Centauri Dreams
Imagining and Planning Interstellar Exploration
Server Problems Resolved
I’m going to keep Alex Tolley’s fine essay (below) at the top for another day, in hopes of re-starting the comment thread that was going along so nicely before the site went down. Then tomorrow we’ll start talking about gravitational lensing, in the first of a series that may extend until next week.
Distinguishing Between Biological and Machine Civilization Techno-signatures
If we ever make a SETI detection, will it be of biological beings or machine intelligence? As Alex Tolley explains in today’s essay, there are reasons for favoring the latter possibility, leading our author to compose what he calls a ‘light-hearted speculation’ about machines searching for other civilizations of their own kind. Life seems to be easy compared to this. We are developing the tools to delve into planetary atmospheres in search of biosignatures, hoping to cull out ambiguities. But is there an equivalent in the machine world of a biosignature, and how would it be found? Interesting implications arise, some of them seemingly close to home.
by Alex Tolley
Curiosity Rover. Credit Nasa.
Terry Bisson’s amusing short sci-fi story “They’re made Out of Meat” [4], is a communication between two individuals who express their disbelief that a biological species (detected on Earth by a galactic survey) can possibly be intelligent. The denouement is to erase the record of discovery from the survey report. It remains one of the few stories where machine entities are dominant in the galaxy. For me, this story is memorable because it is one of so few stories that focuses on the viewpoint of aliens, and moreover, machine aliens. This essay similarly focuses on what a machine civilization would look for when searching for machine intelligence in space.
Until recently, most speculation about extraterrestrial intelligence (ETI) has assumed it will be biological. In science fiction from the venerable H. G. Wells’ novel The War of the Worlds to recent movies like Independence Day and Arrival, technologically advanced ETI is depicted as biological.
SETI starts with the probability that life will appear, first unicellular then complex, leading on Earth led to hominid-level intelligence, which in turn eventually flowered culturally and created civilization and technological societies. SETI assumed there would be some sort of galactic communication between biological species confined to their home systems due to the extreme difficulties of interstellar travel.
Our civilization has placed primacy on our cognitive level to ensure we are the prime agencies, using animals, and later machines, to displace physical labor. Our conceit is that this will always remain so, as our technologies increase their capabilities, but always remain controlled by us.
However, the rapid development of artificial intelligence (AI) since the middle of the 20th century, the continuing rapid performance improvement in computer systems, and the undeniable success and longevity of our robotic explorers in space should be an indication that we are in the throes of a rapid transition to true, artificial general intelligence (AGI) machines that are well adapted to inhospitable environments, especially space.
Sci-fi authors have explored these machine-centric futures, from the novel by Stephen Baxter and Alastair Reynolds The Medusa Chronicles [2], which extended the Arthur C Clarke novelette [3] and has machines building a competing civilization to humans in the outer solar system, to Greg Benford’s Galactic Center novels, where sentient machines dominate the galaxy and humans have to survive like mice in a human world, while the mechs try to eliminate the humans just as we do for small rodents in our buildings.
More recently, James Lovelock wrote that he believed that humans would be replaced by cyborgs, by which he meant not Martin Caidin’s Cyborg (AKA The Six-million Dollar Man) or Star Trek‘s Borg, but intelligent robots [1]. These would be our descendents and would be the explorers of the galaxy. This view has been supported by the Astronomer Royal, Sir Martin Rees, who stated that he believes that if we receive communication from the stars, it will be from a machine civilization [5]. Sir Martin is old enough to have watched the BBC production of A for Andromeda and the sequel The Andromeda Breakthrough, where it is revealed that the source of the radio signal might have been a machine intelligence.
Space advocates continue to argue over human versus robotic exploration of space, which usually proceeds around the superiority of human capabilities compared to robotic probes, especially surface rovers. What is rarely discussed is that this is a dynamic situation, where the improvement in capabilities favors robots far more than humans. Astronomer Royal Rees is surprised this argument still continues, as he sees robotic exploration, primarily for science, as clearly advantageous over human exploration. The farther away that exploration extends from the Habitable Zone (HZ), the more difficult to reach and inhospitable the targets become.
Some, like Jupiter’s inner moons, have radiation levels so high that even robotic probes need specially hardened microchips and circuits. Reaching the outer planets is so time consuming that without drives that are orders of magnitude more powerful than today’s, or hibernation technology, human travel will be particularly arduous. Such trips will make even the global sea voyages in the Age of Exploration seem like child’s play by comparison. The only advantage such travellers will have over Captain Cook is that there will be no hostile natives to meet them.
