Centauri Dreams
Imagining and Planning Interstellar Exploration
SETI: A New Kind of Stellar Engine
The problem of perspective haunts SETI, and in particular that branch of SETI that has been labeled Dysonian. This discipline, based on Freeman Dyson’s original notion of spheres of power-gathering technology enclosing a star, has given rise to the ongoing search for artifacts in our astronomical data. The fuss over KIC 8462852 (Boyajian’s Star) a few years back involved the possibility that it was orbited by a megastructure of some kind, and thus a demonstration of advanced technology. Jason Wright and team at Penn State have led searches, covered in these pages, for evidence of Dyson spheres in other galaxies. The Dysonian search continues to widen.
I cite a problem of perspective in that we have no real notion of what we might find if we finally locate signs of extraterrestrial builders in our data. It’s so comfortable to be a carbon-based biped, but the entities we’re trying to locate may have other ways of evolving. Clément Vidal, a French philosopher and one of the most creative thinkers that SETI has yet produced, likes to talk not about carbon or silicon but rather ‘substrates.’ Where, in other words, might intelligence eventually land, and is that likely to be a matter of chemistry and biology or simple energy?
Vidal is currently at the UC Berkeley SETI Research Center, though he has deeper roots at the Free University of Brussels, from which he created his remarkable The Beginning and the End (Springer, 2014), along with a string of other publications. Try to come up with a definition of life and you may well emerge with something like this: Matter and energy in cyclical relationship using energy drawn from the environment to increase order in the system. I think that was Vidal’s starting point; it’s drawn from Gerald Feinberg and Robert Shapiro in Life Beyond Earth, Morrow 1980). No DNA there. No water. No carbon. Instead, we’re addressing the basic mechanism at work. In how many ways can it occur?
As Vidal reminded the audience at the recent Interstellar Research Group symposium in Montreal (video here), we are even now, at our paltry 0.72 rank in the Kardashev scale, creating increasingly interesting software that at least mimics intelligence to a rather high order. Making further advances that may exceed human intelligence is conceivably a matter of mere decades. If we consider intelligence embedded in a substrate of some kind, it makes sense that our planet may house fewer biological beings in the distant future than creatures we can call ‘artilects.’
The silicon-based outcome has been explored by thinkers like Martin Rees and Paul Davies in the recent literature. But the ramifications go much further than this. If we consider life as critically embedded in energy flows, the notion of life upon a neutron star swims into the realm of possibility. Frank Drake is one scientist who wrote about such things, as did Robert Forward in his novel Dragon’s Egg (Ballantine, 1980). If the underlying biology is of less importance and matter/energy interactions take precedence, we can further consider concepts like intelligence appearing wherever these interactions are at their most intense. Vidal has explored close binary systems as places where a civilization might mine energy, and for all we know, extract it to support a cognitive existence far removed from our notion of a habitable zone.
What about stars themselves? Greg Matloff has pointed to the low temperatures of red dwarf stars as allowing molecular interactions in which a primitive form of intelligence might emerge. Olaf Stapledon dreamed up civilizations using stellar energies in novel ways in Star Maker (Methuen, 1937) and mused on the emergence of stellar awareness. At Montreal, Vidal presented recent work on how an advanced civilization – in whatever substrate – might deploy a star orbited closely by a neutron star or black hole as a system of propulsion, with the ‘evaporation’ from the host star flowing to the compact companion and being directed by timing the pulsations to coincide with the orbital position of each. Far beyond our technologies, but then we’re at 0.72, as opposed to Kardashev civilizations at the far end of Kardashev II.
We have no idea how likely it is that such entities could emerge, but consider this. Charles Lineweaver’s work at Australian National University shows that the average Earth-like planet in the galaxy is on the order of 1.8 billion years older than Earth. The Serbian astronomer and writer Milan Ćirković has made the further point that this 1.8 billion year head-start is only an average. There must be planets considerably more than 1.8 billion years older than ours, and that makes for quite a few millennia for intelligence to develop and technologies to flourish.
