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Is ET Lurking in Our Cosmic Backyard?

Jim Benford is continuing his research into the still nascent field known as SETA, the Search for Extraterrestrial Artifacts. A plasma physicist and CEO of Microwave Sciences, as well as a frequent Centauri Dreams contributor, Benford became intrigued with recent discoveries about Earth co-orbital objects — there is even a known Earth Trojan — and their possibilities in a SETI context. If we accept the possibility that an extraterrestrial civilization may at some point in Earth’s 4.5 billion year history have visited the Solar System, where might we find evidence of it? Two papers grew out of this, one in Astrobiology, the other in the Journal of the British Interplanetary Society (citations below). In the first of two posts here, Jim explains where his work has led him and goes through the thinking behind these recent contributions.

by James Benford

Part 1: How Many Alien Probes Could Have Come From Stars Passing By Earth?

1. Searching for Extraterrestrial Artifacts

Alien astronomy at our present technical level may have detected our biosphere many millions of years ago. The Great Oxidation Event occurred around 2.4 billion years ago; it was a rise in oxygen as a waste product due to organisms in the ocean carrying out photosynthesis. Long-lived robotic probes could have been sent to observe Earth long ago. I will call such a probe a “Lurker,” a hidden, unknown and unnoticed observing probe, likely robotic. They could be sent here by civilizations on planets as their stars pass nearby.

Long-lived alien societies may do this to gather science for the larger communicating societies in our Galaxy. The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.

Here, in part 1, I estimate how many such probes could have come here. This is explained in detail in [1].

In Part 2, titled ‘A Drake Equation for Alien Artifacts’, I propose a version of the Drake Equation to include searching for alien artifacts that may be located on Moon, Earth Trojans and co-orbital objects [1]. I compare a Search for Extraterrestrial Artifacts (SETA) strategy of exploring near Earth for artifacts to the conventional listening-to-stars SETI strategy.

1.1 Observing Earth

From Figure 1, the time over which our biosphere has been observable from great distances, perhaps thousands of light years, due to oxygen in the atmosphere, is a very long time, measured in the billions of years [7,8]. The first oxidation event occurred about 2 .5 billion years ago and the second, largest oxidation event about 0.65 billion years ago, so 0.65 109 < TL <2.5 109 years.

An ET civilization that passes nearby can see there’s an ecosystem here, due to the out-of-equilibrium atmosphere. They could send interstellar probes to investigate.

Figure 1. History of Oxygen content of Earth’s atmosphere is observable from great distances. Dashed line is present value. Horizontal axis is in millions of years before present. (Wikipedia Commons)

2. How Often Do Stars Pass By Our Sun?

It is not widely known that stars pass close to our solar system. The most recent encounter was Scholz’s Star, which came 0.82 light-years from the Sun about 70,000 years ago [3]. A star is expected to pass through the Oort Cloud every 100,000 years or so, as Scholz’s Star did, shown in Figure 2.

Bailer-Jones et al. showed that the number of stars passing within a given distance R, NS (R), scales as the square of that distance [4]. This comes about because Earth is in a flow of stars circling the galactic center, so the cross-sectional area is what matters, which gives an R2 scaling, rather than the volume, ~ R3. Figure 3 shows that several stars have approached or will approach our solar system over 105 years.

Figure 2. Our most recent visitor: Scholz’s Star came within 0.82 light-years from the Sun about 70,000 years ago (NASA).

Bailer-Jones et al., using accurate 3D spatial and 3D velocity data for millions of stars from the Second Gaia Data Release has shown that a new passing star comes within one light year of our Sun every half million years, 100 within 10 light years [4].

With the number of stars passing within a given distance, NS (R), and R the distance of the star from the Sun in light years, the rate of passing stars is:

So a new star comes within 10 ly every 5,000 years [3]: during our 10,000-year agricultural civilization, two new stars have come within 10 ly.

Figure 3. Stars come very close to Earth frequently. About 2 stars come within a light year every million years. An ET civilization that passes nearby can see there’s an ecosystem here, due to the out-of-equilibrium atmosphere. They could send interstellar probes to investigate. (stackexchange.com)

3. How Many Lurkers May Have Come Here?

To calculate the number of Lurkers that could be located at various sites nearby to Earth, such as the Moon, Earth Trojan zone or the co-orbitals, I make the following estimates. The quantities to use in calculating this concept are shown in Table 1.

There are two factors to evaluate: 1) How often do stars get within a given range of Earth? 2) How long would a Lurker reside in a given location near Earth?

Of course, a key factor we do not know is what fraction of the stars have spacefaring civilizations.

Table 1 Passing Stars Parameters

The number of Lurkers that could arrive and now be found, NL, would be fip times TL, the orbital lifetime of the object upon which the Lurker is resident, times the passing star rate, [dNS(R)/dt] from Eq. 1:

We don’t know fip, but we can calculate the ratio

Now we make estimates of NL/fip. Details of these estimates below can be found in [1].

4.0 Locations for Lurkers Near Earth

The time that Lurkers would be in the solar system, TL, will be limited by the lifetime of the orbits they are in. That is determined by the stability of the orbit of the near-Earth object it lands on. This provides an upper bound to how long they could be around. The Moon, Earth Trojans and co-orbitals of Earth lifetimes are:

4.1 The Moon

Searching on the Moon has recently been advocated [5, 6]. Our Moon is thought to have formed about 4.5 billion years ago, long before life appeared. Then the Earth ecosystem would not attract attention. Later, life became evident in our atmosphere.

