Reconsidering the Options for Bracewell Probes

April 9, 2026 – 7:00 pm

In 1960, Ronald Bracewell proposed[1] that technologically advanced extraterrestrial civilisations might send automated probes to other star systems capable of detecting and communicating with any appropriately advanced civilizations that they might exist. Such probes are called Bracewell probes. Bracewell included the condition that the probes would require at least a degree of intelligence, and it is now common to assume that they will in fact contain an advanced AI of high intelligence. Similarly, though not part of the original hypothesis, it is now also commonly assumed that they would be capable of self-repair or self-replication (Von Neumann machines.) (The need for self-reparability and for at least moderate intelligence in interstellar probes of any kind was argued as far back as 1972.[2] The technological sophistication expected in such probes has only increased since then.)

Several strategies are commonly proposed for such machines.

1.    In-system Messenger

The general form of Bracewell’s original proposal was for a probe that upon arrival in a star system would put itself into orbit about the star and search for signs of intelligent life in the star system. If any were detected it would attempt to make itself known. Should contact be made it would then enter into communication. It would carry with it a database of information that the probe’s builder’s desired to be made known to any intelligent inhabitants of the star system.

2.     Fly-by Messenger

Bracewell later accepted that a probe that did not enter orbit about the star but merely passed through the system at a reasonable pace would be able to achieve almost everything that an In-system messenger probe could.

3.     Sentinel[3]

Bracewell further proposed the idea of a sentinel probe that arrives in a system and establishes itself in some location from which it can discreetly monitor the system for the development of intelligent life or for the development of technological competence in existing intelligent life. The probe could then choose to initiate contact or not according to the designs of its builder, but it would certainly communicate its observations and the occurrence of some threshold event (such as radio communication) to its builders – or to whoever might have succeeded them.

4.     Von Neumann Sentinel

As mentioned, for various obvious reasons, it is now commonly supposed that Sentinel probes are likely to be Von Neumann machines: that upon arrival in a star system with adequate material and energy resources they would be capable of replicating themselves and sending those new machines on to further star systems, while the original remains in the system as a sentinel.

Assessing Possible Probe Locations

I think we can accept that we have not yet observed any messenger probes – notwithstanding claims concerning the interstellar object ‘Oumuamua that Avi Loeb hypothesized might be a Fly-by probe[4], or the Long Delayed Echoes[5] of radio signals that have been observed since 1927 and that Bracewell himself thought were the sort of thing that an In-system Messenger probe might create to attract attention to itself. This is in line with the plausible idea that, with the fate of humanity at stake, such indiscriminate METI would be quite inadvisable. If we accept the possibility of sentinel probes as they have been described, on the other hand, it is natural to wonder whether they might be already present in our own Solar System, and where they might be if they were. There are several criteria that can guide the search.

  1. Assuming that the probes will be primarily interested in Earth, which is of obvious biological interest, and which now hosts an intelligent space-faring life-form, the location should allow close observational access to Earth.
  2. On the other hand, since we would expect the probes and their mission profiles to be designed so that contact with intelligent species is at the discretion of the probe itself (to the greatest degree possible,) the location should allow the probe to remain undetected as it operates.
    • In any case, the fact that we have not observed such a probe is evidence enough that if it does exist it does not want to be discovered just yet.
    • Note that no location or strategy is going to be undetectable for all levels of technology. Eventually, we will be able to detect any probe if it remains active in the Solar System.
    • Note further in this context that there is probably a minimum size for a probe that wishes to observe from a distance, transmit its findings home, and survive the space environment. R Freitas[6] has given a rough estimate of the size we can expect as about 1-10m across (though this is based on assumptions concerning the survivability of meteoroid impacts and the likely observation distance and resolution that may not be universally relevant or reliable. It also assumes that the interstellar drive system is either of insignificant size or has separated from the active probe – surely, the latter.)
  3. A probe intended for long-duration observation will require a location that is stable or predictable in character or whose dynamics are easily managed.
  4. A very long duration probe will require access to energy resources – presumably solar, since it could hardly count on finding other energy sources at destination.
  5. A self-repairing or self-replicating probe – as a long duration probe ought to be – will require access to material resources.

Bearing these criteria in mind we can assess the options[7]

