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Interstellar Probe: Pushing Beyond Voyager

Our doughty Voyager 1 and 2, their operations enabled by radioisotope power systems that convert heat produced by the decay of plutonium-238 into electricity, have been pushing outward through and beyond the Solar System since 1977. Designed for a four and a half year mission, we now have, more or less by accident and good fortune, our first active probes of nearby interstellar space. But not for long. At some point before the end of this decade, both craft will lack the power to keep any of their scientific instruments functioning, and one great chapter in exploration will close.

What will the successor to the Voyagers look like? The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has been working on a probe of the local interstellar medium. We’re talking about a robotic venture that would be humanity’s first dedicated mission to push into regions that future, longer-range interstellar craft will have to cross as they move far beyond the Sun. If it flies, Interstellar Probe would be our first mission designed from the start to be an interstellar craft.

Pontus Brandt is an Interstellar Probe Concept Study project scientist, in addition to being principal investigator for two instruments aboard the European Space Agency’s Jupiter Icy Moon Explorer (JUICE) Mission. Brandt puts the ongoing work in context in a recent email:

Interstellar Probe would represent Humanity’s first deliberate step into interstellar space and go farther and faster than any spacecraft before. By using conventional propulsion, Interstellar Probe would travel through the boundaries of the protective heliosphere into the unknown interstellar cloud for the first time. Within its lifetime, it would push far beyond the Voyager mission to explore the heliospheric boundary and interstellar space so that we can ultimately understand where our home came from, and where we are going.

Image: A possible operation scenario, divided into phases and indicating science goals along the way. Credit: JHU/APL, from the Interstellar Probe 2019 Report.

The nature of the interstellar cloud Brandt refers to is significant. But before examining it, a bit of background. APL’s role in Interstellar Probe has roots in principal investigator Ralph McNutt’s tireless advocacy of what was once called Innovative Interstellar Explorer, a report originally funded by NASA in 2003 and often discussed in these pages. The current study began in 2018 and will continue through early 2022, examining the technologies that would make Interstellar Probe possible, with an eye on the coming Decadal Survey within NASA’s Heliophysics Science Division. Bear in mind as well that the space community has been discussing what we can call ‘interstellar precursor’ missions all the way back to the 1960s — an interesting story in itself! — and the Interstellar Probe concept appeared in the 2003 and 2013 Heliophysics Decadal Surveys.

About those Decadals: Every ten years, Decadal Surveys appear for the four NASA science mission divisions: Planetary Science, Astrophysics, Heliophysics and Earth Science, the idea being to provide guidance for the agency’s science program going forward. So the immediate context of the current effort at APL is that it is being conducted to provide technical input that can feed into the next Heliophysics Decadal Survey, which will cover the years 2023 to 2032. But the implications for science across all four divisions are part of APL’s remit, affecting specific targets and payloads.

What can realistically be done within the 2023-2032 time frame? And what kind of science could a mission like this, launching perhaps in 2030, hope to accomplish? Workshops began in June of 2018 and continue to refine science goals and support engineering trade studies in support of what the team calls “a ‘pragmatic’ interstellar probe mission.” The most recent of these, the fourth, just concluded on October 1. You can see its agenda here.

A launch in the early 2030s demands not futuristic technologies now in their infancy but proven methods that can be pushed hard in new directions. This is, you might say, ‘Voyager Plus’ rather than the starship Enterprise, but you build interstellar capability incrementally absent unexpected breakthroughs. That calls for a certain brute force determination to keep pushing boundaries, something Ralph McNutt and team have been doing at APL, to their great credit , for many years now. A spacecraft like this would be a flagship mission (now known as a Large Strategic Science Mission) — these are the most ambitious missions the agency will fly, a class that has included the Voyagers themselves, Cassini, Hubble and the James Webb Space Telescope.