We have had the Lunar Reconnaissance Orbiter in low orbit around the Moon since 2009. It has photographed about 2 million sites at sub-meter resolutions. We can see where Neil Armstrong walked! The vast majority of these photos have not been inspected by the human eye. Davies and Wagner have proposed searching these millions of photographs for alien artifacts, which would require an AI for initial surveys [5]. Development of such an AI is a low-cost initial activity for finding alien artifacts on the Moon, as well as Earth Trojans and the Earth co-orbitals. A recent AI analysis of 2 million images from LRO revealed rockfalls over many regions of the Moon [9]. So we have proof a search for artifacts of ~1-meter scale could be done by AI.

Figure 4 The Apollo 17 site as seen by the Lunar Reconnaissance Orbiter. Note that Moonbuggy tracks can be clearly seen. A study of the >2 million such photos could detect possible artifacts on the Moon (NASA).

4.2 Earth Trojans

Figure 5 shows the many Jupiter Trojans, located at stable Lagrange Points near that planet. There may be many such objects in the Earth Trojan region [11], ~60 degrees ahead of and following Earth. Their lifetime 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 Lagrange Point orbits [11-14].

Figure 6 shows a portion of the orbit of the only Earth Trojan found so far, 2010 TK7. It oscillates about the Sun–Earth L4 Lagrange Point, ~60 degrees ahead of Earth [15]. Its closest approach to Earth is about 70 times the Earth-Moon distance. It is not a primordial Earth Trojan and is estimated to have an orbital lifetime of 250,000 years, when it will go into a horseshoe orbit about the sun. It is clear why there are no other Trojans of the Earth yet found: they are hard to observe from Earth.

There are large stable regions at Lagrange Points, so Trojans may exist for long time scales. It is possible that primordial Earth Trojans exist in the very stable regions around the Lagrange Points. Orbital calculations show that the most stable orbits reside at inclinations <10° to the ecliptic; there they may survive the age of the solar system, so again we use the oxygen time, ~2.5 Gyr. So Trojans’ orbital lifetimes can vary from 2 105 years to 2.5 109 years.

Figure 5. The many Jupiter Trojans, which lead and follow the planet at ~ 60°. (Wikipedia Commons)

Figure 6. Portion of the orbit of the one Earth Trojan found so far, 2010 TK7. (NASA)

4.3 Earth Co-orbitals

See [16] for a discussion of the co-orbitals of Earth. A large number of tadpole, horseshoe and quasi-satellites that approach near to Earth appear to be long-term stable. Figure 7 shows to orbit of the nearest one, 2016 HO3. Morais and Morbidelli, using models of main asteroid belt sources providing the co-orbitals and their subsequent motions, estimate lifetimes to run between 1 thousand and 1 million years. They conclude that the mean lifetime for them to maintain resonance with Earth is 0.33 million years (17).

Figure 7. Orbits around the Sun of Earth and the nearby quasi-satellite 2016 HO3. It comes within 5 million km of Earth (NASA).

5. Conclusions

In [1] the above remarks are quantified. Here I summarize the calculations in the Table, for probes traveling from 10 ly and 100 ly. (Note that, since co-orbitals have a finite lifetime on their orbits near Earth, Table 2 refers to this is the number of probes that may have landed on what was at the time a co-orbital but will now have wandered off somewhere.)

Table 2: NL/fip: The number of Lurkers, from stars that pass by our Solar System that could have arrived and now could be found, for several nearby astronomical bodies, divided by fip, the fraction of stars that have civilizations that develop interstellar probe technology and launch them.

  • Clearly, the Moon and the Earth Trojans have a greater probability of success than the co-orbitals.
  • Of course, fip is the factor we don’t know: how many civilizations develop interstellar probe technology and launch them.
  • The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.
  • Close inspection of bodies in these regions, which may hold primordial remnants of our early solar system, yields concrete astronomical research. It will yield new astronomy and astrophysics, quite apart from finding Lurkers.
  • A suggestion for SETI observers: Look at the specific stars that have passed our way in the last 10 million years and ask how many of them are ‘sunlike’ and/or are known to have habitable planets. Observe those stars closely for possible emissions to Earth [16].

For discussion of approaches to study these objects, starting with passive observations, and going on to missions to them, see Reference 14, section 4, “SETI Searches of Co-orbitals”. The actions and observations are:

1. Launch robotic probes and manned missions to conduct inspections, take samples.

2. Conduct passive SETI observations.

3. Use active planetary radar to investigate the properties of these objects

4. Conduct active simultaneous planetary radar ‘painting’ and SETI listening of these objects.

5. Launch robotic probes and manned missions to conduct inspections, take samples.

This argues for a Search for Extraterrestrial Artifacts (SETA) strategy of exploring near Earth for alien artifacts [2].


