In a long and discursive paper on self-replicating probes as a way of exploring star systems, Alex Ellery (Carleton University, Ottawa) digs, among many other things, into the question of what we might detect from Earth of extraterrestrial technologies here in the Solar System. The idea here is familiar enough. If at some point in our past, a technological civilization had placed a probe, self-replicating or not, near enough to observe Earth, we should at some point be able to detect it. Ellery believes such probes would be commonplace because we humans are developing self-replication technology even today. Thus a lack of probes would indicate that there are no extraterrestrial civilizations to build them.
There are interesting insights in this paper that I want to explore, some of them going a bit far afield from Ellery’s stated intent, but worth considering for all that. SETA, the Search for Extraterrestrial Artifacts, is a young endeavor but a provocative one. Here self-replication attracts the author because probing a stellar system is a far different proposition than colonizing it. In other words, exploration per se — the quest for information — is a driver for exhaustive seeding of probes not limited by issues of sustainability or sociological constraints. Self-replication, he believes, is the key to exponential exploration of the galaxy at minimum cost and greatest likelihood of detection by those being studied.
Image: The galaxy Messier 101 (M101, also known as NGC 5457 and nicknamed the ‘Pinwheel Galaxy’) lies in the northern circumpolar constellation, Ursa Major (The Great Bear), at a distance of about 21 million light-years from Earth. This is one of the largest and most detailed photos of a spiral galaxy that has been released from Hubble. How long would it take a single civilization to fill a galaxy like this with self-replicating probes? Image credit: NASA/STScI.
Growing the Idea of Self-Reproduction
Going through the background to ideas of self-replication in space, Ellery cites the pioneering work of Robert Freitas, and here I want to pause. It intrigues me that Freitas, the man who first studied the halo orbits around the Earth-Moon L4 and L5 points looking for artifacts, is also responsible for one of the earliest studies of machine self-replication in the form of the NASA/ASEE study in 1980. The latter had no direct interstellar intent but rather developed the concept of a self-replicating factory on the lunar surface using resources mined by robots. Freitas would go on to explore a robot factory coupled to a Daedalus-class starship called REPRO, though one taken to the next level and capable of deceleration at the target star, where the factory would grow itself to its full capabilities upon landing.
I should mention that following REPRO, Freitas would turn his attention to nanotechnology, a world where payload constraints are eased and self-reproduction occurs at the molecular level. But let’s stick with REPRO a moment longer, even though I’m departing from Ellery in doing so. For in Freitas’ original concept, half the REPRO payload would be devoted to self-reproduction, with a specialized payload exploiting the resources of a gas giant moon to produce a new REPRO probe every 500 years.
As you can see, the REPRO probe would have taken Project Daedalus’ onboard autonomy to an entirely new level. Freitas’ studies foresaw thirteen distinct robot species, among them chemists, miners, metallurgists, fabricators, assemblers, wardens and verifiers. Each would have a role to play in the creation of the new probe. The chemist robots, for example, were to process ore and extract the heavy elements needed to build the factory on the moon of the gas giant planet. Aerostat robots would float like hot-air balloons in the gas giant’s atmosphere, where they would collect the needed propellants for the next generation REPRO probe. Fabricators would turn raw materials (produced by the metallurgists) into working parts, from threaded bolts to semiconductor chips, while assemblers created the modules that would build the initial factory. Crawler robots would specialize in surface hauling, while wardens, as with Project Daedalus, remained responsible for maintenance and repair of ship systems.
I spend so much time on this because of my fascination with the history of interstellar ideas. In any case, I don’t know of any earlier studies that explored self-reproduction in the interstellar context and in terms of mission hardware than Freitas’ 1980 paper “A Self-Reproducing Interstellar Probe” in JBIS, which is conveniently available online. This was a step forward in interstellar studies, and I want to highlight it with this quotation from its text:
A major alternative to both the Daedalus flyby and “Bracewell probe” orbiter is the concept of the self -reproducing starprobe. Replicating spacefaring machines recently have received a cursory examination by Calder  and Boyce , but the basic feasibility of this approach has never been seriously considered despite its tremendous potential. In theory, each self -reproducing device dispatched by the launching society would become an independent agent, slowly scouting the Galaxy for evidence of life, intelligence and civilization. While such machines might be costlier to design and construct, given sufficient time a relatively few replicating starprobes could search the entire Milky Way.
The present paper addresses the plausibility of self-reproducing starprobes and the basic parameters of feasibility. A subsequent paper  compares reproductive and nonreproductive probe search strategies for missions of interstellar and galactic exploration.
Hart, Tipler and the Spread of Intelligence
These days, as Freitas went on to explore, massive redundancy, miniaturization and self-assembly at the molecular level have moved into tighter focus as we contemplate missions to the stars, and the enormous Daedalus-style craft (54,000 tons initial mass, including 50,000 tonnes of fuel and 500 tonnes of scientific payload) and its successors, while historically important, also resonate a bit with Captain Nemo’s Nautilus, as spectacular creations of the imagination that defied no laws of physics, but remain in tension with the realities of payload and propulsion. These days we explore miniaturization, with Breakthrough Starshot’s tiny payloads as one example.
But back to Ellery. From a philosophical standpoint, self-reproduction, he rightly points out, had also been considered by Michael Hart and Frank Tipler, each noting that if self-replication were possible, a civilization could fill the galaxy in a relatively short (compared to the age of the galaxy) timeframe. Ultimately self-reproducing probes exploit local materials upon arrival and make copies of themselves, a wave of exploration that would ensure every habitable planet had an attendant probe. Thus the Hart/Tipler contention that the lack of evidence for such a probe is an indication that extraterrestrial intelligence does not exist, an idea that still has currency.
