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Interstellar Probe: Prospects for ESA Technologies

The Interstellar Probe concept being developed at Johns Hopkins Applied Physics Laboratory is not alone in the panoply of interstellar studies. We’ve examined the JHU/APL effort in a series of articles, the most recent being NASA Interstellar Probe: Overview and Prospects. But we should keep in mind that a number of white papers have been submitted to the European Space Agency in response to the effort known as Cosmic Vision and Voyage 2050. One of these, called STELLA, has been put forward to highlight a potential European contribution to the NASA probe beyond the heliosphere.

Image: A broad theme of overlapping waves of discovery informs ESA’s Cosmic Vision and Voyage 2050 report, here symbolized by icy moons of a gas giant, an temperate exoplanet and the interstellar medium itself, with all it can teach us about galactic evolution. Among the projects discussed in the report is NASA’s Interstellar Probe concept. Credit: ESA.

Remember that Interstellar Probe (which needs a catchier name) focuses on reaching the interstellar medium beyond the heliosphere and studying the interactions there between the ‘bubble’ that surrounds the Solar System and interstellar space beyond. The core concept is to launch a probe explicitly designed (in ways that the two Voyagers currently out there most certainly were not) to study this region. The goal will be to travel faster than the Voyagers with a complex science payload, reaching and returning data from as far away as 1000 AU in a working lifetime of 50 years.

But note that ‘as far away as 1000 AU’ and realize that it’s a highly optimistic stretch goal. A recent paper, McNutt et al., examined in the Centauri Dreams post linked above, explains the target by saying “To travel as far and as fast as possible with available technology…” and thus to reach the interstellar medium as fast as possible and travel as far into it as possible with scientific data return lasting 50 years. From another paper, Brandt et al. (citation below) comes this set of requirements:

  • The study shall consider technology that could be ready for launch on 1 January 2030.
  • The design life of the mission shall be no less than 50 years.
  • The spacecraft shall be able to operate and communicate at 1000 AU.
  • The spacecraft power shall be no less than 300 W at end of nominal mission.

This would be humanity’s first mission dedicated to reaching beyond the Solar System in its fundamental design, and it draws attention across the space community. How space agencies work together could form a major study in itself. For today, I’ll just mention a few bullet points: ESA’s Faint Object Camera (FOC) was aboard Hubble at launch, and the agency built the solar panels needed to power up the instrument. The recent successes of the James Webb Space Telescope remind us that it launched with NIRSpec, the Near-InfraRed Spectrograph, and the Mid-InfraRed Instrument (MIRI), both contributed by ESA. And let’s not forget that JWST wouldn’t be up there without the latest version of the superb Ariane 5 launcher, Ariane 5 ECA. Nor should we neglect the cooperative arrangements in terms of management and technical implementation that have long kept the NASA connection with ESA on a productive track.

Image: This is Figure 1 from Brandt et al., a paper cited below out of JHU/APL that describes the Interstellar Probe mission from within. Caption: Fig. 1. Interstellar Probe on a fast trajectory to the Very Local Interstellar Medium would represent a snapshot to understand the current state of our habitable astrosphere in the VLISM, to ultimately be able to understand where our home came from and where it is going.

So it’s no surprise that a mission like Interstellar Probe would draw interest. Earlier ESA studies on a heliopause probe go back to 2007, and the study overview of that one can be found here. Outside potential NASA/ESA cooperation, I should also note that China is likewise studying a probe, intrigued by the prospect of reaching 100 AU by the 100th anniversary of the current government in 2049. So the idea of dedicated missions outside the Solar System is gaining serious traction.

But back to the Cosmic Vision and Voyage 2050 report, from which I extract this:

The great challenge for a mission to the interstellar medium is the requirement to reach 200 AU as fast as possible and ideally within 25-30 years. The necessary power source for this challenging mission requires ESA to cooperate with other agencies. An Interstellar Probe concept is under preparation to be proposed to the next US Solar and Space Physics Decadal Survey for consideration. If this concept is selected, a contribution from ESA bringing the European expertise in both remote and in situ observation is of significance for the international space plasma community, as exemplified by the successful joint ESA-NASA missions in solar and heliospheric physics: SOHO, Ulysses and Solar Orbiter.

