Ronald Bracewell’s name doesn’t come up as often in these pages as I might like, but today James Jason Wentworth remedies the lack. Bracewell (1921-2007), active in radio astronomy, mathematics and physics for many years at Stanford University, developed the concept of autonomous interstellar probes. Such a craft would be capable not only of taking numerous scientific readings but of communicating with any civilizations it encounters. His original paper on these matters dates back to 1960 and relies on artificial intelligence, long-life electronics and propulsion methods that don’t necessarily involve high percentages of c. Jason considers these factors from the perspective of 2018 and explains what a program sending such probes to numerous stars might look like. If you’re recalling Arthur C. Clarke’s ‘Starglider’ from The Fountains of Paradise, you’re not alone, but as the author notes, there are quite a few directions in which to take these ideas.

by J. Jason Wentworth

The writer wishes to extend his special thanks to Ellen N. Bouton, Archivist, at the National Radio Astronomy Observatory, for locating and providing a scan of Dr. Ronald N. Bracewell’s June 1, 1974 Stanford University Alumni Conference lecture, “Studies of Extraterrestrial Life” (Reference 25), and to Jim Hassel, Library Technician at the Rasmuson Library, University of Alaska Fairbanks, for providing scans of the Nature and American Journal of Physics articles (References 21, 22, and 23).

Starflight and the Matter of Time

The problems of interstellar spaceflight, more than any other type of travel, whether on Earth or in space, are inextricably bound up with time. This is true of starprobes as well as starships. Everything about starflight—from the choice of onboard power systems to the desires of the mission personnel (and those who fund the missions) to see the final mission results, and everything in between—is determined, or at least heavily influenced, by the lengths of the journeys. Insisting on rapid trips compounds the difficulties and costs of the missions, while being content to accept longer transit times eases their engineering (and financial) challenges, including those posed by cosmic rays and physical erosion. Lower interstellar cruise velocities also make stellar rendezvous missions less difficult. However, settling for slower interstellar passages introduces another possible cause for partial or total mission failure—the breakdown of components and systems due to aging and wear. The planners of interstellar missions must arrive at acceptable compromises between these and other competing, and sometimes diametrically opposed, factors. Longer missions also exacerbate another, all-too-human problem, which would be present even for the fastest possible missions: the long intervals between departure and arrival at distant stellar systems. Fortunately, there are countervailing factors which may—of necessity, in some cases—make slower interstellar missions acceptable, and even desirable.

Just as the Galileo and Cassini spacecraft examined asteroids and/or Venus while in flight to their destination outer planets, there are other, closer interstellar worlds that starprobes may be able to investigate while en route to other stars. If interstellar asteroids like 1I/2017 U1 (‘Oumuamua) are as common as some astronomers suspect, interstellar probes may have more to do while in transit than collecting fields and particles data and making VLBI (Very Long Baseline Interferometry) radio astrometric observations. Indeed, it would be prudent, in the interests of the probes’ safety—and to collect population statistics of use to future starship missions—for them to keep a constant watch for such objects, as well as for ejected comets, rogue planets, and even brown dwarfs, all of which (even if observed from afar) would be scientifically rewarding targets of opportunity. [1]

But before such interstellar probe missions can come to fruition, it will be necessary to develop electronics and power systems that can operate reliably in interstellar space for decades, centuries, or even longer. We will have to create machines which are, for practical purposes, immortal. While no such devices have been developed for consumer use (where “planned obsolescence” appears to often be a design factor), it is not uncommon for electronic devices—including solid-state ones—to remain perfectly functional for decades. As Dr. Ronald N. Bracewell (the electrical engineer and radio astronomer who developed the interstellar exploration/messenger probe concept that bears his name) pointed out in his 1974 book The Galactic Club: Intelligent Life in Outer Space, the lifetimes of electronic components and systems could already, at that time, be accurately predicted (an engineering capability that had existed long before he wrote the book).

