Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


24.3.2á Mission Profile

Assume that an advanced alien technical civilization institutes a major starprobe program and dispatches computerized messenger vehicles to spy out neighboring solar systems. One plausible "standard entry procedure" (Figure 24.11) might run as follows:

1. Preliminary acquisition -- 0.5 light-year away, traveling at 10%c, the starprobe makes its first high-resolution scan of the target and makes slight course adjustments to increase accuracy. Braking engines are activated.

2. Messenger searches for the star’s Zodiacal Light, stray sun light scattered from the dust and debris surrounding the star. All planets lie in this thin blanket of dust, so the probe corrects its course so as to enter the plane of the solar system. Speed has fallen to l%c or less.

3. Approach on cometary orbit until solar irradiance sensors indicate that the midpoint of the local habitable zone (for the desired biochemistry) has been reached -- say, just above the melting point of water. Drop into a circular, circum solar orbit.

4. Seek out and examine any planets in or near the ecosphere, and examine each for spectroscopic evidence of, say, water and oxygen in the atmosphere. Select the first planet having both in appropriate concentrations and move into orbit around it.

5. If the body is accompanied by a large natural satellite (such as Luna in the case of Earth), perturbations will seriously disrupt a simple global orbit. To negate the disturbing influence of the moon, settle into a relatively stable Trojan Point orbit. (For Earth, two of these points lie in the lunar orbital path both 60o ahead and behind.)

6. Activate search sensors to infeed data to look for any signs of intelligent life or technological activity. Collate and compile the information.

7. Record positional star map, including accurate fix on home world. Transmit to home world a preliminary report of sensor findings, including astronomical, biological and technical data gained by scanning and eavesdropping on the target planet.

8. Begin routine signaling or other activity to announce presence and to attract attention. If no intelligence is in evidence, enter dormant mode with specified wake/sleep schedule for periodic resampling of planetary environment and basic self-maintenance functions.


Figure 24.11 Synopsis of an Interstellar Probe Mission718

In 1998, an interstellar spacecraft, which has been drifting in space in the region beyond the Moon, launches itself toward the triple star system a Centauri. It thrusts at high acceleration, its engines running at a power level that is ten times the power of a Saturn V rocket. The exhaust of hot hydrogen plasma glows like a bright star, visible both night and day. After four months the probe has left the Solar System, and has reached 1/3 the velocity of light. It begins its long drift through interstellar space using its bulky first stage shell as a radiation barrier to protect it from the constant rain of high energy particles produced by its high speed through the interstellar hydrogen.

After drifting quiescently for 12 years (it has now covered four light years), it sheds its first stage, turns around, and begins deceleration. As the probe velocity drops well below relativistic speeds and it approaches its target, the spacecraft opens up from a compact mass into a 100-meter-diameter sphere. The sphere is a dense wire mesh embedded with arrays of tiny sensors and transmitters, close coupled to complex digital molecular circuitry, all held together and interconnected with high strength, one-dimensional superconductive fibers.

The array of sensors spaced over the hundred meter sphere collect the light and radiation from the three stars in the a Centauri system and correct the rocket thrust to zero in on Proxima Centauri. As it approaches the small red star, the probe searches for a planet. It is there, along with three others further out. The other three are cold, and probably lifeless, but they will be visited before the probe leaves Proxima to investigate the other two stars in the a Centauri system.

With its thruster at low power, the probe approaches the planet -- Proxima Centauri One -- constantly beaming pictures and sensor data back to Earth using a phased array of solid state lasers scattered densely over its mesh surface. Earth will not see the pictures of the new planet for 4.3 years, long after the probe has completed its survey and moved on to the other planets and stars in the a Centauri system.

Laser pulses from interstellar probe scanning Proxima Centauri One.

With no feedback possible from Earth, the computer circuits distributed through the sphere analyze for themselves the information its sensors collect as it approaches. The probe swings into a near polar orbit of the new planet and begins a survey. Wideband sensors sensitive to the entire electromagnetic spectrum produce imagery in the radio, microwave, infrared, visible, and ultraviolet bands. The one hundred meter size of the detector array gives the picture a resolution of less than 1 meter even from the 1000 km orbital altitude. Picosecond pulses of laser light beam down as a laser radar to measure the height variations of the topography. Certain regions that might have life are interrogated with selected laser wavelengths, and their return light analyzed to look for absorption or fluorescent bands characteristic of organic compounds. A few regions that have the most potential for life are selected. Small portions of the sensor mesh on the sphere detach from the main probe mesh and are driven down into the atmosphere with radiation pressure from the lasers on the probe. The small mesh sections drift down to the surface, collecting and storing images as they descend. The mesh settles on the surface where specialized chemically sensitive molecular circuits react to the various forms of chemical compounds found there. The orbiting probe interrogates the lander mesh with a laser beam, collects the images and chemical data it has stored, combines it with the other information that it has collected, and sends a detailed report back to Earth.

