NASA's twin Voyager spacecraft, launched in 1977, have been traveling for so long that they've left our solar system. Amazingly, these venerable probes still talk to Earth, but their plutonium-powered energy supply is getting ever closer to running out.
That's why NASA asked a team of scientists and engineers to come up with a successor mission that could pick up where the Voyagers left off. The group, which will finish a report on their work within weeks, has designed a practical, doable spacecraft that could go faster than the Voyagers and much farther out into interstellar space.
A mission to last a century — or more
If NASA decides to build this probe, it could launch in 2036 and boldly venture forth for a minimum of 50 years and possibly more than a century. That means it would have the longest planned duration of any NASA mission, by a long shot.
Agency officials would have to cope with all of its technology inevitably going obsolete — to say nothing of all the people involved in the mission getting older and dying.
"It's always hard to talk about transition plans," says Ralph McNutt of the Johns Hopkins University Applied Physics Laboratory, who leads the team working on this proposed mission. "Typically, it hasn't really been happening in most space missions. But on this one, it's going to be front and center."
McNutt, who is about to turn 68 years old, thinks he's the youngest member of the Voyager science team who has been on it since the spacecraft launched. Some of the scientists originally involved with Voyager "did indeed pass away," he says. "Some of those transitions were a little bit rough."
No one could have known that the Voyager spacecraft would last so long; they originally were built to survive for only about five years, so that they could visit Jupiter and Saturn. But the hardware kept working, and so NASA added on flybys of Uranus and Neptune.
And still the plucky spacecraft continued on, faithfully reporting back to Earth. Each made history by leaving the bubble around the sun that's considered the boundary of the solar system, crossing into the unexplored space between stars.
"We're just flat running out of power"
Scientists who weren't even born when the Voyagers launched, like 25-year-old Stella Ocker, a graduate student at Cornell University, now depend on data that the Voyager probes send back as they move through this uncharted territory.
Voyager 1 is now more than 14 billion miles away, and Voyager 2 is more than 11 billion miles away. To the casual observer, it would look like they're moving through black nothingness, but Ocker says that's not exactly true.
"The interstellar medium seems like it's empty, but it's really not," says Ocker. "It's full of gas and dust and cosmic rays, energetic particles."
When it comes to understanding this part of the universe, she says, "there's still huge gaps in our knowledge that can really only be filled by direct sampling."
The Voyagers have been doing that and constantly send back information on the density and characteristics of the interstellar gas they're moving through.
But unfortunately, "we're just flat running out of power," says McNutt. The science team has been thinking of clever ways to conserve what's left to keep the Voyagers going for as long as possible. "We're looking at which instrument gets to have their heaters all turned off first."
Current predictions say that the very last instrument might get shut down around 2030 or 2031, he says, assuming nothing breaks before then.
Knowing this day would come, a few years ago NASA officials asked McNutt and his colleagues to come up with a plan for a new interstellar mission, so that the legacy of the Voyagers didn't just end.
Their proposed probe relies on technology that's either tried-and-true or already far along in development, with a price tag similar to the recent Parker Solar Probe, which was recently sent hurtling toward the sun at a cost of $1.5 billion.
While other groups have dreamed up interstellar missions in the past or are working on them now, those efforts aren't ready for prime time because they're either incredibly expensive or involve serious engineering challenges, says Michael Paul, who serves as project manager for the interstellar probe study team at the Johns Hopkins University Applied Physics Laboratory.
For example, one group called Breakthrough Starshot has an admirable vision of sending out many tiny probes that get pushed forward by powerful lasers, says Paul, but "you and I are probably never going to get to see them operate."
He and his co-workers, in contrast, were determined to devise a pragmatic interstellar mission blueprint for NASA that "wasn't so pie in the sky that it just ended up on people's shelves."
On its way out of the solar system, this proposed spacecraft could swing by a dwarf planet, similar to the New Horizons mission that made the first visit to Pluto in 2015. Kirby Runyon, also at the Johns Hopkins University Applied Physics Laboratory, notes that scientists know little about dwarf planets, even though they are the most common kind of planet in the solar system.
"We have a handful of terrestrial planets, and we have a handful of giant planets, but we've got over 130 dwarf planets," says Runyon. He points out that many of these icy bodies may have started out as ocean worlds that might even have been habitable in the past.
Besides visiting some mysterious dwarf planet, the spacecraft would be able to go about twice as fast as Voyager 1 and travel about 375 astronomical units, or over 34 billion miles, within the first 50 years.
McNutt thinks it's entirely plausible that, like the Voyagers, this spacecraft could just keep going and end up more than 800 astronomical units or 74 billion miles away after traveling for a century.
That may sound far, but Proxima Centauri, the closest star to our own, is about 25 trillion miles away.
Designing a multigenerational space plan
Still, this tiny step out into interstellar space may help in designing future missions that could actually reach other stars, says Ocker.
"We do need to learn how to conduct missions over these very long timescales if we are ever going to come close to achieving any of the aspirations of interstellar exploration that are so often posed in the popular media," she says.
To help understand how to deal with the intergenerational nature of this proposed mission, the research team reached out to Janet Vertesi, a sociologist with Princeton University who has studied the organizational aspects of other projects in space.
"A lot of NASA missions have this as a happy problem. I mean, the Mars rovers that were supposed to last for 90 days lasted for like 12 years," notes Vertesi, who says it's a different matter to make a coordinated plan for a long-term mission.
The researchers already knew they'd need to stockpile computers and other technical components that might become obsolete, she says. And they knew that at some point, they'd have to pass the baton to the next generation of scientists.
"What they hadn't really considered, and where my expertise came in, was how frequently that has to happen for that to be an expected and normal part of the mission operations and not a major breach or big problem," says Vertesi.
In hospitals, for example, frequent handoffs between shifts mean that doctors and nurses have worked out checklists and other standard procedures to make sure the transition can happen seamlessly. "That's the kind of thing they needed to get good at thinking about and planning for," says Vertesi.
She's led discussions with the researchers to help them sort through this. Astronomer Carey Lisse, who is working on the interstellar probe study, said these sessions were "very blunt and made us think a lot."
He's done the math. "I will be 75 in 2036 when we launch. That means that I know I'm not going to be on this mission probably for more than 10 years after launch," says Lisse, adding that the need for handovers is just a fact. "This isn't just theory or just talk. It's going to happen multiple times, probably two or three times at least."
It's challenging to anticipate how the program will need to change over time, says Paul, and to look forward to "what the demographics of the science community, the engineering community, and the whole world is going to look like, so that this is a program for them, and not for us."
Ocker, who doesn't even have a Ph.D. yet, points out that she'll be late in her career by the time this probe reaches interstellar space, if NASA decides to support it and if it launches in the 2030s. "I'm very hopeful that this mission will happen. I really hope it does, in which case I'll be very excited to use the data when it does eventually come down," she says.
She'd also love for it to carry some kind of contemporary take on the Golden Records, the phonograph albums carried by the Voyager probes to share greetings and Earth sounds with any aliens who might stumble across them out there in interstellar space.
"I don't think we should try to copy the Golden Record," she says, "but I think it would be really amazing to have a similar kind of public outreach piece that plays an important part in the mission."