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A Drake Equation for Alien Artifacts

Jim Benford’s study of ‘lurkers’ — possibly ancient probes that may have been placed here by extraterrestrial civilizations to monitor our planet’s development — breaks into two parts. The first, published Friday, considered stars passing near our Sun in the lifetime of the Solar System. Today Dr. Benford looks at the Drake Equation and sets about modifying it to include the lurker possibility. Along the way, he develops a quantitative way to compare conventional SETI with the strategy called SETA — the search for extraterrestrial artifacts. Both articles draw on recently published work, the first in JBIS, the second in Astrobiology. The potential of SETA and the areas it offers advantages over traditional SETI argue for close observation of a number of targets close to home.

by James Benford


“To think in a disciplined way about what we may now be able to observe astronomically is a serious form of science.” –Freeman Dyson

I propose a version of the Drake Equation for Lurkers on near-Earth objects. By using it, one can compare a Search for Extraterrestrial Artifacts (SETA) strategy of exploring for artifacts to the conventional listening-to-stars SETI strategy, which has thus far found no artificial signals of technological origin. In contrast, SETA offers a new perspective, a new opportunity: discovering past and present visits to the near-Earth vicinity by ET space probes.

SETA is a proposition about our local region in the solar system. SETA is falsifiable in its specific domain: ET probes to investigate Earth would locate on the nearest objects down to a specified resolution. SETI, on the other hand, is about messages sent from distant stars. For example, one can falsify a proposition such as “Are signals being sent to Earth at this moment within 100 ly?” But there is the region beyond 100 ly and beyond 1000 ly, etc. So SETI is falsifiable only within larger and larger domains. Of course other factors can also weaken falsification: our sensitivity might be inadequate, duty cycle might be small, and of course frequency coverage will always be incomplete.

Rose and Wright pointed out the energy efficiency of an inscribed physical artifact vs. an EM signal, because the artifact has persistence and the EM signal has to be transmitted indefinitely (Rose & Wright, 2004). Here I point out that artifacts are not only energy efficient, but increase the chance of contact. Rose and Wright did not explore where to locate the artifact so it would be identified; here I suggest there are attractive locations near Earth where they might be readily observable.

In a recent paper, I introduced the term ‘Lurker’: an unknown and unnoticed observing probe from an extraterrestrial civilization, which may well be dead, but if not, could respond to an intentional signal. And/or it may not, depending on unknown alien motivations (Benford, 2019). Lurkers include self-replicating probes, based on von Neumann’s theory of self-replicating machines, which is why they are often called von Neumann probes (Von Neumann, 1966). Recently concepts have appeared for self-replicating probes that could be built in the near future (Borgue & Hein, 2020).

Another pioneering work on this concept was of course famously developed in “2001 A Space Odyssey” (Clarke, 1968). A ‘solarcentric’ Search for Extraterrestrial Artifacts advocated by Robert A. Freitas, who coined the term SETA for it in the 1980s (Freitas & Valdes, 1985). There are also the papers from the mid-1990s by Arkhipov (Arkhipov , 1995, 1998a, 1998b). Scot Stride has shown that autonomous instrument platforms (i.e. robotic observatories) to search for anomalous energy signatures can be designed and assembled using commercial off-the-shelf hardware and software. That provides an economical, flexible and robust path toward collecting reliable data (Stride 2001a and 2001b). Further analysis has appeared recently (Haqq-Misra & Kopparapu, 2012, Lingam & Loeb, 2018, Cirkovic et al. 2019, Shostak, 2020).

Near-Earth objects could provide an ideal way to watch our world from a secure natural object (Benford, 2019). They are attractive locations for extraterrestrial intelligence to locate a platform to observe Earth while not being easily seen.

2. Drake Equations

2.1 The Standard Drake Equation estimates the number of radiating civilizations that are detectable, NC , as the product of the rate of creation of such radiating civilizations (Drake, 1965),

This modified Drake Equation is:

I replace the usual Drake Equation symbol for time over which they radiate L, with TR. And I also multiply by:

fR = fraction that actually do radiate signals that might be observable at Earth. That is, they radiate with the intention of trying to communicate. Leakage radiation is unintentional, but comes in two types: radar, which has no message, and broadcasts, which come from many incoherent sources which cancel out, such as TV.

These parameters are listed in Table 1:

Table 1: Drake Equation Parameters. Subscripts are italicized letters in definitions

2.2 A Drake Equation for Alien Artifacts An equivalent to the Drake equation for the number of Lurkers in our solar system, NL, can similarly be expressed as the rate of creation of radiating civilizations, times the fraction that also develop interstellar probe technology fip, times the sojourn that Lurkers would be in the solar system, TL:

fip = fraction that also develop interstellar probe technology and launch them

TL = time that Lurkers could reside in the solar system

(Note that for such civilizations, fC =1; a civilization with the capability to build such probes surely can build interstellar transmitters.)

Then a Drake equation for alien artifacts is

The new parameters are listed in Table 2:

Table 2 Drake Alien Artifact Equation Parameters

In the ratio of equations 1 and 2, of the number of Lurkers in our solar system to the number of radiating civilizations, most terms, in the first bracket, cancel so:

This initial result is that the ratio of civilizations sending probes that are now resident in our solar system to the number sending messages is the product of two ratios: A ratio of motives:

the fraction that also develop interstellar probe technology and launch them, divided by fraction that only radiate, so fip/fR < 1,

and a ratio of times:

the time Lurkers are present in the solar system/ the time ET civilizations release electromagnetic signals. Surely a civilization with the capability to build such probes can build interstellar transmitters, so I will argue that TL/TR > 1.

Our own civilization has been capable of radiating for about 50 years, including message-free Cold War radar transmissions and inadvertent leakage radiation has been emitted for a long time (Quast, 2018). Intentional messages have also been sent but are difficult to detect with Earth–scale receiver systems (Billingham and Benford, 2014). We cannot yet build interstellar probes capable of traveling to and decelerating into a star system and conducting operations there. But that may be possible in the next century. If so, relatively soon we will be capable of both radiating to the stars and sending probes to explore nearby star systems.

However, equation 4 does not take account of the space volumes that the two groups operate in.

2.3 Space Volume Factor

Another factor must be included: Equation 4 must be modified for VL, the volume over which Lurkers can travel, and its corresponding range RL vs. VB, the volume over which Beacons can transmit and be plausibly detected, and its corresponding range RB. Lurker probes traveling at a small fraction of the speed of light should be compared to the transmissions from an interstellar Beacon propagating at the speed of light. That means that the volumes from which signals can be detected from Beacons is much larger than the volume over which Lurker could travel.

For example, assume that interstellar probes could operate at ~10% c, the speed of light, as contemporary concepts of fusion rockets are designed for. An example: for the Icarus Firefly magnetically confined Z-pinch concept at 4.7% c, traveling 10 ly would take about two centuries (Freeland & Lamontagne, 2015). Starshot, which is a flyby probe concept, at 0.2 c takes more than 20 years to arrive at the Centauri system. Assuming the attention span of the civilization is measured in centuries, a rough estimate of the distance over which probes will be launched is tens of lightyears. (The signal from the probe reporting back to its origin would travel at the speed of light, of course.) If it is possible for probes to move close to c, then the beacon volume to probe volume would be close to unity.

In contrast, the electromagnetic waves of an interstellar Beacon, be it light, millimeter-wave or microwave, propagate ~20 times faster, at the speed of light. For example, we can estimate the range over which a Beacon would be used to be hundreds of light years. By that I again mean that the attention span of a civilization might be measured in centuries.