A variety of methods for reaching beyond the heliosphere in the shortest possible time have been under consideration, including an “Oberth maneuver” (named after scientist Hermann Oberth, who documented it in 1929), where a propulsive burn is performed during a close solar pass that has itself been enabled by a retrograde Jupiter gravity assist. Other Jupiter flyby options, with or without a propulsive burn via a possible upper stage, remain on the table. The plan is to drive the probe out of the Solar System at speeds sufficient to reach the heliopause in 15 years. The participating scientists talk in terms of a flyout speed of 20 AU/year, which translates to 95 kilometers per second. Voyager 1, by comparison, is currently moving at roughly 17.1 kilometers per second.

The Voyagers own our current distance records, with Voyager 1 currently at 154 AU and Voyager 2 at 128 AU. Interstellar Probe would still be returning science at 1000 AU, meaning it would be capable of looking back and seeing not just the Earth in the context of the Solar System, as in Voyager’s ‘pale blue dot’ image, but also taking measurements of the heliosphere from well outside it, helping us understand both the interstellar medium and the effect of our stellar system as it moves through it.

There is much to be learned about the protective magnetic bubble called the heliosphere in which the entire Solar System is embedded. We have to understand that it is anything but static, as Pontus Brandt explains:

During its evolutionary journey around the galaxy, [the Sun] has plowed through widely different environments, witnessed supernova explosions on its path, that have all shaped the system that we live in today. The vast differences in interstellar densities, speeds, charge fractions have been responsible for an extreme range of sizes and shapes of the global heliosphere throughout its history – from many times bigger than today, to a tiny heliosphere below even the orbit of Earth. This, in turn, has had dramatic consequences for the penetration of the primordial soup of interstellar material that have affected several crucial aspects of elemental and isotopic abundances, atmospheric evolution, conditions for habitability and perhaps even biological evolution. Only some 60, 000 years ago, the Sun entered the vast Local Interstellar Cloud (some 30 light years across), and in just a few thousand years the solar system will enter a completely different interstellar cloud that will continue to shape its evolution.

Image: The Sun is on the way to exiting the Local Interstellar Cloud and entering another unexplored interstellar region. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.

An interstellar precursor mission can examine energetic neutral atoms (ENAs) to provide data on the overall shape of the heliosphere. Major issues include how plasma from the Sun’s solar wind interacts with interstellar dust to form and continue to shape the heliosphere.

But a mission like this also shapes our views of time, as the Voyagers have done as we have watched their progress through the Solar System, the heliosphere and beyond. Mission scientists turned the 4.5 year mission into a surprising 45 year one solely on the strength of their design and the quality of their components, not to mention the unflagging efforts of the team that operates them. A mission designed from the start for 50 years, as Interstellar Probe would be, will likely have a lifetime far beyond that. Its components are meant to be functional when our grandchildren are in their dotage. Most of its controllers in 2080 have yet to be born.

So this is a multi-generational challenge, a reach beyond individual lifetimes. Let me quote from the Interstellar Probe Study 2019 Report, which is now available online.

It is important to note that the study does not purport to center on “the one and only” interstellar probe but rather on this mission as a first step to more advanced missions and capabilities… In addition to promising historically groundbreaking discoveries, the Interstellar Probe necessitates a transformation in the programmatics needed to accommodate lifetime, reliability, and funding requirements for this new type of multigenerational, multi-decade operational mission. Paving the way for longer journeys utilizing future propulsion technologies, such as those not invoked here, the Interstellar Probe is the first explicit step we take today on the much longer path to the stars.

Principal investigator Ralph McNutt tells me that the Interstellar Probe team is finishing up a Mission Concept Report for NASA on the progress thus far, incorporating results of the recent workshop. This report should be available on the Interstellar Probe website in early December, with a number of items clarifying aspects of the currently available 2019 report. We need to dig into some of the issues that will appear there, for the concept is changing as new studies emerge. In particular, let’s look next time at the ‘Oberth maneuver’ idea, what it means, and whether it is in fact a practical option. I’m surprised at what’s emerging on this.

