Imagining and Planning Interstellar Exploration
I don’t suppose that Frank Drake intended his famous Drake Equation to be anything more than a pedagogical device, or rather, an illustrative tool to explain what he viewed as the most significant things we would need to know to figure out how many other civilizations might be out there in the galaxy. This was back in 1961, and naturally the equation was all about probabilities, because we didn’t have hard information on most of the factors in the equation. Drake was already searching for radio signals at Green Bank, in the process inventing SETI as practiced through radio telescopes.
The factors here should look familiar to most Centauri Dreams readers, but let’s run through them, because among the old hands here we also get an encouraging number of students and people new to the field. N is the number of civilizations with communications potential in the galaxy, with R* the rate of star formation, fp the fraction of stars with planets, ne the number of planets that can support life per system, fl the fraction of planets that actually develop life, fi the fraction that develop intelligent life, fc the fraction that go on to communicate and L the life time of a technological civilization.
The idea of course is that you can multiply all these things together to derive some idea of how many civilizations are out there whose signals might be detected on Earth. Multiplying all those factors obviously ratchets up the uncertainties, but given that we have been proceeding with our investigations, and fruitfully so, for decades since Drake addressed the 1961 meeting at Green Bank, it’s interesting to see just where we stand today. Bringing us up to date is what Pascal Lee did at the recent symposium of the Interstellar Research Group in Montreal, where he gave the talk you can access here.
Image: M31, the Andromeda galaxy. Are civilizations common in spirals like this one? The Drake Equation is one way of probing the question, with ever-changing results. Credit: Space Telescope Science Institute.
Lee (SETI Institute, NASA Ames) is all too aware that with Earth as our only datapoint, we run the risk of being eaten alive by our own assumptions. So he takes a conservative approach on each of the factors involved in the Drake Equation. I think some of the most interesting factors here relate to the nature of intelligence, which didn’t pop up until halfway through the life of our star. That’s assuming you posit, as Lee does, that the appearance of homo erectus signifies this development, and this datapoint indicates that intelligence is not necessarily a common thing.
After all, the creatures that ran the show here on Earth for well over 200 million years do not seem to have developed intelligence, if we take Lee’s definition and say that this trait involves making technologies that did not exist earlier. A beaver dam is not a mark of intelligence because over the course of time it remains the same basic structure. Whereas true intelligence produces evolving and improving technologies. Thus primitive humans learn how to control fire. Then they make it portable. They start using tools. That this occurs so late causes Lee to give the fi factor a value of 0.0002, derived by dividing the median duration homo erectus has been around, roughly one million years, by the age of our planet. That’s but a sliver in Earth’s geology. We can reasonably argue that intelligence is circumstantial and fortuitous.
And what about intelligence developing into a technological civilization? This one is likewise interesting, and I like Lee’s idea of pegging 1865 as the time when humans became capable of electromagnetic communications because of James Clerk Maxwell’s equations. Thus we go from intelligence to communicative technologies and SETI possibilities in roughly a million years, from homo erectus to Maxwell.
But that’s just Earth’s datapoint. What about ocean worlds, or life forms in gas giant atmospheres or in heavy gravity surface environments where attaining orbit is itself problematic and perhaps the stars are rarely visible? Intelligence may develop without producing advanced civilizations that can become SETI targets. Here we have to get arbitrary, but Lee’s choice for the fraction of intelligent life turning into communicating civilizations is 0.1. It happens, in other words, just one time in ten. He’s being deliberately conservative about this, and the result has a lot of play in the equation.
Take a look at the rest of Lee’s values, based on all the work on these matters since the Drake Equation was conceived. Here’s his summary slide of the current outlook. I won’t run through each of the factors because we’re making progress on refining our numbers for those on the left side of the equation, whereas for these last two points we’re still extrapolating from a serious shortage of data. And that is certainly true in the last factor, which is L.
Obviously how we evaluate the longevity of a civilization determines the outcome, for if it is common for advanced technological cultures to destroy themselves, then we could be looking at a galaxy full of ruins rather than one with a flourishing network of intelligence. Our own threats are obvious enough: nuclear war, pandemics, runaway AI or nanotech, the ‘democratizing’ of potentially lethal technologies and more. Speculation on this matter runs the gamut, from a lifetime of 100 years to over a million, but historically human cultures last somewhere between 500 and 5000 years.
Is the anthropocene to be little more than a thin layer of rust in the geological strata? Lee sees 10,000 years for the lifetime of a civilization as a generous estimate, given that we are global and have more than the capability of self-annihilation. If this estimate is correct, we arrive at the conclusion shown in the slide above: The number of civilizations in our galaxy most likely equals one. And that would be us.
I was once asked in the question session after a talk how many civilizations I thought were in the Milky Way right now. I remember hedging my bets by referring to everybody else’s estimates – the 1961 estimate from the Green Bank meeting had ranged from 1,000 to 100,000,000, whereas one recent paper pegged the number at 30. But my interlocutor pressed me: What was my estimate? My conservative nature came to the fore, and I heard myself answering: “Between 1 and 10.” That’s still my estimate, but as I told my audience then, it’s the hunch of a writer, not the conclusion of a scientist, so take it for what it is.
Now I find Lee reaching the same conclusion, but two things about this stick out. If a single civilization did somehow get past what Robin Hanson calls ‘the great filter’ to emerge as a star-faring species living on many worlds, then their presence could still make all our talk of an Encyclopedia Galactica relevant. We might one day find that there is indeed a thriving network of intelligence, but one based around the work of a single Ur-civilization whose works we have better learn about and emulate. It’s a pleasing thought that such a civilization might be biological as well as machine-based, but all bets are off.
The other point, and this is one Lee makes in his talk: If life is common but technological civilizations are rare, that still leaves room for a value for N that takes in not just the Milky Way but the entire universe. N=1 means that the visible universe should contain 1011 civilizations, a satisfyingly large number and one that keeps our SETI hopes alive. We had better, in this case, concentrate our attention on nearby galaxies to have the greatest chance of success. There are 60 of them within 2 million light years, and over a thousand within 33 million light years. M31, the Andromeda galaxy, may deserve more SETI attention than it has been getting.
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?
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.
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 . 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.
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.
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.
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 .
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).