The probe then moves on to the next planet in the system, more slowly now, for it is no longer as lavishly supplied with fuel as it was at the start of its mission. It will not stop until it has made a complete survey of every planet in the three star system. This will take a long time, but the probe has a lot of time.

It will be at least 30 years before man will arrive to take over.


What sorts of things might the visiting probe look for to determine if intelligence exists on the planet? To be visible from space, the intelligence must manifest itself in artifacts. Direct photographic reconnaissance from the distance of lunar orbit could be very difficult. To resolve the artifacts of human civilization unambiguously would require a visible-light telescope with an effective diameter of many tens of meters. A neutrino detector to pick up evidence of fission of fusion power generation on the surface of the planet probably would be too massive. Atmospheric heat flow and composition analysis should be highly suggestive (e.g., fluorocarbon aerosols in the stratosphere probably cannot be generated naturally in an oxygenic carbon-aqueous biosphere), but may still be too ambiguous.

The two most critical parameters of technological civilizations -- energy usage and information flow rate -- frequently will be directly measurable from space. At night, the waste energy escaping the metropolitan regions of Earth can be measured from orbit, so the starprobe should be able to assemble a fair estimation of global power consumption. As for information flux, the silent alien craft could detect countless powerful radio stations whose emissions seem to wax and wane with the daylight. Assuming the ET spy is smart enough, it should be able to listen in on our transmissions, learn our languages and customs, and tap into our cultural and technological heritage. Before it makes overt contact, it will probably know a great deal about us.

What is the best way to attract attention and to initiate first contact with a planetbound Type I civilization such as our own? The orbiting starprobe could turn on a bright light or explode a bomb, but this would be inefficient, ambiguous, and might not even be seen at all by the intelligent planetary inhabitants. Contact lander craft or robot encounter vehicles could be soft-landed on the surface, but this is a rather tall order for a modest-sized automated starcraft. Ronald Bracewell, a Stanford University radioastronomer and an early advocate of interstellar messenger probes, has suggested that once intelligent radio emissions have been detected by the starprobe the selection of frequency and message is relatively easy:

What frequency will it use? In the case of a messenger probe, this is a nonproblem, since the probe can rest assured that on any frequency where a transmitter can be detected there will also be, somewhere, a receiver! The probe can choose any frequency which is already plainly in use. This automatically guarantees that someone is listening, because no one transmits if nobody is listening.. . . It is true that at least one receiver will be tuned in, and perhaps a large audience, but will they pay attention to an unwanted, interfering signal? If the probe gives out something that you don’t want, you will go away. So a simple procedure would be for the probe to amplify and transmit the same TV program or military communication it was receiving. Its signals would then have the appearance, to us, of echoes exhibiting delays of seconds to minutes depending upon its distance from Earth. For instance, if we were listening to the radio, each word would be heard twice, first by direct transmission from the station and then again a little later via the probe.85,80

In other words, the best frequency for a starprobe to use to attract attention is one which is already in use, since this guarantees a listening audience. The best message to send is a duplicate of whatever was transmitted, since this guarantees an interested audience.* The detailed contact plan of the "Bracewell probe," outlined in Figure 24.12, is one eminent xenologist’s view of the most likely way it may happen. Says Bracewell: "I believe we are on the eve of plugging into the galaxy-wide communication network."1040


Figure 24.12 Bracewell Probes: First Contact Scenario80

The Message

In my opinion, the message will be in television. Television is like sign language; although you and I may not speak the language, we can exchange through signs or pictures. Geometrical furnish a means whereby we learn each other's language. The words in dictionary that can defined by drawings probably run into the thousands and those that can be defined by animated drawings are many more. Not only nouns, but many adjectives, adverbs, and verbs can be depicted through television. Other words are harder, but if one had a dictionary and already knew a few thousand basic words, one could interpret some of the more difficult ones.