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{ 27 comments… add one }
  • John walker October 5, 2021, 8:08

    Off topic question, but does anyone have official word on when Astro2020 is getting released? As little as a few weeks ago the media was stating end of September. I find it unspeakably frustrating that even so close to release date, I’ve not been able to get an official confirmation. The report was scheduled to be released mid 2020, but was initially delayed to Jan 2021. Then spring 21, then summer 21, then Sep 21. Yes, the pandemic is ongoing, but this dangling carrot always just a couple months out is driving me nuts.

    • Paul Gilster October 5, 2021, 12:09

      John, all I have is the current update, which you’ve seen, I think:

      “The Astro2020 steering committee is nearing completion of the report, which they anticipate will be released in Fall 2021.”

  • Henry Cordova October 5, 2021, 10:31

    While reading this entry, I couldn’t help but be reminded of the A.E. van Vogt short story “Far Centaurus”. A sub-luminal earth expedition to Alpha Centauri encounters midway a ship from that star on the way towards them–with tragic results. The Centaureans are not aliens, but humans; the system had long been settled by colonists from earth and the explorers are awakened from cryosleep by automatic systems. just in time to meet the welcoming committee sent out to greet them! The non-relativistic spacecraft took so long to reach its destination that the invention of FTL tech while they were underway meant their mission was for nothing.

    This sort of possibility should be considered when planning long-term deep space missions like this one. There is always the chance that when our probe finally gets to where it is going, subsequent advances in propulsion technology will mean its destination will already be thoroughly explored by faster (and more capable) craft. By the time we arrive we’ll be obsolete. In other words, we will outrun our own technology.

    A related issue may be that these plans all assume we will retain at least our current level of technology, and sufficient long-term societal stability, to have a data reception system still in place when our probe reaches its destination. Can we say for certain we will still be interested in scientific data from deep space a century from now? Or that we will still have the technology available to receive and process it?
    Our probe may make astounding discoveries and transmit them home, but no one will be listening, or worse, no one will care.

    Of course, it would be foolish to suggest all exploration be postponed today just because we may have better methods available tomorrow. But when we start planning centuries in advance, perhaps these issues should be at least considered. To put it another way, we may find more effective ways to spend our resources if we do not invest them in projects that may not be necessary in the future.

    In our historical past, civilizations sometimes embarked on enterprises that would not deliver dividends for a lifetime, generations, even centuries; the Egyptian Pyramids, the Dutch land reclamation, the Medieval Cathedrals come to mind. But history moved at a more gradual pace then. The faster you go, the more critical your steering becomes, and the more precise your navigation must be.

    It may seem contradictory to say so, but the speed of progress now is so fast that it may be shortsighted to plan too far in advance.

    • Alex Tolley October 5, 2021, 11:35

      Your comment hadn’t posted when I was composing my comment, but we have similar conclusions. It may well be better to wait for better propulsion systems as the long time of the mission might mean that a later, faster probe could overtake the proposed probe before it reached its target of 1000 AU.

      • Patient Observer October 5, 2021, 20:20

        True, advancements in propulsion technology could render obsolete a launched mission. However, any probe, regardless of final velocity will also need communications systems and instrumentation suited for the task so those capabilities developed for the earlier, slower, probe will have value to subsequent probes. So, let’s get it on with the best near-term technology as, at a minimum, a path finder for hoped for faster probes.

        • Alex Tolley October 6, 2021, 9:49

          That is a good point. I think you are making a similar argument to the spinoff technologies from Apollo and the 1960s space program. However, those support technologies are not only needed for this mission, but for a range of other missions. Therefore they can still be developed even if this particular mission is not developed.

          [The Parker space pen was a novelty item that consumers could buy, although today all the pens I use need gravity to work. The older technology, – pencils – however, work regardless of gravity. The Russians made do with those on their human spaceflight missions.]