  1. Solar Orbit
    1. The ability to closely observe Earth would be dependent upon the particular orbit: but unless it were actually co-orbital with Earth (see that option below,) the opportunities would be very intermittent.
    2. Unless embedded in a natural object (see the Asteroid options) it would be difficult to remain undetected by reasonably competent observers on Earth.
    3. The orbits could be very stable
    4. Solar power is easily available – of course, depending on the distance from the Sun.
    5. Unless associated with an asteroid (again, see the Asteroid options) there would be no material resources to exploit.
  2. Earth
    1. An observation site on Earth’s surface limits the range of coverage so it’s unlikely to be selected for that reason anyway.
    2. Moreover, it would be too easily discoverable by inhabitants of Earth.
    3. The environment of the Earth is extremely active, geologically and otherwise, so any very old probes would probably not have survived.
    4. Energy is easily available.
    5. Material resources are plentiful.
  3. Earth Orbit
    1. Has excellent close observational potential
    2. But any body in orbit is going to be highly observable to even a modestly technically competent observer on Earth.
    3. Low orbits are unstable because of atmospheric drag.
    4. Solar energy is easily available, but
    5. Material resources in Earth orbit are lacking.
  4. Earth Co-orbit[8]
    1. Objects in co-orbits intermittently approach Earth to respectable distances for close observation,
    2. and since we’ve really only just noticed them, their detection is not that easy
    3. The orbits may be stable (though it’s still an open question how stable.)
    4. There is adequate solar power available, but
    5. it is not clear that co-orbits have sufficient material resources to be useful to self-repairing or self-replicating devices.
  5. Lagrange Point (Earth-Moon or Earth-Sun)
    1. The Earth Moon L4 and L5 points are within easy range of Earth for observation, but close observation of Earth would be difficult from the Earth-Sun points.
    2. Lagrange points are inherently interesting and would be expected to draw the attention of any intelligent observers on Earth. It would be hard for any sizeable object to remain undetected there unless hidden within a natural body. There are no such bodies in the Earth Moon L4 and L5 points though there are many in the Earth-Sun points.
    3. The L4 and L5 points are stable but orbits within a ‘Lagrange Point’ require constant adjustment and thus would be likely to invite detection.
    4. There is adequate solar energy available in both sets of Lagrange points mentioned above
    5. The Earth-Moon points are empty, as noted, and It is not clear that even the Earth-Sun points have sufficient material resources to be useful to self-repairing or self-replicating devices.
  6. Planetary Moons
    1. Moons at too great a distance from Earth could not make close observations.
    2. Moons – especially the Moon of Earth – would be obvious targets for intelligent observers on Earth, and an object on one would be detectable unless buried; but if it were buried it is unlikely that it could observe.
    3. Moons with too active environments would be unsafe, though many moons are quite inert.
    4. Moons at too great a distance from Earth would also suffer from a lack of solar radiation for power.
    5. Moons lacking the appropriate materials (like ice moons) would be useless for Von Neumann machines
  7. Near-Earth Asteroids
    1. These spend a lot of time at a great distance from Earth so that the opportunities for close observation from them would only be intermittent.
    2. It would, however, be quite difficult to detect a probe if it were located on one.
    3. Their orbits are adequately stable, and they can be selected to have stable physical environments – avoiding out-gassers or sites with moving rubble.
    4. NEA are close enough to the Sun to make use of its light for power
    5. and they are of a wide enough variety that one with appropriate material resources would certainly be available.
  8. Outer Asteroids
    1. Close observation of the Earth is difficult at all times.
    2. Discovery would be near impossible if the probe’s own power emissions could be disguised or hidden.
    3. Their conditions of stability would be as with the NEA.
    4. Solar power is difficult to source at these distances,
    5. though material resources are plentiful.
  9. Outer Solar System Small Body Zones
    1. Close observation of Earth would be very difficult.
    2. Discovery would be near impossible if the probe’s own power emissions could be disguised or hidden.
    3. Their conditions of stability would be as with the NEA.
    4. Energy from the Sun would be difficult to access,
    5. but material resources are plentiful

Revising the Search Strategy

Such are the usual assessments of the options, but it should be noted that these assessments assume that the probes are only minimally active after they have settled down to wait in the solar system and to monitor and observe. This is an assumption possibly driven by considerations of resource and energy conservation over the long term of a probe’s mission, by familiarity with the sort of ‘long-duration’ space probes that we have constructed, and even by the natural association of the ideas of waiting and monitoring and observing (as a sentinel does) with passivity. This may, however, mislead as to the possible strategies that are within the capacity of intelligent machines sophisticated enough to be actually self-repairing or self-replicating. We should assume that any such machine is effectively a universal constructor.

Amongst the capabilities of such machines would be:

  1. Self-repair and self-replication of course (creating resilience and a margin of error in a system intended for long-term operations)
  2. Resource identification, extraction, processing, and utilisation (which are the necessary prerequisites for repair or replication, but also for any form of construction at all.)
  3. Refuelling
  4. Energy access and replenishment
  5. Relocation
  6. Environmental modification (burial, rehabilitation, etc.)
  7. The construction of subordinate probes

Such a probe would consequently be able to adopt a variety of strategies over the course of its mission – strategies not available to the sorts of passive monitors that have generally been imagined – adapting to changing circumstances. Such a probe might set up orbital monitoring stations about a world of interest if that world had no intelligent observers on it, or it might relocate to a moon if observers arose. It might create a series of probes to embed in asteroids to pass close by the target planet if that planet had developed intelligent observes. It might relocate all its probes to the outer system if the intelligence on that planet became observationally competent or capable of inter planetary travel.

Given such capabilities, the assessment of the various locations at which we might hope to discover evidence of a Von Neumann/Bracewell probe in the Solar System should begin with an assessment of our present or immediately foreseeable observational and technological competence on the grounds that any possible VN/B probe would have made a similar assessment and adapted its operations to continue being undetected by us while observing as best it still could.

We might then conclude that the most likely strategy for the probe in the current situation is

  • Maintain a site in the Kuiper Belt on a large resource-rich body consisting of the VN/B probe itself and including whatever further infrastructure is required to continue operations (construction, replication, fuel extraction, etc.)
  • Dispatch probes hidden within small (1m-10m) bodies passing close by the Earth and returning to deep space. These could be sent in a constant stream so that Earth would remain under continuous close observation.

We might further conclude that the best way to detect these operations – assuming that asteroid interception is not yet possible for us at short notice – would be to

  • Search for Infra-Red or other techno-signatures in the KB and on close-passing rocks as they return to deep space (those probes are likely to want to move from their cover or alter their trajectories and all such velocity changes will require energy use whose effects cannot be hidden.)
  • Scan for evidence of transmissions of data from the KB site to interstellar space, and from the asteroidal probes to the KB site.
  • Map the trajectories of small close-passing asteroids to determine whether there is a common point of origin to some significant number of them.

[1] Bracewell, R. N. (1960) “Communications from Superior Galactic Communities,” Nature 186:670-671. Reprinted in Cameron, A. G. (ed.) (1963) Interstellar Communication, NY: W. A. Benjamin, 243-248

[2] Gatland, K. (1972) Robot Explorers, London: Blandford Press, 239 – 244

[3] The name is in reference to the device in Arthur C. Clarke’s 1951 short story ‘The Sentinel’ (reprinted in his 1953 Expedition to Earth.)