Until we set up a common language, television also permits us to ascertain quickly the answers to questions of basic importance, such as where the probe came from. The probe, too, would like us to know this without delay. To speculate a bit, I will assume that television is what we are going to see and that the first picture will be of a constellation of stars familiar to us, followed by a zooming in on the home star. At one time I thought the picture of the constellation would have one star winking on and off like an electric sign. but anyone who can make an animated movie to do that can just as readily simulate a zoom lens. The zoom lens technique is a very quick way for a foreign probe to tell us which star it came from without knowing our name for the star, or our coordinate systems, or anything about our language. You might also want to know how we are going to get our TV screen synchronized to its system of transmission. if the probe keeps running its program until we get the number of lines per frame and so on worked out and arrange a so on display it. then we can report our readiness by repeating the program back. Alternatively, since it has been listening in on cur TV transmissions, it might oblige us by adopting one of the numerous standards in use on earth.

Now that we know the home star, the probe will zoom in further until the star grows into a visible disc, perhaps with starspots. From their motion we will know the axis of rotation. The planetary system will then be displayed, and at last we will zoom in on Superon, time home planet Clearly a fantastic travelogue lies ahead, and it is well within the capacity of a modest probe to execute the simple steps required to display this information. After this brief preview there is some rather serious business to attend to, namely: the messenger probe must convey to us the schedule oh listening that is being observed on Superon and the frequency being used this rather tier urgent. Afterward the probe is dispensable but until the schedule and frequency are conveyed the probe's mission is incomplete. I believe this job can be accomplished with pictures, but I surmise the programmers on Superon will surprise us by the brevity and clarity of their particular method of conveying this vital information.

Language Barrier?

The nature of our direct transmission to Superon is a matter of taste. Obviously we can send zoom movies to match those the probe will have shown us, but much more interchange can take place with the probe while our direct transmission is being readied. I believe the probe can learn our language in printed form quite readily, working with an animated pictorial dictionary that we furnish for its computer memory. At first, its expressions might be quaint, but there is no reason why its compositions should not be televised back to it in corrected form. To be sure of getting a point across, the probe can do what we do -- say it again in different words and if we don't understand, we can question.

Knowledge of our language will enable the probe to tell us many fascinating things: the physics and chemistry of the next 100 years, wonders of astrophysics yet unknown to man, beautiful mathematics. After a while it may supply us with astounding breakthroughs in biology and medicine. But first we will have to tell it a lot more about our biological makeup. Perhaps it will write poetry or discuss philosophy. Perhaps the messenger knows how the universe started, whether it will end, and what will happen then. Maybe the probe knows what it all means, but I wonder...I think that is why Superon wants to consult us!

The Limit

In time, the probe's store of knowledge will be used up, as it is only a modest probe. Presumably the computing part need only be the size of a human head, which is, we know, large enough to store an immense amount of information. Meanwhile our transmission to Superon will have commenced. One might ask whether it would be better to use our language or Superese, which we could learn from the probe. Now while I think the probe could learn a functional form of our language. I don't think it would be practical to teach it to the people of Superon. The continual checks and confirmations, corrections and repetitions that are possible between ourselves and the probe resemble face-to-face encounters between humans meeting on language frontiers and would be ruled out by the round-trip delay to Superon and back. It is true that we could transmit our pictorial dictionary followed by text, and hope for the best: if the probe met an untimely demise that would be the only course. However, by the time the probe is well advanced in its mastery of our language, it will possess an on-board translation program that converts our language to Superese. Therefore we can copy its program and either transmit it to Superon or translate into Superese locally before transmission. Perhaps the probe would advise us as to our degree of success.

Mission Accomplished

From what has preceded we can see that the messenger probe scheme for contact overcomes dependence on terrestrial socioeconomic and political stability over centuries, circumvents the problem of determining the proper frequency, and is well adapted to its primary mission of detecting our existence and announcing the location of Superon. If the probe accomplishes no more than this, it has achieved the initial detection that seems so fraught with difficulty if attempted by direct radio; in fact it dos better by reporting to a home base which is ready and waiting to receive the report!

Spheres of exploration expanding around intelligent communities in the galaxy, Superon, our nearest superior neighbor, has just contacted the earth. Some time ago, is found technical life at A, which is now making its own sphere of exploration.

The Galactic Club

Meanwhile, back on Superon, much time has elapsed since the messenger probe set out, so receipt of the probe's report is dependent on the very kind of political and economic stability that Superon should avoid relying on. But this is a relatively mild dependence for several reasons. First, the schedule and frequency of transmission as well as the report's direction of arrival are known, in fact, have been known well in advance. Second, the receiving equipment exists. Third, no financial outlay of great magnitude is required. And, fourth, the report will be interesting because of the new data it contains about another planetary system even if it does not carry the spectacular news that Superon is ultimately seeking.

