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A SETI Search of Earth’s Co-orbitals

One objection to SETI is that it is not falsifiable — there is no point at which a lack of signals can prove that extraterrestrial civilizations do not exist. But there are some aspects of SETI that can be falsifiable. Consider a class of objects near enough for us to investigate not only with listening efforts but with probes, one small enough to be thoroughly covered, and one most people know almost nothing about. Could these offer a listening post for ‘Bracewell probes,’ a way of watching the development of our culture and reporting home to ETI? And if so, could we combine SETI with METI to advance both disciplines without compromising our own security?

If the idea of nearby probes seems far-fetched today, it was even more so when Ronald Bracewell advanced his ‘sentinel hypothesis.’ Bracewell took the question of SETI and stood it on its ear. That was no mean feat in 1960, for SETI was just being born in that year through the efforts of Frank Drake at the Green Bank instrument in West Virginia. While Drake was, reasonably enough, asking whether we might pick up signs of an extraterrestrial civilization around another star, Bracewell had begun to wonder whether there might be a different way to study an alien culture. A long-lived probe could be planted in any system under investigation.

Image: Stanford’s Ronald Bracewell, who in 1960 advanced the idea of long-lived probes investigating other planetary systems. Credit: Stanford University.

Add Von Neumann-style self repair and such an object might stay on sentry duty for millennia, for aeons, all the while returning useful data about the changes occurring on an interesting habitable planet. And if a civilization arose on that planet and reached the level of electromagnetic communications, then the probe could be programmed to make contact, at whatever threshold its builders chose.

Jim Benford has been thinking about Bracewell probes and their possibilities of late because they offer advantages over traditional forms of SETI. For one thing — and this is a huge advantage — a contact once made with a local probe could initiate dialog in more or less real time, without interstellar lightspeed delays, although of course we would be querying an intelligence that was itself subject to those delays if it communicated with its home world.

Image: Plasma physicist Jim Benford (Microwave Sciences).

The question is interesting enough that is has inspired some top-notch science fiction, in particular David Brin’s novel Existence, where the idea is extended to not one but a series of different probes at work in Earth orbit and in the asteroids. But it would be Michael Papagiannis who in 1978 wrote seriously about the asteroid belt as a possible venue for such ‘lurkers,’ (to use Benford’s term). Benford is not sold on the asteroid belt as a target.

For we also have an all but undiscussed body of targets that can be called co-orbital objects with Earth. These small objects approach the Earth closely and on an annual basis, for they have the same orbital period as Earth. We might study them for signs of artificiality through spectroscopy in the visible or near-infrared as well as pinging them with radar or other signals.

What strange orbits these objects occupy, with some in so-called ‘horseshoe’ orbits — these can actually become quasi-satellites for a time before returning to earlier orbital parameters. Have a look at a horseshoe orbit.

Image: A horseshoe orbit. No wonder these objects took so long to find. Credit: James Benford.

And from another view:

Image: Plan showing possible orbits along gravitational contours. In this image, the Earth (and the whole image with it) is rotating counterclockwise around the Sun. Credit: Wikimedia Commons.

As Benford explains, think of a quasi-satellite as an object in a 1:1 orbital resonance with a planet, so that the object stays close to the planet over many orbital periods. Outside the Hill sphere (that region where an astronomical body dominates the attraction of satellites), quasi-satellites cannot be considered true satellites. Instead, while their period around the Sun is the same as the planet, they seem to travel in an oblong retrograde loop around it.

Beyond horseshoe orbits we also find ‘tadpole’ and ‘quasi-satellite’ orbits as shown in the figure below. Here we find stable orbits for centuries and possibly longer, much longer. Co-orbitals include Cruithne (3753), a 5-kilometer object with closest approach to Earth of 0.080 AU — interestingly, this one experienced a close encounter with Mars in historical timescales, around the time of Periclean Athens. Another is Earth Trojan 2010 TK7, which oscillates around the Sun-Earth Lagrangian point L4, and 2016 HO3, which Benford describes as “currently the smallest, closest, and most stable (known) quasi-satellite of Earth,” with a minimum distance of 0.0348 AU. A number of other quasi-satellites are known.

Image: Three types of co-orbital orbits. Credit: James Benford.

How might we investigate these objects with the tools of SETI, and why? Benford calls for a multi-year program of observations in radio and optical wavelengths as well as planetary radars, with the main burden of the work falling upon the Lick Observatory and other platforms involved in the Breakthrough Listen project. Here we’re looking for size, shape, rotation periods and spectra. At the same time, he urges SETI observations of this range of objects.

Planetary radar also comes into play, and with an interesting consequence. From the paper:

These objects have not been pinged or imaged by any planetary radar as yet. Recent developments in planetary radars have shown they can detect the presence and trajectories of spacecraft in lunar orbit, even though their size is a few meters. Whether these radars are sensitive or powerful enough to get a return signal from any of the presently known co-orbital objects requires analysis. In any case, they can ‘ping’ the objects, meaning that a signal reaches there but the return signal may be too weak to detect at Earth.