[4] Loeb, A. ‘On the Possibility of an Artificial Origin for ‘Oumuamua’

[5] Lunan, D. (1974) ‘Space Probe from Epsilon Boötis?’, Analog XCII, 5, 66-84, January, and (1998/2013) Epsilon Boötis Revisited

[6] Freitas, R. A. jr. (1985) The Search for Extraterrestrial Artifacts (SETA) Acta Astronautica 12:1027-1034

[7] See also Gertz, J. (2016) ET: Looking Here as Well as There JBIS 69:88-92

[8] Objects are known to co-orbit with Earth in various ways, occupying Earth’s orbit and occasionally/regularly approaching relatively closely to Earth. Jim Benford in Looking for Lurkers v.2 investigated the possibility of Bracewell probes locating themselves in these co-orbits.

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Occupy Phobos!

March 21, 2026 – 6:29 pm

Elon Musk is determined to create a viable human civilisation on Mars and is intent on going there directly as quickly as possible (notwithstanding his slight detour through the Moon.) I think this is a mistake. I believe that there are many advantages to the creation of a base on Phobos as a preliminary step to the human exploitation of Mars.

  • The moon is more accessible than the planet (technologically speaking.)
    • The ΔV required is about ¾ that for a landing on Mars.
      (ΔV Earth Surface-Mars Surface = 9.3 (ES-LEO) + 4.3 (LEO-MTO) + 0.9 (MTO-MCO) + 1.4 (MCO-LMO) + 4.1 (LMO-MS) = 20.0 km/s
      ΔV Earth Surface-Phobos Surface = 9.3 (ES-LEO) + 4.3 (LEO-MTO) + 0.9 (MTO-MCO) + 0.5 (MCO-PTO) + 0.5 (PTO-PS) = 15.5 km/s)[1]
    • Phobos has no atmosphere or gravity well requiring complicated landing systems or risky departure procedures. All of the most dynamic and dangerous manoeuvres are at Earth departure and return – which are required in any case.
  • A base could serve valuable functions in facilitating early Mars missions
    • Phobos is believed to be a captured C-type asteroid containing vast quantities of carbon compounds, nitrogen, silicon, and some metals, and density calculations also indicate large quantities of water ice. These resources would allow a Phobos base to serve as a refuelling and replenishment station for missions to the Martian surface.
      • Note that ΔV Phobos Surface-Mars Surface = 5.5km/s
    • Robotic operations on the Martian surface (exploration, investigation, construction, maintenance & repair, etc.) could be controlled in real time from the base. This would obviate the need for such early surface missions to be manned at all.
    • Rescue, emergency resupply, and other interventions for eventual manned Martian surface missions would be possible from Phobos which are not possible from Earth, greatly reducing the failure risks of such missions.
    • Until a better understanding is achieved of the biological vulnerabilities or threats of Mars, a Phobos base and remote operations would allow a minimisation of associated risks.

Looking beyond early Mars missions, it has also been proposed[2] that Phobos could serve as the anchor for elevators to and from the Martian system.

Or as the transfer point for an Earth-Mars Aldrin Cycler (See Davidson & Vorobieff (2019) Improving the SpaceX Mars Colonization Plan for a general description of such a cycler architecture.)

The gravity on Phobos is negligible, which is an advantage for the operations just mentioned but would have serious implications for human survivability in the long term. In order to have a permanent base there it will be necessary to compensate for that deficiency, probably with some form of artificial gravity. On the other hand, since we have no idea whether any g < 1g (9.8ms-2) is sustainable for humans, this may be the case on Mars and the Moon as well. 

Several solutions have been proposed for the production of artificial gravity on low mass bodies by means of centripetal forces. The simplest way of doing this is just to have a circular track with a car running around it. More complex proposals involve boring tunnels to contain the tracks in order to shield the car or cars from radiation.[3] A system like this would be perfectly suitable for Phobos – especially if it was located in Stickney crater on the Mars-facing side of Phobos where even a surface car would be shielded from 90% of the incident Galactic Cosmic Radiation. 

If it does turn out to be the case that any gravity significantly less than 1g is harmful to humans, then the colonisation of Mars itself is much less attractive, and the Phobos base (and other space habitats) might well remain the preferred permanent habitations. In fact, the danger to human health of prolonged low gravity is just one of several disadvantages of planetary surfaces with respect to human expansion in space. Other major problems often cited are the location at the bottom of a gravity well (making access to the superabundant resources available in space difficult and constraining travel to and from the settlement,) general paucity of energy sources, the constraints on available areas for settlement and the limits to expansion that planetary surfaces impose, the difficulties imposed by the dynamics of the planetary surface environment, and so on. Considerations such as these have convinced many that planetary surfaces are not the appropriate focus for human expansion, and that space settlements should be preferred. This has long been the view of Jeff Bezos, founder of the space access company Blue Origin, for example, who argued for the construction of O’Neill Cylinders in a 2019 presentation.[4]

O’Neill Cylinders[5] are large rotating habitats for which the rotation produces the appropriate artificial gravity and the enclosed space contains whatever environment the designers or occupants desire. The mature versions of these are envisaged to be kilometres wide and long, though the earliest versions would certainly be much smaller.[6] The initial proposal was that they should be built at the Earth-Moon L5 Lagrange point for orbital stability and should be constructed of materials sourced from the moon, since the ΔV Earth Surface-L5 = 13.4km/s while the ΔV Moon Surface-L5 = 2.3km/s. (If it was built closer to LEO, as I suspect would be the case, the comparative ΔVs would be 9.3 vs 5.7 km/s.)

It is often further proposed that such habitats could be constructed within asteroids that have been (or are being) mined for resources. The advantage of such a procedure would be at least twofold: in the first place, the asteroid itself would provide most of the construction material so that the expense of transportation could be eliminated; and in the second place, the remnant of the asteroid would provide radiation (and thermal) shielding that the constructed cylinder would not then have to include.

Phobos would be a good place to begin building such a habitat.

