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Degrees of Visibility

Alexander Zaitsev’s latest contribution to the debate over sending messages to the stars is a short paper that looks at how visible our planet might be thanks to transmissions from planetary radars like Arecibo, Goldstone or the Evpatoria site from which directed transmissions have already been sent. METI (Messaging to Extra-Terrestrial Intelligence) is broadly dedicated to transmitting messages to stars likely to have habitable planets, but so far the number of transmissions is relatively sparse. The debate over METI discusses the wisdom of continuing them without broader discussion.

But tucked within that debate is the specific question of our civilization’s visibility. For in addition to the messages that have already been sent, beginning with the Arecibo message in 1974 and continuing in the far more targeted transmissions from Evpatoria between 1999 and 2003, we are using our planetary radars to perform crucial astronomical studies. The work these dishes do in refining our knowledge of potentially dangerous asteroids is too significant to stop, but Zaitsev (Institute of Radio Engineering and Electronics, Russia) argues that their radar signature is far more obvious than the METI messages.

The argument goes like this: Roughly 1400 sets of radar transmissions have been produced from the three sites, with a distribution covering a broad swath of sky compared to the small number (16) of METI transmissions, which covered an area 2000 times smaller. Zaitsev then factors in the total duration of the radar transmissions, which exceeds the METI broadcasts by a factor of 500. He concludes that the radar work on asteroids and other objects is one million times more likely to be detectable than the signals sent as communications to other stars.

If this is the case, then making our civilization less visible involves shutting down activities necessary for planetary protection, an obviously dangerous move. But we still have a problem of degrees. A directed signal sent to a star with potentially habitable planets brightens our planetary signature significantly to receivers near that star, a place pre-selected for its astrobiological interest. The fleeting pulses of planetary radar transmissions covering broad areas of sky should likewise be detectable, but I find it hard to agree that this kind of transmission is at the same order of visibility, precisely because it is far more widely and randomly dispersed. [Addendum: See Dr. Zaitsev’s comment below re my misuse of the term ‘pulses’ in this context. These are not pulsed systems.]

The more we learn about how our activities might be detected elsewhere, the better, and I think that raising the issue of ongoing radar transmissions is completely valid. Similarly, we have much to learn about how the radio and television transmissions of the past century might or might not be receivable at various distances. The METI debate takes place in a context of apparent degrees of visibility, and questions whether making a specific attempt to raise that visibility to carefully chosen targets is wise. Settling that debate should involve not just astronomers and physicists but a broad spectrum of informed opinion. It is a debate that Dr. Zaitsev is fully engaged in, but one that most media outlets (with striking exceptions like SEED Magazine) have chosen to ignore.

The paper is Zaitsev, “Detection Probability of Terrestrial Radio Signals by a Hostile Super-civilization,” available online.

Comments on this entry are closed.

  • dad2059 April 25, 2008, 11:28

    So, if someone is concerned about our detection by an aggressive super-civilization (socalled
    METI-phobia, [6]), first of all one has to prohibit not the METI, but the radar astronomy.
    However, one can not prohibit it because the radar astronomy is an important and indispensable
    component of the asteroid hazard and defense system [7]. For this reason, we conclude that all ongoing
    conversations about the ETs danger of METI for our civilization are meaningless…

    So his reasoning is because asteroid plotting and other stray radars make us visible from a distance, we’re sitting ducks anyway?

    He does have a point, but I tend to agree with you Paul, the defence radars are more diffuse, even if they do show up more on his graph.

    The METI messages are targetted and are more intense. Radar signals pointing every which way from the Solar System and would thin out the further one gets I would think.

  • kurt9 April 25, 2008, 11:50

    This may be a little off topic, but I cannot think of anywhere else to say this.

    I have another explanation for Fermi’s Paradox.

    As you all are aware of, the Earth’s climate consists of a cycle of ice age that last 100,000 years, interspersed with a 10,000 year warm period. We created civilization during the warm period. We are likely to have the next ice age in the next 1,000 years or so (unless we use planetary engineering to prevent it). My argument is the following:

    Much of tool making and industrial civilization developed in the temperate to colder regions of the planet (the West and East Asia). This is no accident. The sub-optimal living conditions (cold winters) required that the people living in these regions think and plan ahead in order to assure adequate food supply and shelter to survive each winter. An optimal living environment does not provide the necessary pressure to develop the ability to think and plan ahead. In short, I believe the development of intelligence (and industrialization) requires a “stress” condition that forces the existing life form to evolve the ability to think and plan ahead.

    This concept is reflected in the geographical distribution of IQ (East Asian – 105, European – 100, tropical regions – 85-90). This suggests that advanced technological civilizations require a sub-optimal environment to develop in. History tells us that all of the significant inventions (magnetic compass, large sailing ships, printing press, gun powder, etc.) were invented by the Chinese and independently re-invented by the Europeans.

