It seems a good time to re-examine the venerable Kardashev scale marking how technological civilizations develop. After all, I drop Nikolai Kardashev’s name into articles on a regular basis, and we routinely discuss whether a SETI detection might be of a particular Kardashev type. The Russian astronomer first proposed the scale in 1964 at the storied Byurakan conference on radio astronomy, and it has been discussed and extended as a way of gauging the energy use of technological cultures ever since.
The Jet Propulsion Laboratory’s Jonathan Jiang, working with an international team of collaborators, spurs this article through a new paper that analyzes when our culture could reach Kardashev Type I, so let’s remind ourselves of just what Type I means. Kardashev wanted to consider how a civilization consumes energy, and defined Type I as being at the planetary level, with a power consumption of 1016 watts.
This approximates a civilization using all the energy available from its home planet, but that means both in terms of indigenous planetary resources as well as incoming stellar energy. So we are talking about everything from what we can pull from the ground – fossil fuels – or extract from planetary resources like wind and tide, or harvest through solar, nuclear and other technologies. If we maximize all this, it becomes fair to ask where we are right now, and when we can expect to reach the Type I goal.
Image: Russian astronomer Nikolai Kardashev (1932-2019). Credit: Physics-Uspekhi.
If the Kardashev scale seems arbitrary, it was in its time a step forward in the discussion of SETI, which in 1964 was an emerging discipline much discussed at Byurakan, for the different Kardashev types would clearly present different signatures to a distant astronomer. Type I might well be all but undetectable depending on its uses of harvested energy; in any case, it would be harder to spot than Types II and III, whose vast sources of power could result in stronger signals or observable artifacts.
Carl Sagan was concerned enough about Kardashev’s original definitions to refine them into a calculation, his thinking being that the gaps between the Kardashev types needed to be filled in with finer gradations. This would allow us to quantify where civilizations are on the scale. Sagan’s calculation would let us discover the present value for our own civilization using available data (as, for example, from the International Energy Agency) regarding the planet’s total energy capabilities. According to Jiang and team, in 2018 this amounted to 1.90 X 1013W, all of which, via Sagan’s methodology, takes us to a present value of Kardashev 0.728.
But let’s circle back to the other two Kardashev types. Type II can be considered a stellar civilization, which in Kardashev’s thinking means a ten orders of magnitude increase in power consumption over Type I, taking us to 1026W. Here we are using all the energy released by the parent star, and now the idea of Dysonian SETI swings into view, the notion that this kind of consumption could be observable through engineering projects on a colossal scale, such as a Dyson swarm enclosing the parent star to maximize energy collection or a Matrioshka Brain for computation. Jiang reminds us that the Sun’s total luminosity is on the order of 4 X 1026W.
Again, these are arbitrary distinctions; note that at the level of the Sun’s total energy output, we would need only about a fourth of that figure to reach the figure described in the Kardashev Scale as Type II. Quantitative limitations, as noted by Sagan, beset the scale, but there is nothing wrong with the notion of setting up a framework for analysis as a first cut into what might become SETI observables. Kardashev’s Type III, using these same methods, offers up a galactic energy consumption of 1036W, so now an entire galaxy is being manipulated by a civilization.
Consider that the entire Milky Way yields something like 4 X 1037W, which actually means that a Type III culture on the Kardashev scale in our particular galaxy would have command of at least 2.5 percent of the total possible energy sources therein. What such a culture might look like as an observable is anyone’s guess (searches for galaxies with unusual infrared signatures are one way to proceed, as Jason Wright’s team at Penn State has demonstrated), but on the galactic scale, we are at an energy level that may, as the saying goes, be all but indistinguishable from magic.
Let’s back down to our planetary level, and in fact back to our modest 0.728 percent of Type I status. Just when can we anticipate reaching Type I? The new paper eschews simple models of exponential growth and consumption over time, noting that such estimates have tended to be:
…the result of a simple exponential growth model for calculating total energy production and consumption as a function of time, relying on a continuous feedback loop and absent detailed consideration of practical limitations. With this reservation in mind, its prediction for when humanity will reach Type I civilization status must be regarded as both overly simplified and somewhat optimistic.