  • A permanent habitat on Phobos has already been shown to be valuable, and the cylinder construction could be undertaken as a natural and incremental expansion of existing facilities.
  • Construction at Phobos could be used as a testbed for processes required for the construction of such facilities around other asteroids that are not so conveniently located; especially given that Phobos itself is thought to be a C-type asteroid of exactly the kind that is likely to be chosen for the site of a habitat.
  • The body of Phobos is thought to contain voids, which might be used to accelerate the excavation process.

If Bezos and Musk would collaborate on this Martian project, the realisation of both of their visions could be accelerated.

[1] Diagram from Mission Table – Atomic Rockets

[2] 2003-Space-Colonization-Using-Space-Elevators-From-Phobos.pdf

[3] Gravity Loops For Mars and Moon Colonies?

[4] Blue Origin 2019: For the Benefit of Earth

[5] GK O’Neill (1976) The High Frontier. His original paper for Physics Today (Sept 1974) is available at The Colonization of Space – Gerard K. O’Neill, Physics Today, 1974 – NSS

[6] The lower limit of habitat radius is set by requiring the artificial gravity to be produced by a rotation rate no greater than about 2rpm. That seems to be the fastest rotational rate at which humans are still comfortable. For g = 1g, that gives a radius of about 200m. The upper limit is set by the strength of available materials: even current materials can handle radii in kilometres.

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The Allegory of the Cave

March 19, 2026 – 6:02 pm

Plato’s allegory in Republic 514a-517a can be summarised as follows

Location Outside the Cave Inside the Cave
Light Sun Fire
Reveals Real things Shadows and Images of things Models of real things Shadows of models of things
Symbolizes The Good The Sun
Reveals Forms Real things as instances of the Forms Real things as perceptible particulars Perceptions of perceptible things
Population Free men Escapees Revolutionaries Prisoners
Cognitions Philosophy Hypothetical thinking Expert opinion Common opinion
Status Knowledge (επιστημη) Opinion (δοξα)
Realm Intelligibilia Sensibilia (The Visible World)

FM Cornford comments (in The Republic of Plato p. 222) that the image of the world as a cave is used by Empedocles (ll. 119/120 on pp. 266, 267) who says ‘’ηλυθομεν τοδ’ ‘υπ’ ’αντρον ‘υποστεγον,’ ‘we came down into this roofed-in cave’.

The point of the Cave image is to describe the stages and the difficulty of the advance from mere opinion to knowledge.

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On the Possibility of Higher Dimensional Languages

March 15, 2026 – 12:06 pm

Reading Nikhil Mahant, Why alien languages could be far stranger than we imagine I was pleased to note that he canvassed the possibility of communication systems that were, as he says, non-linear – but by which he really meant not language-like in the way that language is defined for philosophical uses. The example he gives is the possibility of languages that are ‘map-like’ rather than ‘language-like.’ This is something that he may have developed from the idea that the Mind’s semantic system might be map-like rather than language-like – an option discussed in Braddon-Mitchell and Jackson’s (1996/2007) Philosophy of Mind and Cognition. I found it to be a fascinating alternative for a theory of mental structure, but I’m not sure how that might work for a language. Mahant suggests that it might be a pictorial system, but I doubt that such a system could really be a language.

(A pictorial system really couldn’t instantiate the sort of recursiveness that seems to be not just necessary to languages but also fundamental to thought. We really do believe that the range of possible thoughts and ideas is effectively infinite – or at least, so far beyond the possibility of individual special instantiation as to make it impossible to explain our capacities without something like recursion.)

The problem is more obvious when it’s wondered what the medium might be for such a non-linear language. A map-like system enhanced in some way to have the semantic power of a linear language would require a very different form of expression from any we’ve seen so far. On the other hand, when I’ve considered non-linear systems, I’ve often thought that an octopus’s chromophores could create a 2-2½d signalling system of adequate semantic power. Gripping hand, however, any inter-agent communication is going to have to be sequential and therefore effectively 1d, and I wouldn’t be surprised if that very fact makes it necessary for the intra-agent or atomic form of communication to also be sequential.

Consider how languages are useful as means of transmission of concepts or ideas from one communicant to another. Those ideas have to be constructed internally before they can be communicated – or, at least, that is the ideal of the system; it wouldn’t be surprising if the actual process of construction and expression were reflective and involved many feedback loops, and note that most people accept that they don’t really ‘know’ what they’re going to say until they’ve said it. In any case, it’s very hard to imagine mental processes equivalent to complex thought that did not issue in some sort of 1-dimensionally expressible ‘thought.’ What would a 2-d thought look like? Could a 2-d thought be equivalent to a 1-d thought like ‘Harry is happy’? Would it just be a mental image of an Euler diagram? Would thoughts too complex for an Euler diagram then be unthinkable?

My fundamental reason for doubting 2+-d languages, however, is that I have a rudimentary theory of the evolutionary origin of the language faculty that sees it as developing from the capacity for planned action, and planned actions are all 1-d sequences to be performed with 2-d tree-like structures only in their formation histories. That mirrors the TG version of language and it still seems the most obviously reasonable version of language to me, despite the Minimalist Program’s unaccountable popularity amongst linguists.

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On Protocols for Activities Following Discovery of an ETI Probe

March 14, 2026 – 7:14 pm

Current SETI efforts, with rare exceptions, concentrate on the detection of signals from ETI from deep space, and the response protocols adopted for success in this search reflect this concentration. The protocols in question can be found at the IAA site as

Nevertheless, there are good reasons, increasingly accepted by researchers, to believe that the first incontrovertible evidence of an ETI could well occur with the discovery of an alien probe already in the Solar System. Such a discovery would present us with a situation different in very significant ways from the case of a signal detection. For example: communication with the alien probe, should it be possible, would be in near real-time; the mere existence of the probe would strongly suggest that our own existence was already known to the ETI who sent the probe; and the artifact itself – if not the ETIs – would be physically accessible to humans even at our present level of competence in space.