    • Gary Wilson October 6, 2021, 15:05

      I agree with Henry. Extremely long term goals (in human terms) of the order of 100 years are extremely risky and may divert us (and divert money and resources) from much more important short term goals related to ensuring our survival. Research into achieving higher and higher speeds will continue naturally and we can’t stop any government from expending money on extremely long term space exploration goals (although I haven’t really seen evidence of that occurring to any great extent) but my money would be spent on the shorter term. Goals such as those of SpaceX, i.e. getting to Mars with a significant number of people in 10-20 years make much more sense to me. The exploration of nearby star systems (or even reaching the 1000 AU target) probably won’t occur in the next 50 years. I spent a lot of time thinking about how to get to a nearby star years ago and came up with something very similar to Breakthrough Starshot (I in fact sent an email to a group thinking along those lines at the time) but now I believe the resources required to carry out the project should be used on different projects aimed at stabilizing the Earth’s biosphere. Living on Mars would be interesting but I think we’ll need Mother Earth to be viable for quite some time to come.

  • Alex Tolley October 5, 2021, 11:31

    The online document is a long read. Focusing on the propulsion and trajectory sections, it is clear that there is no viable option to reach a velocity 20 au/yr – 95 km/s. Even 10 au/yr is beyond current technology.

    Interestingly solar sails are ruled out (too underdeveloped for the time frame), as well as other high Isp propulsion systems. The preferred technology is cryogenic chemical (e.g. LH2/LOX) with solid rockets for perijove/perihelion maneuvers.

    Interestingly the targeted launch vehicle is the SLS, still not operational and extremely costly. A Falcon heavy could do the lifting IF the payload was reduced to 60 MT, or the probe could be delivered in 2 parted and mated in orbit. No mention of the Starship (document was too early) which could replace the SLS at a fraction of the cost, if it proves viable.

    So at this point, the targeted 20 au/yr velocity is remains out of reach, and even the 10 au/yr might not be achievable with the available technologies. [1000 AU would take 100 years to reach at this latter velocity.]

    While a early launch of an interstellar precursor probe would be interesting, the long time frame to reach its target distance suggests to me that we might be better waiting for a better propulsion system as a later probe might well overtake any probe launched with the propulsion technologies and maneuvers proposed in the document.

    • Antonio October 5, 2021, 15:42

      If we always wait for later probes there will never be later probes. Better do whatever you can now and develop iteratively. Risk is better than stagnation.

      • Alex Tolley October 5, 2021, 18:44

        That may be why the science goals included ones far inside the 1000 AU target. They would be reached earlier and would be unlikely to be overtaken by later, faster probes.

        However, if you want to reach 1000 AU, and the best you can hope for by the 2030s is to reach a final velocity < 10 AU/year, then there is a high probability that a much more potent propulsion system will be available such that a probes launched perhaps 50 years later will reach the target first. For example, low areal density solar sails using a sun diver maneuver should be able to exceed even 20 AU/year and be available within 50 years. Various nuclear powered probes with much higher Isp should also be available. We may not get such propulsion systems due to economic stagnation, or even collapse, but then who might even care that we have a slow probe sending data from 100s of AU out.

        • Antonio October 6, 2021, 1:52

          Who will develop the technology of those solar sail sundivers if probes like this one are cancelled?

          • Alex Tolley October 6, 2021, 10:03

            You may recall that no one was developing solar sail technology at all until the last 30 years. The Planetary Society gamely tried with its Cosmos 1 solar sail in the 1990s. JAXA was the first to develop a real interplanetary sail craft with IKAROS. Now we seem to have solar sail designs from several sources, included a beamed sail proposal by Breakthrough Starshot.

            We are seeing a similar pattern in the private, New Space arena. Mars missions were forever being proposed and cancelled, but just maybe SpaceX or China will put humans on Mars within a few decades.