Our first encounter with another civilization, then, is almost certainly going to be with one far older than our own. What, then, might we find one day in our astronomical data? I’ve quoted Vidal on this in the past and want to cite the same passage from The Beginning and the End today:
We need not be overcautious in our astrobiological speculations. Quite the contrary, we must push them to their extreme limits if we want to glimpse what such advanced civilizations could look like. Naturally, such an ambitious search should be balanced with considered conclusions. Furthermore, given our total ignorance of such civilizations, it remains wise to encourage and maintain a wide variety of search strategies. A commitment to observation, to the scientific method, and to the most general scientific theories remains our best touchstone.
The specific speculation Vidal tantalized the crowd with at Montreal is one he calls the ‘spider stellar engine,’ about which a quick word. Two types of ‘spider engines’ get his attention, the ‘redback’ and the ‘black widow.’ I assume Vidal is not necessarily an arachnophile, but rather a man aware of the current astrophysical jargon about extreme objects and pulsar binaries in particular. A redback refers to a rapidly rotating neutron star in tight orbit around a star massing up to 0.6 solar masses. A black widow has a much smaller companion star, and the term spider simply refers to the fact that the pulsar’s gravity draws material away from the larger star.
The larger star in such systems can be, spider-like, completely consumed, a useful marker as we study effects such as accretion disks and mass transfer between the two objects. Here is the energy gradient we are looking for in the question of a basic life definition, one that can be exploited by any beings that want to take advantage of it. A long-lived Kardashev II civilization, having feasted on the host star for its energies, could use what is left of the dwindling star at the end of its life to move to another host. The question for astronomers as well as philosophers is whether such a system would throw an observational signal that is detectable, and the question at this point remains unanswered.
Image; An illustrated view of a black widow pulsar and its stellar companion. The pulsar’s gamma-ray emissions (magenta) strongly heat the facing side of the star (orange). The pulsar is gradually evaporating its partner. ZTF J1406+1222, has the shortest orbital period yet identified, with the pulsar and companion star circling each other every 62 minutes. Credit: NASA Goddard Space Flight Center/Cruz deWilde.
We do have some interesting systems to watch, however. Vidal cites the pulsar PSR B1957+20 as having pulsations between host star and pulsar that match the orbital period, but notes that of course there are other ways of explaining this effect. We may want to include this particular signature as an item to look for in our pulsar work related to SETI, however. Meanwhile, the question of stellar propulsion (I think also of the ‘Shkadov thruster,’ another type of hypothesized stellar engine), explored by Vidal in his Montreal talk, yields precedence to the broader question with which we began. Are our perspectives sufficient to look for the kind of astronomical signatures that might be pointing toward forms of life almost unimaginably beyond our own?
The Realities of Interstellar Hibernation
Larry Niven played around with an interesting form of suspended animation in his 1966 Ballantine title World of Ptavvs. While the usual science fictional imagining is of a crew in some sort of cryogenic deep freeze, Niven went all out and envisioned a means of suspending time itself. It’s an ingenious concept based on an earlier short story in Worlds of Tomorrow, one that so aggressively pushes the physics that the more subtle delights of characterization and perspective come almost as afterthoughts. Niven fans like myself will recognize it as taking part in his ‘Known Space’ universe.
In the absence of time manipulation, let’s plumb more modest depths, though these can be tantalizing in their implications. In the last post, Don Wilkins described new work out of Washington University on inducing states of torpor – life processes slowed along with temperature – in laboratory experiments involving rodents. The spectrum from torpor to suspended animation has intervals that may suit our purpose. At the Interstellar Research Group’s Montreal symposium, John Bradford examined technologies with interplanetary as well as interstellar implications, though as he pointed out, they differ greatly from our cryogenic imaginings.
No deep freeze, in other words, and an experience for the crew that is far more like sleep – a very deep sleep – than the profound stoppage of metabolism that might keep a crew on ice for centuries. Bradford, who is CEO and principal engineer at Atlanta-based SpaceWorks, has explored the subject in two NIAC studies. His review for the Montreal audience took note of where we are today, when placing humans into a reduced metabolic state is relatively well understood, and therapeutic hypothermia is a medical treatment that can involve inhaled gases, cold saline injection and ice packs.
What particularly intrigued me in Bradford’s presentation (available here) is that there are numerous documented stories of human survival in extreme conditions, including one Anna Bagenholm, who survived submersion in an ice-covered lake for three hours. There are even some indications that some form of hibernation may have been present in early hominins, as evidenced by remains found in Spain involving changes in bone structure and density that imply cycles of use and disuse. Bradford’s NIAC work involved a mission design for Mars and Ceres in which the crew cycled through torpor states alternating with active periods. This notion of cyclic hibernation seems promising if we can discover ways of using it that maintain crew health both physical and mental.