Instead, the authors consider planetary resources, policies and suggestions on climate change, and forecasts for energy consumption to develop an estimated timeframe. The idea is to achieve a more practical outlook on the use of energy and the limitations on its growth. They consider the wide range of fossil fuels, from coal, peat, oil shale, and natural gas to crude oil, natural gas liquids and feedstocks, as well as the range of nuclear and renewable energy sources. Their analysis is keyed to how usage may change in the near future under the influence of, and taking in the projections of, organizations like the United Nations Framework Convention on Climate Change and the International Energy Agency. They see moving along a trajectory to Type I as inevitable and critical for resolving existential crises that threaten our civilization.
So, for example, on the matter of fossil fuels, the authors consider the downside of environmental concerns over the greenhouse effect and changes to policy affecting carbon emissions that will impact energy production. On nuclear and renewable energy, their analysis takes in factors constraining the growth of these energy sources and data on the current development of each. For both fossil fuels and nuclear/renewables, they produce what they describe as an ‘influenced model’ that predicts development operating under historically observed constraints and the likely consequences.
Applying the formula for calculating the Kardashev scale developed by Carl Sagan, they project that our civilization can attain Kardashev Type I with coal, natural gas, crude oil, nuclear and renewable energy sources as the driver. Thus their Figure 6:
Image: Figure 6 from the paper. Caption: The energy supply in the influenced model. Note: Coal is minimal for 1971-2050 and largely coincides with the Natural gas line. Credit: Jiang et al.
Again referring to the Sagan equation, the paper continues:
A final revisit of Eq 1.1, which is informed by the IEA and UNFCCC’s suggestions, finds an imperative for a major transition in energy sourcing worldwide, especially during the 2030s. Although the resultant pace up the Kardashev scale is very low and can even be halted or reversed in the short term, achieving this energy transformation is the optimal path to assuring we will avoid the environmental pitfalls caused by fossil fuels. In short, we will have met the requirements for planetary stewardship while continuing the overall advancement of our technological civilization.
The final estimate is that humanity reaches Kardashev Type I by 2371, a date the authors consider on the optimistic side but achievable. All this assumes that a Type I civilization can be sustained as well, rather than backsliding into an earlier state, something that human history suggests is by no means assured. Successful management of nuclear power is just one flash point, as is storage and disposal of nuclear waste and global issues like deforestation and declining soil pH. That list could, of course, be extended into global pandemics, runaway AI and other factors.
…for the entire world population to reach the status of a Kardashev Type I civilization we must develop and enable access to more advanced technology to all responsible nations while making renewable energy accessible to all parts of the world, facilitated by governments and private businesses. Only through the full realization of our mutual needs and with broad cooperation will humanity acquire the key to not only avoiding the Great Filter but continuing our ascent to Kardashev Type I, and beyond.
The Great Filter, drawing on Robin Hanson’s work, could be behind us or ahead of us. Assuming it lies ahead, getting through it intact would be the goal of any growing civilization as it finds ways to juggle its technologies and resources to survive. It’s hard to argue with the idea that how we proceed on the Kardashev arc is critical as we summon up the means to expand off-world and dream of pushing into the Orion Arm.
The paper is Jiang et al., “Avoiding the Great Filter: Predicting the Timeline for Humanity to Reach Kardashev Type I Civilization” (preprint).
Not long ago we looked at Greg Matloff’s paper on von Neumann probes, which made the case that even if self-reproducing probes were sent out only once every half million years (when a close stellar encounter occurs), there would be close to 70 billion systems occupied by such probes within a scant 18 million years. Matloff now considers interstellar migration in a different direction in a new paper addressing how M-dwarf civilizations might expand, and why electric sails could be their method.
