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).