SunVoyager: A Fast Fusion Mission Beyond the Heliosphere

by | Jan 6, 2023 | Missions | 39 comments

1000 AU makes a fine target for our next push past the heliosphere, keeping in mind that good science is to be had all along the way. Thus if we took 100 years to get to 1000 AU (and at Voyager speeds it would be a lot longer than that), we would still be gathering solid data about the Kuiper Belt, the heliosphere itself and its interactions with the interstellar medium, the nature and disposition of interstellar dust, and the plasma environment any future interstellar craft will have to pass through.

We don’t have to get there fast to produce useful results, in other words, but it sure would help. The Thousand Astronomical Unit mission (TAU) was examined by NASA in the 1980s using nuclear electric propulsion technologies, one specification being the need to reach the target distance within 50 years. It’s interesting to me – and Kelvin Long discusses this in a new paper we’ll examine in the next few posts – that a large part of the science case for TAU was stellar parallax, for classical measurements at Earth – Sun distance allow only coarse-grained estimates of stellar distances. We’d like to increase the baseline of our space-based interferometer, and the way to do that is to reach beyond the system.

Gravitational lensing wasn’t on the mind of mission planners in the 1980s, although the concept was being examined as a long-range possibility by von Eshleman at Stanford as early as 1979, with intense follow-up scrutiny by Italian space scientist Claudio Maccone. Today reaching the 550 AU distance where gravitational lensing effects enable observation of exoplanets is much on the mind of Slava Turyshev and team at JPL, whose refined mission concept is aimed at the upcoming heliophysics decadal. We’ve examined this Solar Gravity Lens mission on various occasions in these pages, as well as JHU/APL’s Interstellar Probe design, whose long-range goal is 1000 AU.

What Kelvin Long does in his recently published paper is to examine a deep space probe he calls SunVoyager. Long (Interstellar Research Centre, Stellar Engines Ltd) sees three primary science objectives here, the first being observing the nearest stars and their planets both through transit methods as well as gravitational lensing. A second objective along the way is the flyby of a dwarf planet that has yet to be visited, while the third is possible imaging of interstellar objects like 2I/Borisov and ‘Oumuamua. Driven by fusion, the craft would reach 1000 AU in a scant four years.

Image: The Interstellar Research Centre’s Kelvin Long, here pictured on a visit to JPL.

This is a multi-layered mission, and I note that the concept involves the use of small ‘sub-probes’, evidently deployed along the route of flight, to make flybys of a dwarf planet or an interstellar object of interest, each of these (and ten are included in the mission) to have a maximum mass of 0.5 tons. That’s a lot of mass, about which more in a moment. Secondary objectives involve measurements of the charged particle and dust composition of the interstellar medium, astrometry (presumably in the service of exoplanet study) and, interestingly, SETI, here involving detection of possible power and propulsion emission signatures as opposed to beacons in deep space.

Bur back to those sub-probes, which by now may have rung a bell. Active for decades in the British Interplanetary Society, Long has edited its long-lived journal and is deeply conversant with the Daedalus starship concept that grew out of BIS work in the 1970s. Daedalus was a fusion starship with an initial mass of 54,000 tons using inertial confinement methods to ignite a deuterium/helium-3 mixture. SunVoyager comes nowhere near that size – nor would it travel more than a fraction of the Daedalus journey to Barnard’s Star, but you can see that Long is purposely exploring long-range prospects that may be enabled by our eventual solution of fusion propulsion.

Those fortunate enough to travel in Iceland will know SunVoyager as the name of a sculpture by the sea in central Reykjavik, one that Long describes as “an ode to the sun or a dream boat that represents the promise of undiscovered territory and a dream of hope, progress, and freedom.” As with Daedalus, the concept relies on breakthroughs in inertial confinement fusion (ICF), in this case via optical laser beam, and in an illustration of serendipity, the paper comes out close to the time when the US National Ignition Facility announced its breakthrough in achieving energy breakeven, meaning the experiment produced more energy from fusion than the laser energy used to drive it.

Image: The Sun Voyager (Sólfarið) is a large steel sculpture of a ship, located on the road Sæbraut, by the seaside of central Reykjavík. The work of sculptor Jón Gunnar Árnason, SunVoyager is one of the most visited sights in Iceland’s capitol, where people gather daily to gaze at the sun reflecting in the stainless steel of this remarkable monument. Credit: Guide to Iceland.

Long’s work involves a numerical design tool called HeliosX, described as “a system integrated programming design tool written in Fortran 95 for the purpose of calculating spacecraft mission profile and propulsion performance for inertial confinement fusion driven designs.” As a counterpart to this paper, Long writes up the background and use of HeliosX in the current issue of Acta Astronautica (citation below). The SunVoyager paper contemplates a mission launched decades from now. Long acknowledges the magnitude of the problems that remain to be solved with ICF for this to happen, notwithstanding the encouraging news from the NIF.