Here the Bracewell idea comes into full view:

If there is an ET probe there, it might sense that it had been noticed by us.

What an interesting campaign Benford has in mind. It includes simultaneous use of planetary radar on the target and SETI observations. Readers at this point may be recalling that Benford is on the record with strenuous objections to METI, the idea of Messaging to Extraterrestrial Civilizations, given the limits on what we know about what is around us in the cosmos, and the need for international agreement on how to proceed, as opposed to sending signals to the stars in random bursts of activity and with wildly varying content.

Yet here we are talking about an activity that, in the unlikely event there is a probe in our own Solar System, could conceivably activate it and cause it to respond. The Bracewell probe is front and center here, recalling Duncan Lunan’s 1974 proposition that a Bracewell probe could be the cause of long-delayed echoes of radio transmissions heard in the 1920s. Benford notes that the phenomena Lunan identified have subsequently been explained as unusual propagation patterns in Earth’s magnetosphere. But it’s interesting to see Benford’s response to the idea of METI in this new context:

This would be ‘Active SETI’, which could solicit a response from a hypothetical probe. This does not incur the objections to sending interstellar messages, messaging to ETI (METI), because any such alien lurkers would already know we are here. Of course, this is at very short range compared to the interstellar ambitions of METI enthusiasts. We presume that Lurkers already know that we have radar, but might not respond to a single radar painting such as we have done to many asteroids. If we want to send a message, as Paul Davies suggested for the LaGrange points in 2010, how would a signal be designed to elicit such a response?

An interesting question indeed, and as the author points out, actually working on a near-term use of METI at a nearby target could benefit research into message creation and drive the field forward. The problem is an easy one to state: What kind of message would one send to a lurking probe that would ‘awaken’ it to the possibility of communicating with us?

As to falsification and SETI:

In my view SETI has suffered from being seen as somewhat nonscientific. That’s because it doesn’t offer itself as a study with falsifiable propositions, which is the very definition of science, as Popper said.

I advance a falsifiable proposition: “There exist in near-Earth space extraterrestrial probes which are observing Earth and it may be possible for us to find and contact them.

This proposition can be disproved. We can observe them, ping them with radar, transmit messages to them, send robotic probes to them and visit them with human spacecraft missions.

What a lively concept. We blend SETI’s listening to the stars with astronomical imaging and spectroscopy, while simultaneously turning METI into what Benford calls a ‘local experiment.’ And as we do this, our efforts at studying co-orbital objects advance the cause of astronomical science, which is engaged in the great process of mapping the entire Solar System.

The paper is Benford, “Looking for Lurkers: Objects Co-orbital with Earth as SETI Observables,” submitted to the Astrophysical Journal (preprint).

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{ 46 comments… add one }
  • David March 22, 2019, 19:21

    Popper was just one view of Philosophy of Science. Some current thinking simply says science is repeated observation and or experiments and accumulation of evidence.
    Let me give ypu an example. Ancient astronauts a priori I cant find anything wrong with it. So lets look at the evidence.
    There just isnt any. There were some tidbits but they all turned out to have very interesting explanation but not ancient astronauts.
    it wasnt the idea of ancient astronauts that was bad its that so many proponents use discredited data.

  • Alex Tolley March 22, 2019, 20:08

    1. It isn’t clear to me that local probes are a falsifiable hypothesis. They may be here but too well hidden to find. We might have to search not just local space, but perhaps all the way out to the Oort cloud. To use Clarke again, they may be one buried on the Moon waiting to be dug up. As Existence has been mentioned, they could even be here on Earth, buried in the rocks or even in artifact collections. The more our technology miniaturizes, the more we have to consider at what scale they may be.

    2. METI concerns. As we move steadily towards an interstellar flyby probe and telescopes capable of imaging exoplanets and detecting artifacts, I seriously wonder how we can believe that advanced ETI hasn’t already established the state of our planet. And if they are the senders of the hypothesized probes in our system, is METI really about shouting into the jungle to an ETI that did not know of our existence?

    3. What to make of ʻOumuamua in this context? Posited as a possible interstellar probe, it ignored all our pings. What if there is a probe in our system, exactly in the co-orbital location…but it is dead? Silent to all our remote probings. We might need our own probe to sidle up and inspect it, showing that it is artificial and not a lump of rock.

    4. In my copy of Frontiers of Science in the section “The Giant Leap” there is an illustration of a possible Bracewell probe. All very SciFi prop. But in reality, why would they not be camouflaged in some way. They could stay in orbit but be unobservable with em radiation. They might even put down landers in inaccessible areas, invisible to all but the closest inspection. There is no need to assume that they stay in our system and continuously monitor conditions. They may make inspection rounds of a number of habitable worlds.

    We should certainly look for them using a number of approaches, but I suspect the results will be as fruitless as the SETI search to date.

    BTW, what do the SETI folks think of this? I recall suggesting that there might be local probes to Shostak over a decade ago, but he dismissed it. Maybe he thought he was being sucked into a UFO discussion and wanted to extricate himself. ;)

    • James Franklin March 26, 2019, 7:43

      We need to address this with logic. Let’s assume we have the technology to create “Bracewell” probes to lurk in a system and report back.