Image: Suspended animation the ‘old-school’ way. This is an image from the 2016 film Passengers, involving a crew that is put into stasis inside hibernation pods.
Can we implement torpor in deep space? It’s a fascinating issue, one that may offer a path to manned interstellar travel if we can reduce trip times down to perhaps 50 years, a number Bradford chooses because, in conjunction with increases in human longevity, it offers missions where the balance of a human life is lived before and after the mission. The challenges are huge and already familiar from our space activities aboard the ISS. Muscle atrophy can be profound, as can bone loss and demineralization. An interstellar crew would confront exposure to galactic cosmic rays, while shielding exacts a mass penalty. There is also the matter of consumables.
Let’s face it, the human body is simply not designed for being off-planet, nor do we have even a fraction of the information we need to really assess such things as interplanetary missions with human crews from the physiological point of view. Until we have a dedicated research lab in orbit, we’re reduced to theorizing based on data from the ISS and manned missions that have never gone beyond the Moon.
If we can induce torpor states, we could drastically reduce the bulk of crew consumables needed for missions to another star, but let’s be clear about the kind of mission we’re talking about. A breakthrough in some sort of Alcubierre-like drive would eliminate the need for hibernation. On the other hand, absent the ability to take humans to 10 to 20 percent of lightspeed, we need to look to generation ships as the only viable way to move adult humans to an exoplanet, perhaps considering the possibility of using AI and human embryos raised at destination.
That’s why Bradford is really looking at ships moving fast enough that an interstellar crossing could be made within decades, and here we can look at such possibilities as induced ‘profound hypothermia’ that drops the body temperature below 20 ℃, a procedure used today in extreme cases and generally as a last resort. Perhaps more useful and certainly less drastic is gene-editing to enhance what may be in-built hibernation capabilities. Combine increased human longevity with some form of induced torpor and you come up with mission scenarios involving cycles of torpor and full wakefulness. Indeed, a 4-week cycle of torpor between periods of wakeful activity can reduce the perception of a 50-year transit to another star to a ten year period.
Plenty of work is going on in terms of longevity extension, ranging from research groups like the Methuselah Foundation and Altos Labs to drug trials involving replacement of molecules that tend to diminish with age and supplements like resveratrol and taurine that have promise in increasing lifespan. I’m not familiar with the details of this research, but Bradford said that there are voices in the scientific community arguing that 150 years is a reasonable goal for the average human, with the current record-holder (Jeanne Calment) having managed a startling 122.
Our scenario, then, is one in which we use induced torpor in a cyclical manner to reduce the time perception of an interstellar crossing. Therapeutic hypothermia (TH) currently involves periods of no more than three days, but as the chart above shows (drawn from Bradford’s slides), we can think in terms of two weeks in stasis with four or five days active as an achievable goal based on current research. Bradford’s NIAC work involved missions to Mars and Ceres using this cycle. Going beyond it raises all kinds of interesting questions about how the body responds to lengthy torpor states and, just as significantly, what happens to human cognition.
Dreaming to the Stars
Suspended animation shows up early in science fiction after a long history in prior literature. In Shakespeare, it’s the result of taking a “distilling liquor” (thus Juliet’s ‘sleep,’ which drives Romeo to suicide). In the SF realm, an early classic is John Campbell’s 1938 story “Who Goes There?”, which became the basis for the wonderful “The Thing from Another World” (1951). Here an alien whose spacecraft has crashed remains in frozen suspension for millennia, only to re-emerge as the barely recognizable James Arness. In the essay below, Don Wilkins points us toward a new study that could have implications for achieving the kind of suspended animation that one day might get a crew through a voyage lasting centuries. A frequent contributor to Centauri Dreams, Don is an adjunct instructor of electronics at Washington University, where the work took place. Echoes of van Vogt’s “Far Centaurus”? Read on. I’ll have another take on this topic in the next post.
by Don Wilkins
Humans have often observed with envy the ability of certain animals to extend sleeping periods from mere hours to months. If bears can do it, why cannot a suitably prepared person do it? Artificial hibernation is often used in science fiction to transport an individual into a distant future without the bother of aging or achieving relativistic speeds or conserving scarce resources. A practical hibernation system, in a more terrestrial function, provides medical support, improving survival by decreasing metabolic activity of a critically ill patient. Some writers hypothesize that a certain number of sleepers, particularly hibernations of decades or more, will suffer disabilities or death.