I define the volumes and ranges in Table 3:

Table 3 Space Volume Factor Parameters

Therefore equation 3 must be multiplied by the ratio of these 2 volumes, VL/VB:

As volume scales as the cube of the distance to them, RL/RB:

This is a ‘Success Ratio’ of searching for artifacts compared to listening to stars. It allows us to quantitatively evaluate their relative merits. Although the volume ratio would argue that long-range Beacons will be much more likely to be detected than probes that come to observe Earth, the time ratio tends to mitigate that advantage.

2.4 Decision Tree Parameters

The ratio of the number of lurkers to the number of radiating civilizations can be estimated using the three factors in equation 5, which have the following’s sizes:

So the ‘Success Ratio’, Eq. 5, will depend on choices for these parameters.

The key parameters making up these factors can be divided into objective and subjective components, where ‘objective’ means it can be quantified or at least estimated and ‘subjective’ means it’s a matter of opinion. Here is a Table of the parameters:

Table 4: Objective and subjective SETA Parameters and determining factors

The issues determining the objective parameters are listed; subjective parameters are a matter of taste and underlying assumptions.

By making choices among the objective and subjective parameters, one constructs a decision tree: A set of parameter choices leads to a conclusion about the success ratio for SETA and SETI strategies, as embodied in equation 6. Because ET civilizations will vary enormously in motivations, we can expect a variety of outcomes for the Success Ratio.

2.41 Estimates of TR, time that ETI Beacons radiate

In the literature, estimates of TR fall between a hundred and 100 million years, a very wide range. Michael Shermer estimated TR by averaging the lifespans of 60 Earth civilizations, getting 420 years, (Shermer, 2002). Using 28 civilizations since the Roman Empire, he gives ~300 years for “modern” civilizations. But Shermer’s number for the lifetime of societies is not relevant if new societies arise to replace old ones.  In that case, one should take the summation of existence times for all the technological cultures on a planet. Note that the longest operating institution still existing on Earth is the Catholic Church, ~2,000 years. We’ll take the times to be 300-10,000 years, an order of magnitude range.

2.42 Estimates of TL, time Lurkers could reside in the solar system

A key point is that Lurkers will still be discoverable even though dead for a long time. That’s not true of an EM transmission, which is simply passing through at the speed of light. That fact weighs to the advantage of the Lurker search strategy.

The time that Lurkers would be in the solar system, TL, will be limited by the lifetime of the orbits they are in, which provides an upper bound. The Moon, Earth Trojans and co-orbitals of Earth lifetimes are:

The Moon

Our Moon is thought to have formed about 4.5 billion years ago. For TL we use the time that life became evident in our atmosphere, 0.65 109 < t1 < 2.5 109 years.

Earth Trojans

There may be many objects in the Earth Trojan region (Malhotra, 2019). Their lifetime in Trojan orbits is likely to be on the order of billions of years, and some objects there may be primordial, meaning that they are as old as the Solar System, because of their very stable orbits about the Lagrange Points (Ćuk et al., 2012, Dvorak et al., 2012, Marzari & Scholl, 2013, Zhou et al. 2019). Orbital calculations show that the most stable orbits reside at inclinations < 0° to the ecliptic; there they may survive the age of the solar system, ~2.5 Gyr.

Earth Co-orbitals

Morais and Morbidelli, estimate lifetimes to run between 1 thousand and 1 million years (Morais and Morbidelli, 2002). With a mean lifetime of 0.33 million years. Morbidelli says that no further studies have been done on their approach (A. Morbidelli, personal communication).

3. Scenarios for Success Ratio Estimates

Here we show several scenarios, some of which show that the two strategies, SETA and SETI, are competitive.

Scenario 1: Choosing via relative costs at equal ranges:

Assume that:

1) The ratio of fractions of ET civilizations would be proportional to the cost of interstellar probes vs. Beacons. The cost of interstellar probes will be substantially more than the cost of interstellar Beacons. Stated differently, Beacons will have substantially longer range for a fixed cost.

2) RL and RC are equal.

If we take as an example a Beacon at 100 ly and a Lurker probe launched from 100 ly, then RL and RC in Eq. 5 cancel out. For Beacons that have a range of 100 ly the cost is of order $1 billion. This is from extrapolations, based on current cost scaling and costs (Benford, 2010, Billingham and Benford, 2014). The Firefly interstellar fusion rocket has an estimated cost of $60 billion. Two thirds of that cost is fuel to accelerate and decelerate (A. Lamontagne, personal communication). Therefore the cost ratio is ~100 in favor of Beacons. If cost is the deciding factor, then fP/fR = 1/100 and Eq. 5 reduces to

Next, one chooses an orbital location for the Lurker: Our Moon is thought to have formed about 4.5 billion years ago, long before life appeared. So we use the time life became evident in our atmosphere, 0.65 109 < TL < 2.5 109 years.

Next, one guesses the transmit time of the Beacon: estimates of civilization radiating times TC vary from ~300 -105 years. Here the ‘dash’ means the range of credible values:

So for these parameter choices, a Lurker search is much more likely to be successful. Note, however, that if we assume the Beacon civilization is at 100 ly, and the probe-building civilization is at 10 ly, a factor of 1/1,000 reduces the ratio to 0.1 to 100.

Scenario 2: What if cost doesn’t matter? That would be at variance with all we know of economics on Earth, but is a hypothetical we could consider. If cost doesn’t matter, then a civilization wanting to investigate the life of Earth or whether civilization was here could build probes to investigate the ecosystem, visible in spectra of our atmosphere, and also build Beacons to broadcast to us. In such a case, fP/fR = 1, and, as we’re talking about a single civilization, RL/RC = 1. Consequently the Success Ratio NL/NC = TL/TC, which would surely be >>1. Again, lurker strategy is likely to be more successful. In this scenario, the time ratio is the important factor.

Scenario 3: Early spacefaring civilizations: A civilization such as ours, which is presently capable of only interplanetary speeds, cannot build interstellar probes as envisioned by some of our starship concepts. Starships are centuries into our future and will always be more expensive than Beacons. They could be only a radiating society and might build Beacons. In this case the success ratio NL/NC = 0, and a listen-only strategy is appropriate.

Scenario 4: Supercivilizations capable of fast interstellar flight: The opposite extreme from scenario 3 is a civilization where starships can travel at a large fraction of the speed of light. In this case, Beacons, although still cheaper, would serve to reveal our civilization only if we respond by sending a message back to them. At about the same time their probes would be arriving and could be reporting the existence of our civilization. This could’ve occurred over geological time frames, so in this case NL/NC >>1, and we would expect to find dead Lurkers on the nearby objects described in 2.42.1, and we would expect to find dead Lurkers on the nearby objects described in 2.42.

Scenario 5: Lurkers in Co-orbitals and short radiating time: Instead of a Trojan or the Moon, we choose one of the co-orbitals, which have a mean lifetime TL ~0.33 million years. 1) For TR , choose the 300-year lifetime estimate of Shermer for the Beacon to radiate. Then TL/TR = 1,000. 2) Let’s assume that starship probes are launched from a civilization 10 ly away. (A probe such as Firefly, traveling at 0.2c and decelerating into our solar system, would take 50 years to come 10 ly.) 2) Assume the Beacon civilization is at 100 ly, and the probe-building civilization is at 10 ly. So RL/RB = 0.1. 3) Further, again assume that the willingness of civilization to undertake the expense would be determined by economics. A continuous Beacon at hundred light-years would cost about $1 billion and a Firefly probe is estimated to cost $60 billion (M. Lamontagne, personal communication), so fP/fR = 0.01. Therefore the Success Ratio, eq. 5, is:

For this case listening-to-stars has a higher success ratio. But if one assumes that the radiating civilization also develops interstellar probes, fR~fp, the two strategies have a roughly equal success ratio:

So one’s assumptions of the parameters in the Table determine the answer.