I’m looking at the latest European white paper on the matter, whose title points to what could happen assuming the JHU interstellar probe concept is selected in the coming Heliophysics Decadal Survey (as we know, this is a big assumption, but we’ll see). The paper, “STELLA—Potential European contributions to a NASA-led interstellar probe,” appeared recently in Frontiers of Astronomy and Space Science (citation below), highlighting possible European contributions to the JHU/APL Interstellar Probe mission, and offering a quick overview of its technology, payload and objectives.

As mentioned, the only missions to have probed this region from within are the Voyagers, although the boundary has also been probed remotely in energetic neutral atoms by the Interstellar Boundary Explorer (IBEX) as well as the Cassini mission to Saturn. We’d like to go beyond the heliosphere with a dedicated mission not just because it’s a step toward much longer-range missions but also because the heliosphere itself is a matter of considerable controversy. Exactly what is its shape, and how does that shape vary with time? Sometimes it seems that our growing catalog of data has only served to raise more questions, as is often the case when pushing into territories previously unexplored. The white paper puts it this way:

The many and diverse in situ and remote-sensing observations obtained to date clearly emphasize the need for a new generation of more comprehensive measurements that are required to understand the global nature of our Sun’s interaction with the local galactic environment. Science requirements informed by the now available observations drive the measurement requirements of an ISP’s in situ and remote-sensing capabilities that would allow [us] to answer the open questions…

We need, in other words, to penetrate and move beyond the heliosphere to look back at it, producing the overview needed to study these interactions properly. But let’s pause on that term ‘interstellar probe.’ Exactly how do we characterize space beyond the heliosphere? Both our Voyager probes are now considered to be in interstellar space, but we should consider the more precise term Very Local Interstellar Medium (VLISM), and realize that where the Voyagers are is not truly interstellar, but a region highly influenced by the Sun and the heliosphere. The authors are clear that even VLISM doesn’t apply here, for to reach what they call the ‘pristine VLISM’ demands capabilities beyond even the interstellar probe concept being considered at JHU.

Jargon is tricky in any discipline, but in this case it helps to remember that we move outward in successive waves that are defined by our technological capabilities. If we can get to several hundred AU, we are still in a zone roiled by solar activity, but far enough out to draw meaningful conclusions about the heliosphere’s relationship to the solar wind and the effects of its termination out on the edge. In these terms, we should probably consider JHU/APL’s Interstellar Probe as a mission toward the true VLISM. Will it still be returning data when it gets there? A good question.

IP is also a mission with interesting science to perform along the way. A spacecraft on such a trajectory has the potential for flybys of outer system objects like dwarf planets (about 130 are known) and the myriad KBOs that populate the Kuiper Belt. Dust observations at increasing distances would help to define the circumsolar dust disk on which the evolution of the Solar System has depended, and relate this to what we see around other stars. We’ll also study extragalactic background light that should provide information about how stars and galaxies have evolved since the Big Bang.

Image: A visualization of Interstellar Probe leaving the Solar System. Credit: European Geosciences Union, Munich.

The white paper offers the range of outstanding science questions that come into play, so I’ll send you to it for more but ultimately to the latest two analytical descriptions out of JHU/APL, which are listed in the citations below. To develop instruments to meet these science goals would involve study by a NASA/ESA science definition team, and of course depends on whether the Interstellar Probe concept makes it through the Decadal selection. It’s interesting to see, though, that among the possible contributions this white paper suggests from ESA is one involving a core communications capability:

One of the key European industrial and programmatic contributions proposed in the STELLA proposal to ESA is an upgrade of the European deep space communication facility that would allow the precise range and range-rate measurements of the probe to address STELLA science goal Q5 [see below] but would also provide additional downlink of ISP data and thus increase the ISP science return. The facility would be a critical augmentation of the European Deep Space Antennas (DSA) not only for ISP but also for other planned missions, e.g., to the icy giants.