Research has focused on inducing torpor, a condition of significantly decreased metabolic rates and body activity, producing hibernation without adverse side-effects or horrifying experiments with cryogenics. A practical system remains within the realm of science fiction.
Torpor, like hibernation, is a physiological state in which various animals, including certain fish, reptiles, insects and mammals, actively suppress metabolism, lower body temperature and slow other life processes to conserve energy and survive fatal conditions and cold environmental temperatures.
A research team led by Yaoheng Yang (Washington University, St. Louis) has demonstrated a novel method for inducing torpor in rodents: Deep ultrasonic stimulation of a mammal’s brain [1]. Animals in torpor states experience reduced metabolism and body temperatures. Ultrasound was selected as the stimuli as it noninvasively and safely penetrates bone, and can be tightly focused with millimeter precision. The team hypothesizes that the central nervous system organizes the multitude of reactions needed to induce torpor.
In the experiments, as shown in Figure 1, a mouse wore a tiny “hat”, a lead zirconate titanate ceramic piezoelectric ultrasonic device with a center frequency of 3.2 MHz. The output was focused on the animal’s brain in the hypothalamus preoptic area (POA). Activating the POA neurons induced a torpor state for periods greater than 24 hours.
Fig. 1: Ultrasound device for inducing a torpor-like hypothermic and hypometabolic state. a, Illustration of ultrasound (US)-induced torpor-like state. b, Illustration of the wearable US probe (top). The probe was plugged into a baseplate that was glued on the mouse’s head. MRI of the mouse head with the wearable US probe shows that ultrasound was noninvasively targeted at the POA (insert). Photograph of a freely moving mouse with the wearable US probe attached is shown at the bottom. c, Illustration of the US stimulation waveform used in this study. ISI, inter-stimulus interval; PD, pulse duration; PRF, pulse repetition frequency. d, Calibration of the temperature (T) rise on the surface (top) and inside (bottom) the US probe. The temperature inside the probe was measured between the piezoelectric material and the mouse head when US probes were targeted at the POA or the cortex.
When the POA was stimulated, the body temperatures of the test animals dropped approximately three degrees C, although the environment was held to room temperature. Metabolism switched from carbohydrates and fat to solely fat. Heart rates declined about 47%.
The system used an automatic closed-loop feedback controller with the animal’s body temperature as the feedback variable. Tests which kept the subject’s body 32.95 ℃ for 24 hours were successfully concluded when the ultrasonic influence was removed and the animal returned to normal body temperature. According to previous studies, the body temperature must be below 34 ℃ to induce torpor.
Increasing the acoustic pressure and duration of the ultrasound stimulus further lowered body temperature and slowed metabolism. Each ultrasonic pulse produced consistent neuronal activity increases together with body temperature reductions in the test subjects.
The team, through genetic sequencing, discovered ultrasound could restrain the TRPM2 ultrasound-sensitive ion channel in the POA neurons. The precise mechanism providing the torpid state is unknown.
In a rat, which does not naturally enter torpor or hibernation, ultrasound simulation of POA neurons reduced skin temperature and core body temperature.
Ultrasonics can, with great spatial accuracy, reach deeply within the brain to stimulate the POA neurons. This approach could serve as the foundation of a system providing long term, noninvasive, and safe induction of torpor.
Reference
1. Yang, Y., Yuan, J., Field, R.L. et al., “Induction of a torpor-like hypothermic and hypometabolic state in rodents by ultrasound,” Nature Metabolism 5, 789–803 (2023). Full text.
Crafting the Interstellar Sail at Delft
Breakthrough Starshot’s concept for a flyby of Alpha Centauri would reach its destination in a single human generation. We’ve discussed sail materials in the last couple of posts, but let’s step back to the overview. Using a powerful ground-based laser, we illuminate a sail on the forward side of which are embedded instruments for communications, imaging and whatever we choose to carry. We need a sail that is roughly 4 meters by 4 meters, and one that weighs no more than a single gram.