It’s an intriguing notion because M-dwarfs are by far the most numerous stars in the galaxy, and if we learn that they can support life, they might house vast numbers of civilizations with the capability of sending out interstellar craft. They’re also crippled in terms of electromagnetic flux when it comes to conventional solar sails, which is why the electric sail comes into play as a possible alternative, here analyzed in terms of feasibility and performance and its prospects for enabling interstellar migration.
The term ‘sail’ has to be qualified. By convention, I’ve used ‘solar sail,’ for example, to describe sails that use the momentum imparted by stellar photons – Matloff often calls these ‘photon sails,’ which is also descriptive, though to my mind, a ‘photon sail’ might describe both a beam-driven as well as a stellar photon-driven sail. Thus I prefer ‘lightsail’ for the beamed sail concept. In any case, we have to distinguish all these concepts from the electric sail, which operates on fundamentally different principles.
In our Solar System, a sail made of absorptive graphene deployed from 0.1 AU could achieve a Solar System escape velocity of 1000 kilometers per second, and perhaps better if the mission were entirely robotic and not dealing with fragile human crews. The figure seems high, but Matloff gave the calculations in a 2012 JBIS paper. The solar photon sail wins on acceleration, and we can use the sail material to provide extra cosmic ray shielding enroute. These are powerful advantages near our own Sun.
But the electric sail has advantages of its own. Rather than drawing on the momentum imparted by solar photons (or beamed energy), an electric sail rides the stellar wind emanating from a star. This stream of charged particles has been measured in our system (by the WIND spacecraft in 1995) as moving in the range of 300 to 800 kilometers per second at 1 AU, a powerful though extremely turbulent and variable force that can be applied to a spacecraft. Because an interstellar craft entering a destination system would also encounter a stellar wind, an electric sail can be deployed for deceleration, something both forms of sail have in common.
How to harness a stellar wind? Matloff first references a 2008 paper from Pekka Janhunen (Finnish Meteorological Institute) and team that described long tethers (perhaps reaching 20 kilometers in length) extended from the spacecraft, each maintaining a steady electric potential with the help of a solar-powered electron gun aboard the vehicle. As many as a hundred tethers — these are thinner than a human hair — could be deployed to achieve maximum effect. While the solar wind is far weaker than solar photon pressure, an electric sail of this configuration with tethers in place can create an effective solar wind sail area of several square kilometers.
We need to maintain the electric potential of the tethers because it would otherwise be compromised by solar wind electrons. The protons in the solar wind – again, note that we’re talking about protons, not photons – reflect off the tethers to drive us forward.
Image: Image of an electric sail, which consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of meters into the surrounding solar wind plasma. Therefore the solar wind ions “see” the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or “flying” the resulting spacecraft electrically. Credit: Artwork by Alexandre Szames. Caption via Pekka Janhunen/Kumpula Space Center.
For interstellar purposes, we look at much larger spacecraft, bearing in mind that once in deep space, we have to turn off the electron gun, because the interstellar medium can itself decelerate the sail. Operating from a Sun-like star, the electric sail generation ship Matloff considers is assumed to have a mass of 107 kg, assuming a constant solar wind within the heliosphere of 600 kilometers per second. The variability of the solar wind is acknowledged, but the approximations are used to simplify the kinematics. The paper then goes on to compare performance near the Sun with that near an M-dwarf star.
We wind up with some interesting conclusions. First of all, an interstellar mission from a G-class star like our own would be better off using a different method. We can probably reach an interstellar velocity of as high as 70 percent of this assumed constant solar wind velocity (Matloff’s calculations), but graphene solar sails can achieve better numbers. And if we add in the variability of the solar wind, we have to be ready to constantly alter the enormous radius of the electric field to maintain a constant acceleration. If we’re going to send generation ships from the Sun, we’re most likely to use solar sails or beamed lightsails.
But things get different when we swing the discussion around to red dwarf stars. In The Electric Sail and Its Uses, I described a paper from Avi Loeb and Manasvi Lingam in 2019 that studied electric sails using the stellar winds of M-dwarfs, with repeated encounters with other such stars to achieve progressively higher speeds. Matloff agrees that electric sails best photon sails in the red dwarf environment, but adds useful context.