Q5, as referenced above, refers to testing General Relativity at various spatial scales all the way up to 350 AU, and the authors note that less than a decade after launch, such a probe would need a receiving station with the equivalent of 4 35-meter dishes, an architecture that would be developed during the early phases of the mission. On the spacecraft itself, the authors see the potential for providing the high gain antenna and communications infrastructure in a fully redundant X-band system that represents mature technology today. I’m interested to see that they eschew optical strategies, saying these would “pose too stringent pointing requirements on the spacecraft.”

STELLA makes the case for Europe:

The architecture of the array should be studied during an early phase of the mission (0/A). European industries are among the world leaders in the field. mtex antenna technology. (Germany) is the sole prime to develop a production-ready design and produce a prototype 18-m antenna for the US National Research Observatory (NRAO) Very Large Array (ngVLA) facility. Thales/Alenia (France/Italy), Schwartz Hautmont (Spain) are heavily involved in the development of the new 35-m DSA antenna.

As the intent of the authors is to suggest possible European vectors for collaboration in Interstellar Probe, their review of key technology drivers is broad rather than deep; they’re gauging the likelihood of meshing areas where ESA’s expertise can complement the NASA concept, some of them needing serious development from both sides of the Atlantic. Propulsion via chemical methods could work for IP, for example, given the options of using heavy lift vehicles like NASA SLS and the possibility, down the road, of a SpaceX Starship or BlueOrigin vehicle to complement the launch catalog. The availability of such craft coupled with a passive gravity assist at Jupiter points to a doubling of Voyager’s escape velocity, reaching 7.2 AU per year. (roughly 34 kilometers per second).

As to power, NASA is enroute to bringing the necessary nuclear package online via the Next-Generation Radioisotope Thermoelectric Generator (NextGen RTG) under development at NASA Glenn. But improvements in communications at this range represent one area where European involvement could play a role, as does reliability of the sort that can ensure a viable mission lasting half a century or more. Thus:

Development and implementation of qualification procedures for missions with nominal lifetimes of 50 years and beyond. This would provide the community with knowledge of designing long-lived space equipment and be helpful for other programs such as Artemis.

This area strikes me as promising. We’ve already seen how spacecraft never designed for missions of such duration have managed to go beyond the heliosphere (the Voyagers), and developing the hardware with sufficient reliability seems well within our capabilities. Other areas ripe for further development are pointing accuracy and deep space communication architectures, thus the paper’s emphasis on ESA’s role in refining the use of integrated deep space transponders for Interstellar Probe.

Whether the JHU/APL Interstellar Probe design wins approval or not, the fact that we are considering these issues points to the tenacious vitality of space programs looking toward expansion into the outer Solar System and beyond, a heartening thought as we ponder successors to the Voyagers and New Horizons. The ice giants and the VLISM region will truly begin to reveal their secrets when missions like these fly. And how much more so if, along the way, a propulsion technology emerges that reduces travel times to years instead of decades? Are beamed sails the best bet for this, or something else?

The paper is Wimmer-Schweingruber et al., “STELLA—Potential European contributions to a NASA-led interstellar probe,” a whitepaper that was submitted to NASA’s 2023/2024 decadal survey based on a proposal submitted to the European Space Agency (ESA) in response to its 2021 call for medium-class mission proposals. Frontiers in Astronomy and Space Sciences, 17 November 2022 (full text).

For detailed information about Interstellar Probe, see McNutt et al., “Interstellar probe – Destination: Universe!” Acta Astronautica Vol. 196 (July 2022), 13-28 (full text) as well as Brandt et al., “Interstellar Probe: Humanity’s exploration of the Galaxy Begins,” Acta Astronautica Volume 199 (October 2022), pages 364-373 (full text).

{ 32 comments… add one }
  • Andrew Palfreyman November 23, 2022, 8:48

    The functional overlap of this mission proposal with the solar gravity telescope mission (was FOCAL) at >550 AU is hard to ignore.

  • Afterthought November 23, 2022, 9:41

    The road to Alpha Centauri runs through the inner solar system.