1. J. Benford, “How Many Alien Probes Could Have Come From Stars Passing By Earth?”, JBIS 74 76-80, 2021.

2. J. Benford, “A Drake Equation for Alien Artifacts“, Astrobiology 21, 2021.

3. E. Mamajek et al, “The Closest Known Flyby Of A Star To The Solar System” ApJ Lett., 8003 L17, 2015.

4. C. A. L. Bailer-Jones et al, “New Stellar Encounters Discovered in the Second Gaia Data Release”, Astronomy & Astrophysics 616 A37, 2018.

5. P.C.W. Davies, R.V. Wagner, “Searching for Alien Artifacts on the Moon”, Acta Astronautica, doi:10.1016/j.actaastro.2011.10.022, 2011.

6. A. Lesnikowski, L. Bickel and D. Angerhausen, “Unsupervised Distribution Learning for Lunar Surface Anomaly Detection”, arXiv:2001.04634. 2020.

7. X. L. Kaltenegger, Z. Lin and J. Madden, ““High-resolution Transmission Spectra of Earth Through Geological Time”, Astroph. Lett., 2041, 2020.

8. Y. V. S. Meadows et al., “Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment“, Astrobiology 18, 620, 2018.

9. V. Bickel V. et al., 2020 Impacts drive lunar rockfalls over billions of years, Nature Communications, 11:2862 | https://doi.org/10.1038/s41467-020-16653-3

10. R. Malhotra, “Case for a Deep Search for Earth’s Trojan Asteroids”, Nature Astronomy 3, 193, 2019.

11. M, Ćuk, D. Hamilton and M. Holman, “Long-term stability of horseshoe orbits”, Monthly Notices Royal Astronomical Society, 426, 3051, 2012.

12. F. Marzari, H. Scholl, “Long term stability of Earth Trojans”, Celestial Mechanics and Dynamical Astronomy, 117, 91, 2013.

13. Zhou, Lei; Xu, Yang-Bo; Zhou, Li-Yong; Dvorak, Rudolf; Li, Jian, “Orbital Stability of Earth Trojans”, Astronomy & Astrophysics, 622, 14, 2019.

14. R. Dvorak, C. Lhotka, L. Zhou, “The orbit of 2010 TK7. Possible regions of stability for other Earth Trojan asteroids”, Astronomy & Astrophysics, 541, 2012.

15. P. Wiegert, K. A. Innanen and S. Mikkola, “An Asteroidal Companion to the Earth”, Nature, 387, 685, 1997.

16. J. Benford, “Looking for Lurkers: Objects Co-orbital with Earth as SETI Observables”, AsJ, 158:150, 2019.

17. M. Morais and A. Morbidelli, ‘The Population-of Near-Earth Asteroids in Co-orbital Motion with the Earth”, Icarus 160, 1, 2002.

{ 50 comments… add one }
  • Talminder singh bhola April 16, 2021, 13:31

    I am a common layman.
    Just curious, can we develop some technology so that we can use the gravitational waves or other cosmics waves or neutrinos to drive our space crafts.

  • Alex Tolley April 16, 2021, 13:52

    While the search for lurkers seems attractive, I find the idea of passing stars a red herring. The frequency of “close” passes and the unknown density of technological civilizations means that if sparse, the passing stars are likely uninhabited. As we are already considering interstellar craft, whether beamed sails or more exotic propulsion technologies, it seems to me that any technological species will simply send out their probes as fast as possible and live with the speed of light limitations if no FTL communication or flight is possible once telescopic observations have indicated likely target worlds. Waiting thousands of years to pass by a star, which may or may not have life, seems like an unlikely scenario, especially if probes can travel at decent fractions of c.

    I am sure that some simple math would indicate at what density ETI must exist to make close passes a better strategy than just sending out probes for the distances required. My intuition is that this density is high. So high that we might just detect them telescopically as soon as we look with instruments of sufficient resolution and capabilities.

    If lurkers are deliberately camouflaged, like terrestrial hides, I suspect they would be hard to detect. They might look like rocks, or be buried in the lunar regolith. They might look like asteroids if lurking in a gravitational stable/metastable position. Or they might be stealthed in other ways. For example, we already have radar stealthed aircraft that if left at the L5 point would be extraordinarily hard to detect either by radar or optical means. If the lurker is dead and inert, it certainly cannot respond to signals, and therefore requires a painstaking search.

    As Douglas Adams said: “Space is big, really big…” The solar system, even from the asteroid belt inwards would take a very long time to search. It would make those police lines to search for evidence across miles of the countryside for evidence seem trivial by comparison.

    Let’s hope that any probes want to be found and make themselves conspicuous even as inert objects. A large magnetic field seems an obvious choice…

    • Mike Serfas April 16, 2021, 19:58

      I don’t think the premise here is that implausible. It is possible that technological civilizations manage to destroy themselves before they have launched probes to more than a dozen systems – probes that seem ambitious enough merely in reaching an orbit in a nearby star system, without many finer considerations of camouflage.

  • Gary Wilson April 16, 2021, 13:53

    A bit of an aside here but we have been discussing in a previous post whether or not the Oort Cloud actually exists. Could Scholz’s star have removed most of the Oort cloud objects as it passed through?

  • Mike Serfas April 16, 2021, 14:11

    What is the best compilation you could find of stars that have passed near Earth? Wikipedia writers use a chart based on “The Close Approach of Stars in the Solar Neighborhood”, Quarterly Journal of the Royal Astronomical Society, volume 35, 1994 … which might as well be the Paleolithic compared to modern astronomical resources.