…a capsule of fusion fuel, typically hydrogen and helium isotopes, must be compressed to high density and high temperature, and this must be sustained for a minimum period of time. One of the methods to achieve this is by using high-powered laser beams to fire at a capsule in a spherical arrangement of individual beam lines. The lasers will mass ablate the surface of the capsule and through momentum exchange will cause the material to travel inward under spherical compression. This must be done smoothly however, and any significant perturbations from spherical symmetry during the implosion will lead to hydrodynamic instabilities that can reduce the implosion efficiency. Indeed, the interaction of a laser beam with a high-temperature plasma involves much complex physics, and this is the reason why programs on Earth have found it so difficult.

Working through our evolving deep space mission designs is a fascinating exercise, which is why I took the time years ago to painstakingly copy the original Daedalus report from an academic library – I kept the Xerox machine humming in those days. Daedalus, a two-stage vehicle, used electron beams fired at capsules of deuterium and helium-3, the resulting plasma directed by powerful magnetic fields. Long invokes as well NASA’s studies of a concept called Vista, which he has also written about in his book Deep Space Propulsion: A Roadmap to Interstellar Flight (Springer, 2011). This was a design proposal for taking a 100-ton payload to Mars in 50 days using a deuterium and tritium fuel capsule ignited by laser. Long explains:

The capsule design was to utilize an indirect drive method, and so a smoother implosion symmetry may give rise to a higher burn fraction of 0.476. This is where the capsule is contained within a radiation cavity called a Hohlraum and where the lasers heat up the internal surface layer of the cavity to create a radiation bath around the capsule; as opposed to direct laser impingement onto the capsule surface and the associated mass ablation through the direct drive approach.

Image: Few images of the Vista design are available. I’ve swiped this one from a presentation made by C. D. Orth to the NASA Advanced Propulsion Workshop in Fusion Propulsion in 2000, though it dates back all the way to the 1980s. Credit: NASA.

SunVoyager would, the author comments, likely use a similar capsule design, although the paper doesn’t address the details. Vista feeds into Long’s thinking in another way: You’ll notice the unusual shape of the spacecraft in the image above. Coming out of work by Rod Hyde and others in the 1980s, Vista was designed to deal with early ICF propulsion concepts that produced a large neutron and x-ray radiation flux, sufficient to prove lethal to the crew. The conical design was thus an attempt to minimize the exposure of the structure to this flux, with a useful gain in jet efficiency of the thrust chamber. SunVoyager is designed around a similar conical propulsion system. The author proceeds to make predictions for the performance of SunVoyager by using calculations growing out of the Vista design as modeled in the HeliosX software.

In the tradition of Daedalus and Vista, SunVoyager explores ICF propulsion in the context of current understanding of fusion. I want to talk more about this concept next week, noting for now that a fast mission to 1000 AU –SunVoyager would reach that distance in less than four years – would take us into an entirely new level of outer system exploration, although the timing of such a mission remains hostage to our ability to conquer ICF and generate the needed energies to actualize it in comparatively small spacecraft systems. This doesn’t even get into the matter of producing the required fuel, another issue that will parallel those 1970s Daedalus papers and push us to the limits of the possible.

The paper is Long, “Sunvoyager: Interstellar Precursor Probe Mission Concept Driven by Inertial Confinement Fusion Propulsion,” Journal of Spacecraft and Rockets 2 January 2023 (full text). The paper on HeliosX is Long, “Development of the HeliosX Mission Analysis Code for Advanced ICF Space Propulsion,” Acta Astronautica, Vol. 202, Jan. 2023, pp. 157–173 (abstract). See also Hyde, “Laser-fusion rocket for interplanetary propulsion,” International Astronautical Federation conference, Budapest, Hungary, 10 Oct 1983 (abstract).

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Gathering the Evidence for Life on Enceladus

by | Jan 4, 2023 | Outer Solar System | 13 comments

With a proposal for an Enceladus Orbilander mission in the works at the Johns Hopkins Applied Physics Laboratory, I continue to mull over the prospects for investigating this interesting moon. Something is producing methane in the ocean under the Enceladus ice shell, analyzed in a 2021 paper from Antonin Affholder (now at the University of Arizona) and colleagues, using Cassini data from passages through the plumes erupting from the southern polar regions. The scientists produced mathematical models and used a Bayesian analysis to weigh the probabilities that the methane is being created by life or through abiotic processes.