As Richard Norte pointed out to the Interstellar Research Group’s Montreal symposium (video here), a US penny weighs 2.5 grams, which gives an idea what we are up against. We need a payload at gram-scale and a sail that is itself no more than a gram. Obviously our sail must be of nanoscale thickness, and able to take a beating, for we’re going to light it up for 10 minutes with that laser beam to drive it to 20 percent of lightspeed. We’re engineering, then, in the realm of nanotechnology, but working on combining our nanoscale components into large objects that can be fabricated at the macro-scale.
This is an uncharted frontier in the realm of precision and microsystems engineering, and it’s one that Norte and his team at Delft University of Technology are pushing into one experiment at a time, with recent funding from the EU and Limitless Space Institute. Things get fascinating quickly at this scale. To make a membrane into a mirror, you puncture your material with nanoscale holes, producing reflectivity at specific wavelengths. The Delft work is with silicon nitride, and in the current thinking of the Starshot team, this material formed as a metagrating is layered between the instruments on the lightsail and the sail body, becoming the means for keeping the sail on the beam and providing attitude control while protecting the instruments.
Image: Delft University of Technology’s Norte, whose lab focuses on novel techniques for designing, fabricating and measuring nanotechnologies needed for quantum optics and mechanics. Credit: Delft University of Technology.
At Delft, as Norte made clear, we’re a long way from achieving the kind of macrostructures that Starshot is looking at, but remember that Starshot is conceived as a multi-decade research effort that will rely on advances along the way. The Delft team is showing us how to make the thinnest conceivable mirror, using machine learning optimization techniques to optimize nanotechnology. The material of choice turns out to be silicon nitride, as we saw in our previous Starshot discussion. Says Norte:
”Of all the material people have used for making photonic crystals, silicon nitride winds up being one of the best. We can make it big, we can make it reflective, we can make it without wrinkles, and it actually has parts per million absorption. This extremely low absorption means we won’t blow this thing up when we shine a laser on it.”
Image: Scanning electron microscope image of a silicon nitride membrane. Credit: Richard Norte/Delft University of Technology. 9:47
The question is how to move to larger mirrors, given that the state of the art when Starshot was announced was at the 350-micron to a side scale. It would be helpful if we could simply ‘stitch’ smaller units together to craft a larger sail, but Norte likens that idea to trying to stitch two soap bubbles together – the structure is so amorphous, filled with the holes of the lattice – that we have to rely on manufacturing techniques that can produce larger wafers rather than combinations of smaller ones.
Scaling up is no small challenge. Norte told the audience at Montreal that his team can now make photonic crystals in the range of 450 mm to the side. The crucial term here is ‘aspect ratio,’ which relates the thickness of a metamaterial to its diameter or width. Interacting with light on the nanoscale means designing around the aspect ratio of these structures to achieve specific nanophotonic effects. Tuning the size and spacing of the holes in the lattice governs the wavelength at which the sail will reflect light.
No less important is the coupling of the laser beam with the sail, because while we are planning to accelerate these sails to speeds that are, by current standards, fantastic, we can only do so by optimizing how they interact with the beam. ‘Alignment resilience’ refers to the reaction of the sail as it is hit by the beam. New ways to arrange the nanoscale holes in the material weigh reflectivity against cost and efficiency, and Norte pointed out that a sail will need to be reflective over a wide range of light, given that it will experience large Doppler shifts in its abrupt change in velocity.
Getting this right will involve considering misalignments between the laser and the sail that can be self-adjusting depending on the design of the mirror lattice, and perhaps faster to accelerate. We seem at the moment to be decades away from being able to make meter-scale photonic crystal lightsails, a daunting thought, but Norte has an exhilarating thought about what we can do today with a sail of the 450 mm size now possible. An Alpha Centauri mission reaches target in centuries, perhaps as few as 200 years. This is assuming a one-gram payload and 70 percent reflectivity.
A wafer size fabrication of 100 mm can be used to build a sail that reaches Voyager 1 distance in 162 days, by Norte’s calculation. Even using the tiny 4.5 cm wafer Delft has already made, we could make that journey in about a year. Using the same 4.5 cm demonstrator alone, we reach Mars with a 1-gram payload in 32 hours, Saturn in 22 days, Uranus in 46 days and Neptune in 74 days. Contrast that with the speed of our fastest flyby probes. Voyager 2 took 12 years, for instance, to reach Neptune.