    Besides probe-builders are checking up on their work, a long history of passing stars might help us understand why light pollution does such little damage to the ecology. Not that it isn’t bad enough already, but I would think that if we hadn’t spent a few millennia with some overly bright star in the sky every now and then, many organisms would be much more hard-wired in their expectations for night.

    • Mike Serfas April 19, 2021, 21:52

      I should thank James Benford for coming back and responding below! It was Ref. 4 and sources citing it. I see why he used the older illustration though – Ref. 4 does a fair amount of ‘fearing to tread’, presenting a table with daggers to indicate which entries are particularly likely to be bogus. Sometimes it is nice to have a canon you can shoot something with, even if you have doubts about its accuracy. In any case, the paper ( https://arxiv.org/abs/1805.07581 ) develops a model with perihelia fairly randomly arranged in the plane, so within a certain distance the odds are proportional to the inverse radius squared: encounter rates of 78.6 +- 8.7 per Myr within 2 pc, and 19.7 +- 2.2 per Myr within 1 pc.

      The authors didn’t say this, but I suppose if you wait a million years and something should come within half a light year – wait four, and cut that in half again. (As it happens, we are nearly into the lucky leap megayear – see https://en.wikipedia.org/wiki/Gliese_710 ) The above is true if you can assume the encounter rate is constant, but they are at 1.5 sigma for a change. Ignoring that, I get that sometime in Earth’s history a star should have passed within 500 AU. Must have been a show!

      A 2019 paper working from this base ( https://arxiv.org/pdf/1911.01735.pdf ) points to C/2002 A3 as a comet perturbed by the passage of HD 7977, a G0 dwarf (1.1 solar masses), passing 1.3 light years away, 2.8 million years ago. A comet that nominally had an eccentricity of 0.25, before the star passed within 740 AU of it!

  • Ron S. April 16, 2021, 14:13

    “The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.”

    I disagree. If these probes are indeed Lurkers there will be steps taken to prevent detection. Like our own geosync satellites that approach their normal end of life, the last of their fuel is used to eject them from their orbit to place them safely out of the way.

    Of course this assumes they don’t want the probes to be found. Or perhaps they don’t care because they see no risk to themselves from accidental discovery of the probes.

    As is usual with all such questions there is an overabundance of assumptions, and that is likely to persist for quite some time. The questions are nevertheless interesting even when they are not immediately addressable.

  • Jeff Wright April 16, 2021, 15:25

    Titan has fuel-and I still half expect front-end loaders on Miranda

    • Robin Datta April 17, 2021, 2:31

      Sans O₂, hydrocarbons ain’t fuel – but in an atmosphere of hydrocarbons, a tank of O₂ is fuel.

  • Astronist April 16, 2021, 15:27

    Two questions.

    (1) If the “lurker” is sent to do scientific investigation of Earth, it will need an orbiter in low polar orbit for global mapping and one or more landers to gather ground truth, including the most pressing science for any investigator interested in life, which will be analysing the DNA of living organisms. Why then do you want to examine the Moon and the Earth-Sun trojan orbits?

    (2) If an alien civilisation was interested enough in the Solar System to do scientific investigation here, what was there to prevent it coming here and colonising the system? Note that natural selection will preferentially populate passing stars with colonising cultures rather than sedentary ones.


    Stephen A.

    • Robin Datta April 17, 2021, 2:40

      Satellites in orbit may have limited lifetimes. Probes placed on the surface of the Earth might corrode away unless made of noble metals such as Au or Pt.

  • Jens Fridthjof Knudsen April 16, 2021, 16:09

    A thought experiment – if there was a star system passing by about 1 light year from us, would we be talking about sending something crewed there? If so, if there is such a thing as an advanced civilization out there, maybe they think the same way? If they have, there might be a few star systems around that might not harbour habitable planets in the usual sense, but there might still be ‘civilized worlds’ present in the system, one way or the other. And they might not be so much in a hurry to reach really distant star systems from theirs, as settling just one might take some time. So they simply take their time and use the opportunities as they present themselves. Anyway – if so, we could be looking at star systems that might not be so habitable, and they might have advanced cicilization still.

    • Robin Datta April 17, 2021, 3:05


      With stone age technology a species evolved for tropical rainforests and perhaps temperate grasslands – Homo sapiens – adapted to every climate but did not inhabit the antarctic, perhaps due to remoteness and a lack of megafauna.

      With different molecular machinery, different evolution and different technology, the uninhabitable might be habitable.

  • Gary Wilson April 16, 2021, 18:57

    As far as lurkers are concerned, I think they are not worth spending resources on. With all due respect if advanced ET’s want us to find an artifact they will make it abundantly easy to do so; and if they don’t we won’t find anything. A crash site of some type or accidental loss of an alien probe that we then find is entirely different and probably doesn’t qualify as a lurker, although we could end up splitting hairs about that. If there are such things and we end up finding one it will be extremely serendipitous and not to be planned on.

    • Alex Tolley April 16, 2021, 22:08

      The Moon survey seems inexpensive to me. Software to scan the images of the surface and flagging possible anomalous objects. At some point, we will have similar large archives of high-resolution images of asteroids and other celestial objects that we can scan and analyze automatically.