  • Alex Tolley November 23, 2022, 13:05

    In the image, both Voyagers, New Horizons, and the possible Interstellar Probe are all headed toward the bowshock.

    Fluid dynamics indicate that there is likely to be turbulence, even vortex shedding, behind the impinged object (the sun and its radiation pressure). Yet there is no suggestion of investigating this region. Is it because it is of little interest, or that the distances involved are too great?

    We can observe such dynamics from the outside by viewing other stars. These hypervelocity stars show show interesting turbulence in their wake. Could we expect something similar even with our far slower moving sun?

  • Michael November 23, 2022, 13:30

    Would a final explosive stage be helpful, a drop towards the sun at perihelion and then a explosive is use to push for delta v with a boost via the oberth maneuver.

    • Paul Gilster November 23, 2022, 14:55

      The boost at perihelion has been extensively studied but as I understand it, the numbers favored the trajectory toward Jupiter with gravity assist there, and no close pass of the Sun. The IP report has a discussion of this.

      • Donald November 23, 2022, 20:30

        I would encourage everyone to watch a recent talk by one of the developers of this mission on the PSW science channel on YouTube. The talk is quite good and, as he explains it, a gravity assist from the sun adds an immense amount of technical risk and because of all of the extra weight from heat and radiation shielding, barely cuts any time off the trip at all, in addition to nearly doubling the cost of the mission. The Jupiter flyby is, hands down, far preferred over a sun gravity assist. https://youtu.be/6CvHzmq_6hM

        • Michael November 24, 2022, 6:10

          An explosive one can be a lot smaller and require less shielding but the probe would need to be acceleration resistant like a artillery shell.

  • Alex Tolley November 23, 2022, 13:42

    The 2000 movie Space Cowboys used the idea that the aging pilots could command a mission to deal with a rogue Russian satellite as its control system was based on long obsolete hardware from Skylab.

    The Voyager space probes used Fortran (5), the first computer programming language (1954) and still in use, although now a relatively niche language. How easy is it to find software engineers knowledgeable in its use?

    Timeline of languages.

    Programming languages have flowered, especially since the advent of ubiquitous programming. While BASIC was the language of choice for many in the 1970s and 1980s, I well recall the agonizing over which language to migrate to C or Pascal. Now there are so many languages there is no universal “best pick” to start with, and so many to try to learn depending on the application.

    The number of languages and how they are used may present a problem as the proposed precursor interstellar probes travel through their half-century journeys, with frozen hardware and software languages that may no longer be in use by the journey’s end.

    The probes’ computers will be quite powerful, and capable of deploying, and even building ML models to make decisions in flight. Certainly creating simple decision trees with changing input fields will be almost trivial, and they may even be using neuromorphic chips to support their connectionist ML models to manage their systems. Whatever the choice of hardware and software language, we can be sure that they will be relics in 50 years. While the hardware cannot be changed [unless it can be rebuilt by an advanced onboard builder] it is quite possible that the software language could be changed if the system supports such changes, and certainly, the software can be updated as we do now by uploading code changes.

    We may need our Frank Corvin’s to manage these obsolete systems if cosmic ray and dust damage renders them in need of repair beyond the capabilities of the onboard systems.

  • Henry Cordova November 23, 2022, 16:54

    I’ll volunteer!

    I learned Fortran programming as a student in the 1960s and 70s, and actually worked as a Fortran scientific and engineering programmer throughout the 1980s. I wrote and maintained computer mapping software and image processing/remote sensing applications.

    I also am a qualified celestial navigator (maritime) and can drive a manual transmission.

  • Cosmicist November 23, 2022, 17:35

    The way things are going, Europe will have trouble keeping the lights and heat on and their industry running in the near future, let alone sending probes into deep space by 2050. I think we have to look at the big picture of civilization and where it is headed, particularly in the West, and not get delusional about what we’re going to be doing. I expect space programs to be dramatically cut in both the EU and the USA in the coming years, and deep space exploration will be one of the first thing to go since it has no immediate utility. No one ever accused me of being an optimist, but I like to think of it as being a realist, which is preferable to being an escapist.