            The 1000 AU mission is really a mapping/exploration mission with no obvious commercial spinoff. This suggests that a public agency will need to do the mission. However, that does not mean that the US is the de facto country to do this, despite its historic achievements in deep space. The mantle may pass to another country.

  • ljk October 5, 2021, 11:55

    Anything new happening with the Innovative Interstellar Explorer (IEE)? Or has this project been merged with the subject of this essay?

    https://www.centauri-dreams.org/2012/01/09/innovative-interstellar-explorer-a-response-to-questions/

    I hope those who are working on Interstellar Probe (this is just a placeholder name, right?) will also take into account what kind of information package to add to their vessel for any future recipients. I do not ever want to see again what was done – or should I say not done – with New Horizons.

    https://www.centauri-dreams.org/2013/01/18/the-last-pictures-contemporary-pessimism-and-hope-for-the-future/

    • Paul Gilster October 5, 2021, 12:07

      To my knowledge, the work on IIE has been folded into the current project.

  • Spaceman October 5, 2021, 12:43

    Does anyone know of any studies that aim to estimate how long the Voyager probes will remain physically intact? I vaguely remember hearing about a study that modeled the long term future of Voyager and they estimated that Voyagers could remain recognizable for billions of years….

    • Paul Gilster October 5, 2021, 15:30

      I don’t know about papers on this, but I do get into the matter in ‘Voyager to a Star’:

      https://www.centauri-dreams.org/2015/12/24/voyager-to-a-star-2/

    • ljk October 5, 2021, 15:39

      What I have read in terms of longevity for the Voyager Interstellar Record is that the side facing outwards from the probe will last a conservative estimate of 1 billion years before interstellar dust and gas wear it down enough to make it unusable.

      Now I am assuming here they mean that the cover will be worn down enough first so that the record itself is left exposed to gradual deterioration. The side facing the probe will last probably as long as the vessel itself, upwards of 10 billion years.

      However long the Voyager probes will last – and let us not forget Pioneer 10 and 11 and New Horizons, along with most of their final booster stages – they will exist far longer than anything left on Earth that is not deliberately preserved over time.

      This is why we should have (as an international law) information packages on every deep space vessel to both preserve a record of ourselves and our world and to help those who find them a way to identify their purposes and their makers.

  • Antonio October 5, 2021, 17:12

    Mmm if it’s launched in the early 2030s and it really travels at 10 AU/year… it could visit the Bernardinelli-Bernstein megacomet. If it only had a camera…

  • James Essig October 5, 2021, 17:50

    I remember seeing a show where you, Paul and Marc Millis were interviewed about Icarus Interstellar and Tau Zero Foundation stuff.

    I remember you talking about a concept for which a very wide and long starship suitable for intra-galactic travel and beyond would be made of drop-off tiles that would serve as local scout ships for landing parties and the like around specific stars.

    I really like the above concepts.

    Perhaps focusing on very mildly relativistic craft such as the tile ship you mentioned could be the way to go initially.

    The tile ship would indeed have greatly reduced collision risks since it would be long and have a very thin aspect ratio.

    Perhaps NASA can consider tile probes that have separable tiles that can be drop-off near stars but where the mother craft velocity would be about 0.1 c.

    As for such tile ships in hyperspace, that would be really cool but we are not there yet in physics and technology.

    The tile probes can likely work with nuclear fission powered systems that have high Q values, i.e, where they extract energy from the background such as magnetic field energy, or perhaps by collecting ions and electrons and neutralizing the atoms in a linear exhaust chamber, This way, drag is greatly reduced and the emissions of the newly formed neutral atoms can be collected and used for spacecraft propulsion systems.

    Ions that are reduced in charge but not neutralized may be employed in electro-hydro-dynamic-plasma-drives, magneto-hydro-dynamic-plasma-drives, electro-magneto-hydro-dynamic-plasma-drives, electromagnetic-hydro-dynamic-plasma-drives, plasma sails, and plasma bottle sails.