      Surely the point is that if we don’t even look, we cannot find anything. I don’t think we need to initiate an expensive search program, but we should piggyback artificial object detection on existing missions, much as SETI piggybacked on radiotelescope observations to look for ETI signals.

      • Gary Wilson April 18, 2021, 13:46

        Actually that seems right Alex. Here I am participating in Planet Hunters TESS and not thinking about how such a resource could be used to cheaply do a Moon survey looking for lurkers. My bad. :)

      • Joe H. April 21, 2021, 17:52

        And if we actually found a lurker on the moon it would justify NASA’s current “make work” project to land a woman on the moon in the next few years.

  • Martin April 16, 2021, 20:41

    At the risk of biting my own tail:
    Assume that NL is greater than or equal to 1, because if it isn’t we may find lots of neat stuff, but we won’t find a lurker.
    This implies that Fip is greater than or equal to 1/500,000 to 1/130,00 i.e. 2e-6 to 7.7e-6.
    Given the geologic time scales involved these don’t seem unreasonable, but the question remains: How many technological space fairing civilizations would do this, that is, what is F(prime directive)?

  • Robin Datta April 17, 2021, 3:22

    Lurkers placed on the moon might be located to give an unrestricted view of the earth and a communications line-of-sight to their home base or relay station away from earth, on a plane perpendicular to the moon’s orbital plane, and tangential to the moon at the earth-moon axis. Such an area on the moon at a fair elevation could be of particular interest.

  • torque_xtr April 17, 2021, 5:14

    SETA is wonderful direction of research. IMO, finding artifacts is by far the most probable way to break the state of knowledge about ETI. And we don’t need to consider “they are hiding”. While it seems reasonable that any exploratory mission would obey some codes (like non-contamination), it is very unlikely that all explorers intend to camouflage their missions and that indeed all of them _succeeded_ in that. An opposite assumption just doesn’t pass Occam’s Razor. We won’t find Lurkers, by their design, but the vast majority of ETAs would be long-dead and inert space debris. So the most straightforward way to estimate the probability of finding ETAs is just to estimate the total accumulated mass of artificial material in an “interesting” stellar system. It depends on many unknowns but impressive numbers can result even from assumption that civilizations use means that are surely feasible, and are constrained by them. Like, they use gravitational lensing to study systems and identify interesting ones, but they can study xenobiology only in-situ and use thermonuclear propulsion to send probes. If any habitable system receives an expedition in ten million years, corresponding to appearance of a civilization with explorative lifespan of 10000 years within 50 light-years every 10 MYr during the last 4 Gyr, then I guess the total mass of ETAs in the Solar system is in the range of thousands to billions of tons. Of course, Archaean Earth was nothing to look at, but it was not alone. There was early warm and wet Mars and still-not-boiled-dry Venus – three worlds that looked habitable from afar instead of the current single one. That does not count possible extinct or abandoned colonies.

    Of course, it is the needle-in-haystack task, but at least the needles and the haystack are not many parsecs away! They are within reach of observation and exploration ranges of our current and near-future instruments. On decameter-scale, LSST or another survey mission could look for objects in the asteroid belt with unusual light curves, suggesting non-natural shape. On cm-scale, as it was said in the article, an automated lunar orbiter can continously take images of surface and look for something weird by an on-board neural (calibrated and/or validated by Apollo and other landing sites), sending images and coordinates to Earth for futher inspection if something shows above threshold. (just occured to me, any object that emits it’s own light in the lunar night would be quite interesting and very easy to detect, though living probes are much rarer thing and a whole another cause). A fleet of small solar-sail or electric powered satellites can disperse in the asteroid belt, wandering from one object to another, scrutinizing them for scientific and mining purposes and looking for something unusual on their surfaces or in their shapes.

    And all of this is near-term even compared to Breakthrough Starshot, and profoundly more promising than search for distant technosignatures. Even if we find CFCs in transit spectrum of earthlike world, or identify solid-state laser emission from a cloud around a distant star, we won’t be able to do anything about it in centuries, but even a single 2 GYr-old piece of alien junk from asteroid belt is a treasure trove in our hands.

  • Mark April 17, 2021, 10:39

    To be able to develop a space travel, an alien civilization
    should develop one feature called “creativity”. Otherwise
    it would be just an animal kingdom. How long they
    could stay in this state nobody knows. ..might be billions
    of years. We cannot say what sparks creativity either.

  • Alex Tolley April 17, 2021, 12:35

    “Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue green planet whose ape-descended life forms are so amazingly primitive that they still think digital watches are a pretty neat idea.”

    ― Douglas Adams, The Hitchhiker’s Guide to the Galaxy

    After a few hundred years of industrial civilization, we seem to think manufactured space craft with semi-smart software are a pretty neat idea. Which makes us think other civilizations will send Bracewell probes (“lurkers”) to our system. We think up ingenious ideas to reduce the effort needed, as well as what assumptions are needed to detect live or dead ones.

    However, the longest living “artifact” is staring us in the face – life iteself. It has a near 4 billion year history of replication that has withstood at least 5 major catastrophes, but each time created new phenotypes that have increased the richness of the terrestrial bisophere. It has even managed to evolve a species that can consider sending machines and life to other star systems.