As a protection against assorted mishaps, such as failure of report to reach Superon, the probe can announce to us the location of the other chain of communication. For there will likely be a galactic club hose members are experienced at finding developing communities such as ours and inducting them into the galactic community. Each will have responsibility in its own sphere of influence and will engage in an ongoing program of launching messenger probes, at an annual rate appropriate to local priorities, in an endeavor to comb their unexplored frontiers for new technical life. An idea of scale may be obtained by referring to the drawing above, where four superior communities -- A, B, C, and Superon -- are shown. The shaded circles, ranging in radius from 10 to 25 light-years represent the volumes of space around those communities that have been rather closely inspected by well-equipped expeditions; the larger circles show the limit now reached by the messenger probe program. Superon has reached the 100 light-year range, finding the earth. Some time ago it located technical life at A, which was inducted into the chain, is now a superior community, and is participating in the messenger probe program as indicated by the circle showing the progress of A's own exploration. Two other members of the chain are B, which is relatively old and has explored more space, and C, which was inducted by B long ago. Superon is in contact with C, not as a result of probe exploration but because Superon became independently aware of the existence of C when news of its discovery was relayed via links in the chain not shown in the diagram because they are not in the plane of the paper. Had this not been so, an interesting situation could have arisen in the lens-shaped region of overlap between Superon and C, where each could have had messenger probes in the field. (It is a whimsical thought to contemplate two automatons, both far from home in the reaches of space, exchanging notes about their builders, though I admit it is not a very likely encounter.)

Many localities remain unexplored in the vast crevices between the expanding spheres. For example by the time Superon expands its probed sphere from the 100-light-year radius to the 125-light-year radius indicated by the dashed line, it will have doubled the number of stars visited. Therefore even this apparently modest expansion will require much time. Of course, the exploration is helped as new communities such as A take over part of the work.

The Time Scale of Interstellar Dialogue

If the messenger probe plan is so good, why do scientific publications to extraterrestrial contact refer mainly to radio? I think the answer is that if a superior community averages one launching per year, 1,000 years will pass before enough probes have been launched to cover the likely stars within 100 light-years. When the first technical life is discovered, decades will elapse before the probe's report filters back. In addition, we have to consider the travel time of the probe to the star's environs. Depending upon the size of engine the probe uses, the travel time could be kept down to centuries or even decades. But a community that is prepared to wait it out for 1,000 years does not need to hurry. Perhaps 10,000 years travel time would be reasonable; it depends on a trade-off involving reliability in transit, cost per unit, number of launches per annum that can be afforded within the budget allocation, and the maximization of the probability of success. This is indeed a long-term project! (By the way, note that interruption of the launch program does not affect the chances of success of probes already launched; the plan is tolerant of diversion of resources to urgent priorities.) Of course, the human life span being what it is, we are reluctant to contemplate programs that stretch over many centuries; we have to realize that interstellar contact is not contact between individuals, but a contact between civilizations. This is a slightly depressing thought for action-oriented people. The arrival of a probe would be exciting. Nonetheless, the individual who directs the launching, be it a probe or a radio signal has to face the reality that it will not be he who receives the answer.


Near indeed -- is it possible that we have already detected a Bracewell probe? Duncan Lunan, former President of ASTRA (Assn. in Scotland for Technology and Research in Astronautics), once advocated that, just possibly, we have.

The story begins in 1927, during research then in progress on round-the-world radio echoes. (These echoes are propagated by reflections between the charged layers of the ionosphere and the Earth itself, and take about 1/7 second to circle the planet and return.) Taylor and Young in the United States reported hearing echoes they couldn’t explain, signals with delays of only hundredths of a second and coming from a point 2900-10,000 kilometers overhead. Today we surmise these came from the Van Allen radiation belt, discovered in 1958 by the Explorer I satellite, but in 1927 the effect was a real mystery.

