In 1949, famed mathematician and physicist John von Neumann delivered a series of addresses at the University of Illinois, where he introduced the concept of “universal constructor.” The theory was further detailed in the 1966 book, Theory of Self-Reproducing Automata, a collection of von Neumann’s writings compiled and completed by a colleague after his death. In the years that followed, scientists engaged in the Search for Extraterrestrial Intelligence (SETI) considered how advanced civilizations could rely on self-replicating probes to explore the galaxy.

As many theoretical studies have shown, self-replicating probes (released from a single planet) could proliferate and explore the entire galaxy within a few eons. According to new research by Professor Alex Ellery of Carleton University (go Ravens!), these probes may have already visited the Solar System, and some could be operating here right now. As he recommends in a recent paper, future SETI surveys should be on the lookout for the telltale technosignatures these probes would produce.

Alex Ellery is an Engineering professor with the Centre for Self-Replication Research (CESER) and the Department of Mechanical & Aerospace Engineering at Carleton University. In the past, Ellery has explored the concept of Von Neumann probes and their potential as a viable method for interstellar exploration by advanced civilizations, as well as the implications this has for SETI. In this latest paper, he revisits the rationale for such probes, their implications for the Fermi Paradox, and how resource requirements would drive their behavior, producing discernible technosignatures in the process.

As a researcher with CESER, Ellery is well-versed in the concept of Von Neumann probes and the technological innovations that will go into creating them. In a previous study, Ellery has detailed how 3D printing, self-replication, and robotics will allow humans to build Von Neumann probes sooner than expected. He also conducted a detailed study on how human engineers could place limits on the number of times each probe could reproduce itself, thus ensuring that they do not run amok (per the Berserker Hypothesis).

In these and other papers, Ellery also argues that the search for Von Neumann probes and the technosignatures they would produce is a focus that SETI researchers should prioritize, rather than the traditional practice of searching the night sky for signs of radio transmissions. These searches, he’s indicated, should consider the Solar System as a good starting point, which echoes similar recommendations made by Professor Gregory L. Matloff in his paper “Von Neumann probes: rational propulsion interstellar transfer timing.”

As he told Universe Today at the time of the paper’s publication:

The Solar System is huge and mostly unexplored, and the probes could be very small. There could be probes everywhere: in craters on the Moon, or lurkers in the Asteroid Belt and Kuiper Belt. There are 100 million objects in the Kuiper Belt alone, and we have examined only two, one of which was very anomalous in its shape.

Similarly, Ellery considers how SETI researchers and future explorers could look for evidence of extraterrestrial probes through a dedicated search. The first step, he argues, is to consider the rationale for sending out self-replicating probes.

Motivations

As he indicated in his paper, the most obvious reason for a species to engage in interstellar exploration is survival. This could be motivated by a desire to live beyond the main sequence phase of its star, the threat of destruction at the hands of a more advanced civilization, or fear that technology advancements will eventually threaten the existence of its creators. As told Universe Today via email, in all cases, self-replicating probes offer the most efficient and plausible means for ensuring the survival of an advanced civilization:

ET probes would be driven by survival of their local environment, be it main-sequence star lifetime, tectonic activity, etc., plus military reconnaissance to assess threats and alliances. Although many of us are motivated [by the desire] for knowledge (of which military reconnaissance is a type), science per se is not much of a driver. Nor is the exploration instinct - usually, greed or escape underlies exploration. Columbus wanted to find a route to the riches of Cathay, but America was in the way.

What’s more, he goes on to demonstrate that such probes would not be hampered by the same restrictions as biological organisms. This is true of propulsion, since advanced probes would be able to withstand acceleration beyond 9.8 m2 (the force of Earth’s gravity) and would not require exotic propulsion methods. In addition, interstellar probes would not require supplies, bioregenerative life support systems, or have to worry about waste disposal. All the materials they would need could be harvested along the way.

This would include extracting resources from star systems (such as asteroid belts and/or smaller rocky-metallic bodies) or from objects found in the interstellar medium, including asteroids, comets, and rogue planets. This desire for exploration and threat assessment, paired with the need for resources, would lead to predictable behaviors that could help guide the search for interstellar probes.

Activities

From the question of motivations, Ellery concludes that the activities of interstellar self-replicating probes would follow a basically pattern that can be broken down into the six likely steps. First, they would target the ubiquitous asteroids and moons in a specific system for their readily accessible raw materials, which are required for universal construction. Second, from these materials, they would build surveying probes to survey the extrasolar system for resources and life-bearing environments comprehensively.