    As far as we know, all terrestrial life has a single common ancestor, there is no “shadow life” detected. This implies that life either originated on Earth, or if seeded from elsewhere, was done once only, and never repeated. The only counterargument I can see that would allow other seedings is that all life in the galaxy is based on the same fundamental biology, or that other civilizations know of our type of biology and send only compatible seeds, and that subsequent seedings just integrate with the existing biology.

    If we become a stable civilization that can live sustainably for millions of years, it might make sense to seed other, sterile worlds, with terrestrial life that can be sent as dormant spores, eggs, and seeds, and let evolution do its work to evolve new forms and build rich biospheres. We could start in the relatively near future, adapting whatever advanced propulsion technologies we have to deliver payloads of dormant life to worlds ready to be “greened”. Such a program won’t be a simple as sprinkling some bacteria to start a culture, as we may want to add multicellular life too, and that may require some effort where species need to be co-existing to survive.

    Lastely, let me return to the idea of multiple seeds of Earth but with compatible biology. Imagine a galactic library with catalogs of worlds and their basic biology, wrested by long ages of exploration by probes. A civilization could select the biology needed for Earth and send some “firmware” updates to enhance the existing life. How might this be done? Using retroviruses that can insert their new instruction sets into the DNA of host species. Hoyle may have been wrong that comets deliver viral epidemics, but just maybe deliberate introduction of viruses into the terrestrial biosphere is possible, to push evolution by “punctuated equilibrium” and shorten the time needed to “uplift” a species to join the galactic club.

    • Robin Datta April 17, 2021, 18:37

      The transition from ape brain and intelligence (chimpanzee IQ ~60, subsaharan human IQ 80+) is a phenomenon that has not been adequately explained. Perhaps a firmware upgrading from afar as suggested by Alex Tolley did the trick.

      • Joe H. April 21, 2021, 18:05

        “The Runaway Brain” by Christopher Wells does a pretty good job of explaining the prehistoric growth in human intelligence without the need for aliens.

    • Joe H. April 21, 2021, 17:56

      We don’t need to go to the stars. We could just seed Venus with only slight advances in our technology.

  • Michael Fidler April 17, 2021, 15:22

    The other side of the coin is that such objects are common and would destroy themselves so as not to give information to immature cultures such as ours. Perhaps all those iron/nickel objects that we keep finding falling from the sky are the smelted remains of ET’s lurkers.

  • Carl Pham April 17, 2021, 15:31

    I don’t think any advanced technological civilization would bother to send physical artifacts. What for? A space-based telescope with an aperture of 150,000 km or so could resolve the Solar System down to 10cm pixels at a distance of 1 ly. That’s certainly a BIG telescope, but (1) building one is probably not a lot harder than building a bunch of interstellar probes (and the scopes to receive their transmissions), and (2) you only need to build ONE, and then you can use it for all kinds of things, and all kinds of observations. Plus you keep your technology right next to you and it’s easy to repair or upgrade.

    • Alex Tolley April 18, 2021, 3:06

      Why do we build probes to inspect other worlds in our system. There are so many things that require physical contact to understand. For example, you cannot find out much about biology just by looking at a planet through a telescope. You need samples to dissect and analyze.

    • EricSECT April 18, 2021, 3:18

      Carl, that is an excellent point!

      Building a crazy big telescope has gotta be orders of magnitude easier than pursuing interstellar propulsion (….with travel times in a reasonable time frame…. lets just say 100 years to Proxima).

  • Henrik Nilsson April 17, 2021, 15:42

    >The vast majority of these photos have not been inspected
    >by the human eye. …develop AI algorithm… …low cost

    What about making a Zooniverse project (https://www.zooniverse.org/) out of them? Then we would not only be talking “low cost”, but virtually “no cost”. I’d be happy to look through a couple of photos per day, and I’m guessing others would be too. The Zooniverse user base is no laughing matter, and many “comparable” photo-based project are available right now.

    Having those Moon photos collecting dust seems like the wrong thing to do.

  • Dimjo April 17, 2021, 16:30

    Is there anyone looking for or planning to look for signs of Lurkers phoning home? My assumption is that Lurkers would lie dormant and either periodically phone home or phone home if they encounter technology like a radar scan of the trojan they are sitting on or nuclear weapons test or use. Is there any way we could look back and see if there was any evidence of such activity coincident with North Korea underground testing? A one time signal would be ignored. But if a signal was picked up twice – each time following a underground detonation?

  • Steve Muise April 17, 2021, 19:51

    Searching the lunar surface imagery database is a fairly cheap thing to do, but what do we tell the computer to look for? Training the deep learning system with images of our own hardware on the surface induces a bias; “I don’t know what that thing is, but it doesn’t look like a LEM descent stage or a Lunakhod, so I’ll keep looking”…

    Lurkers may not even be hidden. Non-interference/staying out of sight are OUR cultural values, they may not necessarily apply to visiting aliens. Especially if, at the time of the visit/observation, there’s no apparent threat from we Earthlings.