Let’s think about generation ships departing from an M-dwarf. Whereas the electromagnetic flux from these stars is far below that of the Sun, the stellar wind has interesting properties. We learn that it most likely has a higher mass density (in terms of rate per unit area) than the Sun, and the average stellar wind velocity is 500 kilometers per second. Presumably a variable electric field aboard the craft could adjust to maintain acceleration as the vehicle moves outward from the star, although the paper doesn’t get into this. The author’s calculations show an acceleration, for a low-mass spacecraft about 1 AU from the Sun, of 7.6 × 10?3 m/s2 , or about 7.6 × 10?4 g. Matloff considers this a reasonable acceleration for a worldship.
So while low electromagnetic pressure makes photon sails far less effective at M-dwarfs as opposed to larger stars, electric sails remain in the mix for civilizations willing to contemplate generation ships that take thousands of years to reach their goal. In an earlier paper, the author considered close stellar encounters, pointing out that 70,000 years ago, the binary known as Scholz’s Star (it has a brown dwarf companion) passed within 52,000 AU of the Sun. We can expect another close pass (Gliese 710) in about 1.35 million years, this one closing to a perihelion of 13,365 AU. From the paper:
Bailer?Jones et al. have used a sample of 7.2 million stars in the second Gaia data release to further investigate the frequency of close stellar encounters. The results of this analysis indicate that seven stars in this sample are expected to approach within 0.5 parsecs of the Sun during the next 15 million years. Accounting for sample incompleteness, these authors estimate that about 20 stars per million years approach our solar system to within 1 parsec. It is, therefore, inferred that about 2.5 encounters within 0.5 parsecs will occur every million years. On average, 400,000 years will elapse between close stellar encounters, assuming the same star density as in the solar neighborhood.
If interstellar missions were only attempted during such close encounters, we still have a mechanism for a civilization to use worldships to expand into numerous nearby stellar systems. It would take no more than a few star-faring civilizations around the vast number of M-dwarfs to occupy a substantial fraction of the Milky Way, even without the benefits of von Neumann style self-reproduction. With the number of planetary systems occupied doubling every 500,000 years, and assuming a civilization only sends out a worldship during close stellar encounters, we get impressive results. In the clip below, n = the multiple of 500,000 years. The number of systems occupied is P:
At the start, n = 0 and P = 1. When 500,000 years have elapsed, the hypothetical spacefaring civilization makes the first transfer, n = 1 and P = 2. After one million years (n = 2), both the original and occupied stellar systems experience a close stellar encounter, migration occurs and P = 4. After a total elapsed time of 1.5 million years, n = 3 and they occupy eight planetary systems. When n = 5, 10 and 20 the hypothetical civilization has respectively occupied 32, 1024 and 1,048,576 planetary systems.
With M-dwarfs being such a common category of star, learning more about their systems’ potential habitability will have implications for the possible spread of technological societies, even assuming propulsion technologies conceivable to us today. What faster modes may eventually become available we cannot know.
The paper is Matloff, “The Solar?Electric Sail: Application to Interstellar Migration and Consequences for SETI,” Universe 8(5) (19 April 2022), 252 (full text). The Lingam and Loeb paper is “Electric sails are potentially more effective than light sails near most stars,” Acta Astronautica Volume 168 (March 2020), 146-154 (abstract).
I’m always interested in studies that cut across conventional boundaries, capturing new insights by applying data from what had appeared, at first glance, to be unrelated disciplines. Thus the news that the ice shell of Europa may turn out to be far more dynamic than we have previously considered is interesting in itself, given the implications for life in the Jovian moon’s ocean, but also compelling because it draws on a study that focused on Greenland and originally sought to measure climate change.
The background here is that the Galileo mission that gave us our best views of Europa’s surface so far showed us that there are ‘double ridges’ on the moon. In fact, these ridge pairs flanked by a trough running between them are among the most common landforms on a surface packed with troughs, bands and chaos terrain. The researchers, led by Stanford PhD student Riley Culberg, found them oddly familiar. Culberg, whose field is electrical engineering (that multidisciplinary effect again) found an analog in a similar double ridge in Greenland, which had turned up in ice-penetrating radar data.
