Interstellar Probe mission would go twice as fast and far as the Voyagers

Kirby Runyon leads a project designed to take humanity farther than anyone has gone before, no technological breakthroughs required. Here's what he has to say about it.
By | Published: February 10, 2021 | Last updated on May 18, 2023

Breakthrough Starshot isn’t the only interstellar plan around. Planetary geomorphologist Kirby Runyon leads the working group for the Interstellar Probe concept.


We are all living in a bubble. In pandemic times, we’ve been stuck in our little social bubbles for nearly a year. Politically speaking, we’ve been living in bubbles for decades, probably. But astronomically speaking, this thing has been going on for about 4.5 billion years, ever since the Sun ignited its internal nuclear reactor and began blowing out a steady flow of charged particles, the solar wind.

The blowing of the solar wind creates an enormous magnetic bubble around the Sun, known as the heliosphere. The interstellar medium — the wisps of atoms and molecules that flow between the stars — is pushed away by the heliosphere. Earth and all of the other major planets are embedded deep inside, cut off from the rest of our galaxy. The team that has developed the Interstellar Probe concept want to escape from the trap.

Kirby Runyon, a planetary geomorphologist at Johns Hopkins University, is the planetary science working group lead for the Interstellar Probe. It’s a concept that has been in discussion and in development for decades, but recently the idea has been getting more serious attention and a more fully developed plan for execution. Upcoming heavy-lift rockets from NASA and SpaceX could finally provide the kind of oompf that Insterstellar Probe would need to reach interstellar space in a reasonable amount of time.

But even at the best-available speed, time will still be a huge challenge. The soonest a mission like this could plausibly launch is in the 2030s. From there, the primary mission would last 50 years, and the extended mission could run much longer if the hardware survives. For anyone working on such a mad venture, Interstellar Probe would be a multi-generational effort. It might reach fruition in the lifetime of people working on it now, but it would certainly exceed the span of any researcher’s active career.

And yet Runyon is cheery, optimistic, and utterly driven to make Interstellar Probe happen. It’s one of the most appealing attributes of the people who work on long-distance, long-timeline planetary missions. By necessity, they have to think about goals larger than themselves, and about efforts that will continue long after they are gone. Thinking inside the bubble is simply not a meaningful option.

I spoke with Runyon about the goals and challenges of launching Interstellar Probe. An edited version of our Zoom conversation follows.

Pay attention to those powers of 10: It’s a long, long way from here to the interstellar medium.
“Interstellar Probe” sounds like something from science fiction, but I know that you’re quite serious about this as a near-term mission. What are your actual goals?

Thanks for asking about Interstellar Probe. It’s one of my favorite things to talk about. Interstellar Probe is actually a heliophysics mission to understand the furthest reaches of the Sun’s influence, the heliosphere, and to dip our toes into interstellar space. It would be the first purpose-built mission to get to interstellar space — not to another star, but to that realm of space just beyond the Sun’s particles and the influence of its magnetic field, all of the atoms and plasma moving out from the Sun.

How far are you aiming to go? Could you get out to 1,000 AU, that is, 1,000 times Earth’s distance from the Sun?

The goal is to leave the solar system quickly on a 50-year prime mission, and to get as far as we can. There’s an independent criterion to be able to operate at 1,000 AU, even if it takes longer than the prime mission to get there.

The goal is to get into interstellar space, which we think begins roughly around 120, 130 AU, as shown by the Voyager 1 and 2 spacecraft. With the speeds we’re talking about, we think we can get Interstellar Probe up to carry out a 50-year mission. With that timespan, we think we can get beyond 350 AU, which is well into interstellar space. We want to get to hundreds AU with Interstellar Probe.

NASA has sent five spacecraft on interstellar trajectories, but they were all planetary missions that are leaving the solar system basically as a side-effect. Interstellar Probe would be different, right?

Yes, with Interstellar Probe we hope we can create a large, strategic mission that is shared between at least two divisions, and maybe three divisions, within NASA. This would be unprecedented. We want to break down the pigeonholes between being heliophysics or astrophysics or planetary science. As much as we want the science to be interdisciplinary, we also hope that the money can flow between different parts of NASA. That might be even harder.