Let me be clear, I do not expect humans to be displaced by robots on Earth, at least not in the foreseeable future, nor will there be a binary pure biological human vs robot future. Humans will take advantage of modifications using technologies with increasing capabilities that will help us compete with robots, as well as modifications at the biological level incorporating genetic engineering. As computers have moved from dedicated buildings to the desktop to mobile devices, wearable devices will eventually become implanted, interfacing with the appropriate neural circuitry, and in some cases, replacing human organs. Genetic engineering is at its infancy and we can expect rapid developments once the moral objections are overcome.
I would argue that most biological extraterrestrial intelligences (ETIs) will follow a similar path, as they have evolved to live in a biological environment and not an off-world one. In other words, technological evolution will converge on embodied machine intelligence.
A Machine Diaspora
In the short term, well in advance of human brains becoming artificial, artificial minds will be rapidly deployed in many settings. They will likely be the only types of minds in deep space vehicles. Such minds will not just be embodied in probes and rovers, but also in industrial facilities to mine resources from asteroids and planets. They will likely be specialized and interact with other specialized robots to build industrial ecosystems and eventually their own colonies and civilizations. The barriers to humans colonizing space so easily will allow such robot civilizations to develop [semi]independently from humanity.
If robots are the best embodiment of minds to travel in space, explore and colonize it, then it seems most probable that they will be the first emissaries to other star systems. They may well prove the only travelers, with biological species trapped within their home systems, and possibly just to their homeworld, a few planets and moons, and space habitats. If the Breakthrough Starshot project ever launches sailcraft, the crude minds in the vessel will be the first of many AI interstellar emissaries.
World ships or seed ships carrying humans to the stars may eventually happen, but the populations may find robots have fully developed the possible target systems and are not particularly interested in “carbon-based units” potentially parasitizing their artificial environments.
If these prognostications prove right and machine intelligences become our descendants and dominate the galaxy, it seems reasonable to speculate that the same has happened on other worlds where biological intelligence has evolved. Whether this has happened elsewhere or not, machine descendants will also be searching space for others like themselves. If so, the question I want to pose is:
How would such a machine civilizations look for similar signs of a machine civilizations in the galaxy?
Because machine life is dependent on the earlier evolution of intelligent biological life, any technological signature we detect, from electromagnetic wave signals to manufactured artifacts, could be the result of either a biological or machine intelligence.
For a machine intelligence looking for other machine intelligence in the galaxy, this presents an ambiguity over agency. For techno-signatures from a world in the HZ, the earlier evolution of biological intelligence may indicate a reduced probability of machine intelligence compared to biological intelligence. However, over the long term, if machines inevitably displace biological intelligence, then the probability rises. Once interstellar exploration is under way, then the probability of any civilization being machine-based rises very quickly towards unity, as suggested by Sir Martin Rees.
For a machine civilization looking for other machine civilizations, are there ways to rule out biological civilizations from machine ones, or are the two indistinguishable?
The range of possible techno-signatures would be ones we already know to look for. Planetary surface structures, platonic shapes, processed surface materials like metals, radio emissions with spectrum spikes, signals with non-random patterns, space-based structures, artificial structures that require energy to move in space, industrial gases in the atmosphere such as chlorofluorocarbons. All these techno-signatures may be accompanied by biosignatures, especially from a habitable planet in the HZ with an atmosphere.
The foregoing should make it clear that sentient machines will have a harder time searching for their machine cousins than humans have for searching for life and intelligence of any sort. Biosignatures will indicate life. Techno-signatures can indicate technological civilization of either biological or artificial origin. Just as we cannot separate biological and machine civilizations remotely today or even in the near future, neither can a machine civilization, unless their technology allows remote observations to make these distinctions. Below I outline some scenarios, many of which require a local probe.
Machines Searching for Machines
So let us assume a machine civilization that is colonizing the galaxy is looking to make contact with other machine civilizations. This civilization will know that it was preceded by at least one biological intelligent species that developed a technological civilization that spawned its ancestors before being replaced.