Image: This is Figure 1 from the paper. Caption: a Europan double ridge in a panchromatic image from the Galileo mission (image PIA00589). The ground sample distance is 20?m/pixel. b Greenland double ridge in an orthorectified panchromatic image from the WorldView-3 satellite taken in July 2018 (© 2018, Maxar). The ground sample distance is ~0.31?m/pixel. Signatures of flexure are visible along the ridge flanks, consistent with previous models for double ridges underlain by shallow sills. Credit: Culberg et al.
The feature in Greenland’s northwestern ice sheet has an ‘M’-shaped crest, possibly a version in miniature of the double ridges we see on Europa. The climate change work used airborne instrumentation producing topographical and ice-penetrating radar data via NASA’s Operation IceBridge, which studies the behavior of polar ice sheets over time and their contribution to sea level rise. Where this gets particularly interesting is that flowing ice sheets produce such things as lakes beneath glaciers, drainage conduits and surface melt ponds. Figuring out how and when these occur becomes a necessary part of working with the dynamics of ice sheets.
The mechanism in play, analyzed in the paper, involves ice fracturing around a pocket of pressurized liquid water that was refreezing inside the ice sheet, creating the distinctive twin peak shape. Culberg notes that the link between Greenland and Europa came as a surprise:
“We were working on something totally different related to climate change and its impact on the surface of Greenland when we saw these tiny double ridges – and we were able to see the ridges go from ‘not formed’ to ‘formed… In Greenland, this double ridge formed in a place where water from surface lakes and streams frequently drains into the near-surface and refreezes. One way that similar shallow water pockets could form on Europa might be through water from the subsurface ocean being forced up into the ice shell through fractures – and that would suggest there could be a reasonable amount of exchange happening inside of the ice shell.”
Image: This artist’s conception shows how double ridges on the surface of Jupiter’s moon Europa may form over shallow, refreezing water pockets within the ice shell. This mechanism is based on the study of an analogous double ridge feature found on Earth’s Greenland Ice Sheet. Credit: Justice Blaine Wainwright.
The double ridges on Europa can be dramatic, reaching nearly 300 meters at their crests, with valleys a kilometer wide between them. The idea of a dynamic ice shell is supported by evidence of water plumes erupting to the surface. Thinking about the shell as a place where geological and hydrological processes are regular events, we can see that exchanges between the subsurface ocean and the possible nutrients accumulating on the surface may occur. The mechanism, say the researchers, is complex, but the Greenland example provides the model, an analog that illuminates what may be happening far from home. It also provides a radar signature that future spacecraft should be able to search for.
From the paper:
Altogether, our observations provide a mechanism for subsurface water control of double ridge formation that is broadly consistent with the current understanding of Europa’s ice-shell dynamics and double ridge morphology. If this mechanism controls double ridge formation at Europa, the ubiquity of double ridges on the surface implies that liquid water is and has been a pervasive feature within the brittle lid of the ice shell, suggesting that shallow water processes may be even more dominant in shaping Europa’s dynamics, surface morphology, and habitability than previously thought.
So we have a terrestrial analog of a pervasive Europan feature, providing us with a hypothesis we can investigate with instruments aboard both Europa Clipper and the ESA’s JUICE mission (Jupiter Icy Moons Explorer), launching in 2024 and 2023 respectively. Confirming this mechanism on Europa would go a long way toward moving the Jovian moon still further up our list of potential life-bearing worlds.
The paper is Culberg et al., “Double ridge formation over shallow water sills on Jupiter’s moon Europa,” Nature Communications 13, 2007 (2022). Full text.
We’ve talked about the ongoing work at the Jet Propulsion Society on the Sun’s gravitational focus at some length, most recently in JPL Work on a Gravitational Lensing Mission, where I looked at Slava Turyshev and team’s Phase II report to the NASA Innovative Advanced Concepts office. The team is now deep into the work on their Phase III NIAC study, with a new paper available in preprint form. Dr. Turyshev tells me it can be considered a summary as well as an extension of previous results, and today I want to look at the significance of one aspect of this extension.