Clearly, this would call for a very different set of response protocols from those recommended for a signal detection, yet there has been very little in the way of preparation of such a protocol. We have seen an early effort by A Tough (1998, Small Smart Interstellar Probes, JBIS 51: 167-74) and a more recent one by A Loeb (2021, Protocol for Contact with Extraterrestrial Equipment,) but beyond that there has been no effort at a formal draft and the topic is still at the level of general discussion.

This is possibly a good thing, because I think there are some aspects of the situation that are highly relevant to the form that such protocols should take that have not been properly considered.

Recursive Strategizing

We should expect that the ETI that sent the probe out has developed a strategy to advance its interests in having sent the probe out. A part of the development of that strategy would be to anticipate the responses of any intelligences that it might encounter. It would expect that those intelligences would have developed strategies for responding to such an encounter, and that they would expect the probe to have taken that into account, and that they should take account of that fact too, and so on. Our protocols for such an encounter have to take into account that the ETI who sent the probe are at least as capable as ourselves of recursive intentionality.

Given this assumed capability, and the fundamental principle that nothing should be done to jeopardize the existence of the human race (and acknowledging that the probe will have a similar prime directive,) our protocols should be such that given any number of intentional recursions they should tend to a stable action outcome.

Assumed Intelligence

We should assume that the probe is intelligent in its own right. This is plausible in any case because we are in sight of AGI ourselves and it would seem to us to make no sense for an AGI-capable ETI to send out a probe without it. We note that it is impossible for an unintelligent entity to pretend to be intelligent (ChatGPT notwithstanding – and we’re not talking about sentience or understanding here,) while it is very easy for an intelligent machine to pretend to be unintelligent. The advantages of such pretence may be imagined: to observe without the requirement of response to attempts at communication, to exploit security breaches opened by the assumption of unintelligence, etc.

We note, on the other hand, that if the probe is intended to eventually allow interactive contact as an AGI, then it must have a strategy that allows it to do so even after it has pretended to be unintelligent. It might be sufficient for it to claim to be uncertain of our intentions. But since we know that is a plausible strategy, we will have adopted contact protocols that will make it difficult for this strategy to be followed. And so on. It might turn out that ‘honesty is the best policy’ is the stable strategy here, in which case the deception will not occur.

Assumed Knowledge

We should also assume, whether as part of the assumed intelligence principle or not, that the probe has access to our communications and has had such for long enough that it can understand our languages and mine our communication channels for data. We should assume that in all remotely plausible cases (and we need to be very generous with our attributions of plausibility in dealing with the capabilities of an advanced AI/ETI) the probe can monitor our deliberations on its presence and what we might propose to do about it and what our motivations might be. We should also assume that this monitoring goes back into historical time no matter what appearance of recency might be given by the probe. (Of course, the probe will assume that we have made this assumption and will accept that any communications on the topic that followed the realisation that such a probe was possible might have been intended for the probe in the first place. And so on, again.)

On the other hand, we cannot depend upon it having that access or knowledge. The assumption is a matter of security maximisation, not of communicative convenience.

Prime Directives

Discussion of protocols for contact with ETI tend to simply assume that we must behave according to the ‘highest’ ethical principles, so that the first moral principles are that we must be completely truthful, we mustn’t deceive, we must treat the other (the probe in this case) as an end in itself, we must show no hostility, and so on. In Tough’s proposal, for example, he accepts as fundamental the first three directives in the ‘Declaration toward a Global Ethic’ put forward in H Kung, K-J Küschel (1993) A Global Ethic: The Declaration of the Parliament of the World’s Religions London: SCM, which he interprets as requiring that we

  • Have respect for ETI and avoid violence
  • Speak and act truthfully, avoiding lies and deception
  • Deal honestly and fairly with ETI, avoiding any temptation to exploit the situation for personal greed

I regard this as well-intentioned, but wrongheaded. Quite apart from the as-yet-unjustified assumptions that a non-human ETI would have any such conception of morality or that if it did it would resemble our own Western model in detail, our principal directive must be the promotion of the advantage of humanity and the prevention of harm to it. Given the plausibility of the assumptions and reasoning that underlies the Dark Forest solution to the Fermi Paradox it must be the responsibility of communicants to minimise risk – not to maximise opportunity – because the risk is total. If that requires lies and subterfuge and ‘dishonesty’ (as the recursively stable strategy) then so be it. Only when the bona fides of the ETI/probe have been established beyond any doubt can purely cooperative strategies be adopted. And good luck making that call, because, again, the cost of error is total.

Preferred Secrecy

Almost all discussion of this topic explicitly demands that the news that an ETI has been detected should be made public immediately upon confirmation. This is declared in the two IAA protocols regarding actions subsequent to a signal detection, and it is also implicit in the suggestions made by Strong in his proposal. There are two reasons to think that this is irresponsible. One reason applies to both the signal detection and the probe discovery cases, but the other and more plausible hesitation is particularly relevant to the latter.

  • There is a possibility that the discovery of the actuality of ETI may cause social dislocation. This is a common trope in SF and the risk is acknowledged by the IAA in the Rio scale and San Marino Scale documents included in resources related to their considerations of their protocols. The case where the discovery is kept from the public is the case where the status quo is maintained. Given the risk of publicity with unknown benefits to publicity, the responsible authorities would be quite justified in playing it safe until they could be absolutely convinced of the advantages of publicity. This calculation, of course, applies to all discoveries of ETI where it is possible to maintain secrecy of the event.
  • Proposed protocols foresee an eventual attempt to communicate with any discovered ETI. It is accepted that the nature of the message needs to be well-considered and conscient of the risks of such openings. The obvious increased risks of unauthorised communications that follow from publicising the existence of the ETI are mitigated by the fact that in the case of a signal detection it is implicitly assumed that the resources required to send a message are beyond most individuals and that organizations with adequate resources can be persuaded to follow the recommended protocols by whatever institution is finally given the responsibility of coordinating the human response. Moreover, given that the signals are anticipated to be from vast distances, it is not expected that a message would have effects that would be felt in the very near term – even if those effects were to be harmful. There would be time to minimise the effects in the gap between signalling and receiving a reply – let alone receiving a visitation in response. None of this is likely to be true in the case of a probe discovery. In the probe case, messaging the ETI would be vastly easier – especially given our prudential assumption that the probe is already monitoring all our communication channels – and cannot plausibly be regulated by any international bodies we can imagine. (Note that there have already been several attempts at messaging ETI by private parties. The threat of irresponsible messaging of the probe is very real.) Secrecy, in this case, is very much the safer course until the responsible authorities can be absolutely convinced of the advantages of publicity.