      No matter how advanced you are there is a cost, perhaps not in financial terms we use, but in resources. You would not build a million probes and launch to a million planetary systems, you would do basic observations, as we do with wild life, but if you find a world with a developing society, one that is moving beyond natural behaviour to organised farms, infrastructure etc, even at pre-industrial levels, then you might install one or more probes to observe that society and interpret how they may develope as time passed.

      Alternatively, you may decide to wait for them to become technically advanced, you may have probes in interstellar space monitoring hundreds or thousands of planetary systems, thus, when an artificial signal is detected from one, such as basic radio transmission leakage, a “Bracewell” probe is dispatched to the system to monitor the society.

      In this way you are using resources intelligently and efficiently,thus, whilst discussion of and looking for advanced tech in our solar system is both sensible and achievable, such a probe may not be here yet.

      However, there is another issue, when we set up camera traps and monitoring equipment for animals, we camouflage it, it would make sense that a suitably advanced society would use a natural body for the probe, it can be large or small, depending on needs. If it is designed to work for a few hundred Earth years then a small probe makes sense on or in a very small body, however, if the probe is capable of self repair or replication, then the larger body you utilise increases the potential life span of the probe due to the materials available.

      I agree that Earth’s co-orbital bodies and Lagrangian points need full investigation, we should also look at larger, especially metal rich, inner solar system asteroidal bodies that are significantly larger and have orbits stable for tens of thousands of years in relation to their relationship with the Earth.

      We need to put ourselves in the shoes of the alien, be the alien, we may not have the tech yet, but we have the ability, uniquely in history, to see where our tech is going and drive it forward, but consider how we would use this technology if we had it.

    • ljk March 26, 2019, 11:30

      The SETI “folks” have long disdained the idea that aliens might actually be able to make starships that could reach us. Most of that is due to the smell of UFOs in that mix, and that is understandable. However, sadly, they often went overboard in regards to interstellar travel, declaring that aliens would only use radio waves – and okay, maybe lasers, maybe – to reach out to the stars.

      The 21st Century has arrived and concepts like Breakthrough Interstellar have shown that not only is interstellar travel possible, but that it will involve multiple tiny probes and not just hulking big vessels with equally big probes such as Daedalus, which was innovative in the 1970s but technology has progressed.

      By the same measure, it seems that SETI is also finally breaking out of its own paradigms and radio is now just one of the ways we might find them, or they us. Took long enough.

    • J. Jason Wentworth March 27, 2019, 14:04

      Jim Benford’s point seems to be that his probe METI strategy is a “no-lose” one. If we radar (and/or lidar) ping–and/or otherwise illuminate–those peculiar objects and get a reply, GREAT! But even if we get no responses, but only echoes (natural ones, not classic Bracewell probe “record and immediately re-broadcast” ones [consistently–even slightly–“late” echoes would suggest that the object doing the echoing might be artificial…]), we will more accurately pin down the orbits, and perhaps even get some shape, size, and composition data, on the natural co-orbiting objects. We might even serendipitously find more natural objects–co-orbiting or otherwise–during such a program. Also, regarding our recent interstellar interloper (no more appear to have been detected, despite predictions that they should be very abundant):

      I know that we *listened* to ‘Oumuamua with at least one radio telescope (and perhaps more), but I never saw any articles that mentioned anyone *transmitting* radar, radio, or lidar signals to it. Did someone try that, with a sufficiently powerful radar/radio telescope? Broadcasting a handful-of-watts signal at it with a 2 meter mobile or handheld ham radio rig transmitting through an AMSAT/OSCAR-type Yagi-Uda antenna doesn’t count. :-)

  • Anton M. March 22, 2019, 20:44

    It really depends on the planned probe longevity. If it is hundreds or maybe thousands years then yes, some near-Earth orbit will do. The short range will be helpful for the smaller probe too.

    If we are speaking about million years or so then no, the probe should stay somewhere reasonably near the raw materials needed to repair it unless the ET figured out how to make the completely invincible stuff. So the asteroid belt it is.

  • Adam March 22, 2019, 22:35

    Fascinating. Maybe Phil Dick’s VALIS awaits our call.

  • Michael C. Fidler March 22, 2019, 23:15

    AI would figure prominently in any probes and the number one rule would be self destruction for security reasons. Any race would fear the possibility of the technology being used for their demise, just as our military drones would have.
    The best place to put Bracewell probes would be at Mercuries L4 and L5, in the solar glare with plenty of power and easy observations of the earth.
    David, there is plenty of historical evidence of objects inside the earth’s orbit. See this;
    https://www.centauri-dreams.org/2019/03/19/exploring-our-systems-dust-lanes/#comment-191353

    Earth’s Co-orbitals and any other objects inside of earth’s orbit should have been observed in the imaging search patrols for comets and asteroids. This is being done by both amatuer and profesional astronomers on an hourly basis. Even objects closer then Mercury to the sun should of been picked up, but what could be happening is short bright flashes like some earth satellites that would be hard to track except in long exposures.
    A serious look into the imaging archives may bear fruit if the specific orbit of such objects is calculated and any signs, such as bright flashes, are looked for. This would be a good job for AI deep learning;

    https://blogs.nvidia.com/blog/2016/07/29/whats-difference-artificial-intelligence-machine-learning-deep-learning-ai/

  • Andrew Palfreyman March 23, 2019, 0:44

    Might as well trigger it with high-powered radar. What could possibly go wrong?