Scenario 6: Lurkers in Co-orbitals and long radiating time: If we use the band of estimates in the literature for co-orbital lifetime, ~105 years, and estimates of civilization radiating times TC vary from 102 – 105, then TL/TR varies from 1 to 1,000. For the previous 100 ly/10 ly distance ratio, Eq. 5 then gives a Drake Equation ratio of

And the listening strategy will be preferred.

It is clear from these scenarios that 1) the two strategies, SETA and SETI, are competitive, 2) the Moon and the Earth Trojans have a greater probability of success than the co-orbitals.

5. Research for Finding Alien Artifacts

I advocate a sequence of tasks:

  • We have had the Lunar Reconnaissance Orbiter in low orbit around the Moon since 2009. It has taken about 2 million images at high sub-meter resolution (M. Revine, personal communication). We can see where Neil Armstrong walked! The vast majority of the photos have not been inspected by the human eye. Searching these millions of photographs for alien artifacts would require an automatic processing system. Development of such an AI is a low-cost initial activity for finding alien artifacts on the Moon, as well as Earth Trojans or the co-orbitals (Davies & Wagner, 2011, Lesinkowski et al., 2020). Note the recent AI analysis of 2 million images from LRO which revealed rockfalls over many regions of the Moon (Bickel et al., 2020).
  • Conduct passive SETI observations of these nearer-Earth objects in the microwave, infrared and optical.
  • Use active planetary radar to investigate the properties of these objects
  • Conduct active simultaneous planetary radar ‘painting’ and SETI listening of these objects.
  • Launch robotic probes to conduct inspections, take samples of Earth Trojans and the co-orbitals. The low delta-V, 3-5 km/sec, make this an attractive early option, is well within present capability (Stacey & Conners, 2009, Venigalla et al., 2019). China plans a mission to co-orbital 2016 HR 3 in the middle of this decade (Zxiaojing, et al., 2019).

6. Conclusion

Clearly looking for alien artifacts in the region of the solar system near Earth is a credible alternative approach, a strategy of ETI archeology. The formulation given here is a way of discussing the SETA strategy and comparing it to SETI.

The listening-to-stars strategy that SETI researchers have been following for over 50 years, is now being pursued very vigorously by Breakthrough Listen. What has SETI learned so far about life in the universe? Only that there is no intelligent life broadcasting signals toward Earth at the time we’ve listened, within the sensitivity levels, duty cycles and frequencies we have observed. If the ongoing SETI listening program continues to not hear a signal, the case for looking for Lurkers will grow ever stronger.

The SETA strategy was not pursued after it was suggested in the 1980’s, because listening to stars is easier and observing technologies and spacecraft were not sufficiently developed to pursue it. But now SETA is more attractive:

  • Close inspection of bodies in these regions can now be done with 21st Century observatories and spacecraft.
  • The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.
  • The Moon and the Earth Trojans have a greater probability of success than the co-orbitals.
  • There are differences in detection in the two strategies: in the artifact case we should listen to those objects and image them in the optical or radar from Earth or send probes to visit them. In SETI, we can only listen.
  • SETA is a concept that can be falsified, a fundamental requirement for a science. SETA can be falsified or verified in practice by precisely specifying what one is looking for. For example, the statement “No artificial objects larger than 1 m exist on the surface of the Earth Trojan” can be verified by observing that object at that resolution. Smaller objects wouldn’t be resolved. If we conduct the efforts described in Section 5, and don’t find artifacts, the SETA concept is disproven for the near-Earth region, where it is most credible. If we find them, it’s verified.


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Comments on this entry are closed.

  • Alex Tolley April 20, 2021, 13:31

    The logic is clear, and in general, I agree with it. However, it appears that you are confusing the presence of a lurker to be found and actually finding it.

    Let me give you some terrestrial examples:

    1. Archaelogy. Ruined cities clearly are analogous to lurkers. They exist long after the civilization that built them disappeared. They may have inscriptions to communicate with us [c.f. Shelley’s “Ozymandias”], or just be piles of stones. However, many such cities became buried. Despite billions of humans tramping all over the globe, buried cities continue to be found, thousand of years later. There are likely to be more to be found and therefore the found/total cities ratio is >> p(lurkers), at least with current technologies, although this, in turn, depends on assumptions.

  • Harold Shaw April 20, 2021, 14:12

    Cost is a strong predictor of choice when the utility offered by different items is the same. The utility provided by beacons and probes is very different. I would caution against using the cost difference as a strong predictor. For example, even though the utility of bicycles and boats overlaps, the fact most bicycles are cheaper than most boats is irrelevant if there is a demand for floating.

    Probes offer valuable utility to a people demanding communication with other peoples. A probe within your neighbor’s system provides the security of verification. The cost of a probe would need to be compared to the cost of a beacon, giant telescopes, and the risk posed by revealing a home world. The difference in utility may make the difference in cost irrelevant.

    Advances in robotics, Ai, and biotech could fundamentally reshape economics, especially the economics of super projects. The transformation of a planet into billions of probes or a giant telescope could be bootstrapped from a tiny “seed capital”. These same advances may also increase civilization lifetime.

    The advantages of probes increases with time. An ancient people will be more likely to invest in probes and over time, will focus investment where it has a high return. Assuming an ancient player 1, the likelihood player 2 makes contact with a bespoke probe increases with time. Imo, the surveillance advantage provided by probes is too big for player 1 to ignore. Arguably, surveillance, verification is the foundation of diplomacy. MAD is impossible without it. However, as player 2 matures, the probability of player 1 being discovered increases. The location of living probes will be important. Perhaps proximity to Earth would be proportional to the general aggressiveness of galactic diplomacy.

    Thank you James Benford for the last two articles. Your discipline provides a place for me to be less disciplined.

  • Mike Serfas April 20, 2021, 15:55

    I don’t find very much meaning in the Drake equation. It makes reasonable assumptions which, taken collectively, are immensely confining.

    To begin with, we assume the lifespan of a civilization is finite, and indeed, more evanescent than the Milky Way. Yet as we consider the subjective time of organisms in high energy environments, such as Forward’s cheela, we realize that they should be expected to live very fast. The universe is not exactly an ideal gas, but at a brutish approximation, the smaller it was, the hotter it was, and the faster things happened. Give or take a few orders of orders of magnitude, a voyage across the universe was as long and interesting an affair when that universe was the size of a grapefruit as it is today. There may only be a finite amount of “t”, with our chemistry of atoms, but there may have been an infinite amount of “tau” through which the minds of the cosmos have developed.

    Parallel with this is the possibility of alternate regimes of physics, where different laws are of foremost importance, whether at earlier times or in strange environments. Some earlier version of the Drake equation might have discussed the possibility of life evolving in a ripple of quark-gluon plasma, and science fiction writers may glumly have pondered ways to survive the bitter cold end when triplets of quarks would be ripped away from one another by immeasurable distance.

    If our conditions were not the first conditions of life, we can’t be sure that our observations are free of “supernatural” interference (at least by our standards). What if vast and complex intelligence lurks in the unnoticed entanglements of nucleons or the magnetic field lines of the sun? What if the positive terms of the Drake equation are offset by negative terms as advanced cultures destructively interfere with radio noise to protect the peace of their evolutionary reserves? What if Lurkers can be reverted to the original source rock of a planet, complete with sediments and fossils, with flawless accuracy? Few who have seen old mining country on Earth would not wish for such. What if the messages between aliens, or the aliens themselves, fly with warp drives like the one profiled in your recent article, hidden from our sight and detectable only as the overwhelming mass of dark matter and energy that lies somewhere outside of our nature preserve? Where does the Drake equation calculate the odds that we are living in the Matrix? We really can’t predict where the breakthrough will be made.