The extra ambiguity faced by such a civilization is distinguishing between a biological and machine civilization. Because of the length of galactic time, I will assume that any period of transition will be transient and therefore has a low probability of being encountered. Either the biological intelligence will have retained control [11] or the transition to a machine civilization will be complete. The current view of techno-utopians that humans will use advanced AI technologies to increase their capabilities to stave off any machine takeover will therefore be relegated to a transient transition period, one that will eventually either have to be abandoned or will lead to a machine civilization that will supplant human civilization.
With this in mind, what signatures will a machine civilization look for that will lead it to conclude that it has found a machine civilization that is independent of any previous biological civilization?
We start with the assumption that a techno-signature of some type has been detected [7].
The most convincing support for a machine civilization would be the absence of any biosignature in the system, or the planet nearest the source of the signature. A sterile planet with a techno-signature would indicate that any biological intelligence was either never located there, or that it has been systematically eliminated with all other life. Such a sterile planet would have an atmosphere gas composition in equilibrium, which would also eliminate unseen microbes. However, there could still be some ambiguity as to whether the techno-signature implies an extant civilization or not. Structures and even a transmitting beacon might imply a dead civilization that had disappeared with all other life. If there are biosignatures elsewhere in the system, it could indicate that the techno-signature is a product of a biological intelligence on that world, with machines providing the needed capabilities elsewhere in the system. Humans might have METI transmissions from the lunar farside as an example of such a scenario.
Now suppose that the source of the techno-signature is from or near a planet that has been confirmed as having no complex life forms. This lack of complex life forms might be determined telescopically (spectroscopically and visually) by noting a barren continental surface devoid of plants. An absence of plants also implies an absence of a terrestrial food chain and therefore no intelligent biological intelligences. It would take a local probe to eliminate oceanic complex life, and eliminate any possibility of an intelligent technological species that lived in the ocean, but came out onto the land to develop a fire-based technology, perhaps as the Europans may have been doing in Clarke’s Odyssey series. As with the lifeless planet scenario, there remains the issue of whether the civilization is extant or not.
The next case is that there is a planet in the system that has a biosignature and clear signs of complex life such as biomes with plant-based ecosystems. Human civilization to date, that is the last ten millennia or so, has required agriculture. This has resulted in field cultivation, primarily of monoculture crops. Often these fields are regular in shape, and may form a patchwork of different monocultures. Field boundaries also tend to be straight. Even if this is not a universal method of farming (e.g. hillside rice paddies, or domesticated animal ranching), any evidence of such monocultures in what appear to be unnatural delineated areas would be a probable indication of the presence of biological intelligence.
This biosignature would still be ambiguous and need further exploration. On Earth, our human population is limited by food production, a Malthusian condition that we seem to be coming up against again after a brief period of being free of that condition. We have extended the productivity of land for food production with artificial fertilizers, and we are just starting to increase it much farther using artificial light in vertical farms. Earth could, in theory, support a much larger population if traditional farming in spaces open to sunlight was replaced by these vertical farms, and even factory food production using other fast replicating food sources such as single celled organisms, insects, and cell culture. In extremis, the agriculture signature would disappear, leaving just the techno-signature of extensive cities.
The other possibility is a machine civilization that has allowed human populations to remain in existence, but removed from control. We might think of this as the movie version of Planet of the Apes, but where machines are the dominant civilization, and humans reduced to either a wild or early agrarian state.
Nothing Beats Propinquity
The next ambiguities will need local probe involvement to be resolved, or at least a technology that substitutes for this.
A planet with biosignatures, signs of both complex life and techno-signatures, might distinguish between biological and machine civilization if there is evidence of widespread active machine use but without the presence of biological entities, especially of a common type being associated with them. Human civilization on Earth applies human cognition in close proximity to operate machinery and transport vehicles, as well as being passengers. While an ETI might not readily be able to distinguish between intelligent human passengers on a bus and domesticated animals being driven to a slaughterhouse, it will notice that only humans are operating and controlling machinery behind the wheel in a moving vehicle, and it will notice that horses are never seen doing those things.
In the event of a catastrophe leaving abandoned cities, many different animal species will be seen in the presence of machines, but none will be able to operate them. If all observations of active machines indicate no operation by biological entities, then it is most likely that they are controlled by machine intelligence. However, we should also be aware that we are developing autonomous machines managed by humans.
It is possible that in some future scenario, human civilization may have humans living in pods and controlling or just managing semi-autonomous and autonomous machines. Philip K Dick’s autofacs may be the primary sources of goods, possibly even following the paperclip apocalypse [10]. The BBC’s Doctor Who series also offers another difficult to interpret scenario – are daleks machines or biological? Early on it was intimated they were just robots, but later their nature was shown to be degenerate biological entities living in mechanical carapaces. As before, closer exploration of such a world would be needed.