“It’s a compelling thought,” says Norte. “We can. send microchips to Mars the way we send international mail, just shotgunning them out there in 32 hours. Or we can get them to Titan’s oceans in less than a month. This is possible in nanotechnology today.”
Experimental work at Delft involves developing a sail that can be fabricated in a plasma etcher that allows the team to remove the silicon underneath, suspend the structure, and move it (without breaking vacuum) for lift by a laser. The dynamics of the sail under the beam can be explored, as can the question of beam-riding. Out of all this, Norte said, should come new levels of optical levitation, novel structural engineering, a new generation of sensors and detectors. In other words, new material science ahead.
Aerographite and the Interstellar Ark
The science fiction trope that often comes to mind in conjunction with the interstellar ark idea is of the crew that has lost all sense of the mission. Brian Aldiss’ Non-Stop (1958), published in the US as Starship, is a classic case of a generation ship that has become the entire world. The US title, of course, gave away the whole plot, which is sort of ridiculous. Have a look at the British cover, which leaves the setting mysterious for most of the book, and the American one, which blatantly tells you what’s going on. I wonder what Aldiss thought of this.
Be that as it may, interstellar arks are conceived as having large crews and taking a lot of time to move between stars, usually on the order of thousands of years. We can trace the concept in the scientific literature back to Les Shepherd’s famous 1952 paper on human interstellar travel, a key paper in the evolution of the field. An interesting adaptation of the paper appeared in Science Fiction Plus in April of the following year (see The Worldship of 1953). Alan Bond and Anthony Martin, whose names will always evoke Project Daedalus, likewise discussed interstellar arks, and Greg Matloff, whose presentation we looked at in the last post, has been working the numbers on these craft for much of his career.
Let’s look, then, at what Matloff and Joseph Meany say in their paper on aerographite. Here we’re talking about a sailcraft driven by solar photons rather than beamed energy, one that is based on an inflatable, hollow-body sail (itself a concept that goes back at least to the 1980s). Working with Roman Kezerashvili, Matloff has in the past addressed hollow-body sails made of beryllium as well as graphene, last discussing the latter in an interstellar ark concept in 2014. Here he and Meany set up an aerographite-graphene variant using a 90% absorptive and 10% reflective layer of aerographite that effectively pushes against the Sun-facing surface of graphene.
We’re in the realm of big numbers again. The radius of the sail is 764 kilometers, with the sail massing 5.49 X 106 kilograms. The as yet unpublished paper on this work shows that the payload mass (2.56 X 107 kilograms) is considerably higher than would be possible using a hollow-body sail made only of graphene. It’s interesting that for the close solar pass envisioned in the ‘sundiver’ maneuver for the sail, Matloff chooses a perihelion of 0.1 AU in order to hold down the g forces for the human crew. The point came up in the question session after his Montreal talk, for there do seem to be technologies for sustaining (for a short period) g-forces of 3 g and beyond, which would allow for a closer perihelion pass.
In any case, our ark reaches Alpha Centauri in about 1375 years carrying a crew of several hundred. If that figure seems exasperatingly high, consider that in the past few decades we’ve gone from the routine assumption that an interstellar mission would take millennia to the now plausible suggestion that we can get it down to a century or so. Massive arks, of course, take much longer, but this number isn’t bad. NASA’s Les Johnson told me in 2003, when I mentioned a thousand-year mission, that he would love to see a plan for one that could make the journey that fast. But here we are, discussing materials and techniques that can go well below that for unmanned probes. And then there is that Breakthrough Starshot concept of 20 percent of lightspeed…
We are, in other words, making progress. But so much remains to be done with regard to this particular material. Indeed, the work on graphene reminds us how little we know about the physical properties of aerographite. The paper lays out some large questions:
- Will what we know of aerographite’s properties be sustained when we reduce the thickness to a single micron?
- Will aerographite be stable at the temperatures demanded by our perihelion calculations?
- Will aerographite equal the performance of graphene when exposed to the space environment?
- What about trajectory adjustment for a non-reflecting surface like aerographite?