Would any exploring civilization turn to self-replication? The author sees many reasons to do so:
There are numerous reasons to send out self-replicating probes – reconnaissance prior to interstellar migration, first-mover advantage, insurance against planetary disaster, etc – but only one not to – indifference to information growth (which must apply to all extant ETI without exception). Self-replicating probes require minimal capital investment and represent the most economical means to explore space, interstellar space included. In a real sense, self-replicating machines cannot become obsolete – new design developments can be broadcast and uploaded to upgrade them when necessary. Once the self-replicating probe is established in a star system, the probe may be exploited in various ways. The universal construction capability ensures that the self-replicating probe can construct any other device.
Probes that can fill the galaxy extract maximum information and can not only monitor but communicate with local species. Should a civilization choose to implement panspermia in systems devoid of life, the capability is implicit here, including “the prospect of exploiting microorganism DNA as a self-replicating message.” Such probes could also, in the event of colonization at a later period, establish needed infrastructure for the new arrivals, with the possibility of terraforming.
Thus probes like these become a route from Kardashev II to III. In fact, as Ellery sees it, if a Kardashev Type I civilization is capable of self-reproduction technology – and remember, Ellery believes we are on the cusp of it now – then the entire Type I phase may be relatively short on the way to Kardashev Types II and III, perhaps as little as a few thousand years. It’s an interesting thought given our current status somewhere around Kardashev 0.72, beset by problems of our own making and wondering whether we will survive long enough to establish a Type I civilization.
Image: NASA’s James Webb Space Telescope has produced the deepest and sharpest infrared image of the distant universe to date. Known as Webb’s First Deep Field, this image of galaxy cluster SMACS 0723 is overflowing with detail. Thousands of galaxies – including the faintest objects ever observed in the infrared – have appeared in Webb’s view for the first time. This slice of the vast universe covers a patch of sky approximately the size of a grain of sand held at arm’s length by someone on the ground. If self-reproducing probes are possible, are all galaxies likely to be explored by other civilizations? Credit: NASA, ESA, CSA, and STScI.
Early Days for SETA
The question of diffusion through the galaxy here gets a workover from a theory called TRIZ (Teorija Reshenija Izobretatel’skih Zadach), which Ellery uses to analyze the implications of self-reproduction, finding that the entire galaxy could be colonized within 24 probe generations. This produces a population of 424 billion probes. He’s assuming a short replication time at each stop – a few years at most – and thus finds that the spread of such probes is dominated by the transit time across the galactic plane, a million year process to complete assuming travel at a tenth of lightspeed.
Given this short timespan compared with the age of the Galaxy, our Galaxy should be swarming with self-replicating probes yet there is no evidence of them in our solar system. Indeed, it only requires a civilization to exist long enough to send out such probes as they would thenceforth continue to propagate through the Galaxy even if the sending civilization were no more. And of course, it requires only one ETI to do this.
Part of Ellery’s intent is to show how humans might create a self-replicating probe, going through the essential features of such and arguing that self-replication is near- rather than long-term, based on the idea of the universal constructor, a machine that builds any or all other machines including itself. Here we find intellectual origins in the work of Alan Turing and John von Neumann. Ellery digs into 3D printing and ongoing experiments in self-assembly as well as in-situ resource utilization of asteroid material, and along the way he illustrates probe propulsion concepts.
At this stage of the game in SETA, there is no evidence of self-replication or extraterrestrial probes of any kind, the author argues:
There is no observational evidence of large structures in our solar system, nor signs of large-scale mining and processing, nor signs of residue of such processes. Our current terrestrial self-replication scheme with its industrial ecology is imposed by the requirements for closure of the self-replication loop that (i) minimizes waste (sustainability) to minimize energy consumption; (ii) minimizes materials and components manufacture to minimize mining; (iii) minimizes manufacturing and assembly processes to minimize machinery. Nevertheless, we would expect extensive clay residues. We conclude therefore that the most tenable hypothesis is that ETI do not exist.
The answer to that contention is, of course, that we haven’t searched for local probes in any coordinated way, and that now that we are becoming capable of minute analysis of, for instance, the lunar surface (through Lunar Reconnaissance Orbiter imagery, for one), we can become more systematic in the investigation, taking in Earth co-orbitals, as Jim Benford has suggested, or looking for signs of lurkers in the asteroid belt. Ellery notes that the latter might demand searching for signs of resource exploitation there as opposed to finding an individual probe amidst the plethora of candidate objects.
But Ellery is adamant that efforts to find such lurkers should continue, citing the need to continue what has been up to now a meager and sporadic effort to conduct SETA. I’m going to recommend this paper to those Centauri Dreams readers who want to get up to speed on the scholarship on self-reproduction and its consequences. Indeed, the ideas jammed into its pages come at bewildering pace, but the scholarship is thorough and the references handy to have in one place. Whether self-reproducing probes are indeed imminent is a matter for debate but their implications demand our attention.
The paper is Ellery, “Self-replicating probes are imminent – implications for SETI,” International Journal of Astrobiology 8 July 2022 (full text). A companion paper published at the same time is “Curbing the fruitfulness of self-replicating machines,” International Journal of Astrobiology 8 July 2022 (full text).