For a number of more subtle cases, local exploration will be necessary.
A probe that has landed can sample the sounds within and around structures. If the sounds show complex structure with a high information content, and they are associated with a single, or few species, then the likelihood is that this biological species is intelligent. In addition to other evidence of this species controlling machines, then the civilization is likely biological.
If video transmissions are detected and can be decoded, then the presence of a dominant species and depictions of biological activities such as feeding and sex will indicate that this is a biological civilization rather than a machine one. A wide sampling of video will be required to prevent an unfortunate limited sampling of only nature videos.
Transmissions that appear to be made by machines would be ambiguous. They could be due to machines in a machine civilization communicating, or machines in a biological civilization communicating. Currently most communication and information creation on Earth is by computers, although video transmissions still dominate bandwidth. How long this will last is unknown. Computing machines are certainly increasingly consuming more of the available electrical energy produced. It is possible that at some point in the future they may become the dominant consumers of electrical power, making the determination of whether Earth is a biological or machine civilization more ambiguous.
A space probe encountering space-based or even surface structures on sterile worlds that are open to vacuum might well imply a machine civilization. But as before, are these for a machine civilization, or for machines controlled by a biological civilization? This particular scenario will be particularly difficult to determine if machines are the first to cross interstellar space and set up production facilities in a lifeless star system. This scenario would at first seem to be the most unambiguous of situations: Techno-signatures in a star system devoid of any biosignature on any of the planets in the HZ or even beyond. The machines would seem to be autonomous, working to replicate themselves and build facilities that are clearly not intended to support biological entities. Any Von Neumann replicators [9] operating in such a system would have all the apparent hallmarks of a machine civilization. Such an observation could be due to a true machine civilization, a machine operation controlled by a [distant] true machine civilization, or a distant biological civilization.
A last confounding situation is detailed in the novel, The Medusa Chronicles [2]. There may be both biological and machine civilizations that exist in the same milieu, neither dominant entirely, but both dominant locally in their part of the solar system. A machine civilization might well want to communicate with the machine but not the human civilization in that scenario. Determining the true status of such a situation may require exploration and even interaction before making the determination to communicate with the machines. At this point, the machine civilization is having to emulate the explorers during the Age of Exploration, making contact with natives and interacting with them.
Conclusion
Jill Tarter said that SETI is not directly searching for ETI, but rather looking for technological proxies using our radio (and now optical) telescopes [8]. While astrobiologists are searching for life, any life, SETI does not make the distinction between biological or machine intelligence. SETI scientists may talk as if they assume that ETI is biological, but their methods cannot distinguish between the two types. If we wanted only to communicate with biological civilizations, we would face the same difficulties as a machine civilization only wanting to communicate with a civilization of machines. To determine whether a techno-signature was from one particular type of civilization would require other observations, some of those necessarily local to the source of the techno-signature.
If ever there was a case for a Lurker in the solar system monitoring Earth over a long period, this might be it.
References
1) Lovelock, J. (2020). NOVACENE: The coming age of hyperintelligence. MIT Press.
2) Baxter, S, & Reynolds, A. (2016). The Medusa Chronicles. New York: Saga Press.
3) Clarke A, (1971) “A Meeting with Medusa”, Playboy December 1971.
4) Bisson T (1991) “They’re Made Out of Meat”, first published in Omni.
5) Rees, M (2015) “Why Alien Life Will Be Robotic”, http://nautil.us/issue/29/scaling/why-alien-life-will-be-robotic accessed 11/16/2020
6) Klaes, L (2020) “The People’s Space Odyssey: 2010: The Year We Make Contact” https://www.centauri-dreams.org/2020/07/31/the-peoples-space-odyssey-2010-the-year-we-make-contact/ accessed 11/16/2020
7) Lemarchand, G (1992) “Detectability of Extraterrestrial Technological Activities” accessed on 11/17/2020 http://www.coseti.org/lemarch1.htm
8) Tarter, J (2019) “Technosignatures vs. Biosignatures: Which Will Succeed First?” SETI Institute talk, https://www.youtube.com/watch?v=ES5zt7JsJtQ&list=PL7B4FE6C62DCB34E1&index=12
9) Dvorsky, G. (2008) “Seven ways to control the Galaxy with self-replicating probes”, http://www.sentientdevelopments.com/2008/03/seven-ways-to-control-galaxy-with-self.html (accessed November 24, 2020)
10) Bostrom, N (2003) “Ethical Issues in Advanced Artificial Intelligence Cognitive, Emotive and Ethical Aspects of Decision Making in Humans and in Artificial Intelligence,” Vol. 2, ed. I. Smith et al., Int. Institute of Advanced Studies in Systems Research and Cybernetics, 2003, pp. 12-17
11) Herbert, Brian, and Kevin J. Anderson. (2019) Dune. The Butlerian Jihad. Tor, 2019.