    • Henry Cordova November 23, 2022, 23:55

      Things weren’t going that smoothly before the Ukraine invasion, either.

      Covid, economic distress, mass migrations, a fascist revival in the Western Democracies and accelerating climate change. And Russia, China, Iran and N Korea are all competing for our undivided attention.

      For those of us who don’t give a flying fig for the Super Bowl, the World Cup or keeping up with the Kardashians, a little interstellar escapism is just a harmless perversion I feel absolutely no guilt about indulging in.

      “History repeats itself; first as tragedy, then as farce.”

      • Patient Observer December 1, 2022, 10:41

        Very well said Mr. Cordova!

    • Gary Wilson November 24, 2022, 17:49

      A lot of American space facilities are placed close to coastlines and very little above sea level. I know NASA is aware of this but immediate attention should be paid to moving the most endangered facilities. For example how long will Kennedy Space Center be about water? The Thwaites Ice Shelf in Antarctica is very close to breaking up which will release an enormous ice sheet behind it. Greenland is in a very similar state (a lot of liquid water has gotten under the ice sheet and is destabilizing it). We have possibly a decade or less to begin preparing for these massive flooding events. If we want a future in space we better make the correct moves now.

      • Alex Tolley November 24, 2022, 22:17

        Not to mention that most cities are coastal and flooding may undermine the economy which pays for these missions. Just look at the problems trying to protect Venice from periodic flooding. New York would require massive structures to protect the city from tides and flooding. Miami is already flooding with no indication of how mitigation will be made. Maybe these cities will either try to migrate to higher ground where possible, or the roads will eventually become canals (although how would freshwater and sewage be managed, as well as relocating power lines?).

        If southern Florida floods, maybe the space center moves to Texas?

    • Jeff Wright November 26, 2022, 21:52

      Don’t be so sure. Europa Clipper only got funded due to SLS enjoying widespread support.

      It has flown:

      —where SuperHeavy almost collapsed:


      I suggest that the Interstellar Probe guys hitch their wagon to Nuclear Thermal guys, but have the probe itself be nuclear electric. The NTR can separate and be a separate mission of some kind at Jupiter as the NEP follows Ouamuamua from behind.

      The one-two nuclear punch will give you greater speed and political support…along with a defacto mission to an extraterrestrial object. A simpler probe with whip-antennas atop Falcon Heavy might seem more cost effective…but it will also have less broad support.

  • Jeff Stopple November 23, 2022, 18:17

    If you want a catchier name for Interstellar Probe, I suggest ‘Sagan’.

    • Andrei November 28, 2022, 8:39

      I concur with that name suggestion.
      Though I don’t remember if Carl S was part of the group that proposed the original TAU mission (Thousand AU).
      As for Mr Wright comment above, NEP is the way to go for a mission like this. But no ion engines have been rated to run constantly for such a long time, and none exist for the preferred power level. So even though the proposed launch date sound distant, a lot of development will have to be done and get started already now if this bird will be able to fly.

  • David November 23, 2022, 20:50

    I have found the paper’s Abstract under the title ‘Stella: Europe’s contribution to a NASA interstellar probe’ (minus the ‘-led’).

    Quote… ‘I’m interested to see that they eschew optical strategies, saying these would “pose too stringent pointing requirements on the spacecraft.”’ I too find this interesting. It does seem that proposed technologies align more closely with European vendor interests. (I write this as someone who lives close to an ESA spacecraft communications facility and would love to see its further development.) We have a group locally studying optical free-space communications – I wonder what they might think of that view?

    • Paul Gilster November 23, 2022, 21:09

      David, I’ve been talking to Ralph McNutt today about this very issue. There are solid reasons why laser is problematic for this particular mission, and I hope to write them up in the not distant future. The problem is maintaining pointing accuracy at these distances and it turns out to be much more difficult than I would have dreamed. I don’t think Dr. McNutt would mind my quoting him on this:

      “The problem is multifaceted (including very accurate clocks to point ahead to the Earth’s position with a very tight, downlinked beam). The largest problems are with the reaction wheels: (1) they tend to fail after ~10ish years – 50 years is not doable now, and (2) even if that were solved, they require power – likely increasing with time as pointing would become more exacting with distance – while the power available continues to drop with time from the RTGs”

      But there’s more involved and I do hope to be writing about this here before long.