    • Gary Wilson April 18, 2021, 13:52

      To the first part of your post Steve, we could use volunteer citizen scientists to manually look at millions of images of the Moon just as we do when hunting for exoplanets. I don’t have all the answers as to how it would be set up but I would imagine looking for any object with geometric shapes such as right angles, perfect circles or spheres, antenna-like structures etc. etc. I would gladly participate. :)

  • Abelard Lindsey April 17, 2021, 23:46

    What about the dyson swarm out in the Kuiper belt? Can we see that, if it existed?

  • Pathcoin April 18, 2021, 7:36

    The answer: 0 to essentially infinity. As our understanding of the evolution of life on Earth increases, and assuming that this is the prime template for the evolution of complex organism, the likelihood d of finding intelligent life on other planets markedly decreases.

    More practically speaking: if there is an advanced civilization that can find a way to traverse the universe in real time, we had better hope they are friendly.

  • Henry Cordova April 18, 2021, 11:48

    As with so many other issues aired on this forum, the question we should be discussing here is not whether or not this scenario is possible,
    (it certainly is), but whether or not its existence or resolution is probable.

    The Solar System is a very big place, and on astronomical time scales it is constantly changing and evolving, usually in locally destructive circumstances. Even with fast and cheap technology, it would take eons to explore it thoroughly. Unless a Lurker has been deliberately designed to be long-lived and easily found, its ruins or derelict is not likely to be discovered by our level of technology. It it has survived, it is probably deep inside a planetary crust, at the bottom of an ocean, encased under kilometers of ice, or drifting in orbit billions of kilometers away in the cold and dark, indistinguishable at a distance from any of an infinity of other lumps of ice, metal or rock.

    Any search for these guys should be based on these realities, and hopefully piggy-backed onto some other research program with a higher probability of success.

    We need a space program because one day we will likely stumble onto just such a discovery of immense practical, scientific, or philosophical value. But we should not be planning our explorations with the idea of turning any one of them up specifically.

    It would be counter-productive and a squandering of resources. And worse, to our impatient species, it would provide us within a very short time an excuse to cease all our explorations. The Fermi Paradox comes to mind. Is it a valid point? Certainly. Is it a good reason, after less than a century of cursory and superficial searches, to cease searching altogether? Certainly not.

    • Ron S. April 18, 2021, 13:55

      This is needles in haystacks where there are many large haystacks and there may be no needles. If there are needles there is no reliable reason to believe they are hanging out in haystacks. But, all we can see is haystacks and so…

      “piggy-backed onto some other research program”

      …that’s about the best to be hoped for under the circumstances.

  • James Benford April 18, 2021, 15:51

    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 that will be given here on Tuesday is a way of discussing the SETA strategy and comparing it to SETI. I maintain that SETA is a credible strategy.

    Many suggestions here make a false assumption: assuming that all aliens act the same way and that you know what that way is. Alex and others should have a look in the Drake equation paper, reference 2, a version of which will appear on this site on Tuesday 20th April. You’ll see a number of scenarios worked out using varying assumptions about alien technology and intentions. Then perhaps you can reconsider what you’ve written here.

    As for camouflage, as several others as suggested, think of it this way: how many of our probes have had camouflage? Answer: none. Why? Because we don’t care if someone finds it and we don’t want to waste mass on unnecessary items. Bringing camouflage across interstellar distances is unlikely to happen.

    Alex appears to be assuming I’m suggesting searching the entire inner solar system. Far from it. I’m proposing specific locations, mostly small objects, which can be inspected with existing technology.

    Others say Lurkers would reside close to Earth as is practical, low earth orbit or Geo. They are not attractive possibilities. Low Earth orbit is stable for a time of less than a century, so would be gone soon. Geosynchronous is stable 1000-10,000 years. So both are short-term orbit on the long times we could have been visited for. Moreover, we have a complete map of everything in geo orbit. If there’s something alien there, we already know it. That would’ve leaked out by now so it’s not credible. And landers will be destroyed quickly by Earth’s environment.

    Mike Serfas: Consult reference 4 and later citations of it.

    Torque xtr: I completely agree!

    Steve Muise: Software inspections of LRO images are already being done for rockfalls (reference 8) and for artifacts (reference 5).

    Henry Cordova: Studies have shown that the artifacts we have on the Moon will last millions of years.

    • Alex Tolley April 18, 2021, 19:15

      As for camouflage, as several others as suggested, think of it this way: how many of our probes have had camouflage? Answer: none. Why? Because we don’t care if someone finds it and we don’t want to waste mass on unnecessary items.

      As a physicist, you don’t consider that what you study will respond to your experiments with agency. AFAIK, not one of our space probes was expecting to meet with any organism or artifact with agency. Under those circumstances, camouflage is a waste of mass.

      However, this changes if you want to study anything with agency. If you have ever fished by a river or lake, you know that you wear drab, brown/green clothing, and stay very still. If you want to observe wildlife up close, you sit in a hide, make no noise, and position the hide downwind. Any probe that is intent on closer study of a living Earth will want to camouflage itself in some way to prevent its presence from being detected by the organisms it wishes to study. If course, if you want to fish with explosives, or in an analogous case the probe can paralyze or kill anything it wants to sample and ignore other study types, then it probably doesn’t need camouflage. [C.f. fictional UFO abductions.] If the camouflage is active, like cephalopod or flatfish skin, then a dead probe will possibly no longer be camouflaged.