Could Interstellar Probe be a high-frontier planetary mission as well, visiting the dwarf planets of the outer solar system?

We hope that, if NASA decided to go through with Interstellar Probe, that they would strongly consider what we would call the “planetary augmentation” to Interstellar Probe. We can put a camera — or, really more accurately, an imaging spectrometer — on board the spacecraft and fly by one of the solar system’s 130 dwarf planets. I want to break a misconception about dwarf planets. Whether or not you think dwarf planets are planets, whether or not you think Pluto’s a planet, we need a different way of thinking about the solar system as opposed to eight or nine planets.

Dwarf planets are the most common type of planet in the solar system, but we’ve seen only one of the up close. Interstellar Probe could rendezvous with another on its way off to the stars.
Oh man, you’re willing to dive into the “what is a planet” debate. You’ve spoken with Alan Stern about this, I expect?

Alan Stern and I pretty much 100 percent agree with each other on this. Forget about eight or nine planets that are in concentric circles around the Sun. Both planetary scientists and the IAU [International Astronomical Union] consider Pluto to be a dwarf planet. Okay. What bothers me is when people say that dwarf planets are not planets. That is a grammatical absurdity.

To me, we have terrestrial planets. We have an adjective in front of a noun. We have giant planets. There’s four of those. And dwarf planets, they’re just like other planets, except that they’re little. I don’t buy that IAU “clear its neighborhood” bullcrap. They’re just little. Pretty much everyone agrees that Pluto is a dwarf planet. It just depends on whether you think dwarf planets are planets or not. Pluto is the largest known dwarf planet, and it is the archetype for trans-Neptunian dwarf planets. So that’s my soapbox on why Pluto still a planet. Sorry, how’d I get off on that?

I led you off-track, asking about Interstellar Probe as a mission to the dwarf planets. 

Right! There’s a hundred thirty known dwarf planets in the trans-Neptunian region. They’re the most common type of planet. If you accept dwarf planets as a type of planet, dwarf planets are the most common type of planet in the solar system, far outnumbering giant planets and terrestrial planets and even satellite planets. And yet, they’re also the least explored and least understood category of planet.

If NASA headquarters decides to (a) do Interstellar Probe and (b) choose the planetary augmentation where we can put an imaging spectrometer on, we can fly by another one of these dwarf planets, probably either Quaoar or Gonggong, formerly 2007 OR10 —

Wait, the IAU finally gave a formal name to that poor dwarf planet? It had been sitting there with nothing but a number for years.

Thank God, it got a real name. 2007 OR10 is now Gonggong. It’s a Chinese creation deity.

Both of these dwarf planets right on that line between either having lots of bizarre ices, like frozen methane and nitrogen, or not. So they may or may not be active. They’re right on that line between geologically really, really interesting and maybe geologically subdued. Interstellar Probe, hopefully, would be able to fly by Quaoar or Gonggong or another dwarf planet. With even one more flyby of another trans-Neptunian planet [besides Pluto and Neptune’s moon Triton, which is probably a captured dwarf planet], we can revolutionize the field of comparative planetology among dwarf planets.

A 50-year mission is awfully long compared to a human lifetime. If Interstellar Probe launches in the 2030s, you’ll be in your 90s by the end. Do you dream of faster rockets that could speed things up?

Let me start with our engineering philosophy with Interstellar Probe. It’s a pragmatic mission. The idea is that nothing new has to get invented for us to fly. We have a very pragmatic, almost conservative, engineering approach where were talking only about existing technology that can be tweaked a little bit. We want to actually go do it. We don’t need to invent anything new.

The only thing that we would call a miracle, so to speak, is that we need is a super heavy launch vehicle like NASA’s SLS or Starship Super Heavy from SpaceX [neither of which is ready yet, but should be soon]. We’re using SLS as a baseline, because NASA has some pretty good engineering performance numbers on that rocket. Using that rocket, we think we can achieve speeds of between 7 and 8 astronomical units per year. That compares to Voyager 1, our next fastest spacecraft, at 3.6 astronomical units per year. We’d be more than doubling the current record.