      • Ron S. November 23, 2022, 22:07

        Another way of stating it is that you invite disaster when the beam width is reduced to the point where it is smaller than the pointing accuracy. It is optimal to have them roughly equal, but in practice the beam width must be wider than the pointing accuracy for reliable communication.

        • Paul Gilster November 24, 2022, 6:40

          Well said!

          • David Herne December 1, 2022, 0:40

            Paul and Ron – thank you for these replies and thoughts.

      • Mike Serfas November 27, 2022, 20:36

        That promises to be an interesting article! While you’re at it … are there inherent limits on the angular precision of reconfigurable transmitarray antennas? Also, what are the obstacles involved in taking them from terahertz to visible light? I know virtually nothing about this, mind, but I would daydream of near-innumerable micron lasers on a chip, responding directly to a guide beam from Earth to more or less 3D-print a laser beam in space that is headed straight back in our direction no matter which way the spacecraft is facing.

  • Robin Datta November 26, 2022, 21:53

    Some thoughts on nomenclature for the interstellar medium, borrowing terms and their nuances from anatomy…

    just outside the outer boundary of the helio/stello pause:


    VLISM: peri(h_/s_)pause

    ISM: inter(h_/s_)pause, where specifying the region

  • Robert November 26, 2022, 23:33

    “I should also note that China is likewise studying a probe, intrigued by the prospect of reaching 100 AU by the 100th anniversary of the current government in 2049.”

    I prefer to think that it might be the 25th anniversary of Chinese democracy they would be celebrating in 2049.

  • Mike Serfas November 27, 2022, 11:02

    We do have a nice big interstellar probe, which can go out to 900 AU on a 5000-year mission, though it has been derided in the past as “The Flying Dutchman”. It has a pretty red paint job which NASA really ought to inspect more closely. Question is … what is a (minor) planet doing in interstellar space?

    • Paul Gilster November 27, 2022, 20:46

      Mike, the reference to minor planets is to objects the probe might encounter along the way; i.e., before leaving the system.

      • Mike Serfas November 28, 2022, 11:41

        According to Wikipedia, Sedna is a candidate to be a dwarf planet, but it also has an aphelion of 937 AU.

        • Paul Gilster November 28, 2022, 12:30

          Good point!

  • Thomas R Mazanec December 1, 2022, 12:36

    This of course can be used for Solar Focus missions with a slight stretching.

  • ljk December 6, 2022, 16:00
  • ljk December 14, 2022, 13:19

    The longer our deep space missions last, the greater the necessity to keep all of their documentation intact and available, hardcopy as well as electronic. You can always open up a printed book and read it.


    To quote:

    Unearthing old spacecraft documents

    Voyager 1 was designed and built in the early 1970s, complicating efforts to troubleshoot the spacecraft’s problems.

    Though current Voyager engineers have some documentation — or command media, the technical term for the paperwork containing details on the spacecraft’s design and procedures — from those early mission days, other important documents may have been lost or misplaced.

    During the first 12 years of the Voyager mission, thousands of engineers worked on the project, Dodd said. “As they retired in the ’70s and ’80s, there wasn’t a big push to have a project document library. People would take their boxes home to their garage,” Dodd added. In modern missions, NASA keeps more robust records of documentation.

    There are some boxes with documents and schematic stored off-site from the Jet Propulsion Laboratory, and Dodd and the rest of Voyager’s handlers can request access to these records. Still, it can be a challenge. “Getting that information requires you to figure out who works in that area on the project,” Dodd said.

    For Voyager 1’s recent telemetry glitch, mission engineers had to specifically look for boxes under the name of engineers who helped design the attitude-control system — which was ” a time consuming process,” Dodd said.

    Some technical manuals on Voyager:






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