Deep Future: The Next Supercontinent
Science fiction writers range freely through time, making many scientific papers fertile ground for plot ideas and settings. So here’s an extraordinary one. We know that Earth’s continents used to be packed into a single large land mass called Pangaea, which is thought to have broken apart about 200 million years ago as tectonic plates shifted. Interestingly, we can expect a remote future in which the continents will have once again come together, as Michael Way (NASA GSFC) has pointed out at an online poster session at the ongoing virtual meeting of the American Geophysical Union. And such a supercontinent has ramifications for habitability.
Let’s talk about those because they have a bearing on astrobiology as we examine exoplanets and consider their suitability for life. We’re a decade or so (at minimum) away from being able to determine how land and sea are distributed on a nearby world, but climate modeling is useful as we look toward estimating habitability. That involves, as this work shows, investigating how land masses are positioned on a planetary surface and their effects on climate in the habitable zone.
Working with Hannah Davies and Joao Duarte (University of Lisbon) and Mattias Green (Bangor University, Wales), Way has run 3D global climate models which are, according to Columbia University’s Earth Institute (where Way is an affiliate) the first models made on a supercontinent in the deep future. Out of this the scientists derive two likely outcomes. The first, occurring in the modeling in about 200 million years, is a merging of all continents except Antarctica around the north pole, forming the supercontinent ‘Amasia.’
The second: The formation of the supercontinent ‘Aurica,’ as all the continents come together around the equator in about 250 million years. The effects are significantly different. The formation of Amasia around the north pole produces a planet about 3 degrees Celsius cooler than the one resulting from the formation of Aurica around the equator. What happens is that the movement of heat from the equator to both poles is disrupted with all the land around the poles.
With heat not being conveyed as efficiently from equator to pole, the poles become colder and remain covered in ice all year long, reflecting significant heat into space. Amasia, according to Way, produces “a lot more snowfall. You get ice sheets, and you get this very effective ice-albedo feedback, which tends to lower the temperature of the planet.”
You also get lower sea levels in the Amasia scenario, with more water trapped in the ice caps. Less land is available for agriculture in a supercontinent with predominantly snowy conditions.
Image: How land could be distributed in the Aurica supercontinent (top) versus Amasia. The future land configurations are shown in gray, with modern-day outlines of the continents for comparison. Credit: Way et al. 2020.
Aurica turns out to be a more clement place, absorbing the stronger sunlight at the equator and, without ice caps at the poles to reflect heat, having a higher global temperature. The setting sounds like it would be ideal save for the fact that, according to the 3D models, the inland areas would be dry. What the scientists have not yet examined is the kind of precipitation patterns that might emerge. Large lakes would offset the effect, so it would be useful to know how likely they are.
All told, the work is pointing to temperatures suitable for liquid water on about 60 percent of Amasia’s land, while 99.8 percent of Aurica’s terrain should be available. We come back to land mass arrangements as a factor in planetary habitability, given our reliance on habitable zone models that insist on the presence of liquid water on the surface. Building a library of land mass distributions and examining their varying effects may help us tune our notions of habitability.
I’ll add that Way’s investigations using the GISS [Goddard Institute for Space Studies] General Circulation Model and expanding it to model paleo- and now future Earth have also extended to models of early Mars and Venus, with plans to examine Titan’s atmosphere. What our own planet has to teach us about climate, habitability and continental arrangement is thus extended to other worlds as the model evolves. One day soon we’ll add exoplanets into the mix.
The paper is Way et al., “Deep Future Climate on Earth: effects of tectonics, rotation rate, and insolation,” in process at Geophysical Research Letters (abstract).