Thus the paper’s conclusion:
It is wise to consider the above discussion as very preliminary. There are many unknowns regarding aerographite and graphene that must be addressed before the missions discussed can be considered feasible.
One unknown is the closest feasible perihelion distance. Just because the Parker Solar Probe will likely survive its close perihelion pass does not mean that a carbon hyper-thin sail will do the same. It is necessary for some researcher to perform an exhaustive study of the effects of the near-Sun space environment upon these substances. A good consideration of the issues to be addressed is the exhaustive study for beryllium solar-photon sails performed by Kezerashvili [9].
One last note on early aerographite sails: What interesting problems they pose for tracking. We’d have to use infrared to follow their course, and a space-based telescope to do that because of infrared absorption in Earth’s atmosphere. Heller and team figured out in their aerographite paper that JWST could track a 1 m aerographite sail out to 2 AU. But swarm configurations (and we’ll be talking about this concept again in the near future) produce a signature that could be detected well beyond the Kuiper Belt. An onboard laser would greatly simplify the problem if we could find ways to power it up aboard the highly miniaturized craft that would be our first experiments.
The paper is Matloff & Meany, ”Aerographite: A Candidate Material for Interstellar Photon Sailing,” submitted to JBIS and ultimately to be published as part of the proceedings of the Interstellar Research Group’s 2023 symposium. The 2014 paper on graphene arks is “Graphene Solar Photon Sails and Interstellar Arks,” JBIS Vol. 67 (June 2014), 237-246 (abstract). The paper on beryllium sails by Roman Kezerashvili is “Thickness Requirements for Solar Sail Foils,” Acta Astronautica 65 (2009), 507-518 (abstract).
Interstellar Sails: A New Analysis of Aerographite
A material called aerographite offers options for solar sails that transcend the capabilities of both beryllium and graphene, the latter being the most recent candidate for fast sail missions outside the Solar System. Developed at the Technical University of Hamburg and refined by researchers at the University of Kiel, aerographite came to the attention of the interstellar community in 2020 thanks to a groundbreaking paper by René Heller (Max Planck Institute for Solar System Research, Göttingen), working with co-authors Guillem Anglada-Escudé (Institut de Ciencies Espacials, Barcelona), Michael Hippke (Sonneberg Observatory, Germany) and Pierre Kervella (Observatoire de Paris).
I’ve written about aerographite before, in Aerographite: An Advance in Sail Materials with Deep Space Implications and Solar Sails: Deeper into the Aerographite Option, both of which are in the archives along with several other posts on the subject. But here I need to pause for a brief administrative moment: The recent changes to the website inadvertently resulted in a data overwrite in the archives that replaced some specialized characters used in scientific notation with question marks. Not good! I have a crack programmer working the fix using my backups, but at the moment the articles I’ve just mentioned do contain several missing characters. This will be remedied soon.
Back to aerographite, where I’m pleased to see this work receiving the further scrutiny it deserves, for this is a highly unusual material, not what you would expect when conceiving deep space missions. As Gregory Matloff and Joseph Meany explain in a new paper discussed at the Interstellar Research Group’s Montreal symposium, aerographite is both extremely low density and utterly opaque. The normal assumption is that an effective solar sail will be reflective (and indeed, graphene concepts include ways to introduce reflectivity, which could be achieved by adding substrates or doping graphene with alkali metals, thus increasing mass).
Image; A detail of the world’s lightest material: aerographite. Open carbon tubes form a fine mesh and offer a low density of 0.2 milligram per cubic centimetre. The picture was taken with a scanning electron microscope (SEM). Credit: TUHH.
But the startlingly black aerographite so effectively absorbs photons that in sail configuration it will be pushed into interstellar space. Indeed, Guillem Anglada-Escudé told me three years ago that absorbance works quite well for solar sailing, less effective than a highly reflective material by no more than a factor of 2. As Matloff (New York City College of Technology, CUNY) and Meany (Savannah River National Laboratory) explain in the paper growing out of their work, aerographite is produced by a chemical vapor deposition process that yields a synthetic foam connected by carbon microtubes, one whose opacity is complemented by its light weight. Indeed, the teams that developed it called aerographite “the lightest known material.”