There are numerous reasons for getting a spacecraft to the distance needed to exploit the Sun’s gravitational lens – where the mass of our star bends the light of objects behind it to produce a lens with extraordinary properties. The paper, titled “Resolved Imaging of Exoplanets with the Solar Gravitational Lens,” notes that at optical or near-optical wavelengths, the amplification of light is on the order of ~ 2 X 1011, with equally impressive angular resolution. If we can reach this region beginning at 550 AU from the Sun, we can perform direct imaging of exoplanets.
We’re talking multi-pixel images, and not just of huge gas giants. Images of planets the size of Earth around nearby stars, in the habitable zone and potentially life-bearing.
Other methods of observation give way to the power of the solar gravitational lens (SGL) when we consider that, according to Turyshev and co-author Viktor Toth’s calculations, to get a multi-pixel image of an Earth-class planet at 30 parsecs with a diffraction-limited telescope, we would need an aperture of 90 kilometers, hardly a practical proposition. Optical interferometers, too, are problematic, for even they require long-baselines and apertures in the tens of meters, each equipped with its own coronagraph (or conceivably a starshade) to block stellar light. As the paper notes:
Even with these parameters, interferometers would require integration times of hundreds of thousands to millions of years to reach a reasonable signal-to-noise ratio (SNR) of ? 7 to overcome the noise from exo-zodiacal light. As a result, direct resolved imaging of terrestrial exoplanets relying on conventional astronomical techniques and instruments is not feasible.
Integration time is essentially the time it takes to gather all the data that will result in the final image. Obviously, we’re not going to send a mission to the gravitational lensing region if it takes a million years to gather up the needed data.
Image: Various approaches will emerge about the kind of spacecraft that might fly a mission to the gravitational focus of the Sun. In this image (not taken from the Turyshev et al. paper), swarms of small solar sail-powered spacecraft are depicted that could fly to a spot where our Sun’s gravity distorts and magnifies the light from a nearby star system, allowing us to capture a sharp image of an Earth-like exoplanet. Credit: NASA/The Aerospace Corporation.
But once we reach the needed distance, how do we collect an image? Turyshev’s team has been studying the imaging capabilities of the gravitational lens and analyzing its optical properties, allowing the scientists to model the deconvolution of an image acquired by a spacecraft at these distances from the Sun. Deconvolution means reducing noise and hence sharpening the image with enhanced contrast, as we do when removing atmospheric effects from images taken from the ground.
All of this becomes problematic when we’re using the Sun’s gravitational lens, for we are observing exoplanet light in the form of an ‘Einstein ring’ around the Sun, where lensed light from the background object appears in the form of a circle. This runs into complications from the Sun’s corona, which produces significant noise in the signal. The paper examines the team’s work on solar coronagraphs to block coronal light while letting through light from the Einstein ring. An annular coronagraph aboard the spacecraft seems a workable solution. For more on this, see the paper.
An earlier study analyzed the solar corona’s role in reducing the signal-to-noise ratio, which extended the time needed to integrate the full image. In that work, the time needed to recover a complex multi-pixel image from a nearby exoplanet was well beyond the scope of a practical mission. But the new paper presents an updated model for the solar corona modeling whose results have been validated in numerical simulations under various methods of deconvolution. What leaps out here is the issue of pixel spacing in the image plane. The results demonstrate that a mission for high resolution exoplanet imaging is, in the authors’ words, ‘manifestly feasible.’
Pixel spacing is an issue because of the size of the image we are trying to recover. The image of an exoplanet the size of the Earth at 1.3 parsecs, which is essentially the distance of Proxima Centauri from the Earth, when projected onto an image plane at 1200 AU from the Sun, is almost 60 kilometers wide. We are trying to create a megapixel image, and must take account of the fact that individual image pixels are not adjacent. In this case, they are 60 meters apart. It turns out that this actually reduces the integration time of the data to produce the image we are looking for.