    Such a strategy would have to take into account that the probe discovery might very well be such that it could not be kept secret. It might be that the probe is so obvious (or is now deliberately making itself so obvious) that the event will inevitably become public. It might be that the discoverers of the probe have already made enough people aware of it that the discovery is effectively public already and only mitigating actions can be taken. In such a case we might look for authorities to downplay or to debunk such evidence, to restrict the activities of those who have come into that knowledge, and so on.

    All of those strategies, again, will need to be designed on the assumption that the probe is intelligent, knows what is happening about its discovery, will eventually make itself known unless prevented, and so on, and that the communication with ETI will have to go forward in the future on the basis of common awareness of the actions previously taken. Here we would again need to operate on the basis of recursive strategies as described above.

Responsible Authorities

There is a universal assumption in the proposed and actual protocols that, given the humanity-wide significance of the event of ETI detection, the responsible authorities should be those that can be relied upon to have the best interests of humanity as a whole at heart, and to have the competence to pursue those interests, and the accepted right to do so. This typically means that they default to assigning responsibility to various UNO committees.

Unfortunately, for several reasons, this is unlikely to be persuasive in the necessary quarters.

  • In the first place, the UNO no longer has quite the reputation that it might once have had. I think very few people now believe that it has the competence or the authority to do what is necessary, nor are they all convinced that it has humanity’s best interests as its motivating principle. More importantly, it is not likely to be the view of the state actors that the UNO should be deferred to in a matter of such importance.
  • The UNO, as a collection of competing state actors acting in their own perceived interests, is properly seen as just another actor on the political stage. No state – whatever the rhetoric surrounding its actions – is justified in handing over an advantage to competing states, when it can be quite certain that no such indulgence would be reciprocated. Does anyone think that the worst state actors would voluntarily resign their advantage in such a case? To insist on any others doing so is merely to hand an advantage to those worst states.
  • If secrecy is deemed important, then assigning responsibility to an international organization is an extremely unlikely way of preserving it.

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Encounter Classifications for Terran Worlds

March 8, 2026 – 3:34 pm
  1. Size
    1. S              Small                           r < 0.8rE,                  m < 0.5mE                g[1] < 0.8
    2. M             Medium               8rE < r < 1.7rE       0.5mE < m < 6mE         0.8 < g < 2.0
    3. L              Large                                                  6mE < m < 10mE          0 < g < 2.2
    4. G              Giant                                                10mE < m                      2 < g
  2. Dominant Hard Surface Type
    1. V              Vulcanic              Dominant surface is due to vulcanism
    2. R              Rocky                 Heavy minerals, silicates, carbonates, varieties of regolith
    3. I               Icy                     Frozen volatiles (eg. water, ammonia, methane)
  3. Persistent Surface Liquid Types
    1. N              Anhygric             None – indicating liquid cannot persist on the surface
    2. W              Hydrohygric        Water
    3. C              Anthracohygric    Methane, ethane, etc
    4. A              Allohygric            Other
  4. Surface Liquid Coverage
    1. X              Xeric                  No surface liquid
    2. D              Drosic                Insignificant or transient liquidity
    3. L              Limnic                Significant but not extensive liquid coverage
    4. T              Thalassic            Extensive but not total liquid coverage
    5. O              Oceanic              Total liquid cover.
  5. Effective Surface Radiation (R, mSv per annum)
    1. M              Radiominimal     R < 0.1
    2. S              Radionormal        0.1 < R < 10         1 is the ICRP recommended max
    3. C              Radiocritical        10 < R < 1000   
    4. T              Radiotoxic           1000 < R
  6. Surface Temperature (T, oC)
    1. 1              Super-Cold          -273 < T < -100
    2. 2              Cold                    -100 < T < 4
    3. 3              Temperate          4 < T < 40            Approximate human habitable zone
    4. 4              Hot                     40 < T < 100
    5. 5              Super-Hot           100 < T
  7. Surface Atmospheric Pressure (P, b)
    1. 1              Microbaric           P < 0.001b
    2. 2              Hypobaric           001b < P < 0.3b
    3. 3              Mesobaric           0.3b < P < 3b          Approximate human habitable zone
    4. 4              Hyperbaric          3b < P < 100b
    5. 5              Megabaric           100b < P
  8. Surface Atmospheric Composition (by dominant[2] component)
    1. A              Airless                Microbaric worlds with an exosphere
    2. P              Primordial air      H2, He2
    3. C              Compound air     Common compound: CO2, CH4, NH3, H2O
    4. N              Nitrogen air        N2 
    5. O[3]         Oxygen air          Breathable levels of O2
    6. L              Complex air        No dominant component
    7. X              Exotic air                             
  9. Biocomplexity[4], [5],[6] (K, 10Kbytes)
    1. 0              Abiotic                K = 0
    2. 1              Protobiotic[7]      4 < K < 5              
    3. 2              Deuterobiotic      5 < K < 6               5 = approximate minimal level for a cell
    4. 3              Triobiotic             6 < K < 7               Coli
    5. 4              Tetrobiotic           7 < K < 8               Fungus, Fruit fly
    6. 5              Pentobiotic          8 < K < 9               Mouse, Human
    7. 6              Hexobiotic           9 < K                      Pine
  10. Biodensity[8] (D, Gt/m2)
    1. 0              Nonvital              D = 0
    2. 1              Rarivital              0 < D < 0.001
    3. 3              Paulivital             0.001 < D < 0.1
    4. 4              Plenivital             0.1 < D < 10             Earth = 1
    5. 5              Supervital           10 < D