At Montreal, Matloff explored how the material might be deployed in two classes of interstellar missions, looking at such factors as the maximum temperature of the sail at various perihelion distances (for possible ‘sundiver’ missions), the sail’s thermal emissivity, and the peak acceleration that can be achieved, along with payload mass limitations for a 1-micron spherical sail shell and a thin-film payload. The work also probes the characteristics of aerographite under laser beaming conditions, and goes on to examine how it might be deployed in futuristic manned interstellar ‘arks.’ You can see Matloff’s presentation at Montreal here.
Aerographite’s visible photon absorption approaches 100 percent, with high tensile strength and an extremely high melting point. Matloff and Meany’s research involves a hypothetical sail with maximum operational temperature of 3,500 K and a payload mass that is one-tenth of the sail’s. For the purposes of their calculations, they lower the sail’s absorptivity to sunlight to a perhaps more realistic 0.9. Here Matloff’s experience in graphene sails comes in handy, allowing him to use the same analytical tools he and colleague Giovanni Vulpetti have worked out over years of solar sail analysis. Of particular note is the ‘lightness factor,’ which measures solar radiation against acceleration, and which for aerographite works out to an exceptionally high value.
An aerographite sail, in other words, is extremely efficient at converting sunlight into acceleration. The numbers are striking in comparison to previous estimates for solar sailing (as opposed to beaming) technologies. The performance figures in the table below are for an interstellar probe whose sail is unfurled at perihelion during a close solar approach. If you check the perihelion figures used for the analysis, you’ll see that the 0.04 AU figure matches the closest approach of an existing spacecraft, the Parker Solar Probe. And it turns out that 0.06 AU is close to the closest perihelion distance assumed for a beryllium sail. Matloff’s previous analysis of graphene (in a 2014 paper) had assumed a 0.1 AU perihelion for a graphene sail in the same kind of mission.
Our probe reaches Proxima Centauri within a millennium for all cases, with the 0.04 AU perihelion probe cutting the travel time to two centuries, a striking figure for a solar sail. The further good news is that according to these calculations, the aerographite at no point exceeds its melting point. Note the huge peak acceleration for the 0.04 AU perihelion pass: 319 g! A sail that makes it through the perihelion pass at 0.04 AU achieves an interstellar cruise velocity of roughly 0.02 c, which we can then stack up against a laser-launched sail along the lines of what Breakthrough Starshot envisions.
Here we run into trouble. From the paper:
It is not clear that an aerographite sail could withstand the enormous accelerations necessary to propel a Project Starshot terrestrial-launched laser-photon sail. Also, such a sail must either have an appropriate curvature to remain within the beam because the beam source moves with Earth’s rotation or be implanted with an appropriate diffraction pattern to optically simulate an appropriately curved sail surface. Also, because aerographite is absorptive rather than reflective, the enormous required beam power on the sail to achieve an ~0.2c interstellar cruise velocity might be fatal.
Which is why Matloff and Meany studied the effects of a sail powered by the beam from a space-based laser array rather than a terrestrial one, using a 100 meter sail for the analysis. I will send you to the paper (or the video) for the details of these calculations, but a laser transmitter of approximately 1.8 kilometers is modeled, with the Sun-orbiting laser at 1 AU from the Sun. Here the craft achieves a velocity of 0.033c given the constraints applied to the beaming technology, which the authors note may be fewer than those imposed on the Starshot array. Indeed:
Constructing sail, sunlight-collection optics and the laser/transmitter are challenging as is the necessity of keeping the sail within the beam during the ~3-hour acceleration run. But these challenges are considerably less than is the case for the Project Starshot relativistic-velocity sails accelerated by a terrestrial laser array.
Those who know Greg Matloff’s work know how he rejoices in stretching ideas out to their maximum potential, much in the mode of Robert Forward. Thus it’s no surprise that the next idea considered here is an aerographite sail capable of carrying humans aboard an interstellar ark. That’s a discussion in itself, and so is the question of the best path forward for aerographite research, two subjects I’ll be taking up in the next post.
The paper is Matloff & Meany, ”Aerographite: A Candidate Material for Interstellar Photon Sailing,” submitted to JBIS and ultimately to be published as part of the proceedings of the Interstellar Research Group’s 2023 symposium. The Heller, Anglada-Escudé, Hippke & Kervella paper is “Low-cost precursor of an interstellar mission,” Astronomy & Astrophysics Vol. 641 (September 2020), A45 (abstract).
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