He gave an extreme example, to demonstrate how even the lifetimes of large, distributed electronic systems operating in hostile environments can be known. A transatlantic submarine telephone cable contains many amplifiers built into the cable along its length, with each amplifier containing many vacuum tubes (not transistors, he found interesting to note). The entire cable is required to operate as a functional whole for twenty years under water (and hopefully longer). But even though none of these cables’ components had been subjected to a twenty-year test, engineers were able to simulate such tests for entire cables with confidence. The service lifetimes of transistors, he noted, could also be determined, because their deterioration depends in a calculable way on their temperature. If they were maintained at very low temperatures, which is easily done between the stars (and even, with known techniques, closer to a star), electronic devices could easily have indefinite lifetimes. Bracewell also suggested that if it proved necessary, some components could even—in a manner foreshadowing today’s 3D printing—be produced aboard interstellar probes as they neared their destination stars. [2]

Another enemy of such probes (and their electronics) is radiation and particle impact erosion, but these obstacles also appeared to be straightforwardly solvable to him. Referencing an article by I. R. Cameron in the July, 1973 issue of Scientific American (“Meteorites and Cosmic Radiation”), Bracewell noted in his book that the laboratory-measured rates of erosion on meteorites in space (between 0.2 to about 10 millimeters per 1 million years, depending on the meteorites’ compositions—and undoubtedly, where in interplanetary space they spent their lives after being exposed to space) indicated that a quite thin coating would suffice for protecting interstellar probes from erosion. [3] More recently, NASA and the Korea Advanced Institute of Science and Technology (KAIST) have been developing—and have working prototypes of—self-healing electronics for “spacecraft on a chip” vehicles. These devices can accept cosmic ray damage and then heal themselves (between 1,000 and 10,000 times, so far). [4]

Image: Ronald Bracewell (left), with Stanford’s Von Eshleman, a key figure in early research into gravitational lensing. Here the two are examining the horn antennae that Bracewell used in 1969 to determine that the Sun is moving relative to the cosmic background radiation. Credit: Linda Cicero/Stanford University.

Examples of Long-Life Spacecraft

Even today, there are examples of unexpectedly long-lived spacecraft, and this bodes well for the prospects of developing essentially immortal interstellar probes. The Sun-orbiting Pioneer 6, 7, 8, and 9 spacecraft, which were launched between 1965 and 1968, lasted for decades past their six-month design lifetimes; indeed, all of them except Pioneer 9 (which failed in 1983) may still be functioning. These probes have seldom been listened to since the 1980s (the spacecraft were last monitored between 1995 and 2000). [5] The Pioneer 10 and 11 outer planet probes (launched in 1972 and 1973, respectively) operated until 2003 and 1995, respectively. The Pioneer Venus Orbiter functioned in the hostile thermal and radiation environment near Venus between 1978 and 1992 (when it burned up in Venus’ atmosphere), and Voyager 1 and 2 are in their 41st year of operation. Even older spacecraft continue to function, and two—despite decades of exposure to Van Allen belt radiation—came back to life after having fallen silent many years before.

Three of the U.S. Air Force’s LES (Lincoln Experimental Satellite) spacecraft have exhibited this unexpected longevity. LES 1, launched in 1965, was last heard from in 1967—until a British amateur radio operator heard its signal in 2013. [6] LES 8 and 9, launched together into geosynchronous orbit in 1976, are still operating 42 years later. [7 and 8]. In 1974, the AMSAT-OSCAR 7 (AO-7) ham radio satellite was launched into a near-polar, Sun-synchronous orbit as a “hitch-hiker” payload, from Vandenberg Air Force Base. It fell silent in 1981 when its battery shorted, and 21 years later it was heard again (after its short-circuited battery went open, allowing the satellite to operate on its solar cells when in sunlight). Despite its decades of passages through the Van Allen radiation belt, AO-7 remains fully functional, with all of its beacons and transponders operational when it is in sunlight, which is most of the time. [9]

Another long-lived spacecraft is ISEE-3 (the third International Sun-Earth Explorer, launched in 1978, which was re-named ICE—International Cometary Explorer—for the occasion of its 1985 encounter with Comet Giacobini-Zinner). After its many adventures (including multiple lunar flybys), it was re-contacted and operated by the private ISEE-3 Reboot Project in 2014. It may still be operable when it passes near the Earth again in 2031. [10]

Interstellar Probes that Can Learn and Make Decisions

While pre-programmed digital electronic computers would likely be sufficient for “fly-through” interstellar probes, stellar rendezvous probes would likely need to be able to learn and to make decisions for themselves (for seeking exoplanets around their destination stars, entering circum-stellar orbit in the proper plane and orbital direction, computing flyby trajectories to enable close examination of the system’s planets, etc.). This would especially be the case for Bracewell probes, which would also listen for any local, intelligently-produced radio and/or laser signals and attempt to contact any such civilizations, then learn their language(s) in order to act as local scientific and cultural emissaries of humanity. This would include informing the “local aliens” about how and where to contact the Earth directly, via interstellar radio and/or laser transmissions.