There are experimental technologies under development, at JPL for instance, that might be able to double that speed again. Are they worth waiting for?

If we could double that speed, getting up to 14 to 16 AU per year, for the science objectives for this mission, it actually wouldn’t be that useful. Going 7 or 8 AU per year is a nice sweet spot where you leave the heliosphere pretty quickly, get into interstellar space within about 15 years or so, but go slowly enough so that the instruments on board the spacecraft can sense the Sun breathing.

The solar wind puffs the heliosphere out and then it comes back in. That “breathing” happens slowly, and we want to be going slowly enough that we can record it. If we just go out of the heliosphere too fast, we’re going to miss a lot of those solar dynamics.

So slow is bad, but too fast is bad as well?

Yeah. Now if we wanted to go to explore the dwarf planet, Sedna, which currently is at around 90 AU or so from the sun and gets out to almost 950 AU on this really long elliptical orbit, then a factor of two faster would be useful. If we didn’t care what the heliosphere is doing and we wanted to explore farther out into the interstellar medium or explore a more distant trans-Neptunian planet, then faster would be useful. If Planet X exists — this planet hypothesized by Mike Brown and Konstantin Batygin — then we would want to go really, really fast to get out to it on a reasonable time scale.

Regardless, there really doesn’t seem to be any viable technology available in the next decade that we could use to do that. Rather than constantly waiting for the next decade to get here for new technology to be developed, we want to do this mission basically now. We’ve been talking about it since the early ’60s. Let’s go do it!

I have to ask one more question about high-speed spaceflight, because I am excited by the idea of exploring the far, far outer solar system. What do you think of the idea of a solar-thermal rocket: flying close to the Sun, then letting solar heat boil your fuel for a big rocket boost?

With solar thermal and a solar Oberth maneuver [getting maximum boost closest to the Sun] the problem is the mass of your heat shield and the mass of all the plumbing you need for your rocket system. By the time you’ve got all that together, you might as well go with a more conventional propulsion system in a much, much lighter spacecraft. The engineering trade-off studies tend to favor more pragmatic, near-term technology.

Well that’s kind of depressing.

Not at all! I’m a scientist, but I’ve found that my brain likes to think like an engineer. It’s such a puzzle to solve: We could go faster, but if we do this, then we need to bring more fuel along, which means our fuel tanks have to get bigger, which means we’re more massive to begin with. Then that has implications for the thermal problems on the spacecraft…

It’s a whole optimization problem for every component: the communications, the radio, the thermal protection, the mass, how big the fuel tanks are. Do you use just hydrazine [propellant] or do you use both hydrazine and nitrogen tetroxide? Are you a spin-stabilized or can you have three-axis control to orient the spacecraft in any direction? I find these problems really fascinating. And optimizing and trying to find the right solution is a pretty cool problem to try to solve.

It’s striking that the problems you are trying to solve are about the very hardest ones in all of space exploration. Given the difficulty, the expense, and the time involved, why are you so drawn to a far-frontier mission like Interstellar Probe?

When I was an undergrad, I shifted my focus from wanting to do a PhD in astrophysics to wanting to do a PhD in planetary geology. One of the reasons was that I want to be able to, by proxy through a robotic spacecraft, physically go to these places. Astrophysics is awesome, don’t get me wrong, but we’ll never go to a neutron star. That will never happen.

There’s something satisfying of going to the far-off place that you’re studying. Maybe someday we’ll send a spacecraft to the closest star system, to the Alpha Centauri star system. For now, this is the best we can do. I guess I’m just a romantic and I’m drawn to the frontier.

Beautifully said. And I love that there’s a cat prancing around behind you as you’re as you’re talking about this.

Yeah, that’s Nixie. She’s named after Nix, a moon of Pluto.

For more science news and ideas, follow me on Twitter@coreyspowell