    The problem with sub-optimal environments is that they may be unstable. A sub-optimal environment could very quickly become non-optimal and kill off the civilization that is developing there. An example would be if the ice age returns in the next few decades, before we have the chance to colonize space. Such an event could result in the end of industrial civilization an even, perhaps, our extinction.

  • Alexander Zaitsev April 25, 2008, 12:55

    Dear Colleagues,

    Please do not foget that the matter concerns SUPER-CIVILIZATION!

    I guess, it is not serious and forcible argument (again, in the case of SUPER-CIVILIZATION) that radar sounding signals are more arduous for detection then METI transmissions.

    Please remember that for such super-power civilization, which can overcome such huge interstellar distances and reach us, such hardships with signal detection are not a problem…

  • dad2059 April 25, 2008, 17:25

    Prof Zaistev,

    We’re assuming K-Type II-III, correct?

    Chances are if they’re interstellar capable, they won’t be using radio or any EM medium unless it’s used as a legacy back-up.

    Unless they sent a Bracewell probe. Such a device would be AI to a high degree and would have such a legacy system to detect stray radio or radar from nascent cultures. But they would have to have a suspicion we exist first.

    Let me ask a question sir, how do you pick your target stars, other than being G-K type suns? Is that the only criteria?

  • Richard S. April 25, 2008, 18:02

    Unless the hostile super-civilization has warp drive, it won’t be any easier for them to get at us than it is for us to colonize the stars. But we know that asteroid bombardment of the inner planets can occur. Even if radar detection of asteroids is a risk with respect to aliens, I think the cost-benefit analysis in terms of asteroid strikes more than compensates.

  • Adam April 26, 2008, 2:23

    Hi All

    Dr. Zaitsev, we presently have the technology to launch interstellar missions if we so desired, yet we’re far from super-civilization level. A small extrapolation from our level makes interstellar travel even easier and yet there is no reason we have to advance to K-2 or 3 to do so.

    Imagine we developed room temperature superconductors and carbon-cycle fusion. Such would allow Bussard ramjets which would make any star accessible within a subjective lifetime. An unresolved biological drive to exponential growth could compell us to invade/infest the whole Galaxy in search of new worlds.

    Now consider ETIs… would they deliberately target already inhabited worlds knowing there will be a suitable world at the other end?

  • Alexander Zaitsev April 26, 2008, 3:00

    To dad2059:
    >>Let me ask a question sir, how do you pick your target stars,
    >>other than being G-K type suns? Is that the only criteria?

    I am not an authority on target star selection. I stated something about it in my paper

    Messaging to Extra-Terrestrial Intelligence


    The selection for Cosmic Call 1999 & Cosmic Call 2003 was made by Richard Braastad, and for Teen Age Message 2001 — by Lidia Filipova.

  • george scaglione April 26, 2008, 9:25

    kurt,about the above – yes a little stress is probably a great help in survival planing! also…the next ice age 1000 years in the future!? well by then i don’t think it will be much of a problem.by then my friend i sincerely believe that we will be able to do something about it.not to mention that in 1000 years i expect the human race will have outposts all over the solar system and maybe the galaxy as well! when i consider all the possibilities and there are quite a few… i don’t see that i (we),should worry or would have to worry one way or another. respectfully buddy your friend george

  • dad2059 April 26, 2008, 9:30

    I am not an authority on target star selection. I stated something about it in my paper: Messaging to Extra-Terrestrial Intelligence

    The paper answered my question about the selection process, logical considering we only have one sample, us.

    As for hostile K-Type III’s tracking us down and silencing our baby-bird cheepings, while I agree with Paul on radar dispersion, I think that’ll be the way we could be detected. Targetted METI signals are too tight to be intercepted randomly.

    Hopefully by the time we might be detected, we’ll have developed a capability of self defense if need be.

  • dad2059 April 26, 2008, 9:39

    Richard S. has a good point also, but I’m reminded of Arthur C. Clarke’s last Odyssey novel ‘3001’. The Black Monolith is viewed as a threat because of a super-nova observed in Scorpii that could have been artificially induced, possibly by the same super-intelligence of a Black Monolith, or its equivalent.

    No FTL was involved there.

  • ljk April 26, 2008, 20:03

    Note that if we are making radar beams to analyze and
    monitor NEOs, other civilizations – especially ones who
    are colonizing their solar systems – will likely require
    even more radar monitoring of space debris, so perhaps
    that is how we will find THEM first.

    And don’t forget all those military radar beams that
    have let our presence be known as they scanned for
    enemy missile threats. Maybe other civilizations have
    their own versions of the Cold War, too, if they are as
    young and paranoid as we.