Analog computers, as the British biologist Rupert Sheldrake has pointed out, “enable complex, self-organizing patterns of activity to develop through sometimes chaotic, oscillating circuits.” He also noted that in 1952, William Ross Ashby, a British cybernetics researcher, published a book titled Design for a Brain, in which he showed how analog cybernetic circuits could model brain activity. More recently (as Sheldrake also noted), Mark Tilden developed insect-like robots that demonstrated self-organization—and even learning and memory—despite the fact that these devices contain fewer than ten transistors and have no computers in them. [11 and 12] BEAM (Biology Electronics Aesthetics Mechanics, or Biotechnology Ethology Analogy Morphology) robotics, a “reaction-based” type of machine building, was inspired by Tilden’s work. (In the nearer term, analog logic circuits-containing robots such as Tilden’s would be useful as rovers, “hopper” rovers, winged and aerostatic aerobots, instrumented boats, and submersibles for exploring planets, moons, asteroids, and comets in our own Solar System. In the future, stellar rendezvous starprobes could deposit similar robots on the worlds orbiting their target stars, and relay the robots’ findings to Earth.)

A network of such analog devices might also possibly (perhaps in combination with some digital subsystems—such devices are called hybrid computers) function together to form a type of STAR (Self-Testing And Repairing) computer, which could control interstellar spacecraft. Since about 1961, NASA’s Jet Propulsion Laboratory had conducted research on a digital STAR onboard computer, which later in that decade found favor for the planned four-spacecraft Grand Tour mission, for which a non-flight “study model” called TOPS (Thermoelectric Outer Planet Spacecraft) was built. [13, 14, and 15] Kenneth Gatland, and the Soviet engineer B. Volgin (as was mentioned on page 244 of the former’s book, Robot Explorers, see Reference 13), both discussed the need for interstellar spacecraft to have self-repairing computer systems that could also learn and make decisions for themselves. To ensure acceptable mission risks, the computer systems of interstellar probes (and of any robotic sub-probes that they might carry, in the case of some stellar rendezvous—or even “fly-through”—missions) would have to be able to repair themselves in some way, regardless of how rapidly or slowly the vehicles traveled. Fast starprobes would face more intense impact and erosion damage by high-velocity atoms and dust particles (and induced cosmic rays, at high relativistic speeds), while slow probes would be subjected to long-term bombardment by galactic cosmic rays during their decades-long or centuries-long journeys. Self-healing electronic components and STAR-type features appear to be promising solutions to ensure that the vehicles’ electronic brains would remain sharp during transit, and upon and after arrival.

Getting There—Practically and Affordably

Over the years, many interstellar probe concepts have been studied and advocated. Among the earliest ones were ion propulsion, which Soviet scientists discussed at the 1973 International Astronautical Congress in Baku, Azerbaijan. At that meeting, a paper written and endorsed by members of the USSR Academy of Sciences concluded that ion-drive starprobes using then-current technology were feasible. [16] They predicted a flight time of about four hundred years to Barnard’s Star (six light-years away), and a journey duration of six hundred years to stars approximately twelve light-years away. Elsewhere in the 1974 book that discusses the Soviet paper (Is Anyone Out There?, by Jack Stoneley with Anthony T. Lawton, see Reference 16), it is mentioned that ion-drive probe velocities of 5% of c, the speed of light, are possible. The researchers envisioned ion-drive probes about the size of a Saturn rocket, which would enter orbit around their target stars.

Nuclear pulse propulsion—using fission or fusion bombs, or laser- or electron beam-triggered fusion micro-explosions occurring at higher rates—has also been studied extensively. The designs of the Orion starship and the Project Daedalus starprobe (a 0.12 c stellar system fly-through probe) utilized both nuclear pulse methods. [17] While both the bomb-type (Orion) and the fusion micro-explosion-type (Daedalus) designs were enormous and extremely expensive, recent dramatic reductions in payload size and mass would make much smaller interstellar probes feasible. Dr. Mason Peck’s “Sprites”—chip-size spacecraft weighing just a few grams—could be accelerated to high interstellar transit velocities (and be decelerated for relatively slow flybys or circum-stellar orbit insertion) by much smaller propulsion systems. [18] In fact, a much smaller Daedalus-like starprobe could release dozens or even hundreds of Sprite probes, which could be targeted to fly by and examine all of the planets in the destination stellar system.