From the paper [italics mine]:
We estimated the impact of mission parameters on the resulting integration time. We found that, as expected, the integration time is proportional to the square of the total number of pixels that are being imaged. We also found, however, that the integration time is reduced when pixels are not adjacent, at a rate proportional to the inverse square of the pixel spacing.
Consequently, using a fictitious Earth-like planet at the Proxima Centauri system at z0 = 1.3 pc from the Earth, we found that a total cumulative integration time of less than 2 months is sufficient to obtain a high quality, megapixel scale deconvolved image of that planet. Furthermore, even for a planet at 30 pc from the Earth, good quality deconvolution at intermediate resolutions is possible using integration times that are comfortably consistent with a realistic space mission.
Image: This is Figure 5 from the paper. In the caption, PSF refers to the Point Spread Function, which is essentially the response of the light-gathering instrument to the object studied. It measures how much the light has been distorted by the instrument. Here the SGL itself is considered as the source of the distortion. The full caption: Simulated monochromatic imaging of an exo-Earth at z0 = 1.3 pc from z = 1200 AU at N = 1024 × 1024 pixel resolution using the SGL. Left: the original image. Middle: the image convolved with the SGL PSF, with noise added at SNRC = 187, consistent with a total integration time of ?47 days. Right: the result of deconvolution, yielding an image with SNRR = 11.4. Credit: Turyshev et al.
The solar gravity lens presents itself not as a single focal point but a cylinder, meaning that we can stay within the focus as we move further from the Sun. The authors find that as the spacecraft moves ever further out, the signal to noise ratio improves. This heightening in resolution persists even with the shorter integration times, allowing us to study effects like planetary rotation. This is, of course, ongoing work, but these results cannot but be seen as encouraging for the concept of a mission to the gravity focus, giving us priceless information for future interstellar probes.
The paper is Turyshev & Toth., “Resolved imaging of exoplanets with the solar gravitational lens,” available for now only as a preprint. The Phase II NIAC report is Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II (2020). Full text.
A recent paper in Acta Astronautica reminds me that the Mission Concept Report on the Interstellar Probe mission has been available on the team’s website since December. Titled Interstellar Probe: Humanity’s Journey to Interstellar Space, this is the result of lengthy research out of Johns Hopkins Applied Physics Laboratory under the aegis of Ralph McNutt, who has served as principal investigator. I bring the mission concept up now because the new paper draws directly on the report and is essentially an overview to the community about the findings of this team.
We’ve looked extensively at Interstellar Probe in these pages (see, for example, Interstellar Probe: Pushing Beyond Voyager and Assessing the Oberth Maneuver for Interstellar Probe, both from 2021). The work on this mission anticipates the Solar and Space Physics 2023–2032 Decadal Survey, and presents an analysis of what would be the first mission designed from the top down as an interstellar craft. In that sense, it could be seen as a successor to the Voyagers, but one expressly made to probe the local interstellar medium, rather than reporting back on instruments designed originally for planetary science.
The overview paper is McNutt et al., “Interstellar probe – Destination: Universe!,” a title that recalls (at least to me) A. E. van Vogt’s wonderful collection of short stories by the same name (1952), whose seminal story “Far Centaurus” so keenly captures the ‘wait’ dilemma; i.e., when do you launch when new technologies may pass the craft you’re sending now along the way? In the case of this mission, with a putative launch date at the end of the decade, the question forces us into a useful heuristic: Either we keep building and launching or we sink into stasis, which drives little technological innovation. But what is the pace of such progress?
I say build and fly if at all feasible. Whether this mission, whose charter is basically “[T]o travel as far and as fast as possible with available technology…” gets the green light will be determined by factors such as the response it generates within the heliophysics community, how it fares in the upcoming decadal report, and whether this four-year engineering and science trade study can be implemented in a tight time frame. All that goes to feasibility. It’s hard to argue against it in terms of heliophysics, for what better way to study the Sun than through its interactions with the interstellar medium? And going outside the heliosphere to do so makes it an interstellar mission as well, with all that implies for science return.
