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Introduction: what is a Von Neumann probe? Why build them?
The possibility of self-replicating interstellar probes was introduced by John Von Neumann and has been further developed by other researchers (Von Neumann, 1966; Rose and Wright, 2004; Borque and Hein, 2021). Conceptually, the Von Neumann probe is a very simple device (Tipler, 1994). It is a self-sufficient spacecraft endowed with sufficient intelligence to cross from the planetary system of the civilization that constructs it to a neighbouring planetary system. At this destination, one of its functions is to reproduce itself using in-space resources or resources available on planets, dwarf planets and satellites. These ‘daughter’ probes would then depart the planetary system of the ‘parent’ Von Neumann probe to ultimately expand through (and perhaps) beyond the galaxy.
Because interstellar transfers by nuclear fusion (Kezerashvili, 2021) and photon sailing seem feasible (Vulpetti et al., 2015) in the near future, the vast interstellar distances may not deter the designers of such devices. Nanotechnology may certainly allow for very-low mass intelligent interstellar spacecraft (Lubin, 2016).
From a technological point of view, there seems to be no obstacle to the ultimate terrestrial construction of Von Neumann probes. But why should a space-faring society decide to attempt the robotic occupation of the entire galaxy? What are the advantages and disadvantages of various possible interstellar propulsion options for these constructs? Is there a potential launch strategy to minimize the complexity and duration of second-generation and later Von Neumann probes? What locations in our solar system might telescopes and spacecraft explore for signs of local Von Neumann probes constructed by non-humans? The next section of this paper considers propulsion options. Other issues are addressed in subsequent sections of this paper.
Propulsion options for Von Neumann probes
This section begins with unpowered giant-planet gravity assists, the only interstellar propulsion techniques that have been applied so far to human-constructed extra-solar probes. Progressively more advanced propulsion techniques are considered in turn.
It must be noted that one major issue must be addressed by a civilization constructing Von Neumann probes, no matter which propulsion method is selected. Because no terrestrial spacecraft has survived in the deep-space environment for more than about five decades, the difficulties in designing probes that can survive the temperature extremes, cosmic-ray impacts...