Musings on Fusion and the Interstellar Ramjet
Proton-proton fusion produces 99 percent of the Sun’s energy, in a process that begins with two hydrogen nuclei and ends with one helium nucleus, releasing energy along the way. We’d love to exploit the fusion process to create energy for our own directed uses, which is what Robert Bussard was thinking about with his interstellar ramjet when he published the idea in 1960. Such a ship might deploy electromagnetic fields thousands of kilometers in diameter to scoop up atoms from the interstellar medium, using them as reaction mass for the fusion that would drive it.
Carl Sagan was a great enthusiast for the concept, and would describe it vividly in the book he wrote with Russian astronomer and astrophysicist Iosif S. Shklovskii. In Intelligent Life in the Universe (1966), the authors discuss a journey that takes advantage of time dilation, allowing a lightspeed-hugging starship powered by these methods to reach galactic center in a mere 21 years of ship-time; i.e., time as perceived by the crew, while of course tens of thousands of years are going by back on Earth. If you also hear echoes of Poul Anderson’s Tau Zero here, you’re exactly on target.
Shklovskii and Sagan assume proton-proton fusion as the reaction, as Bussard originally did, but Thomas Heppenheimer was able to show in 1978 that it would take more power to compress the protons gathered from the interstellar medium than the reaction would produce. Ramscoops are tricky, and this is just one of their problems — gathering interstellar materials is another, dependent as it is on the density of the gases where the starship travels. Drag is yet another issue, making interstellar ramjets a segue into magsail deceleration rather than starship-enabling speed, though it’s a segue I’ll follow up on another occasion.
But the fusion itself is still interesting. If Bussard assumed proton-proton, it wouldn’t be long before Daniel Whitmire was able to show that a different reaction could produce far more power. The Carbon Nitrogen Oxygen cycle (CNO cycle) came to mind this morning because of word that the team working on the Borexino experiment in the Laboratori Nazionali del Gran Sasso (Italy), which studies the Sun’s fusion reactions through the neutrinos it produces, has been able to identify the CNO cycle as a small component of the Sun’s production of energy.
Image: The Borexino research team has succeeded in detecting neutrinos from the sun’s second fusion process, the Carbon Nitrogen Oxygen cycle (CNO cycle) for the first time. Credit: Borexino Collaboration.
That’s interesting in itself and confirms work by Hans Bethe and Carl Friedrich von Weizsäcker from the 1930s, the first experimental confirmation of their independent investigations. But I cycle back to Bussard’s ramjet. The Carbon Nitrogen Oxygen cycle involves four hydrogen nuclei combining to form a helium nucleus using carbon, nitrogen and oxygen as catalysts and intermediate products in the reaction. Maybe ‘catalysts’ isn’t the right word — I was reminded by reading Adam Crowl’s thoughts on the matter some years back that we’re not talking about chemical catalysis and should perhaps refer to all this simply as ‘nuclear chemistry.’
What boggles the mind about the CNO cycle, which I’ve read is the dominant energy source in stars more than 1.3 times more massive than the Sun, is the degree of energy unlocked by it, far exceeding uncatalyzed proton/proton fusion. And it would take something highly energetic to work on Bussard’s ramscoop, for Whitmire’s 1975 paper showed that a proton-proton reactor built in the fashion originally suggested by Bussard would need a scoop 7,000 kilometers across to make the reaction work.
Isn’t that odd? You would think that a reaction that powers the Sun would be perfectly sufficient to drive the Bussard ramjet, but it turns out that the rate of proton-proton fusion is too low. Looking back through my materials on the problem, I find that the Sun produces less than 1 watt per cubic meter when averaged over its whole volume, which means that the energy produced in a light bulb filament is more powerful. Whitmire realized that the Sun’s vast energy output could occur because of its size. Making equally massive starships is out of the question.
It turns out that Whitmire and Centauri Dreams regular Al Jackson were friends at the University of Texas back in the 1970s, and I’ll remind you of Al’s reminiscence of Whitmire that can be found here — it was actually Al who introduced the Bussard ramscoop idea to Whitmire. Bussard would write to Whitmire that his 1975 paper offered a solution to the proton-proton fusion problem and would “become an enduring classic in this field.”