    • Robert March 26, 2019, 13:47

      Oh, they could be Berserkers instead of Lurkers…

    • J. Jason Wentworth March 28, 2019, 20:18

      Well…in the context of this blog, it is implicit that interstellar spaceflight, whether undertaken by probes or starships, is a means to an end, and that end is learning about what is out there, including (if any exist) other forms of life, including–and especially–intelligent ones. To your comment:

      A whole lot could “go wrong” if we got an alien interstellar probe’s attention–it might self-destruct, or destroy (with other weapons besides a self-destruct charge) a nearby human-made spacecraft that “pinged” it with a signal, or send a message back to its makers that this Solar System is ideal for colonization, except for a minor inconvenient race on its third planet… (I won’t suggest a reaction like that of the NOMAD probe in the 1960s “Star Trek” episode “The Changeling,” because it [as it found them] was an accidental–and excessively unlikely–hybrid between the meteoroid-damaged Earth probe and an alien probe that encountered it, “the other,” whose programming to sterilize returned soil samples became warped to “destroy all imperfect life.”) But it’s not out of the question–although it’s unlikely, I hope–that a race seeking more or new living space might purposely dispatch large, powerful probes with the programming and equipment needed to remove troublesome beings from otherwise-ideal new homes for the probes’ makers. However:

      If we one day found an alien probe–either willing to communicate with us, lurking, or inoperative–in our Solar System, our curiosity simply could not be dammed up forever. Having, right before us, tangible physical proof of the existence of intelligent extraterrestrials, would make examining the probe (and interacting with it, if it was still functioning, and inclined to respond) irresistible, despite all of the negative consequences that doing so might set into motion, and:

      It would be like having a *film* camera (a 35 mm box camera or a Polaroid camera, say [digital pictures are easily faked, much more so than film photographs]), having a real UFO land nearby, with its alien pilot being willing to let you photograph him/her/it and the UFO–could anyone *not* accept such an offer, even if one suspected that it might possibly be a trick to facilitate a CE4K (an abduction)? Likewise with an alien probe, especially one that was willing to talk to us, or at least would not object to non-destructive examination of it.

  • Alex T. March 23, 2019, 3:16

    It looks like mr. Benford knows some secret “password” , that can activate every ETI probe with probability 100% , so this fact allows him to be sure (?) that this idea is falsifiable. But in reality it is not, as well as whole SETI concept.
    The main SETI problem that it is not a science and the “new” idea, like discussed in this article , is the significant prove that SETI in serious crisis.

    • NS March 25, 2019, 17:02

      Right now ETI is a more or less plausible hypothesis (depending in part on your view of what evolution is likely to produce), and since it can be searched for inexpensively and would have enormous implications if found it makes sense to do so (SETI). The objection that SETI is not a science strikes me as a red herring. What in your view could make it one?

      • Alex T. March 28, 2019, 5:29

        Nothing can make from SETI as it is science.
        Mr.Benford understand this fact (he is bothered by SETI non falsifability) , but cannot accept it.
        SETI – It is popular hobby that involves some radio engenering efforts, i.e. it is branch of Amateur Radio.
        To be clear – I am licensed Radio Amateur too, so like and respect this hobby , but it is not science.

    • J. Jason Wentworth March 28, 2019, 21:12

      AlexT., some scientists and others have criticized the whole science of exobiology (the biology of non-terrestrial life) because it is “a science without a single specimen to study,” but none of those critics, to my knowledge, have ever charged that exobiology is NOT a science because it has (so far) no specimen to study. If they don’t dismiss exobiology on those grounds, then SETI cannot be dismissed for that reason (having no specimen to study) either. On the contrary, both exobiology and SETI are entirely scientific, for one simple reason:

      They both posited (even before other solar systems were discovered to definitely exist [^suggestive^ evidence–the unusually slow rotations of dwarf stars like our Sun and dark, possibly planetary, companions of a few nearby stars that were deduced from those stars’ motions–was available for some time before then]) that there is nothing unique about our Solar System or the life on Earth–that other solar systems, some containing life-hosting planets, almost certainly exist. Also:

      Their first postulate–that other solar systems almost certainly exist–has since been found, and abundantly so, to be true and correct. Their second postulate, concerning the existence of other life in other solar systems, has not yet been found to be true, but–as was the case with the existence of other solar systems decades ago–we have ^suggestive^ evidence that it is true. Great clouds of organic molecules have been found–via radio astronomy–in interstellar space, and organic molecules have also been found in meteorites. Also, other solar systems have been found which have conditions–and planets–that are friendly toward life as we know it (carbon-based life, utilizing water [as a solvent]-facilitated chemical reactions), and:

      Non-technological life is far more likely to be common than technological life (in other words, planets with microbes, plants, and animals are more common than planets with intelligent beings who have civilizations and technologies [radio, lasers, etc.] that we could detect, or vice-versa). In a few years, we will be able to spectroscopically examine exoplanets to see whether they have oxygen co-existing with methane in their atmospheres, chlorophyll on their surfaces, etc. (which are signs of the presence of life). Now:

      It *is* possible that we won’t see signs of life (life is admittedly an overly-complicated, chemically unnecessary thing, as all of the elements comprising life as we know it can exist together–and reach minimum energy states–^without^ forming life), but it seems unlikely that life, which obviously exists here, hasn’t arisen elsewhere as well, at least on a few worlds. However:

      Intelligent life that can create High Technology could be quite rare (only one species on this Earth managed to do it), and even if a race *is* that intelligent, it’s no guarantee that it will channel its capabilities into developing science, radio telescopes, lasers, interstellar probes, and other means of making itself known over interstellar distances (Freeman Dyson said that it is easy to imagine intelligent civilizations that have no particular interest in technology). Here the SETI researchers must take solace in statistics–the Galaxy contains so many stars, and is so old, that it appears to be statistically likely that we aren’t the only thinking beings here. In this they could be wrong, and we may indeed be alone (we might be the first, or maybe we’re the only such civilization in the Galaxy in ^this^ epoch of time). But if we never look and listen for others (by searching for distant signals, and alien probes that might be here in our Solar System), we will never be able to find out.

      • AlexT April 3, 2019, 9:11

        [blockquote]some scientists and others have criticized the whole science of exobiology[/blockquote]
        Agree with those critics, same about astrobiology, those two disciplines are not science exactly like SETI.
        [blockquote]Their first postulate… Their second postulate…[/blockquote]
        All proves you give in protection to those Sci-Fi disciplines – are not their achievement, but – results of astronomic researches and efforts, those “exo-SETI” postulates also have history much-much longer than exobiology or SETI exists, i.e. not their too… Both (exobiology and SETI) are parasites, not science.
        Me too suppose that exo-life exists and very probably exo-intelligent species too, but I do not call me scientist :-)
        Most probably there will be time that exobiology (or astrobiology) will became real science, but it is not time yet – no object for observation, study and researches. Situation with SETI even worst…
        By the way if there (somewhere) is exo-life and exo-inteligence – it will be found, by space exploration (human or robotic) and using astronomy instruments and methods, not by any other pseudo …

  • Harold Shaw March 23, 2019, 10:13

    I think a message should be modeled on Tic-Tac-Toe. Demonstrate that we are aware communication and diplomacy can be competitive but that we are interested in non-zero sum outcomes.

  • Abelard Lindsey March 23, 2019, 14:17

    A bracewell probe is going to be hard to find. Depending on the level of technology, it could be as small as a baseball. I think such a problem would be on a small asteroid (say, 100meter diameter) in order to have access to raw materials to maintain itself over long time periods. I’m assuming the bracewell probe will have some kind of self-regenerative capability that requires raw materials input from time to time.

    • J. Jason Wentworth March 28, 2019, 21:31

      But how could a baseball-size Bracewell probe transmit its findings back to its home star, light-years (or maybe tens or hundreds of light-years) away? It simply couldn’t pack all of the radio or even laser communication equipment, plus instruments (and don’t forget the all-important power supply!) into such a small volume, for fundamental physical reasons that no amount of High Technology magic can get around. The notion–which others have advanced–that Bracewell probes could be as tiny as grains of dust is even sillier. BUT:

      If a larger, “mother” probe with communication relay capability brought baseball-size sub-probes into our Solar System, *that* arrangement could work (even on sounding rockets carrying auroral research payloads, it is now common for the main payload to eject tiny, hockey puck-size–and shaped–sub-payloads, to gather data on the aurora in three dimensions). Interstellar fly-through probes could also utilize small sub-probes to good effect, targeting them for close flybys of the target system’s planets and relaying their data and images (via the large “mother” craft) to the launching planetary system.

  • Astronist March 23, 2019, 17:39

    Why would a Bracewell probe be expected to be found in orbit, and not on Earth’s surface? Why would a civilisation take all the trouble to send it to our planetary system and to the vicinity of Earth, and then position it such that it would be impossible for it to catalogue terrestrial and aquatic species, analyse their DNA, unravel the geological history of Earth and so on?

    • Michael C. Fidler March 24, 2019, 10:47

      The idea that it would come close to earth’s surface would make for many technical problems. The technology would be developed to a state that observations from great distances would be able to see at many freqs down to the quantum level. This is not as far fetched as it sounds since concepts such as quantum entanglement and quantum interferometric telescopes are already in the development stage. That would make sense for probes but where you may see direct interaction is with a living entities.