    Dad2059, if there is some sort of device sitting in the
    Sol system waiting to go off if we are perceived as a
    “bad” species, why hasn’t it gone off yet? Are we just
    not the galactic threat we think we are, only to ourselves?
    Are they waiting to give us time to grow up?

    Seeing as we could spread into the galaxy one day and
    compete for resources, what are they waiting for? And
    how do you make a sun explode on purpose? Why not
    just lob a ship at relativistic speeds at Earth? Or put
    rockets on a few NEOs and aim them at our planet?
    That way you take out our species without ruining the
    whole Sol system in the process.

  • Alexander Zaitsev April 27, 2008, 12:09

    >The fleeting pulses of planetary radar transmissions
    >covering broad areas of sky should likewise be detectable,
    >but I find it hard to agree that this kind of transmission
    >is at the same order of visibility, precisely because it is far
    >more widely and randomly dispersed.

    But all three terrestrial radars in Arecibo, Goldstone, Evpatoria are not pulse-signal systems!

    And I proceed from the assumption that all well educated readers know that trivial fact that all three terrestrial radars radiate continuous time signals, either monochromatic, when estimate the Doppler shift, or modulated, when estimate both Doppler shift and delay.

    The second trivial fact, and again I proceed from the assumption that all well educated readers know this fact, is that under transmission radar beam directed to given asteroid or planet, and therefore move very, very slowly relative to stars.

    And the third, also trivial fact, is that signal-to-noise ration in the event that we use COHERENT signals and realize optimum filtering, does not depend on the bandwidth, which have such coherent signals
    and depend on energy of such signals, ONLY!

  • Administrator April 27, 2008, 16:00

    Dr. Zaitsev writes: “The second trivial fact, and again I proceed from the assumption that all well educated readers know this fact, is that under transmission radar beam directed to given asteroid or planet, and therefore move very, very slowly relative to stars.”

    The parameters at work here may indeed strengthen your case, and I would recommend they be addressed more fully in the paper.

    However, the difference between a signal sent randomly with regard to the background heavens and a directed signal beamed at a star of high astrobiological interest seems substantial, whether or not the signal is pulsed.

  • Dr. Alexander Zaitsev April 28, 2008, 1:18

    I think, it is not serious adversary’s argument to consider 15 nearest Sun-like

    >stars of high astrobiological interest

    which were selected by us as a targets in 1999, 2001, and 2003 METI
    by way of hostile SUPER-Civilization’s home.

    Especially since if above 15 nearest Sun-like stars are the hostile
    SUPER-Civilization’s home, THEY for a long time know about us, without

  • dad2059 April 28, 2008, 6:57

    ljk: I just used Clarke’s ‘3001’ as an example, he posited that the Monolith in the story could turn hostile if it considered the human experiment a failure. If you read the series, the reason Monolith super-intelligences turned Jupiter into the mini-star Lucifer is because they detected pre-intelligent creatures under the Europan ice. Heating Europa would melt the ice and help the nascent intelligence develop. So they could start from scratch another ‘experiment’ if need be.

    As to why it would make a star go nova to destroy an ‘experiment’, consider that the experiment had already spread through its solar system, simply lobbing an asteroid at the home planet wouldn’t suffice to destroy the beings that were uplifted. Only a nova would wipe the whole experiment out.

    And why hasn’t a super-intelligent Bracewell probe wiped us out yet if we might be an experiment? It wouldn’t have to, look around you. Why should it waste it’s energy if the experiment is willing to terminate itself?

  • Ron S April 28, 2008, 10:43

    Radar is not all alike. Terrestrial radar is (mostly) constructed to deal with the rapid return time due to the closeness of targets, common antenna for the transmitter and receiver, and a noisy environment. So the signal/pulse has to be very brief and perhaps with an identifiable signature. Astronomical radar can be more inventive since these restrictions don’t apply, especially if the intent is to map a surface rather than simply determine existence and range.

  • ljk May 2, 2008, 13:18

    Mirrors on the Moon as a Galactic Greeting Card -A Smart Move?

    Proponents of making contact with advanced ET life forms have come up with a new way to attract their attention—mounting mirrors on the Moon and using them to signal across space. It’s sort of like a bigger version of Batman’s bat-signal to shout out, “Hello aliens! We’re here! Come on over!”

    Since radio broadcasts haven’t had much luck drumming up a clear response, it’s time to step it up to improve our chances of being found reasons Shawn Domagal-Goldman and Jacob Haqq-Misra of Pennsylvania State University. They say the mirrors could be angled to catch the Sun’s rays, which would increase the amount of light the Earth-moon system reflects by 20%. That could be more than enough to attract the attention of an astute alien astronomer. Domagal-Goldman proposes stealing ideas straight out of Carl Sagan’s book Contact, where a code of prime number flashes let aliens know the signal is intentional and not just natural variations in brightness.