      Whether a probe is camouflaged or not will depend on its purpose and whether it is still active or not. Given our current engineering technology and your assumption about probes arriving with close approaching stars, the logic is that any visiting probe is long dead and will be an inert artifact. Change those assumptions and the camouflage issue is not moot.

      I am very much looking forward to part 2.

  • Michael Simmons April 18, 2021, 21:04

    I’d like to know how many ort clouds our sun has passed through and if the two systems trade comets and other ort cloud bodies.
    The earth is visible as a transiting body for stars lying along the ecliptic. How many nearby stars and for how long? is this a more likely source for contact than passing stars.

  • Alex Tolley April 19, 2021, 11:45

    Reference 6 is an interesting start. An unsupervised ML algorithm was trained on Apollo 17 images from the high-res LRO images (0.5-1.5 m/pixel). The algorithm was able to detect the Apollo 15 LM descent vehicle from images around that site. Their metric suggests that they can reduce the search requirements for human inspection 50x. Clearly, this is a promising start to do the proposed lunar analysis for lurkers.

    Reference 5 is a review of possible technosignature features to look for. Davies suggests both ML to prune the image set, and crowdsourcing human volunteers – albeit that needs work to define what to look for.

    There is a website Moon Zoo that crowdsources volunteers to look for specific natural features. Adding features to look for might be bolted onto this site’s search.

  • Alex Tolley April 19, 2021, 12:38

    This paper is a report on a NASA workshop on technosignatures. Section 4.3 pp31-32) deals with alien probes to our system and includes “lurkers”.

    N. Technosignatures Workshop Participants, NASA and the Search for Technosignatures: A Report from the
    NASA Technosignatures Workshop, arXiv e-prints (2018) arXiv:1812.0868

  • Harold Shaw April 19, 2021, 14:33

    Yes, ET could be in “our” backyard. The probability of a 100% explored Milky Way is higher than a 100% colonized. The energy required to explore the galaxy is less then that required to colonize. Colonization would require some amount of prior exploration. If time is less of a concern, the cost to produce 1 pixel to 10 cm resolutions at 1 light year with a probe is less than a telescope. Telescopes could never deliver a probe’s broad spectrum of data. One of the greatest potential threats to a people will be other people. It is rational game play to observe other players. Survive its evolution and mature biotech will allow for self-replicating probes, mass production of non-replicating probes or space ship people

    Stealth can not be considered a strictly anthropological motivation. Too many non humans employ stealth. It is used so frequently, it can’t be considered a niche strategy either. Remaining unseen offers many practical benefits. The behavior of the observed isn’t polluted. Once the observation tools are discovered, the observed will potentially end the experiment by interfering with the tools. If you are a space ship person, do you think humans would respect your person hood?

    I wouldn’t simplify modeling by assuming all probes sent by a people would be similar. The probe sent to a system with a high likelihood for life or intelligent life could have different capabilities, like stealth. I would even make the case that our existence increases the odds a probe is in the Solar system.

    Local probes or space ship people are a safer target for METI. The pro METI arguments that are actually gaslighting, ring true for local observers. Keep the message quiet and there is no risk of revealing our presence or of influencing another people.

    Lastly, the possibility a probe could be a person can not be discounted. Any action plan for searching and interacting with probes must deal with the potential.

  • Harold Shaw April 19, 2021, 14:43

    The novel “Existence” by David Brin offers an interesting take on the variety of probes and motivation possible. It is difficult to discuss the treatment of probes without spoiling. Definitely recommend it.

  • Alex Tolley April 20, 2021, 14:04

    Re: Stealth and culture.

    I’m reminded that the iconic view of interstellar travel and contact in the US is Star Trek. [Semi] military vessels, crewed like warships, with weapons. Crew members on away teams carry hand weapons (phasers) that are used liberally. The starships have shields against weapons but make no attempt to shield themselves from sensors.

    In the UK, the iconic series is Doctor Who. The TARDIs is small (on the outside), has few passengers, and the Doctor has a rule about no weapons. A working TARDIS has a chameleon circuit to make the ship blend in with its surroundings. It seems to have broken after selecting a 1950s/1960s police box. The Doctor’s nemesis, the Master, usually travels in a TARDIS that has a working chameleon circuit.

    I just wonder if the difference in consideration of probe stealth might not be cultural?

    • Joe Hale April 21, 2021, 18:20

      I suspect production budgets are also a reason why one show got a believable starship and one got a telephone booth.

  • ljk May 4, 2021, 11:31

    Breakthrough Listen Searched for Signals From Intelligent Civilizations Near the Center of the Milky Way


    The Breakthrough Listen project has made several attempts to find evidence of alien civilizations through radio astronomy. Its latest effort focuses attention on the center of our galaxy.

    The idea behind Breakthrough Listen is that if alien civilizations are out there, they probably emit radio signals either intentionally or unintentionally. Most of their work has focused on observing stars with potentially habitable planets, the idea being that just as we emit radio signals, so do they. But by looking at the center of our galaxy, they’ve begun to search for more ambitious aliens.

    The central region of our galaxy is a great place to point your telescope if you want to listen for signals across thousands of stars. It’s the region of the Milky Way where stars are most densely clustered. The downside is that the center of the Milky Way falls outside the galactic habitable zone.


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