    • Alex Tolley March 24, 2019, 15:14

      Our technology can already control insect navigation. I would suggest that an advanced probe could make small animals like cockroaches move under command and transmit both images and sound back to a base transmitter that in turn retransmits to the probe, wherever it might be. I don’t imagine ETI need to use such crude technology a a large orbiting d4evice with radi transmitters and receivers as well as scopes to monitor Earth.

      [Of course all those “UFO abductees” could be human versions of this plan. ;) ]

      • Alex T. March 25, 2019, 4:33

        [quote]…would suggest that an advanced probe could make small animals like cockroaches move under command and transmit…[/quote]
        To do such things probe have to visit the Earth and explore and study it very well.

        • Alex Tolley March 25, 2019, 14:54

          1. We are hypothesizing advanced ETI technology
          2. The point is that its presence may be all but invisible to us and also very local. The technology could be simple, but tiny, bugging equipment scattered widely, like the insect sized drone swarms we are starting to build. I want to get away from assuming some largish object doing the probing where no attempt is made to hide it.

  • Harry R Ray March 24, 2019, 14:30

    Two intriguing objects for Dr Benford to consider as targets, although both of them are not currently in horseshoe, tadpole, or quasi-satellite orbits(one could possibly attain a quasi-satellite orbit in the near future). FIRST: ZTFO2Rm, which hamilton1 introduced to me a couple of months ago on this website. Its orbit ranges from 0.26AU to exactly 1.0AU! It was originally thought to be the third stage of the Parker Solar Probe, but its orientation and node(both ascending and descending, I assume) are very different than what they should be. Its real claim to fame is a possible rotation rate of ONLY ten SECONDS! SECOND: A10bMLz is currently in a RETROGRADE trans-lunar orbit ranging between 372,000 miles and 334,000 miles, but not for long! That’s because it is several meters in diameter, but weighs LESS THAN TWO KILOGRAMS! Dismissed as a man-made piece of trash left over from a lunar probe(Chang.e4 most likely, or possibly TESS, which is not a lunar probe, but has an orbit influenced by the Moon)launch. If, however, it IS a still functioning ET probe, it could definitely ATTAIN a quasi-satellite orbit in the near future, using solar radiation pressure! MOST IMPORTANT: It may have already TRIED TO ATTRACT OUR ATTENTION! The VERY FIRST observation of this object showed it to be MUCH BRIGHTER than ANY SUBSEQUENT OBSERVATIONS, leading astronomers to believe that its perigee was only 600 kilometers instead of 334,000 miles. Brian Lacki recently put up a preprint on ArXiv stating that this would occur IF a mirror-like surface were deliberately pointed in a way that sunlight would be reflected back to Earth’s surface, magnifying glass style.

    • Michael C. Fidler March 25, 2019, 0:08

      This is the orbit of ZTF02Rm on Tony Dunn http://www.orbitsimulator.com. The object came as close as 3 earth moon distance on February 1, 2019.

      http://orbitsimulator.com/gravitySimulatorCloud/simulations/1549035587213_ZTF02RM.html

      There is another designation for this object; 2019 BE5

      Object ZTF02Rm (ZTF02Rm object) = 2019 BE5 _ K19B05E ( MPEC 2019-C10 )

      • Harry R Ray March 25, 2019, 9:47

        In that case, I’d like to really really REALLY know how close 2019 BE5 and A10bMLz came to EACH OTHER on that date!

        • Harry R Ray March 25, 2019, 16:19

          I finally had a chance to get a really good look at the orbit. What strikes me about this orbit is ONE: It is not EXACTLY 1.0AU at periastron, but instead, just a tad greater than 1.0AU, and, TWO: It passed BEHIND the Earth on Feb 1. Because its orbit is ~178 days, and Earth’s orbit is ~365 days, it is moving slightly FASTER than Earth, meaning that it SHOULD have had ANOTHER close pass to the Earth just a few days later, that time passing just in FRONT of the Earth!

    • Ronald March 27, 2019, 6:31

      With regard to TESS: there is a TESS Habitable Zone Star Catalog (Kaltenegger et al.) just out;
      https://www.sciencedaily.com/releases/2019/03/190326160505.htm
      Pity is contains only cool dwarfs up to 5000 K, which, if I am not mistaken, is only M plus K, no G. But ok, the K stars are also very promising, according to recent research.

  • Gary Wilson March 24, 2019, 14:44

    I think we should not worry about a Bracewell probe in our system. If one or more exist they can be anywhere, at any size, and with any level of sophistication. If we truly find one, that would be incredible but don’t hold your breath. It’s more important to decide how to send messages outward (or in fact whether to do so). We have already created some EM headed outward but it’s coherence at greater and greater distances will become questionable. Do we want a shotgun approach to communication attempts? Personally I consider that quite risky. Let’s look, quietly and carefully, not scream out our presence before we know anything at all about what or who is out there.

    • ljk March 25, 2019, 9:13

      Humanity has already been “screaming” its presence since the invention of radio. Much earlier if you count the ability for sufficiently advanced ETI to detect artificial elements in Earth’s atmosphere.

      Even if they do not know of our existence, an ETI could still determine there is abundant life on this planet and has been for a long time with spectroscopy and similar detection methods.