  • Robin Datta April 20, 2021, 16:32

    Brilliant analysis. Astrobiology is an subdiscipline of biology without a specimen. SETA and SETI are sadly similar — to date. But the next picture beamed back from the Perseverance rover could shake up many if not most contemporary world-views. Keepin’ muh fingers crost!

  • DavidB April 20, 2021, 17:43

    Very interesting articles, thank you. In comparing searching for lurkers versus the more traditional radio searches, I have always wondered whether one could get a better idea about the likelihood of success for the latter by constructing various distributions for the radio luminosity versus the number of civilizations emitting signals with that luminosity, that is the luminosity function as it is called.

    This would inform what kind of searches to conduct.
    For example, if easy to detect radio signals are rare, than an all sky search search would probably be likely to succeed than a targeted search.
    This may be why an additional factor why searchiarlng for lurkers may well prove to be more successful than traditional SETI as we have reasons for knowing where they will be so the time spent searching the obvious ones would be the same if you searched for all lurkers. Hope this makes sense.

    I can see one using various Monti-Carlo simulations for trying to get a handle on this.

  • Ioannis Kokkinidis April 20, 2021, 19:23

    My comment is more on the lifetime of civilizations. If we are to consider the Roman Catholic Church as the oldest institution, then we must also consider the Greek Orthodox Church and the other 4 churches claiming to be Apostolic as the oldest institutions too. Of course there can only be one Apostolic church, but truth is that all 6 can claim a succession of bishops all the way back to the Apostles. That being said, I would not quite consider them as the oldest institutions per se. There are several horse races taking place on St George’s day in Greece that claim to be the uninterrupted Christianized descendants of ancient sporting events. The Tuesday bazaar of the city of Serres has been taking place every Tuesday for centuries, earliest known occurrence is in the 13th century and is likely older and I doubt that it is unique in Greece or the rest of the world. For a more informal event I would note the Carnival celebrations that have been taking place since time immemorial and we incorporated into the Church calendar and Christianized. There are quite a large number of similar folk events dating to time immemorial such as the event of Arapis in Nikisiani on Mt Paggaion taking place on either Epiphany or St John’s that shows its obvious ancient origin of a ritual to bring the end of winter:


    Greece is a place where civilizations and states have risen and fallen and the theme is both of continuity and disruption. After prehistory and protohistory which in Greece are important, think Mycenean civilization and the Trojan War, Greek history is divided into Ancient (ca 800 BC-330 AD), Byzantine (330 AD – 1453 AD) and Modern (from 1453 on). We do have a continuity of culture, the whole bickering and backstabbing council that led to the Battle of Salamis, whose 2500 years we commemorated last year, is quite familiar and ordinary, yet also breaks, and not just of the technological kind. Stuff done millennia ago is known to come back and bite us, and I have no doubt that something launched a very long time ago, if it returned, would still be quite recognizable.

  • James Benford April 20, 2021, 19:40

    Alex: I’m not seeing the connection you’re making; please amplify upon it.

  • Alex Tolley April 20, 2021, 23:39


    Let me state up front that I agree that we should look for lurkers. I would go further and suggest that they may even be on Earth. They could have very small sensors. With miniaturization, “smart dust” sensors could be like grains of sand and maybe infiltrated over deep time into the geology.

    Now to your question.

    I have no argument about the Drake Equation, nor your modified versions for SETA. Both provide a possible number of civilizations or artifacts that could exist. The Drake Equation makes the most sense as a civilization isn’t divisible, and the SETI folks assumed no star travel or colonization. Artifacts however are more problematic. In your modification for SETA, you substitute the time to radiate with the time lurkers are in the system. But your value for numbers assumes 1 lurker per system per civilization. Each civilization might launch N probes and we have no way to make estimates on N. This is important for finding them, as your “Success Ratio” is dependent on this.

    Clearly, if there are more than 1 lurker per civilization, then your equation underestimates the “success ratio” – assuming that if any are present they will be found.

    With beacons, an all-sky radio survey is possible, and with a wide enough bandwidth, any beacon should ultimately be detectable. [That we haven’t detected a signal so far is blamed on insufficient coverage of the sky and bandwidth limitations.]

    However, with lurkers, the problem is much more difficult. A dead lurker does not announce its presence. By analogy, a live bird can be heard singing from some distance, so you know it is present, even if unseen, whilst a dead bird is just a small object somewhere on the ground, perhaps covered in leaf litter. Then there is the issue of stealth. Almost all complex organisms utilize stealth whether they are prey or predator. This is because animals have agency and can respond to other animals. Assuming lurkers would not be stealthed to evade detection just assumes that they will not expect to do any close observation of animals, just make standoff remote observations, preferably of phenomena that have no agency. Whatever the reason, lurkers could be very hard to detect. They may be very small, they may be camouflaged, they may be buried in regolith or sediments.

    My point is that any use of numbers to compute a “success ratio” is not suitable to determine the likely success of a search. Even if we assume there is a lurker somewhere nearby and has been so for millions of years, ie N=1, there is no reason to assume that it could ever be found. My analogy to archaeologists finding cities lost for centuries and millennia despite being underfoot is indicative of the problem. They are being found now as a result of droughts on the landscape, and new detection techniques such as aerial lidar.

    Now I don’t want to give the impression that the search is so hard we shouldn’t try. I am all for trying for the “low hanging fruit”. A search of the lunar images makes absolute sense. While I doubt we would find anything other than manmade probes and ships, the cost is so low that we should try, just in case there is a shiny, crashed probe on the surface, or a transparent pyramid as depicted in Clarke’s “The Sentinel”. Looking beyond the Moon gets much more expensive. If a lurker is alive and responds to pings, that would be nice, but that may indicate that we were lucky to find one that was relatively recently arrived and not millions of years old.

    In summary. There could be a lot more lurkers in the system, most of which are dead, launched successively by a civilization to monitor our system. This would be analogous to our launches of probes that last years and then die, like the rovers on Mars. On its face, this increases the numbers for the success ratio, although it is a guess what the multipliers are. OTOH, despite their numbers, they may be very hard to detect for a variety of reasons. By analogy with the bird, a single person can detect a singing bird, but a village might need to carefully scour an area to find the dead bird. The ease of detecting beacons vs lurkers is not a numbers game as per your equations, but more about the probability of detecting either. That is subject to available technology and a lot of guessing. My sense is that the difficulty of detection is more important than the computed number guesstimates of the modified Drake equations.

    This opinion may be colored by my limited fieldwork looking for organisms that should be present, but seem to evade a careful search.

  • Spaceman April 21, 2021, 10:36

    Fascinating analysis!

    Question: Is it still a matter of debate as to whether or not self-replicating space probes are possible?

    To what extent is our current era of technological stagnation related to our ability to access energy? Technological progress increased dramatically as a result of the increased access to energy that came about as a result of the Industrial Revolution. And, as we know, most of the energy that we have used to propel ourselves into the current era has been derived from fossil fuels. Fossil fuels being so energy dense and easily accessible represented a sea-change in our ability to carry out projects and do work as well as synthesize a variety of products such as pharmaceuticals. That said, in addition to being problematic from an environmental standpoint, fossil fuels are finite and represent a very small fraction of the energy available from other potential sources.