If you know your science fiction, you’ll recall that Greg Benford uses the CNO cycle in his 1984 novel Across the Sea of Suns, where he gives a poetic description of the process at work as perceived by his protagonist via the ultimate in futuristic telepresence:
He watches plumes of carbon nuclei striking the swarms of protons, wedding them to form the heavier hydrogen nuclei. The torrent swirls and screams at Nigel’s skin and in his sensors he sees and feels and tastes the lumpy, sluggish nitrogen as it finds a fresh incoming proton and with the fleshy smack of fusion the two stick, they hold, they wobble like raindrops — falling together — merging — ballooning into a new nucleus, heavier still: oxygen.
But the green pinpoints of oxygen are unstable. These fragile forms split instantly. Jets of new particles spew through the surrounding glow — neutrinos, ruddy photons of light, and slower, darker, there come the heavy daughters of the marriage: a swollen, burnt-gold cloud. A wobbling, heavier isotope of nitrogen….
Ahead he sees the violet points of nitrogen and hears them crack into carbon plus an alpha particle. So in the end the long cascade gives forth the carbon that catalyzed it, carbon that will begin again its life in the whistling blizzard of protons coming in from the forward maw of the ship.
And there you are: Carbon – Nitrogen – Oxygen in a cycle that makes starship fusion work. And all of this reminiscing suggested by the results of an experiment deep below the the Italian Gran Sasso massif which has turned up evidence for the CNO cycle within the Sun, a small but ongoing component of its output. If you want to read more on what turned up at Borexino, the paper is The Borexino Collaboration, “Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun,” Nature 587 (2020), 577-582 (abstract). The Whitmire paper is “Relativistic Spaceflight and the Catalytic Nuclear Ramjet,” Acta Astronautica 2 (1975), pp. 497-509 (abstract).
A Centaur in Transformation
There was a time when the Solar System seemed relatively well defined, with nine planets including Pluto and an asteroid belt that orbited in a niche between Mars and Jupiter. These days, in addition to the Kuiper Belt and Oort Cloud, we have to factor in all the objects that move on unusual orbits. We have a mission in the works, called Lucy, to the Jupiter Trojans, those asteroids that share the giant planet’s orbit around the Sun. And today we’re looking at Centaurs, which cross the orbits of giant planets and are in rapid dynamical evolution.
The subject comes up because a newly discovered comet — 2019 LD2 (ATLAS) — is not only a Centaur, but a Centaur that is rapidly on its way to becoming another class of object, a Jupiter Family Comet (JFC). The latter are short-period comets with an orbital period of less than 20 years, largely under the influence of Jupiter. A paper by Jordan Steckloff (Planetary Science Institute) and team lays out the case: Centaurs are objects in transition.
Image: Comet 2019 LD2 (ATLAS) as seen by the Hubble Space Telescope. Credit: Hubble Space Telescope/Bryce Bolin.
According to the paper, the new comet is in the earliest parts of a transition that is common to Centaurs. Their orbits, between Jupiter and Neptune, are unstable. Their fate takes them in one of two directions: They may be ejected from the Solar System entirely, or they may evolve into Jupiter Family Comets. They first emerge beyond the orbit of Neptune, putting them into the category of trans-Neptunian Objects, with gravitational interactions in the outer system pulling them into the population of Centaurs.
The transition to Jupiter Family Comet is, in astronomical terms, a fleeting affair, lasting from a few million to a few tens of millions of years, according to the researchers. And tracing their evolution produces what the authors refer to as a ‘gateway’ that helps them make the transition into JFC status. Thus Steckloff:
“We find that 2019 LD2 is currently in the vicinity of a dynamical ‘Gateway’ that facilitates the majority of transitions from the Centaur population into the Jupiter Family of Comets. The dynamical gateway is a region beyond Jupiter, extending to just inside of Saturn’s influence. Our previous work (Sarid et al. 2019) found that the majority of JFCs first pass through this dynamical gateway as Centaurs immediately prior to transitioning into the JFC population; indeed this ‘Gateway Region’ facilitates the majority of transitions between the JFC and Centaur populations. Currently, there are a handful of objects in the gateway, including LD2, and the much more famous object 29P/Schwassmann-Wachmann 1.”
Let’s pull up short for a moment with that last reference. 29P/Schwassmann-Wachmann 1, known since the 1920s, displays frequent outbursts that cause a sudden change in magnitude, brightening in some cases as much as a thousand times. The outbursts are sudden and tend to fade within days, evidently the result of cryovolcanic activity. Like 2019 LD2, 29P/Schwassmann-Wachmann 1 is a Centaur in transition into a Jupiter Family Comet.