      So we can either make a futile attempt to hide from the galaxy, or we can try to better understand who and what is out there, because if an alien intelligence is deliberately headed our way for whatever reason, being surprised and ignorant and having the usual reactions to those symptoms our species often has is not going to help matters.

      • Gary Wilson March 25, 2019, 17:48

        Good comments ljk. I think you may have misinterpreted part of what I said and ignored some things as well. I did say we have been emitting EM radiation for a considerable time now but that those signals lose coherence with distance and so may not have alerted ET to our presence and exact location. And of course it is true that an advanced civilization may already have determined that this planet contains life by the same methods that we are beginning to try to apply. My main point about METI is that it promotes the idea of contacting possible extraterrestrial intelligences without knowing anything about them. I would just advise we think carefully about METI before going forward, and that may require us to refrain from sending out messages without carefully measuring the possible consequences. Possibly that helps you understand what I was trying to say. If ET already knows we are here there is nothing we can do to change that. I also agree with trying to understand what might be out there. Knowledge is power and it may help us make wise decisions about future contact scenarios.

        • ljk March 26, 2019, 11:23

          I did not ignore some of your comments so much as whenever I hear about METI needing to be regulated, all I can think of is good luck with that.

          Not that I am against the wisdom to send out “nice” messages to the galaxy, but that I find it about as likely to regulate as many other aspects of human behavior. Plus there is a dictatorial side to it involving free speech that I do not care for.

          If China wants to do METI with the FAST radio telescope, who is going to stop them? If anything this will teach those ETI what we are like, that humans are a motley tribal bunch with a million different sets of rules, behaviors, and actions, even when a self-selected few try to determine them for the rest of us.

          METI will always happen, one way or another. Now that CubeSats and general space launch costs are going increasingly commercial, there will be even more ways for so-called “regular” folks to send their own physical messages to the stars.

  • Michael Fidler March 24, 2019, 16:03

    Cockroaches would be great if you want to see the inside of a kitchen cupboard. But for continuous monitoring of a planet the best way is from a distance with tools that give detailed data about it.

    • ljk March 25, 2019, 9:17

      Our just sprinkle nanotech (or picotech or some other form of reallytinytech) all over our globe. Most humans will never know and the few that might find them and suspect will not be taken seriously by either the so-called authorities or the general public. The fringe types might, but that of course will just make it worse for their case.

      An advanced ETI in our Sol system has many ways they can monitor us without suspicion or detection. They will figure that out soon enough.

  • Ezoteryk March 25, 2019, 16:57

    A. E. van Vogt ” Defence ” !
    :-)

  • Michael March 25, 2019, 17:18

    It would easier just sticking them on the moon which has a great view of our world.

  • J. Jason Wentworth March 27, 2019, 14:47

    Another potential problem (it would be our problem, and possibly the same problem for inhabitants of at least some systems that we decided to send Bracewell probes to) is that even an apparently friendly probe, programmed to share information about its makers in exchange for similar information about its “hosts,” might be doing so for sinister reasons. John Macvey discussed this possibility in his 1979 book, “Space Weapons, Space War” (see: http://www.abebooks.com/servlet/SearchResults?sts=t&cm_sp=SearchF-_-home-_-Results&an=Macvey%2C+John+W.&tn=Space+Weapons%2C+Space+War&kn=&isbn= ). While I am loathe to use examples from science fiction to illustrate points, one episode of “Star Trek: The Next Generation” did treat this issue clearly and well:

    Its theme was (Captain Picard actually said much of the following to a distrustful alien contactee): “Every first contact, no matter how it is conducted, can always be construed as being an initial reconnaissance and intelligence-gathering mission preparatory to an invasion, and there is no way that we can prove that that *isn’t* our purpose. We’ve been doing it for a long time, and no one has ever been able to think of a better way than carefully making contact and then endeavoring to build mutual trust between both parties.” Now:

    Even a very “chatty and friendly” Bracewell probe in our Solar System, if we found one, could be here for less-than-friendly reasons, and the intelligence agencies and militaries of most if not all nations would advise great caution regarding any proposed communication with the alien Bracewell probe (rather like a portion of the Miranda Rights recited to criminal suspects by American police officers during arrests, “Anything we say could and might be used by the probe’s makers to exploit perceived security weaknesses on the Earth”), and:

    A Lurker might arouse even darker suspicions about its purposes (“Why has that alien contraption been hiding here silently, and for so long? It’s as if its makers decided to bug our Solar System, so that they could monitor the goings-on here without us being aware of them!”) It seems–to us, at least–that a Bracewell probe is “damned if it does, and damned if it doesn’t” communicate with us. The alternative might be for it to make contact, and risk saying too much rather than too little, in order to hopefully establish mutual trust between it and its “hosts.”

    • ljk March 28, 2019, 14:51

      These and other comments in the trilogy of essays on alien probes has given further evidence as to why these devices will likely remain hidden and silent for a long time to come. :^) Either when we will grow up or be replaced.

