50 years now. And he said, it was hard to think about turning it off for the whole team. It’s kind of like losing a best friend, or someone that’s been a part of your life for so many years, and then suddenly, it’s silent.

At the same time, there’s this pride that you were part of that, and your instrument got so much great data—so it’s a mix of emotions.

Gizmodo: What are the challenges that come with operating a mission for this long?

Spilker:  The spacecraft was built in the 1970s, and so that’s the technology that we had in those days. And we didn’t have very much computer memory, so we had to be very careful and think through what we could do with this tiny amount of computer memory.

So the challenge with these aging components is, how long until a key piece fails? We’re well past the warranty of four years. We also have less power every year, about 4 watts less power so we have to find 4 watts per year to turn off on the spacecraft. The spacecraft had a lot of redundancy on it, so that means two of every computer and two of all the key components. We’ve been able to turn off those backup units, but we’re now at the point where, to really get a significant amount of power, all that’s left are some of the science instruments to turn off. So, that’s where we’re at.

Then, of course, if you have less power, the temperature goes down inside. There’s something called a bus that has all the electronics inside, and that’s getting colder and colder. Along the outside of the bus are these tiny lines of hydrazine that go to the thrusters, so we started to worry about the thermal constraints. How cold can the lines get before they freeze? How cold can some of these other components get before they stop working? So that’s another challenge.

Then there are individual tiny thrusters that align the spacecraft and keep that antenna pointed at the Earth so we can send the data back, and they’re very slowly clogging up with little bits of silica, and so their puffs are getting weaker and weaker. That’s another challenge that we’re going through to balance.

But we’re hopeful that we can get one, possibly two, spacecraft to the 50th anniversary in 2027. Voyager’s golden anniversary, and perhaps even into the early 2030s with one, maybe two, science instruments.

Gizmodo: What about the language that the spacecraft use?

Spilker:  They use something called machine language, and I think it’s a language that’s unique to Voyager’s program. There are three different computers, an attitude control computer, another computer for commands, and another computer that basically configures the data and sends it back to the ground.

So you have to configure these very tiny memories, and it’s in a machine language that nobody really uses anymore. We got some experts to come back and help us solve some of the problems we’ve had on the spacecraft, or other engineers who have had to learn the machine language. We had a chip failure on one of the computers, so we had to reprogram that computer and so we brought in some experts, and they really enjoyed it, trying to troubleshoot and figure out what’s wrong. And it was like a detective story, you know, what can we do? And they figured it out, and it worked.

With Voyager, what often happens is, everything looks really good and then something goes wrong on the spacecraft. And in this case, all of a sudden we went from data coming back every day to just a tone, a signal that said the spacecraft is still there.

One good analogy is going from getting letters from Voyager—you open them up and read about what’s happening every day—to now getting a letter, opening it, and finding it blank. You have no information coming back from Voyager. Imagine your computer fails, and the screen is dark

We were sending up commands and trying to figure out what happened, and ultimately got something called a memory readout, and we found that a chip had failed. We knew which parts of the computer programs were on that chip, and then it was a matter of taking those pieces and then finding enough free space on the rest of the computer to reprogram it and get it to work again. But in bringing in those people, where do you start? In the 70s, we didn’t have the computers we do today. A lot of Voyager material is in memos, and sometimes the memos are scanned in a PDF file. And so you have to go on, literally, a sort of a hunting, like, which would be the most useful for me to look at. Some of the engineers had a big diagram up on the wall of what the computer looked like and all the paths that it had to go through to figure it all out. And they just stuck sticky notes all over as they were figuring it out.

It was a mix of bringing in people who really knew and understood that computer—one of the retirees really understands the flight data system computer—and subject matter experts, and we would get them up to speed and have them work with the Voyager team. Meanwhile, the scientists are patiently waiting for their data to come back.

Gizmodo: You mentioned that the team has gotten smaller over the years. Is it basically the same people that have been working on the mission all along or do you have to bring in new people and fill them in?