    Technological stagnation has clearly set in as Peter Thiel and others have identified such that besides the internet and information technology, many areas of technology have stalled. Let’s step back and imagine the possibilities for what might happen when and if the next energy sea-change occurs:

    A Solar System Energy Infrastructure & What It Could Accomplish

    The greatest source of energy available within the solar system is the Sun. Once space travel finally takes off, we can imagine the construction of huge arrays of space-based solar power that could be used to fuel antimatter production by the ton or power particle accelerators that would make the LHC look miniscule. Advances in basic physics, historically, have come about as a result of experiments carried out at progressively higher energies. So, imagine the technological progress that could result from a massive particle accelerator constructed by robots in the inner solar system. With energy no longer being a limiting factor in technological progress and our ability to conduct experiments and make inroads into fighting entropy, then the Great Stagnation could lift. Right now, we are stuck in the valley of a bimodal distribution of technological progress brought about by our inability to access space cheaply.

  • Alex Tolley April 21, 2021, 12:02

    Technological stagnation has clearly set in as Peter Thiel and others have identified such that besides the internet and information technology, many areas of technology have stalled.

    This may be illusory. It is certainly the case that technologies tend to have a logistic development course which makes highly visible technologies like aircraft appear to have reached their upper limits. However, less visible technologies, like biotechnology, are still undergoing exponential development.

    The economist, Brian Arthur, argues that technologies are multiplicative, that each additional technology multiplies the space of development. It is certainly true that information technologies, the software probably more than hardware, have been a very important enabling technology.

    The coupling of energy and technology development is more likely an artifact, IMO. Early technologies, like steam power, were about creating work, which requires energy. However, the technology development and use of fossil fuels were mainly correlated with growing GDP. With the oil shock of the 1970s, while this energy/GDP correlation still held, the slope dramatically decreased. Miniaturization of hardware, and a focus on information-rich technologies, rather than work generating ones, have decoupled this relationship even further.

    So while our aircraft have stopped traveling faster, they have become more efficient. Our computers are less energy-consuming, and software more capable. Biology manipulation development is still racing ahead (recall how fast the new mRNA vaccines were applied?) and there is a open road ahead in that arena.

    As for space technology, the old ELV technology has matured, although it is getting a rebirth with reusability. Electric engines and solar sails are becoming almost “off the shelf”, and fusion engines are looking like possibilities, Human spaceflight seems to have stalled, but we will see whether reducing the cost of access to space and private enterprise leads us to a renaissance in space or not. Clearly, robotic spacecraft and machines are advancing nicely, and I for one am amazed at how fast telescopic techniques and capabilities are advancing.

    Overall, I don’t see technology development slowing down, just the obvious visible ones that we experienced in the first half of the 20th century. We just need to attune ourselves to new directions in technology development.

    • Spaceman April 25, 2021, 12:20


      The decoupling of technological development and energy to which you refer is well-captured and critiqued in Peter Thiel’s quote:

      “We wanted flying cars, instead we got 140 characters.”

      So, yes, software and hardware have steadily/reliably advanced in recent decades with the internet being the single biggest product of this trend, but IT is only one (albeit important) realm of technology. Think about it this way: we were sending people to the Moon when supercomputers were less capable than the processors we have on smartphones today. “Visible” technologies that do work as opposed to merely process information clearly matter greatly in terms of expanding our presence in the solar system let alone beyond, would you agree? And further, it is precisely in the realm of technologies that do work where we have witnessed the greatest stagnation. In order to terraform Mars or build a solar system infrastructure, all of the IT and Apps in the world aren’t going to substitute for fundamental advances in material science, energy storage and generation, and rocket propulsion. My point is that, right now, we are stuck in the valley of a bimodal distribution of technological progress with respect to technologies that do work brought about in part by our inability to access space cheaply. Accessing space cheaply is one of the biggest ways in which I see us breaking out of this period of stagnation in the realm of technologies that perform work. The fact that advances in IT have not resulted, so far, in great strides in technologies that do work is part of the reason why we are still in the valley, so to speak. Our ability to model certain physio-chemical phenomena using classical computers may be part of the reason, in addition to socio-political reasons, why it has been so hard to break out of this holding pattern. To me, an interesting question is that why, despite the continued advances in IT has this not yet translated into a new logistic curve in the realm of technologies that do work? I also wonder to what extent Richard Feynman may be on to something when he talks about the limitations in our ability to model nature:

      “Nature isn’t classical, dammit, and if you want to make a simulation of nature, you’d better make it quantum mechanical, and by golly it’s a wonderful problem, because it doesn’t look so easy.”

      Could it be that part of the reason why advances in “visible” technologies that do work has slowed is because the advances in IT despite their evolutionary improvements do not represent a revolutionary leap in our ability to model nature quantum mechanically? In other words, perhaps quantum computers, due to their ability to model nature at the quantum level, are what will reignite a period of advancement with respect to technologies that do work– advances in energy generation and storage, material science, propulsion, chemistry, and even fundamental physics? Yes, there is a fair amount of hype when it comes to quantum computers, but the advances in this field are undeniable.

      • Alex Tolley April 25, 2021, 15:16

        This is not a counterargument, more of an exposition of a counter view based on what I see happening.

        “We wanted flying cars, instead we got 140 characters.”

        Theil’s cute meme is clearly an exaggeration. But consider for a moment. “Do we even want flying cars”?
        Personal air transport has been an idea going back to the late 19th century. The “cities of the future” were full of brutalist skyscrapers with the air filled with improbably winged transport. When helicopters became a reality, many cities (like London) banned their use over populated areas keeping their use over the R. Thames. In the more liberal US, police, and traffic helicopters flew over cities with the inevitable misuse issues, not to mention the odd crash. But now we are getting those flying cars – passenger-carrying drones. Let’s just see what happens when the sky is full of these things. How many crashes onto the ground will happen? How much misuse will be evident? We have that pleasure/displeasure to come. Note that cheap IT will make these drones possible, perhaps by constraining their flight paths and maintaining autopilot at all times. What other tech were we promised? Nuclear power “too cheap to bill for”. Not only did this not happen, but nuclear power is also expensive and we have experienced reactor failures, most notably Chernobyl. Luckily, wind and solar power have proven far more benign, and are now cheaper than most fossil fuels and vastly cheaper than nuclear. Just as well we don’t have vast nuclear plant assets that need to be decommissioned and the waster disposed of somehow.

        IT OTOH has hugely increased our capabilities. Job categories like typists and telephone operators have disappeared. Many jobs involved in driving will likely disappear. Skilled, but repetitive cognitive work jobs will be reduced. All these changes benefit the end user in providing capabilities that were once expensive, but can now be done cheaply. In the previous post, we noted that machine learning will replace human eyeballs to find potential non-terrestrial artifacts on celestial bodies.
        While we haven’t reached Star Trek levels of medicine, it is IT that has increased our understanding and manipulation of biology. The time to develop the vaccine for Covid-19 was largely due to clinical trials. Computer models of human physiology reduce that time and reduce costs by eliminating failures before expensive testing. It may be possible that new drugs and vaccines can be developed in weeks. That seems like a very worthwhile goal. The development of therapies that involve the immune system is already considered breakthroughs in the treatment of common diseases like cancer. Seems like a better outcome than dying earlier but having a shiny flying car to drive.

        If you read the popular science press and the journal literature, you will note the rapid advances in many areas due to IT. Much of it is for dealing with the small scale. These improvements and even breakthroughs are not obvious like flying cars, but they have an impact that is cumulative.

        You mention terraforming planets and building space infrastructure. I think the model in your head is that of lots of spaceships and astronaut workers. It is the sort of future depicted in Allen Steele’s short story collection: “Sex and Violence in Zero-G”, itself a future that assumes the building of O’Neill habitats and solar power sats. But the future won’t be like that at all. Instead, IT will allow robots to do the work, supervised by humans, mostly on Earth. Interstellar flight will be robotic for the foreseeable future. Terraforming planets (if we must be so crass reinventing colonization and “Manifest Destiny”) will be much more subtle, using robotics to plan out the needed ecosystem successions using engineered organisms designed by IT systems, rather than “bulldozers and tractors driven by Heinleinian super-competent men plowing up the regolith and planting trees and crops”.