These tiny “spacecraft-on-a-chip” probes also make laser-pushed lightsails—and even solar sails—attractive as potential high-velocity propulsion systems. The Breakthrough Starshot lightsail starprobe project, and NASA’s notional 2069 0.1 c solar sail interstellar probe project (NASA is also considering other propulsion systems), have both become serious contenders thanks to Sprites. [19 and 20] Analyses of laser-pushed lightsails indicate that as the sail velocity approaches c, the beam’s effectiveness in imparting momentum to the sail falls sharply, because a visible light laser’s Doppler-shifted light descends to infrared frequencies (as the sail “sees” the beam’s light). [21, 22, and 23] For sail velocities of 0.1 – 0.2 c or so, these “wavelength-stretching” Doppler shift effects aren’t large enough to cause serious beam thrust drop-off.

G. Marx proposed a variation of the laser-pushed lightsail concept which could reduce the overall complexity and cost of such interstellar probes. [21] He pointed out that an X-ray laser located above the Earth’s atmosphere could emit a much more powerful collimated beam (for the same beam aperture) than an ultraviolet, visible light, or infrared laser. Since X-rays can only be reflected by grazing incidence (shallow-angle reflection) nickel reflectors (like those used in space-based X-ray imaging telescopes), an X-ray laser-pushed lightsail starprobe could be in the shape of a narrow cone of thin nickel foil, with the payload located in its rearward-facing tip. If spin stabilization proved necessary, the conical sail could have several spiral, ridged “flutes”—rather like the spirals depicted on a unicorn’s horn—running from its pointed tip to its base; the flutes would reflect some of the beam, imparting spin to the conical sail. Such X-ray lightsails could be quite small. A SETI “bonus,” for any aliens looking in our direction, would be that such an anomalous, directional X-ray laser beam emission coming from near our Sun should grab their attention (and vice-versa, if anyone out there launched X-ray laser-pushed lightsail starprobes—or starships—in our direction).

Sending slower probes would ease many of the challenges of designing such spacecraft, which would lower their unit cost and enable larger numbers of them to be launched. Cyril Ponnamperuma and A. G. W. Cameron pointed out that it would be extremely wasteful of economic and energy resources to design probes that would travel faster than 1 percent of the speed of light, advocating that technological resources should instead be devoted to ensuring that the probes would remain reliable for long periods. [24] At such a velocity, their propulsion requirements (including braking to enter circum-stellar orbits) and interstellar material (stray atoms and dust particles) erosion shielding requirements would be greatly reduced. Ronald Bracewell also supported this “longevity over speed” strategy, based on his conclusion that the closest spacing between civilizations was probably (except for rare, random close spacing) on the order of at least 100 light-years.

Such spacing would make radio and laser SETI searches problematic because we—and the nearest other technological civilization—would each have about 1,000 surrounding promising stars to check, yielding maximum odds of success of 1 in 1,000,000. The actual odds would be significantly lower than this due to both parties’ unavoidable complete ignorance of what frequencies to use, and when—and toward which star—each society was transmitting or listening at any time. These limitations led Bracewell to develop his interstellar messenger probe concept, which would avoid these problems (and would return data and images from all stellar systems visited—including those without resident intelligent life—making every probe mission scientifically worthwhile). He proposed that in order to examine their 1,000 closest stars, and to establish contact with intelligent beings found around any of them (or to at least inform the “seeking” planet of their existence), another technological civilization would dispatch 1,000 modest interstellar probes, launching at least one probe (and more, if finances permitted) per fiscal year. [25] Bracewell also suggested that humanity would one day engage in such interstellar exploration and SETI searches by means of starprobes. Frederick Ordway suggested that interstellar probes could also monitor for intelligent signals while in transit between the stars. [26]

The nanotechnology scientist Robert A. Freitas, Jr. also advocates the use of Bracewell probes, in a complementary fashion with traditional SETI searches. [27 and 28] Like Bracewell, he points out the relative insensitivity of a probe program’s effectiveness to interruptions in the probe launch rate (because probes already launched will continue with their missions). Freitas is also in favor of the development of self-replicating starprobes (Von Neumann probes), whose general principles were developed by the mathematician John Von Neumann. [29] While this is an economically and logistically attractive concept (because just a few probes, once launched, would multiply, at zero additional cost to the funding government), this approach has a potential ethical problem. Would an intelligent race—including our own—appreciate it if an alien spacecraft suddenly showed up and began harvesting the worlds of its stellar system to produce copies of itself? Its transmitted assurances that its purposes were entirely peaceful might ring very hollow. Also, the highly sophisticated technology that will be necessary for such self-replicating probes is nowhere near fruition. When we can build a device that—if set on the ground—can move around, find and process iron, and make tenpenny nails all by itself, there may be some hope for progress in this field; but even then, it must be remembered that even the simplest spacecraft are far more complex than carpentry nails.