Later that year Carl Størmer, a Norwegian mathematician, chanced to meet a telegraphist by the name of Jorgen Hals, who told him that the 10,000-kilometer delay was no so astounding since he, Hals, had heard echoes of three full seconds which he believed were coming from the moon. Størmer, fascinated by this, conducted his own experiments on the phenomenon with the assistance of Balthus van der Pol, a telecommunications specialist at Philips Radio, Eindhoven. In November and December of 1928, Størmer and van der Pol published two letters to the editor in the prestigious British science weekly Nature.211,210 In these letters they described their work which confirmed Hals’ claims. The first sequence reported by van der Pol, and confirmed by Størmer, consisted of pulse delays at the following intervals: 8, 11, 15, 8, 13, 3, 8, 8, 8, 12, 15, 13, 8, and 8 seconds. This was one of the first reports on the so-called Long Delayed Echoes (LDEs), and many more such sequences were later recorded by Størmer and van der Pol, and others.2869

Following the suggestion by Bracewell that LDEs are remarkably similar to the general kind of message we might expect to receive from an alien starprobe, Duncan Lunan3132 puzzled over the meaning of the delay times. He decided to start with the assumption that LDEs were signals from an extraterrestrial spacecraft, an evidentiary technique which once worked for Heinrich Schliemann in the discovery of the ancient city of Troy. The delay times, Lunan noted, were not the sequence of prime numbers that many had expected would accompany the first alien call signal. Yet if they were artificial they must have some meaning. Lunan decided to make a graph plotting delay time against sequence number. The result, to his surprise and delight, was the pictogram reproduced in Figure 24.13.


Figure 24.13 Space Probe from Epsilon Bo÷tes?3132

First van der Pal sequence, evening of 11 October 1928 (tentatively identified as an incomplete map of Bo÷tes).

This diagram can be interpreted as demanding an intelligent reply. By moving the 5th pulse (delayed 3 sec) to a position where it is delayed by 13 sec (marked X), the constellation Bo÷tes is completed.

This is the required answer and if transmitted back the probe should transmit further information. Note the 8-second "barrier" dividing the diagram into 2 parts. The position of a Bo÷tis -- "Arcturus" -- can be interpreted as tentatively identifying the map as compiled 13,000 years ago.

A tentative conclusion is that the probe arrived here from Epsilon Bo÷tis 13,000 years ago.


According to Lunan’s original interpretation, the drawing represents a picture of the brightest stars in the constellation Boötes. The positions of the stars are shifted as they might have appeared 13,000 years ago (due to proper motion across the sky), so this may be when the probe first arrived in our system and compiled the original sky map. One star, Epsilon Boötis, is displaced to the left of the vertical line a distance equidistant from its true position on the right. To Lunan, the message seemed clear: The home star of the alien craft was Epsilon Boötis. Finally, since LDEs were known to follow the path of Luna’s orbit,1124 the starprobe presumably was located in Earth orbit at one of the two Trojan Points.

Lunan’s interpretation of LDEs was immediately questioned by the scientific community.172,1125 Occam’s Razor demands that the simplest explanation of phenomena should prevail, and experiments by the American researcher F.W. Crawford and his colleagues3133 and others1089 have led to the conclusion that LDEs probably are the result of shock wave propagation and amplification in the F-layer of the Earth’s ionosphere. Also, LDEs appear to occur seasonally, whereas an interstellar messenger would he expected to transmit on a continuous basis. A.T. Lawton and his coworkers in Great Britain have even beamed call signals at the trailing lunar Trojan Point in an attempt to elicit some response from the hypothetical starprobe, but no intelligent return signals were ever detected.

What is the latest word on Lunan’s theory? Lunan himself apparently abandoned his early hypothesis when it was discovered that the Epsilon Boötis double star system has suns much more massive than Sol, and thus probably too short-lived for life to have evolved. Lunan’s new hypothesis is that the starprobe’s point of origin was Tau Ceti. This has received support from others:

[Lunan] has since claimed that Epsilon Boötis would be a prime navigational reference for a starflight from Tau Ceti to the Sun; as seen from Tau Ceti, our Sun would lie in Boötis. In 1975 a Russian astronomer, A.V. Shpilevski, published an alternative interpretation of the 11 October 1928 LDEs in the Polish magazine Urania. By plotting the same dot series in a different way he found a star map of the constellation Cetus, with the star Tau Ceti indicated as the star of origin. If the star map has any reality, it shows us that the probe came from the nearby star Tau Ceti, from which no radio noise has ever been detected.3257


* Of course, the probe could simply broadcast high-powered radio interference on the appropriate frequencies. However, instant replays of local broadcasts would appear more friendly than indiscriminate pamming, and would also indicate that the starprobe was prepared to interact and communicate with the indigenous civilization.


Last updated on 6 July 2013