        Want an anti-matter drive? That will probably need a supercomputer to make sure that the engine doesn’t destroy any nearby infrastructure in a “glitch”. Want a space manufacturing city the size of an O’Neill, churning out hi-tech product and even Enterprise-class spaceships? Again, mostly built by robots, supervised by humans on Earth, perhaps not even commuting to work, a precursor social system to Forster’s “When the Machine Stops”. And those Mars colonists? Less like Heinlein’s colonist ideal, and more like Asimov’s Solarians, I think.

        The key to economic growth on earth, for most people, is growing a low energy economy. That means building and making things that use little energy over their life cycle. One of the best ways to do this is to make them “lighter”, i.e. smaller and more information-rich. Yes, a solar system-wide economy would be great (and probably needed for real starships), but most of this work will be done by robots, including the manufacture of more robots.

        I really don’t see technological stagnation. Just the more visible things of the 20th century reaching their limits while less visible technologies continue to develop, in some cases, explosively.

        The 1960’s vision of space development was exemplified by the movie: “2001: A Space Odyssey”. I would have loved that future to have happened by 2001. In reality, we don’t really want nuclear-powered Orion spaceplanes. A Moonbase would be nice, but we don’t really need Space Station V or even the Discovery. Discovery’s mission could have been completed by HAL, saving a huge amount of hardware and resources for life support. The Aries 1B Earth-Moon ship would be nice, but do we need a Clavius base of the size and complexity shown? Not a single robot to be seen doing surveys and resource extraction. Just people (men only) pottering about in spacesuits. Should we mine the lunar regolith for He3, it will be more like the scenario in “Moon”, but without the cloned Sam Rockwell, because in reality, he would be on Earth, supported by a team to manage repair robots for the excavators.

        As Mies van der Rohe said: “Less is more”. Today I view that as using elegance rather than brute force to achieve goals. Those goals will not be those we once had, and Peter Thiel still seems to want.

        • Spaceman April 26, 2021, 14:16


          I am not necessarily an advocate for flying cars and you certainly make a number of valid points in your response. That said, how can we access space without more advances in these “visible” technologies that do work? And yet, without access to space, it may be harder to advance these visible technologies because we will not have access the the “free energy” of the solar system. So, this represents a paradox of sorts and the irresolution of this paradox partly, IMHO, explains why we have had a hard time expanding our presence in the solar system. The resolution of this paradox, if achieved, will allow us to expand our presence in the solar system and it will increase our ability to make even further advancements in the “visible” that do work. Interstellar travel and even fast interplanetary travel will require substantial advancements in technologies that do work, would you agree? What do you think about the idea of a fully-autonomous solar powered antimatter factory in the inner solar system? Without advances in materials science, energy storage and generation, and rocket propulsion, how can we– without advances in these “visible” technologies that do work– expand our presence into the solar system?

          • Alex Tolley April 26, 2021, 22:01

            What do you think about the idea of a fully-autonomous solar powered antimatter factory in the inner solar system?

            That might be like having an explosives factory nearby. No thank you, they occasionally go boom. Did you see the recent fertilizer explosion in Beirut?

            Without advances in materials science, energy storage and generation, and rocket propulsion, how can we– without advances in these “visible” technologies that do work– expand our presence into the solar system?

            If flying pigs are not possible (except as air cargo), don’t waste effort trying to engineer them?

            I think we will have some of the visible technologies, but they may largely suitable only for machines. I’m reminded of Asimov stories where robots are used to test new technologies. We once tolerated a high human loss rate in test pilots. Today, test flights are far safer. If new aircraft was tested by a semi-autonomous machine (or telechiric?) there would be no need to put a meat person in the pilot’s seat. If the human pilot isn’t needed, why not design the vehicle without the intention of having a human pilot at all? Hence drones aircraft.

            If a machine can tolerate 10,000s of g, maybe the best way to build launchers and interplanetary spaceships is to assume machine control instead. Instead of humans running factories in space, PK Dick autofacs.

            IOW, perhaps the paradox is solved by taking humans out of the equation. Accept that our “mind children” will inherit the future in space, not we meat humans.

            I have used this analogy before: Are we like intelligent Devonian fish trying to build mobile aquaria to colonize the land? Evolution found a better way. I wouldn’t rule out technologies to transfer [copy really] human minds to synthetic brains and bodies, although I suspect AGI embodied in machines will be first.

  • Alex Tolley April 21, 2021, 16:46

    The latest SETI talk on “Vanishing Stars” had this viewer poll:

    “What’s the best way to search for ET?”

    1. Search the observable universe for billions of possible ET candidates – 43%
    2. Search nearby star systems for a radio signal or flashing lasers – 48%
    3. Search for technologies in our solar system – 9%

    Not that the survey was indicative of expertise, but it might suggest that the lurker search has some selling to do. I think Seth Shostak was surprised that option 3 ranked so low, so he may be more on board with the idea than I assumed.

    [I also think the poll was poorly designed. It seems to me that option 2 is just a subset of option 1, which implies that its score should be lower than option 1.]

    I think the problem for searching for lurkers in our system is analogous to the problem of finding a cat, any cat. Is it best to walk the neighborhood and look out for cats, or to do a more intense search of your garage or basement? The article basically argues for the latter as there is the advantage that a long-dead cat’s skeleton may be found in the basement at anytime you search, whilst the neighborhood cats may be out of sight sleeping when you walk the neighborhood in the daytime, rather than for a few hours at night when they are active. ;)

  • Joe H. April 21, 2021, 17:10

    If any alien probes had explored our solar system in the past it seems to me that they would have had good enough AI to select Earth as an obvious destination. It would be relatively easy for an advanced robotic spacecraft to enter and maintain an orbit around Earth. The probe could photograph our planet and send high quality images and videos back to whoever launched it. From the dawn of life on Earth up until the early 20th century the probe would be undetectable so there would be no danger of discovery or destruction. For the past 100 years or so we’ve had the ability to find such a probe and it’s not there. Since Earth is the obvious candidate for exploration in our solar system, the idea that there might be probes or artifacts elsewhere seems far fetched.

  • James Benford April 21, 2021, 20:11

    Alex: Thank you for pointing out that my estimate for the number of Lurkers we could find is in fact an underestimate! Yes, I expect an ET civilization would send several probes to look at aspects of that life once it is discovered. If it saw evidence for civilization, which could happen only in the last few centuries, it would then send other probes with sufficiently large apertures to investigate many aspects of Earth, not just biology.

    Saying that aliens will disguise themselves or adopt camouflage is one possibility, but we can’t assume that all aliens will!

    I do expect there would the problems with swarms of nano probes acting together. Robert Freitas points out that the processing requirements for them to work together would take energy, and with their small mass they’d be bright in the IR.

    I don’t think probes would be found on Earth because the active weather and geology would disassemble it or bury it. However, on the moon NASA estimates that our own artifacts will survive for at least millions of years. But on co-orbitals or Earth Trojans the lifetime will likely be longer, unlike the Moon, because they will not be suffering from a rain of micrometeorites falling into their weak gravitational well.

    I would like to emphasize that the Chinese are likely to explore the new earth region before we do. That’s because they already are there. They now have a probe at L1 and are sending it on to the Earth Trojan region to explore for Trojans. The cost of such missions is relatively small because the delta-V required is small. Several mission studies have been done to estimate the probe and operations cost for such expeditions and they turn out to be quite straightforward. For example, Mason Peck of Cornell estimated that, with a free piggyback launch by SpaceX, the cost of a probe and operations on Earth would be of order $10 million.