Spilker: As you can imagine, most of the people are new. There are really only a handful that helped build the instruments in the 1970s, and a few of the scientists that are left have worked on the mission from the beginning until now.

We’ve actually brought back some people who retired, who were there in that time frame of building and coding Voyager, so they have come back and now work part time. Retirees are very happy to come back and help us. And then, of course, a lot of younger people that have come on and bring their own experiences, and so we’ve been training several new people recently into the roles that we need to operate.

On the science side, there’s a series of guest investigators—basically modelers and theorists—who work with the scientists on the Voyager teams to help pass that knowledge forward. In other words, to mentor the next generation of scientists who might want to work with the data in the future.

Gizmodo: As a scientist, what have been the most important things that you’ve learned from the Voyager mission?

Spilker: Voyager left breadcrumbs, clues for future missions to come. One of Voyager’s goals was to see through to the surface of Saturn’s Moon, Titan. We didn’t know if it could have liquid oceans on the surface, or what the surface looked like. During Voyager’s close flyby of Titan, we found that none of its instruments or camera filters could penetrate through the haze. It looked like a bad day in a smoggy city.

It was Voyager’s discovery, or non-discovery, of not being able to see the surface of Titan, that led to the Cassini mission. After Voyager’s flyby, NASA and the European Space Agency got together and said, “we need to go back.”

I had a chance to go work on Cassini. I got in very early, and helped formulate the mission concept. I spent around 30 years on Cassini, and then the mission ended in 2017. At that point, I was thinking of retiring but then I got the opportunity to go back to Voyager and work with Edward Stone [who served as project scientist for Voyager from 1972 to 2022] and the science team, and go back to the mission where I first started.

I went home and I told my husband, “I don’t think I’m going to retire.”

Gizmodo: How does it feel now that the mission is approaching its end?

Spilker: We’re hoping to get one or both spacecraft to Voyager’s golden anniversary, and that’s going to be in 2027. As we get closer to the end of the mission, for me personally, it’s kind of like wrapping up my career in a way—because I’ll probably retire once the Voyager mission ends. I’m just really, really happy to have been a part of it.

Gizmodo: There’s always this debate of whether we should launch another interstellar probe. I’m wondering how you feel about that?

Spilker: I think it would be a great idea, it could even go further than Voyager.

We know that material mostly comes from supernova explosions, and that those explosions create bubbles in space filled with material that came from the exploding star. Earth and the rest of the planets are inside this heliopause [the outer edge of the bubble that surrounds our solar system]. But there are other bubbles.

You can imagine, every time you have a supernova, you get a new bubble, and those bubbles are all there in space. How far do you have to keep going to reach another bubble? And what is it like to get farther and farther away from the Sun? One of the questions of the Voyager mission is, how far does the Sun’s influence continue into interstellar space?

We’re still working and thinking about an interstellar probe that would go much, much farther than Voyager. You’re talking about a multi-generation mission.

Gizmodo: Should we have already launched one?

Spilker: There’s so many interesting places to go. Prior to Voyager, we had no idea what the heliopause was like. Then getting this sort of taste of interstellar space makes us want to go back.

It’s like going to so many places, you get to answer all these questions and make tremendous discoveries, but you leave behind a list of questions that’s much longer than the ones you answered.

Gizmodo: Do you worry that we won’t be able to recreate a mission like Voyager again under the current circumstances at NASA?

Spilker: We’re entering a new and interesting era. You have the private industry wanting to play a bigger role in getting us to space. In a certain sense, some of these bigger rockets could deliver a mission to Uranus or Neptune in a much shorter time.

I see hopeful signs, but it’s always tough when you have budgets to balance and other things to look out for. But if you look at when I started at NASA’s Jet Propulsion Laboratory to now, the number of missions that are flying in space— whether they’re missions to planets or to study our Sun—there are so many more missions today. There’s just been sort of a blossoming of scientific missions and our understanding of our place in the universe.

So I’m hopeful, there’s always tough times to weather. We’ve been through tough times before, and I think we’ll weather this one.