    Also, for those who think mounting mirrors on the Moon as an intergalactic greeting card isn’t worth the investment, they’ve come up with a win-win solution for the dreamers and the pragmaticists. The underside of the mirrors could be covered with photovoltaic cells. When the mirrors aren’t busy signally they could be reversed to allow the cells to generate electricity, which would be then be beamed via microwaves back to Earth.

    “You could help solve the climate crisis, too,” says Domagal-Goldman, who presented his idea last week at the 2008 Astrobiology Science Conference in Santa Clara, California.

    It sounds good, but there is a surprising amount of controversy over the topic of whether or not we should be actively trying to contact ET life forms. Feelings run deep and rampant on the subject ranging from “alien contact would be the best thing to ever happen to planet Earth” to a very serious “hell no, the aliens will eat us!”

    Full article here:


  • Alexander Zaitsev May 3, 2008, 1:12

    It is not
    >a new way to attract their attention—mounting mirrors on the
    >Moon and using them to signal across space.

    Konstantin Tsiolkovsky wrote about mirrors, which reflect sunlight, for signalling to marsits (he used this word to mark those who live on Mars), in XIX century.

    I guess, undoubtedly similar “way” was suggested also many people long before
    >Shawn Domagal-Goldman and Jacob Haqq-Misra of Pennsylvania State University…

  • Administrator May 3, 2008, 12:49

    Absolutely right about Tsiolkovsky and other ideas for signaling the stars from the 19th and 20th Centuries. This latest seems to be a re-examination of an old idea.

  • ljk May 3, 2008, 23:06

    Believe it or not, Albert Einstein advocated signalling Mars
    using tight beams of light aimed at the planet from Earth
    in the 1930s. I guess you could say Einstein advocated
    Optical METI long before it became widely accepted.

    Robert Goddard envisioned sending a rocket to Mars which
    when it was entering the planet’s atmosphere, various engraved
    images of the constellations would appear as successive
    layers of the entering craft burned away due to friction with
    the Red Planet’s atmosphere. Apparently the Martians
    would notice these patterns and realize they were not
    witnessing a typical meteor.

  • ljk July 30, 2008, 19:14

    Bebo tries to contact Earth-like planet

    Jemima Kiss guardian.co.uk

    Tuesday July 29 2008

    An image of the galaxy comprising red dwarf Gliese 581 and the earth-like planet Bebo is trying to contact. Photograph: AFP

    Bebo and the TV company behind Wife Swap have teamed up with one of the world’s experts in interstellar radio communication, Dr Alexander Zaitsev, to beam 500 messages from users into space in a digital time capsule.

    Dr Zaitsev will use a Ukrainian radio telescope, normally used to identify and track asteroids that pose a threat to Earth, to beam the messages to a planet orbiting the star Gliese 581c – 20 light years from Earth – because it is believed to be capable of supporting life.

    The Message From Earth project, which launches on August 4, will invite Bebo users, celebrities and politicians to submit messages and pictures that “consider the planet from a fresh perspective” and raise awareness of environmental pressures on our planet.

    Whether there is intelligent life elsewhere in the universe remains “one of the greatest unanswered questions”, said Dr Zaitsev.

    But he added that it would be exciting to see how young people chose to represent humanity and the planet.

    “I understand that in the majority of cases these messages may be naïve, but I also hope that we will receive a creative and fresh look at the subject,” Dr Zaitsev said.

    Full article here:


  • ljk October 25, 2008, 10:48


    Why SETI will have missed any cost conscious ET civilizations

    October 24, 2008 | by KFC |

    If we want to contact any of those other civilizations out there, we’ll need a beacon to send messages with. But what to build?

    Gregory Benford at the University of California Irvine and family (?) have done a cost/benefit analysis on the types of microwave generators out there that can produce the 10^17 W necessary to reach a significant proportion of the galactic habitable zone.

    There are various ways that the cost can be optimised and the Benfords summarise them like this:

    “Thrifty beacon systems would be large and costly, have narrow searchlight beams and short dwell times when the Beacon would be seen by an alien observer at target areas in the sky. They may revisit an area infrequently and will likely transmit at high microwave frequencies, ~10 GHz. The natural corridor to broadcast is along the galactic spiral’s radius or along the spiral galactic arm we are in.”

    This has implications for the search for ET civilisations (as opposed sending messages for them). If ET civilisations are as cost conscious as we are, then they may well have built beacons in this way.
    And if so, say the Benfords, nearly all SETI searches to date would have missed them.


    arxiv.org/abs/0810.3964: Cost Optimized Interstellar Beacons: METI

    arxiv.org/abs/0810.3966: Cost Optimized Interstellar Beacons: SETI