    • torque_xtr April 24, 2021, 7:23

      The articles and the comments led me to further thoughts. First, the more I think about search for probes by their own emission in the lunar night, the better it looks.

      A region beyond the lunar farside is well shielded against Earth’s emissions. The cone includes L2 point of Earth-Moon system, near which there already is a Chinese relay satellite. During the lunar eclipses (not only total because of ~3.1 deg lunar angular diameter from L2), it is shielded also from the Sun, and it is safe both to take very long exposure images of the far side, achieving extremely low SNR over UV-Vis-IR ranges, and to listen in the longwave (radio to terahertz) to sensitivities not achievable anywhere else in the near-Earth space. Maybe SNR in the ionizing range (X-Ray and gamma) is also reduced to considerable extent, which may reveal a nuclear-powered probe even if it is no longer active. The surveyed area is very big – looking at the surface is much simpler than at the interior, and the lunar far side has more surface area than a million of one-kilometer asteroids. Yet, from near lunar L2 a sub-kilometer-scale resolution can be achieved with a compact camera, with resolution increase available by low-orbit satellites. Starlight background from a 100 m-wide area is on the order of 1 milliwatt per visible range. So the lunar far side night reconnaissance indeed looks a very attractive and low-cost program, with a range of possible activities and scientific returns beyond SETA.

      The long-exposure photos of farside in the visible range during the coming eclipses could be taken by relay satellites at virtually no cost. At the high end, specialized instruments could be put on some future sats, surveying full electromagnetic domain and yielding additional scientific results, which in this extremely low-noise environment may well be valuable by themselves even in the abscense of ETA detection.

      The sooner – the better, the noise floor is rising due to increasing human presense in cislunar space.

      Here comes the other side. The Lunar Night Reconnaissance is strongly biased towards the living probes, and there is the very important distinction. If we find a dead artifact, we can relatively safely study it, but finding a living probe would raise all the issues of ethics and safety to their fullest. I don’t believe much in the Dark Forest State, but just not recognizing a warning sign could be enough, be it megayear-old-tech analog of “Danger! High voltage” picto or “Watch the traffic lights!” warning.

      But if we find a living probe, it absolutely does not mean that we need to come down on it, because, within this framework, it would almost guarantee that there are many more dead ones.

      Products of a technology live orders of magnitude less than the technology itself. A “no-FTL” probe into another stellar system needs to live for centuries or millenia, but it’s hard to imagine multi-million-year-long lifespan – for most purposes it is just too much overhead. But within the initial assumptions it is quite reasonable to assume that first ETIs capable of interstellar exploration appeared billions of years ago, and Earth biological “interestingness metric” was above threshold since at least Cambrian Explosion, so there were many visits in the pasts by probes that are no longer active.

      So if we do find a living probe one, we could study it from a safe distance while intensifying our searches elsewhere, assured by the knowledge that there is much, much more to find and we can do it. The way actualy looks straightforward: after finding a living probe, search for dead artifacts, find and study them, gain knowledge that will help to understang the living one, then try to study the probe and/or interact with it if it is considered safe or needed, based on what we learned.

      PS I think it’s quite wrong to call all xenoartifacts Lurkers. Aside of ominous connotations, a lurker is the most hard case of xenoarchaeology – something that is deliberately camouflaged and thus hard to find, and demanding most care during studies. Likely also the least common. Two other classes could be identified, which can be described as “active” and “passive”, or “probes” and “artifacts”. First is something that is likely not stealthy, but is living or actively decaying (whatever that means). These are the most easy to find, but demand almost as much care as Lurkers. The last is something that is surely dead – quite hard to find (but not as hard as a true Lurker), but the most common and the most safe to study.

      • Alex Tolley April 24, 2021, 12:16

        My only issue is why would a probe be on the lunar farside, rather than nearside if the purpose is earth observation? If it was, that would seem to imply that it is/was “hiding” and collecting data from other sensors that were able to view the earth directly.

        The Haqq-Misra paper suggests a number of options for detecting both surface and buried technology on the Moon, including anomalous IR signatures in the lunar night [reminiscent of how the buried moon bus ​Selene was detected in Clarke’s “A Fall of Moondust”].

        The obvious place to start is the inspection of the LRO images of the Moon’s surface. If nothing, then consider collecting different data that might expose anomalies. This should probably be piggybacked on surveys for other reasons, like detailed mineral prospecting. Unlike the search for life on Mars, we cannot hope to limit the search area by inspecting the “most promising” places as Perseverance is doing.

        • torque_xtr April 26, 2021, 16:45

          Far side is just easier to search because of lower backgrounds. While it is of little use for direct Earth observations, it could be a location for something that does not require Earth in sight, like data processing module of transmitter. Of course there is no way yet to tell which bias is stronger, but instruments designed to touch noise floor on the far side could both expand “search space” and yield more scientific results in areas other than SETA.

          Lunar poles are another good location – they could attract ETI explorers for reasons similar to our own. They would go there if they need something to build, produce or repair on the Moon and need CNO elements for that. For the locations on the Moon, the lunar polar ice deposits are by far the closest and most convenient place to obtain volatiles. Maybe to the lesser extent even for Earth coorbitals where they are likely very scarce. In addition, the search area is much smaller than the whole lunar surface, and a search for artifacts themselves and for evidence of past activity could be piggybacked on the already-envisioned in-situ exploration.

          https://arxiv.org/pdf/1111.1212.pdf does not go into much details, only proposes search of Diviner data. And I haven’t found yet any papers on comprehensive search of anomalous sources in it. But I can imagine it could be tricky. Nightside thermal emission is not influenced by Earth interference on the near side (like radio and visible, latter because of ashen light), but the background is high and varying with mineralogy and subsurface structure, on all scales. 100 K equals to several watts per meter, enough to swamp even multi-kilowatt source at the Diviner resolution. But it may be much better for non-thermal emission or sources like radiators at 300 K, heated enough to give output in the shorter-wave bands where thermal background is low.

  • Alex Tolley April 22, 2021, 11:24

    I don’t think probes would be found on Earth because the active weather and geology would disassemble it or bury it.

    I would interpret this as saying that the probe would send landers to the Earth’s surface to do the necessary observation, sampling, and analysis of life, but that unless they can take off again, they will eventually be destroyed or buried and this will make them next to impossible to find except serendipitously.

    This may well be a case of looking where the light is available rather than where you dropped your keys (a long time ago).

    As a commentator mentioned elsewhere, acquiring an alien probe would be immensely valuable, far more so than a short radio transmission sending prime numbers, or a low-resolution picture of some sort, even if it was in a poor condition like the Antikythera device that has recently been reconstructed. However, such a device on an airless world[let] would be in far better condition to be investigated.

    I do think looking at the hi-res LRO images of the Moon is a good first step. Both machine learning methods and citizen science would be a good way to start, especially as this could be combined with other searches. I imagine that locating all the manmade objects on the Moon would be a nice project, especially if we found an unexpected or unreported terrestrial probe on the surface. With humans on the Moon again in this decade, what a great opportunity to plan an exploratory trip to investigate a possible artifact up close and bring it back to base for examination.