Image: This is Figure 2-1 from the Mission Concept Report. Caption: During the evolution of our solar system, its protective heliosphere has plowed through dramatically different interstellar environments that have shaped our home through incoming interstellar gas, dust, plasma, and galactic cosmic rays. Interstellar Probe on a fast trajectory to the very local interstellar medium (VLISM) 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. Credit: Johns Hopkins Applied Physics Laboratory.
Crossing through the heliosphere to the “Very Local” interstellar medium (VLISM) is no easy goal, especially when the engineering requirements to meet the decadal survey specifications take us to a launch no later than January of 2030. Other basic requirements include the ability to take and return scientific data from 1000 AU (with all that implies about long-term function in instrumentation), with power levels no more than 600 W at the beginning of the mission and no more than half of that at its end, and a mission working lifetime of 50 years. Bear in mind that our Voyagers, after all these years, are currently at 155 and 129 AU respectively. A successor to Voyager will have to move much faster.
But have a look at the overview, which is available in full text. Dr. McNutt tells me that we can expect a companion paper from Pontus Brandt (likewise at APL) on the science aspects of the larger Mission Concept Report; this is likewise slated for publication in Acta Astronautica. According to McNutt, the APL contract from NASA’s Heliophysics Division completes on April 30 of this year, so the ball now lands in the court of the Solar and Space Physics Decadal Survey Team. And let me quote his email:
“Reality is never easy. I have to keep reminding people that the final push on a Solar Probe began with a conference in 1977, many studies at JPL through 2001, then studies at APL beginning in late 2001, the Decadal Survey of that era, etc. etc. with Parker Solar Probe launching in August 2018 and in the process now of revolutionizing our understanding of the Sun and its interaction with the interplanetary medium.”
Image: This is Figure 2-8 from the Mission Concept Report. Caption: Recent studies suggest that the Sun is on the path to leave the LIC [Local Interstellar Cloud] and may be already in contact with four interstellar clouds with different properties (Linsky et al., 2019). (Left: Image credit to Adler Planetarium, Frisch, Redfield, Linsky.)
Our society has all too little patience with decades-long processes, much less multi-generational goals. But we do have to understand how long it takes missions to go through the entire sequence before launch. It should be obvious that a 2030 launch date sets up what the authors call a ‘technology horizon’ that forces realism with respect to the physics and material properties at play here. Note this, from the paper:
…the enforcement of the “technology horizon” had two effects: (1) limit thinking to what can “be done now with maybe some “‘minor’ extensions” and (2) rule out low-TRL [technology readiness level] “technologies” which (1) we have no real idea how to develop, e.g., “fusion propulsion” or “gas-core fission”, or which we think we know how to develop but have no means of securing the requisite funds, e.g., NEP (while some might argue with this assertion, the track record to date does not argue otherwise).
Thus the dilemma of interstellar studies. Opportunities to fund and fly missions are sparse, political support always problematic, and deadlines shape what is possible. We have to be realistic about what we can do now, while also widening our thinking to include the kind of research that will one day pay off in long-term results. Developing and nourishing low-TRL concepts has to be a vital part of all this, which is why think tanks like NASA’s Innovative Advanced Concept office are essential, and why likewise innovative ideas emerging from the commercial sector must be considered.
Both tracks are vital as we push beyond the Solar System. McNutt refers to a kind of ‘relay race’ that began with Pioneer 10 and has continued through Voyagers 1 and 2. A mission dedicated to flying beyond the heliopause picks up that baton with an infusion of new instrumentation and science results that take us “outward through the heliosphere, heliosheath, and near (but not too near) interstellar space over almost five solar cycles…” Studies like these assess the state of the art (over 100 mission approaches are quantified and evaluated), defining our limits as well as our ambitions.
The paper is McNutt et al., “Interstellar probe – Destination: Universe!” Acta Astronautica Vol. 196 (July 2022), 13-28 (full text).