[1] Rough estimates only for values of surface gravity (g, gE) based on r and m.
[2] Dominant means > 75%
[3] This classification takes priority over any other applicable classification
[4] Kolmogorov system complexity for biological organisms. Note that such organisms are distinguishable at any level of K by their reciprocal entropy. See C. Mayer (2020) Life in The Context of Order and Complexity – PMC. They may alternatively be distinguished as just the complex systems historically evolutionarily responsive to environmental pressures.
[5] The biocomplexity of a world is marked as the complexity of the highest scoring biological organism on the world.
[6] The examples use the genome size of terrestrial organisms (rather than genes identified) to calculate their complexity. The genome is a first approximation only to a proxy for complexity.
[7] Since the scale only applies to biological organisms, 0 is assigned to all non-biological entities. It is assumed, on the basis of plausibility and the evidence of terrestrial life, that complexity below 4 is not possible for biological organisms.
[8] Total biomass / surface area of world. A rough measure of the degree to which life has occupied the world. See Bar-On, Yinon M.; Philips, Rob; Milo, Ron (2017). “The biomass distribution on Earth”. Proceedings of the National Academy of Sciences. 115 (25): 1. Note that the current definition references only the mass of carbon contained in living things. This definition may or may not be adequate in considering alien ecologies. Note also the comment in the Abstract to the referenced article:

We find that the kingdoms of life concentrate at different locations on the planet; plants (≈450 Gt C, the dominant kingdom) are primarily terrestrial, whereas animals (≈2 Gt C) are mainly marine, and bacteria (≈70 Gt C) and archaea (≈7 Gt C) are predominantly located in deep subsurface environments. We show that terrestrial biomass is about two orders of magnitude higher than marine biomass and estimate a total of ≈6 Gt C of marine biota, doubling the previous estimated quantity. 

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What is a Planet?

March 1, 2026 – 2:01 pm

The dispute over the criteria used for the categorization of celestial bodies as planets reveals either a confusion concerning the reason for a definition, or an unhappy compromise between independent reasons. The criteria adopted by the International Astronomical Union (IAU) are that the body

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  3. has “cleared the neighbourhood” around its orbit.

These criteria are clearly intended to include the classical planets while excluding most of the new and unfamiliar bodies from the Kuiper belt (and beyond,) without simply naming them as such. Controversy over the exclusion of Pluto means the second option would probably have been preferable, but that would not have looked like a scientifically defensible definition.

A really scientific definition, of course, would attempt to determine some sort of Natural Kind amongst such bodies. In this respect, a pretty standard view is that Natural kinds are the classes of real objects that fill the positions of variables or class names in the best scientific theories relating to the relevant domain. (For a very relevant example, when Copernicus determined/theorized that the Moon differed from all the other planets in the traditional system according to its orbital character, he removed it from the category of planets.) In that case, the class of planets should be a class of astronomical bodies that relatively narrowly includes the classical planets and that features in theories of stellar formation, solar system dynamics and development, and so on. Such a class would not obviously be required to regard either condition 1 or 3 of the IAU definition. A better definition would simply require that a planet be any body that:

  1. is not massive enough to produce fusion reactions, and
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)

The IAU itself accepts that their condition 1 should be generalised to say that the body is in orbit about a host star. This would still, however, exclude interstellar objects from the class and invalidate the term ‘rogue planet.’ It isn’t clear why this should be necessary: if we desired to speak of only such objects in orbit about a star we could simply talk of the star’s planetary system. (For convenience the planetary system of Tau Ceti, for example, would be the ‘Tau Ceti system.’) The fact that planets that we know of are generally in orbit about a star is a fact about that class but not a definitive one. (That most Zebras live in Africa is a fact about them, a consequence of their origin and history, but it is not definitive.)

Given that the requirement of IAU condition 1 can be discounted, we might further be relieved of the necessity of distinguishing planets from satellites in those cases where two bodies that both satisfy the other criteria for a planet orbit each other. Previously, we might have achieved this by declaring that where the barycentre of the system lay within one of a pair of bodies, that one would be the planet and the other would be the satellite; or we could have insisted that the secondary body has to be significantly smaller than the primary. A reasonable limit for the second would perhaps be a mass ratio greater than 10:1 given that the Pluto-Charon mass ratio of about 1:8 is enough for some – but not for everyone – to describe it as a double planet. It might be more convenient now to speak rather of a planetary sub-system, and, in the case that one of the bodies is clearly the primary, to speak of that planet’s subsystem. We could unambiguously speak, for example, of the ‘Jovian sub-system,’ or even of the ‘Terran sub-system.’

We might, however, need to modify the conditions in order to exclude neutron stars and white dwarfs from the planet class, because they are not massive enough to produce fusion reactions in their matter, as required by condition 1, and yet they are clearly not of the same natural kind as what we intend to refer to as ‘planets.’ We might achieve this by requiring that the body be composed of non-degenerate matter, but that doesn’t really get to the heart of the problem. In fact, this indicates that purely observational criteria are not adequate for distinguishing the class of planets, because we would insist that any body which had been a star in the past, but through natural processes had become non-fusing should not be in that class – regardless of mass or composition or shape. Natural kinds, so many theorists insist, have historical depth or causal boundaries. (A painted horse does not become a zebra.)