  • James Benford April 22, 2021, 14:21

    Alex: Recall that my piece referred to ‘ET lurking in our Backyard’. Using that analogy, the standard SETI staring-at-stars method is to look not in the backyard but in some other country. Are there cats there? Better to start by looking locally.
    Seth Shostak actually favors SETA, looking locally. As my reference shows, he wrote a paper about it a year or two ago. However he did not propose any strategy, tasks for doing so or locations to search.
    The psychology of SETI practitioners, who I have observed for decades now, is to listen for a message. That’s a passive activity. What I advocate is active exploration, look at photos of nearby sites, then send probes exploring our near space. This is not in the comfort zone of SETI practitioners that I know.Their passive psychology is uncomfortable about missions beyond the Earth. So I’ve found that they do accept that SETA makes sense, but they are not going to engage it themselves. That’s why I think that the SETA concept will be pursued outside of the SETI community, perhaps by the Chinese, who are already exploring Near-Earth Space.

    • Mike Serfas April 22, 2021, 22:06

      SETA takes a higher budget, and it’s mostly just plain space exploration. Which should be a great thing. Put to effective use in construction, rock in an Earth Trojan orbit or even one of the near asteroids could be worth more than gold on the ground. Prospecting for alien artifacts helps to keep things interesting, even though I doubt anyone can really calculate the odds.

      Now when something looks at first photograph like a spaceport in the asteroid belt (the Cererian faculae), it may turn out to be a natural formation – but if that formation happens to be a massive salt deposit and perhaps an entryway to an underground water reserve on Ceres, it still bears further examination.

      Even when alien ruins are scarce, these prospecting expeditions test us. Can rival nations and groups possibly reach a less wasteful philosophy than competitive claims of ownership, and work together to bring knowledge and resources that no human made to assure the wealth of all humanity? We’ll need such thinking to avoid woe if they do turn up.

  • Richard Burke-Ward April 23, 2021, 5:20

    For what little it may be worth, I had an article proposing
    a revised Drake Equation for alien artefacts in the Journal of the British Interplanetary Society in 2000, vol 53, No 1/2: https://www.jbis.org.uk/paper.php?p=2000.53.2

    • Alex Tolley April 23, 2021, 13:17

      Sadly, I don’t have a subscription for JBIS so I cannot reach around the paywall. Can you flesh out the abstract a little to provide a flavor for the relevant parts that impact the Benford paper?

    • Alex Tolley April 23, 2021, 13:57

      As your paper was cited by the Haqq-Misra & Kopparapu paper in the references, I gave that paper a read as well. It does an interesting Bayesian estimate of probabilities of probe discovery, assuming a probe of 1-10m in size that is not camouflaged and is present.

      I tend to think the authors have vastly overestimated the detection probability as they assume the probes are not buried. This is reasonable for the Moon and asteroids, but not the Earth. That we are now discovering buried caches of coins and other metal objects by magnetic detection should indicate that a probe is unlikely to be on the Earth’s surface in plain sight. Even on the Moon, dust might cover a crashed artifact over time.

      In the discussion they state:
      [NTA = Non-terrestrial Artifact]

      However, it has been suggested that NTAs might be identified by their emission of anomalous microwave or infrared radiation [8], so the discovery of an unexpected temperature anomaly on the lunar surface could be a signature worthy of further investigation. Likewise, the chemical composition of an NTA would presumably be different from the surrounding lunar regolith, so an anomalous spectral signature on the lunar surface could also be a potential indicator of an NTA. This suggests a method by which surface temperatures and mineralogies, such as those available from the LRO Diviner Lunar Radiometer Experiment [41, 42, 43], could be examined for anomalies that are consistent with the presence of an NTA. Such an analysis is worthy of future investigation but is beyond the scope of the present study.

      Going beyond a visual search makes sense to me. Just as terrestrial searches for buried metal objects with magnetic detectors is employed by “detectorists”, and other techniques have exposed hidden artifacts, buried cities with LIDAR, satellite spectral photography, and surface vegetation changes with drought. Buried probes on Earth must be far rarer than fossil animals, and yet we have barely scratched the surface for fossils. Locations on celestial bodies or free in space might be easier to detect – if we can effectively search those locations.

      I do wonder how easy it would be to detect any one of a swarm of 1 gm Breakthrough Starshot probes that might have crashed into a world in the target system. Would they be detectable at all? Burned up as meteors on planets with atmospheres, or vaporized on impact with an airless moon. What technology would be needed to detect any residual anomalous metals around an impact?

  • James Benford April 24, 2021, 14:07

    Richard: I’ve gotten your paper from my JBIS library (1996 till now). You do propose strategies for finding artifacts and tasks for doing so. You don’t cover much about where to look, but at that time Earth co-orbitals and Earth Trojans had not been discovered. I’m going to reread it carefully!
    It strikes me that Freitas proposed SETA in the 1980’s, you did so in 2000 and I’m doing so now. My proposals are more specific and quantitative, as befits our greater knowledge base. And there is a constant in this: the SETI community continues to ignore SETA!

    • Richard Burke-Ward April 27, 2021, 4:07

      Hi Jim. Thanks for giving it a read. Your paper is more sophisticated and massively more up to date. I like that these things come round in circles – each time they are more refined and move closer to potential implementation. For me, the most important thing is that the possibility of new ‘local’ forms of SETI gain more credence. The chances of success may be remote, but they are non-zero. Your paper is a huge step in the right direction. Best wishes, Richard

  • Dale Smith April 30, 2021, 19:45

    I’m very interested in Alex Tolley’s comments about the use of aerial lidar to find archeological sites on Earth.Is it possible we could do the same on the Moon?Apollo 15 mapped the lunar surface with lasers.Could any possible artifacts be hidden this data?

  • ljk May 5, 2021, 13:16

    ‘Library of the Great Silence’ invites E.T. to share long-term survival strategies

    By Mike Wall 6 days ago

    We have a lot to learn.

    Intelligent aliens will soon have a space here on Earth where they can share how they made it through their technological adolescence.

    We haven’t yet heard from any such beings, of course. Some researchers find this “Great Silence” puzzling, given how old the universe is and how many potentially habitable worlds dot its vast expanse.

    One possible explanation is that civilizations tend to destroy themselves once they become “advanced” enough to explore the cosmos in a meaningful way. Such power is inherently hard to control and can burn you to the ground more easily than it can fuel an outward push, the idea goes.

    Full article here:


    To quote:

    “Although interstellar exchange could take time, a material archive of transformations will have immediate global value that may be sufficient to extend the lifespan of human civilization in the interim,” reads a description of the project, which is a collaboration with the SETI Institute in Mountain View, California. (Keats is currently an artist in residence there, and Hat Creek is the site of the Allen Telescope Array, which SETI Institute researchers use to scan the skies for possible signals from ET.)

    “Manipulating existentially significant objects without the use of words — and without the underlying assumptions of language or limitations on who participates in the conversation — may facilitate comprehension of human behaviors that has previously eluded us, or even directly encourage beneficial practices such as cooperation,” the description adds.

    Hat Creek may not be the only repository of such artifacts, either.

    “We’re starting to reach out to libraries that already exist about whether they would host, potentially, branches,” Keats told Space.com. “And, simultaneously, we’re actually looking off the planet and starting to look at what it would take to have a branch on the moon, for instance.”

    You can learn more about the Library of the Great Silence here:


    And the project is getting a formal kick-off of sorts this afternoon (April 29), via a conversation between Keats and his chief advisor on the project, SETI Institute senior astronomer Seth Shostak. The 30-minute discussion starts at 5 p.m. EDT (2100 GMT), and you can watch it live online.

  • ljk May 13, 2021, 15:38

    SETI: microbes may already be communicating with alien species – new research

    May 12, 2021

    by Predrag Slijepcevic and Nalin Chandra Wickramasinghe


    More information: Predrag Slijepcevic et al. Reconfiguring SETI in the microbial context: Panspermia as a solution to Fermi’s paradox, Biosystems (2021). DOI: 10.1016/j.biosystems.2021.104441