The result: The plume data are consistent with both possibilities, although it’s interesting, based on what we know about hydrothermal chemistry on earth, that the amount of methane is higher than would be expected through any abiotic explanation. So we can’t rule out the possibility of some kind of microorganisms under the ice on Enceladus, and clearly need data from a future mission to make the call. I won’t go any further into the 2021 paper (citation below) other than to note that the authors believe their methods may be useful for dealing with future chemical data from exoplanets of a wide variety, and not just icy worlds with an ocean beneath a surface shell.

Now a new paper has been published in The Planetary Science Journal, authored by the same team and addressing the potential of such a future mission. A saltwater ocean outgassing methane is an ideal astrobiological target, and one useful result of the new analysis is that it would not take a landing on Enceladus itself to probe whether or not life exists there. Says co-author Régis Ferrière (University of Arizona):

“Clearly, sending a robot crawling through ice cracks and deep-diving down to the seafloor would not be easy. By simulating the data that a more prepared and advanced orbiting spacecraft would gather from just the plumes alone, our team has now shown that this approach would be enough to confidently determine whether or not there is life within Enceladus’ ocean without actually having to probe the depths of the moon. This is a thrilling perspective.”

Image: This graphic depicts how scientists believe water interacts with rock at the bottom of Enceladus’ ocean to create hydrothermal vent systems. These same chimney-like vents are found along tectonic plate borders in Earth’s oceans, approximately 7000 feet below the surface. Credit: NASA/JPL-Caltech/Southwest Research Institute.

Microbes on Earth – methanogens – find ways to thrive around hydrothermal vents deep below the surface of the oceans, in regions deprived of sunlight but rich in the energy stored in chemical compounds. Indeed, life around ‘white smoker’ vents is rich and not limited to microbes, with dihydrogen and carbon dioxide as an energy source in a process that releases methane as a byproduct. The researchers hypothesize that similar processes are at work on Enceladus, calculating the possible total mass of life there, and the likelihood that cells from that life might be ejected by the plumes.

The team’s model produces a small and sparse biosphere, one amounting to no more than the biomass of a single whale in the moon’s ocean. That’s an interesting finding in itself in contrast to some earlier studies, and it contrasts strongly with the size of the biosphere around Earth’s hydrothermal vents. But the quantity is sufficient to produce enough organic molecules that a future spacecraft could detect them by flying through the plumes. The mission would require multiple plume flybys.

Actual cells are unlikely to be found in the plumes, but detected organic molecules including particular amino acids would support the idea of active biology. Even so, we are probably going to be left without a definitive answer, adds Ferrière:

“Considering that according to the calculations, any life present on Enceladus would be extremely sparse, there still is a good chance that we’ll never find enough organic molecules in the plumes to unambiguously conclude that it is there. So, rather than focusing on the question of how much is enough to prove that life is there, we asked, ‘What is the maximum amount of organic material that could be present in the absence of life?'”

An Enceladus orbiter, in other words, would produce strong evidence of life if its measurements were above the threshold identified here. Back to the JHU/APL Enceladus Orbilander, thoroughly described in a concept study available online. The mission includes both orbital operations as well as a landing on the surface, with thirteen science instruments aboard to probe for life in both venues. The mission would measure pH, temperature, salinity and availability of nutrients in the ocean as well as making radar and seismic measurements to probe the structure of the ice crust.

Image: Artist’s impression of the conceptual Enceladus Orbilander spacecraft on Enceladus’ surface. Credit: Johns Hopkins APL.

Here the chances of finding cell material are much higher than in purely orbital operations, where survival through the outgassing process of plume creation seems unlikely. The lander would target a flat space free of boulders at the moon’s south pole with the aim of collecting plume materials that have fallen back to the surface. The team points out that the largest particles would not reach altitudes high enough for sampling from orbit, making the lander our best chance for a definitive answer.

The paper, indeed, points to this conclusion:

…cell-like abiotic structures (abiotic biomorphs) that may form in hydrothermal environments could cause a high risk of a false positive… Assuming that cells can be identified unambiguously…, we find that the volume of plume material that needs to be collected to confidently sample at least one cell might require a large number of fly-throughs in the plume, or using a lander to collect plume particles falling on Enceladus’s surface (e.g., the Enceladus Orbilander; MacKenzie et al. 2021).

The paper is Affholder et al., “Putative Methanogenic Biosphere in Enceladus’s Deep Ocean: Biomass, Productivity, and Implications for Detection,” Planetary Science Journal Vol. 3, No. 12 (13 December 2022), 270 (full text). The paper on methane on Enceladus is Affholder at al., “Bayesian analysis of Enceladus’s plume data to assess methanogenesis,” Nature Astronomy 5 (07 June 2021), 805-814 (abstract).

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