    For really high Q values (i.e., measures of the ratio of the carried aboard fuel derived propulsion power and propulsion power sourced from the background), we might plausibly obtain sub-canonical ensemble Lorentz factors given enough ship time. By sub-canonical ensemble, I mean values such as 10 EXP 40 to about 10 EXP 1,000 or more. Currently, these Lorentz factors are a mere pipe dream but given trillions of years of technological innovation, perhaps these values can be realized.

    • Paul Gilster October 5, 2021, 19:42

      Probably our appearance on a History Channel show some years back introducing one of the Star Trek movies. That was a wonderful gig, and they asked a lot of intelligent questions.

      • James Essig October 7, 2021, 11:23

        I remember that show now more clearly. You folks gave wonderful presentations. I have to give you my gratitude especially for bringing up the tile idea which is now one of the starship concepts that interests me the most.

        A key point will be maintaining governance, order, and mission directorates for very long periods of time.

        However, I think we can do such travel itineraries with current science and technology levels.

        As you mentioned previously, some of the great cathedrals in Europe were multi-century projects.

        As I consider the U.S. Constitution and how long it has lasted even under modification with amendments, I think the political science folks and psychologists can put together programs to ensure that such missions can succeed.

        Cryogenic sleep or low temperature alive and wake hibernation states might also be very useful when developed. Nano-technology might be used to maintain the integrity of the human body at temperatures just above freezing for periods perhaps measured in eons.

  • P October 5, 2021, 22:30

    Opportunities for escape missions like this are so rare that surely a fast flyby of a suitable large TNO on the way out could be studied and would help bolster the science rationale. That means an imaging instrument, not just field instruments.

    P

    • Ivar Brouwer October 6, 2021, 13:32

      My thought exactly, there are concepts for flyby missions for the Ice Giants and a re-visit of Pluto in investigative phases.. I always thought these flyby’s a bit frivolous, a lot of money for a short return of value. but, with larger objectives of investigating the space beyond the heliosphere in mind it suddenly looks like flyby missions have a lot more value to offer.

  • torque_xtr October 6, 2021, 9:22

    A probe for 1000 AU launched in 2030-s with 20 AU/yr terminal speed really calls for at least some solar gravitational lens science. If not exoplanets themselves, then a look in the young universe, something with which Hubble Deep Field compares like a smartphone night sky photo.

    PS There are many next-gen propulsion options, but sundiving Oberth maneuver is what comes in mind most often. Many interesting exoplanet targets are off-ecliptic and not favorable if Jupiter is used. And there is a beauty of solar thermal propulsion. Why go for combustion of high-performance fuel/oxidizer pair to obtain high chamber temperatures, when the outer side of graphite/carbide sunshield is heated to 3000 degrees and working fluid could be passed through channels in it? Even hydrazine acquires Isp on par of H2/LOx after such a boost. Then there is lithium hydride, which is easily storable, and decomposes to gaseous Li and atomic H with average molecular weight of 4 at such temperatures. And all this – deep within the deepest gravitational well of Solar system!

  • Alex Tolley October 6, 2021, 9:54

    @P, @torque_xtr.
    The online document makes it clear that there are a number of ancilliary targets on the way to 1000 AU. This includes TNO, and the reaching the sun’s gravity focal line. Whether the instrumentation to do anything useful at these targets is another question that will be resolved if the project ever goes ahead.

  • Gregor October 6, 2021, 13:32

    I’ve been wondering…
    Why not use a fast rocket to match orbits with a comet while it’s passing through the inner solar system, land an instrument package on the comet, and let it be carried out yonder?
    Best to do so when the comet is on the outward passage and not out-gassing as energetically.
    Pick a long-period comet from the Oort Cloud and hitch a ride way far out…

  • Jens Fridthjof Knudsen October 6, 2021, 16:20

    Has anyone had a closer look at the nuclear saltwater rocket? https://www.youtube.com/watch?v=cvZjhWE-3zM

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