*NASA’s Psyche mission to a distant metal asteroid will carry a revolutionary Deep Space Optical Communications (DSOC) package. Credit: NASA/JPL-Caltech/ASU*

Third, they would select and secure resource-rich locations to establish bases for self-replication operations. Fourth, they would begin replicating more copies of themselves, including surveyor probes and sentinels. The fifth step would consist of the long-term and detailed exploration of the extrasolar system using these same probes. The sixth and final step would see the probes execute specific task instructions, which could include building O’Neill Cylinders for future settlers while (hopefully) not encroaching on potentially life-bearing planets.

Another possible task, and a highly controversial one, would be the seeding of planets with simple or more complex organisms (aka. Directed Panspermia). Based on this breakdown of activities, self-replicating probes would produce technosignatures that future surveys could look for.

Possible Signatures

Beyond returning to the Moon by the end of this decade and sending crewed exploration missions to Mars in the next, numerous plans exist for the commercialization of Low Earth Orbit (LEO), cislunar space, and beyond. These efforts, it is hoped, will facilitate the “Great Migration” to space, possibly leading to settlements on other bodies and “Islands in Space” (a la O’Neill Cylinder and Stanford Torii). As Ellery explained, for human exploration of the Solar System to consolidate into commercial industrialization, we must know where to find the resources we need to make that transition happen.

First, Ellery considered asteroid resources, which are likely to be as abundant in extrasolar solar systems as they are in our own. Beyond the Copernican Principle, which states that Earth and our Solar System are typical of the norm, there is also considerable evidence to support this idea. This includes studies of presolar interplanetary dust particles, which have been observed to contain metal nitrides, carbides, oxides, silicates, and iron-nickel alloys—the building blocks of planets, planetesimals, and asteroids.

Furthermore, extrasolar planetesimals observed accreting onto white dwarfs within a 650 light-year (200 parsec) radius of Earth were predominantly composed of oxygen, magnesium, silicon, and iron, with smaller concentrations of volatiles such as carbon, sulfur, and nitrogen—similar to asteroids and moons in our Solar System. In addition to asteroids, self-replicating probes would likely be attracted to the Moon and similar rocky bodies because of their composition (silica, nickel-iron, and other metals), which is partly due to asteroid impacts that occurred over several billion years.

*Artist’s impression of Artemis astronauts operating on the lunar surface. Credit: NASA*

Ultimately, Ellery concluded that asteroidal processing would be difficult to distinguish from natural processes and that the Moon would be an ideal base for self-replicating probes to center their manufacturing operations. Moreover, he suggests that nuclear reactors would likely power these operations, as they are a highly fuel-efficient power source that provides virtually unlimited energy density. This could include Magnox models, a type of gas-cooled reactor that utilizes natural uranium, graphite, and carbon dioxide gas as the heat exchange coolant.

These, he concluded, could be constructed using lunar resources, and that the reactors would have left isotopic ratio signatures of Thorium-232, Neodinium-144, and/or Barium-137. “We further suggest that in anticipatory economic trade for resources, a self-replicating probe may have left artifacts buried with asteroidal resources on the Moon,” he wrote. “Such gifts would be detectable and accessible only once a threshold of technological sophistication has been achieved.” All of this makes the Moon the ideal place to begin searching for possible indications of associated technosignatures.

These searches could take advantage of NASA’s and other space agencies’ plans to create “a sustained program of lunar exploration and development.” As he summarized:

I think measurable technosignatures will be on the Moon - unusual surface Uranium or Thorium isotope ratios and subsurface magnetic anomalies. If we begin to settle the Moon, we will search for resources to utilize, specifically nickel, cobalt, and tungsten, which are delivered by asteroids to the lunar subsurface. I think that if we have been visited, a gift in return for mining of our resources may be hidden among those asteroidal metals. The gift could be a universal constructor machine that would be of immediate and future utility to any civilization settling its solar system before interstellar ventures.

In the near future, it is predicted that human exploration of the Solar System will give way to commercial development and the establishment of permanent outposts in space, as well as on other planets and celestial bodies. Before we establish footholds beyond Earth and LEO, perhaps we should consider exploring for more than just resources and building sites. A dedicated technosignature search could reveal something far greater, such as evidence that humanity is not alone in the Universe. Similarly, the Solar System has been largely overlooked when it comes to SETI efforts in general and technosignature searches in particular.

Further Reading: arXiv