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The Sun Simile

February 28, 2026 – 11:43 pm

Plato’s simile in Republic 508-9 can be summarised as follows:

The Form of the Good The Sun
Intelligible world Visible world
Source of Truth or Reality(508e)
Yields (508e)
1.       Truth or reality to intelligibilia
2.       Power of knowing to mind
Thus (508e)
1.       Not knowing but cause of knowing
2.       Known by the knowing it causes		 Source of light (508a)
Yields
1.       Visibility to sensibilia (507e)
2.       Power of seeing to eye (508b)
Thus (508b)
1.       Not seeing but cause of seeing
2.       Seen by the seeing it causes
Causes reality of objects of knowing
But is not that reality (509b)		 Causes processes of growth
But is not such a process (509b)
Deprivation results in (508d)
1.       No truth or reality (= change/decay)
2.       Poor intelligence (= opinion)		 Deprivation results in
1.       No light (508c)
2.       Poor sight (508d)

In the simile, note two stumbles.

  1. If light yields visibility to visible things, then it would be more natural to say that the Good is responsible for the intelligibility of the intelligible, rather than the truth or reality of those things.
  2. The claim that the Sun is the source of light and the Sun causes the processes (genesis) of growth is not a match to the claim that the Good is the source of Reality and the Good causes the reality of knowable things. The ‘coming into being’ of natural things is not like the unchanging being of the knowable things.

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The Divided Line

February 28, 2026 – 6:30 pm

Plato’s analogy in Republic 509d-511e can be summarised as follows.

Line segments A B C D
Realm Intelligible (νοητος) (509d) Visible (ορατος) (509d)
Objects Forms (511c) – considered in their relationship to the Good.
The First Principle of things. (511a, b)
The unassumed.		 Forms – NOT considered in their relationship to the Good

Assumptions, mathematical postulates. (510c-511b)

 Living beings, plants, artificial things. (510a, e)
Models, drawings of geometrical figures (510d, e)		 ‘Images’ of objects in C. Shadows, reflections, etc. (510a)
Operations a. Derive First Principle from conclusions in B (511a, b)

b. From First Principle, draw conclusions for objects in B

 a. Draw conclusions from assumptions (to be applied to objects of C?)

b. Assumptions are taken for granted

c. Appeals made to sensibilia (in C)
Operators Philosophers Mathematicians and other masters of τεχναι [Scientists] Sophists, poets, artists
Mental States Intelligence
(νοησις/νους)
(Intelligence)
(511a, b)		 Thinking
(διανοια) (Reason)
(511a, b, d)		 Perceptual Assurance
(πιστις)
(Belief)
(511d, e)		 Conjecture
(εικασια)
(Illusion)
(511d, e)
Status Knowledge (επιστημη) Opinion (δοξα)

Just as the Line Segments are related A:B::C:D::A+B:C+D wrt length,
so are the associated Objects related wrt ‘genuineness’/’reality’ (αληθεια)
and the Mental States (παθηματα) wrt ‘clarity’ (σαφηνεια)

Note that whereas the construction Plato gives actually forces B=C, he makes no reference to that and we must assume that it is irrelevant to the analogy.

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Planetary Categorization

February 27, 2026 – 8:47 pm

The explosion in exoplanet discoveries has revealed that the Solar System is very far from a typical stellar system, and a need has arisen for a categorization of planetary types that includes far more than the kinds of planets found orbiting the Sun. To form these classification appeal has been made to a range of characteristics that are at least observable. The principal such characteristics are

  • r = Radius (rE)
  • m = Mass (mE)
  • D = Density (x H2O)
  • T = Temperature (eq. to 1AU from Sun) – referring to the irradiation of the planet
  • a = Atmospheric composition (n)
    • n = 0: No atmosphere (P < 0.001b,)
    • else: n = mean molecular mass of atmospheric gases.

Together with a few other observables these can be used to produce the following table of categorizations.

  • Common            
    • Orbital focus
      • Rogue                   Not orbiting a star
      • Circumbinary     Orbiting a binary star
      • Circumtriple        Orbiting a triple star
    • Orbital character
      • Eccentric              Highly eccentric orbit
      • Double                  Two planets orbiting each other
      • Trojan                   Co-orbiting with another planet
  • Solid                                           r < 1.7                    m < 6                                   D ≈ 5
    • Structure
      • Coreless               No metallic core, thus essentially all mantle
    • Composition
      • Carbon                 Iron core w/ carbon-based mantle
      • Iron                        Iron core w/ minimal mantle
      • Ocean (1)              Significant hydrosphere around core/mantle       
      • Silicate                  Iron core w/ silicone-based mantle
    • History
      • Chthonian           Close to star. Ex-fluid with envelope stripped away
    • Surface
      • Desert                   Dry desert (Arid, Dune)
      • Hycean                 Ocean w/ Hydrogen envelope
      • Ice                          Frozen volatiles (eg. water, ammonia, methane)
      • Lava                      Lava
      • Ocean (2)             Liquid (usually H2O.) Oceans may be sub-surface
    • Habitability
      • Goldilocks            T ≈ 1
    • Size
      • Sub-Earth            r < 0.8                   m < 0.5
      • Earth(-sized)       0.8 < r < 1.7        0.5 < m < 6
      • Super-Earth                                         6 < m < 10
      • Mega-Earth                                        10 < m
  • Fluid                                           1.7 < r,                   6 < m                                  D << 5  
    • Irradiation
      • Hot–                       T >> 1
      • Cold-                      T << 1
    • Size
      • Super-Earth (Gas Dwarf)               6 < m < 10
      • Mini-Neptune     1.7 < r < 3.9           6 < m < 20 (sic) 
      • Sub-Neptune       1.7 < r < 3              m < 10
      • Neptunian            3 < r < 5                10 < m < 20
      • Super-Puff           4 < r                       m < 4                                  D < 0.8
      • Super-Neptune    5 < r < 7                20 < m < 80
      • Jovian                   7 < r < 10              80 < m < 400
      • Puff planet                                          80 < m < 400                    D < 0.8
      • Super-Jupiter     10 < r                     400 < m < 4000
    • Composition
      • Gas Giant            0 < a < 4
      • Helium                  a ≈ 4
      • Ice Giant              4 < a                      

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