The presence of super-volatile ices on 2019 LD2 points to the pristine nature of the object, which according to Steckloff is unlikely ever to have been in the inner Solar System. Here we are looking at a comet whose ices are subliming for the first time, driving cometary activity. The astronomer says this will be a rapid process: “…this transition is likely to finish in only 40 years from now, which is a blink of an eye for astronomy. This means that people alive today will be able to follow this object all the way through its transition into the JFC population.”
As part of that change in perspective that has taught us so much about the wide range of outer system objects, we’re learning that our definitions have to be malleable enough to incorporate objects that, on the range from active comets to inactive asteroids, can take on characteristics of either, presenting different aspects as they proceed through this dynamical evolution. Fast-changing 2019 LD2 is a chance to witness such change within our lifetimes.
The paper is Steckloff et al., “P/2019 LD2 (ATLAS): An Active Centaur in Imminent Transition to the Jupiter Family,” Astrophysical Journal Letters. Abstract.
Exoplanet Imaging via a Fast New Camera
The world’s largest superconducting camera by pixel count has been deployed at the Subaru Telescope at Mauna Kea in Hawaii. This is a technology we’ll want to watch, for it assists the effort to image exoplanets directly from the surface of the Earth, a goal that not so long ago would have seemed impossible. But it can be done, and we have a new generation of extremely large telescopes (ELTs) on the way, so the progress in support technology for such installations is heartening.
The new device is called the MKID Exoplanet Camera (MEC), with the four-letter acronym standing for Microwave Kinetic Inductance Detector. A superconducting photon detector was first developed as far back as 2003 at Caltech and the Jet Propulsion Laboratory, paving the way for devices that can operate at wavelengths ranging from the far-infrared to X-rays. The MEC comes out of the laboratory of Ben Mazin at the University of California at Santa Barbara as part of an effort that includes contributions from both US and Japanese scientists.
The MKID Exoplanet Camera operates in the optical and near infrared, running at 90 millikelvin, or 1/1000th of a Kelvin, which is close to absolute zero. The technology involved can read out data thousands of times per second, according to the MEC’s developers, which plays directly into the success of adaptive optics systems that are designed to correct for atmospheric distortions. Current adaptive optics methods bend a telescope’s mirror at a rate of thousands of times per second, using complex algorithms to produce an image as it if were taken in space.
The problem: Planets most likely to be found with today’s adaptive optics are young worlds still glowing with the heat of their formation. Mazin points to HR 8799, a system with four gas giants, each of which is more massive than Jupiter, as the kind of catch currently available, and indeed, HR 8799 has been confirmed by direct imaging with the Keck and Gemini telescopes in Hawaii. The planets are still hot and glowing as the system matures. Moving into the range of smaller, cooler worlds will take exquisite collaboration between adaptive optics and the camera.
The MKID technology allows Mazin’s MEC to determine the energy of each photon as it hits the detector. Sarah Steiger is a UC-Santa Barbara doctoral student who worked on the project:
“This allows us not only to determine a planet’s brightness, but also to get a spectrum (the brightness as a function of energy), which can reveal additional information about an exoplanet’s properties, such as its age, mass and potentially atmospheric composition.”
Image: The 20440 pixel MKID device designed for MKID Exoplanet Camera is the highest pixel-count superconducting detector array at any wavelength. Credit: UC-Santa Barbara.
The fast data rates available with an MKID mean that the technology can work interactively with an observatory’s adaptive optics system to remove scattered and diffracted starlight, which allows the detection of exoplanets much fainter than can currently be imaged. In terms of astrobiology, says Olivier Guyon, that means we can one day turn the MEC to nearby exoplanets that can be characterized in greater detail than before. Guyon is the project scientist in charge of the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument:
“We’re not going to be able to do that with Subaru, or with any of the current telescopes, because they’re just a bit too small. But we’re preparing for the next big step, which is to deploy exoplanet imaging cameras on larger telescopes such as the Thirty Meter Telescope. When those telescopes come online, the same technologies, the same camera, the same tricks will allow us to actually look for life.”
Ahead for Mazin’s team is the refinement of the software and algorithms that make MEC effective, with fast optical correction being the focus for the next several years. For more on the camera and its ongoing development, see Walter et al., “The MKID Exoplanet Camera for Subaru SCExAO,” Publications of the Astronomical Society of the Pacific Vol. 132, No. 1018 (17 November 2020). Abstract.
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