      • J. Jason Wentworth March 28, 2019, 22:23

        I wish I had grown up in a neighborhood like yours; in mine, street smarts, listening to gut feelings, and yes–paying attention to my suspicions–was an eminently “grown up” thing to do. Being a bookish, partially latch-key child, I had the same “higher,” “enlightened” notions until the day I got beaten up by a gang of neighborhood boys who were waiting for me, before I could get access to the hidden key and get inside my house, and:

        *NOT* carefully staking stock of the situation (including not watching out for any signs of possible nefarious intent behind its presence here, and simply trusting in “the benevolence of the highly intelligent”), should an alien probe turn up in our Solar System, would be the childish thing to do. Humanity did not become the top dog on this Earth by being nice, and by lacking in cunning–I don’t like that fact, but it’s madness to deny it, as many other creatures here are larger, stronger, and faster than human beings. Also:

        Unless they were naturally apex predators on their world by virtue of their size and/or strength, or if the circumstances on their home world were radically different from those on ours, any intelligent aliens we may meet (via direct or remote means, such as probes) likely also had to claw and fight their way to the top of their local food chain. (That would be advantageous to us in a way, because we would have some insight into their psychology and possible motivations from the very start; they would not be totally inscrutable to us.) Astonishingly:

        Even Carl Sagan, who was one of the most ardent supporters of the notion that more advanced aliens would probably be benevolent, having long since cured all of their social ills, advised caution and careful consideration before replying to any CETI message we might receive. Sagan even said–and John Macvey quoted him in “Space Weapons, Space War”–that we can’t rule out an alien evangelism, that some race might conquer us in the sincere belief that they are spiritually “saving” us. We should not be–and need not be–paranoid; but we should remain alert and wary, asking (in all such interactions between societies with greatly dissimilar capabilities [like the Europeans and American Indians, for example]): “What’s in this for us? And what’s in it for them?”

        • ljk April 1, 2019, 9:44

          I do not think we were discussing at cross-purposes here. My comment was that an ETI would be very cautious before contacting us, if ever, due to the potential dangers of dealing with our species, especially if they were in our neighborhood.

          If the ETI were the deliberate threat and were capable of coming to Earth, they would have a number of ways to dispatch us with little to no resistance from humanity.

          Did I miss something here? I would like to think that a more advanced society would also be more benevolent, but if Earth is a literally universal example, then that would not necessarily be the case. However so far all we have is one data point.

  • Michael C. Fidler March 29, 2019, 7:22
  • ljk April 2, 2019, 11:56

    https://arxiv.org/abs/1904.00536

    Possibility of a coordinated signaling scheme in the Galaxy and SETI experiments

    Naoki Seto

    (Submitted on 1 Apr 2019)

    We discuss a Galaxy-wide coordinated signaling scheme with which a SETI observer needs to examine a tiny fraction of the sky. The target sky direction is determined as a function of time, based on high-precision measurements of a progenitor of a conspicuous astronomical event such as a coalescence of a double neutron star binary. In various respects, such a coordinated scheme would be advantageous for both transmitters and receivers, and might be widely prevailing as a tacit adjustment.

    For this scheme, the planned space gravitational-wave detector LISA and its follow-on missions have a potential to narrow down the target sky area by a factor of 10 3-4 , and could have a large impact on future SETI experiments.

    Comments: 4 pages, 3 figures, accepted for publication in ApJL

    Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Popular Physics (physics.pop-ph)

    Cite as: arXiv:1904.00536 [astro-ph.IM]
    (or arXiv:1904.00536v1 [astro-ph.IM] for this version)

    Submission history

    From: Naoki Seto [view email]

    [v1] Mon, 1 Apr 2019 02:05:40 UTC (138 KB)

    https://arxiv.org/pdf/1904.00536.pdf

  • ljk April 2, 2019, 12:10

    https://arxiv.org/abs/1903.11599

    Solving the Fermi paradox without assumptions

    Alexander Berezin

    (Submitted on 27 Mar 2019)

    This paper suggests that a universal solution to the Fermi paradox exists and can be derived directly from the definition of life and/or intelligence, therefore eliminating the need for any questionable assumptions and even for the anthropic principle.

    The proposed solution puts an upper limit on the growth of civilizations that is independent of resource availability or biological factors.

    Subjects: Popular Physics (physics.pop-ph)

    Cite as: arXiv:1903.11599 [physics.pop-ph]
    (or arXiv:1903.11599v1 [physics.pop-ph] for this version)

    Submission history

    From: Alexander Berezin [view email]

    [v1] Wed, 27 Mar 2019 11:55:04 UTC (61 KB)

    https://arxiv.org/pdf/1903.11599.pdf

  • ljk April 30, 2019, 9:52

    The article does not mention Radio SETI, but you know that will also be negatively affected by all these satellites circling Earth. Dollar signs over science once again.

    Satellite constellations and radio astronomy

    Companies are proposing a number of satellite constellations, some with thousands of spacecraft, intended to provide broadband communications. Adam Kimbrough notes that such systems also create new headaches for radio astronomers.

    Monday, April 29, 2019

    http://thespacereview.com/article/3702/1

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