Approach, Arrival, and Post-Arrival Activities

What terrestrial interstellar probes will do as they close in on their targets will depend not only on humanity’s technological capabilities, but also on economic and political considerations. These latter two factors are, of course, interdependent, and may be influenced by the probes’ transit times. (Modern-era governments are less enthusiastic about funding projects which will come to fruition long after the legislators involved are out of office, or even dead, and high-cost projects of this nature are even less popular among politicians.) Depending on the tradeoffs between these factors, interstellar probes may be either stellar system fly-through vehicles or stellar rendezvous (star-orbiting) spacecraft. While probes having cruise velocities of 10% – 20% of c would be more popular with the project scientists and enthusiasts, 1% of c probes would be considerably simpler and cheaper, and they would be less likely to meet premature ends due to in-transit debris impacts. From the point of view of the politicians who would appropriate the funding for the spacecraft, the difference between 0.1 c and 0.01 c probes would make little difference as far as their career durations were concerned, but the much lower price tag of each 0.01 c probe would be more attractive. Also, they could attract political merit by “investing in the future” and “voting to find new worlds for humanity to explore and, perhaps, discover other civilizations with whom we might one day communicate.”

Fly-through probes would be the least challenging in terms of size, complexity, propulsive capability requirements, and cost, while stellar rendezvous probes would be able to make more detailed observations of their target exoplanetary systems. Essentially immortal fly-through starprobes could also conduct “open-ended,” multi-target flyby missions, passing from star to star by utilizing stellar gravity assists to propel them. Such probes would be slow by human standards, as they moved in trajectories similar to those of comets that were ejected from stellar systems by encounters with gas giant planets. But in compensation, they would make leisurely passes through their destination stellar systems. This would give them time to examine the planets in some detail, and—if the probes were so equipped—to listen for intelligent electromagnetic signals and engage in communication with any “local aliens.” Ronald Bracewell initially advocated stellar rendezvous probes exclusively, but he later concluded that fly-through probes would also be sufficient for carrying out these functions. As David Darling wrote some years ago:

“More recently, Bracewell has suggested that it would be sufficient merely for a messenger probe to pass through a planetary system to achieve its goals (as in the case of Project Daedalus). Without the need for retrorockets, such a probe could be made smaller and at much lower cost. Our contribution to the success of attempts by alien races to establish contact in this way might be to construct a sophisticated space watch system, possibly an extension of one designed to search for Near-Earth Objects.” [30]

In his 1978 novel The Fountains of Paradise, Arthur C. Clarke described a stellar gravity assist-propelled, alien fly-through Bracewell probe called Starglider, which passed through the Solar System in 100 days after having been detected moving through the outer Solar System at six hundred kilometers per second. [31] Faster fly-through starprobes (particularly non-Bracewell ones intended only to examine stars and exoplanets) could make more rapid journeys, then slow down before arrival in order to conduct data-collecting and image-taking flybys.

A stellar rendezvous probe would require a greater delta-v capability, so that it could both accelerate to cruise velocity and later brake into orbit around its target star. Limiting such probes’ maximum velocity to 1% of the speed of light would reduce the amount of energy required for braking upon arrival. Ion or plasma propulsion could power the probes. Sail propulsion (using either a laser-pushed lightsail or a solar sail, with the latter utilizing a “Sun-diver” trajectory) could produce a departure velocity of 0.01 c, but braking into circum-stellar orbit upon arrival might limit the target list to multiple star systems (and only at certain times), where photo-gravitational braking could be used. An E-sail (Electric sail, which is pushed by solar wind or stellar wind ions) could also be employed for braking. One advantage of photon sails and E-sails for rendezvous starprobes is that after circum-stellar orbit capture, either type of sail would enable the spacecraft to change its orbit to visit interesting exoplanets in the system (especially in the star’s habitable zone), without expending any fuel.

The spin-rigidized E-sail could also potentially serve as a probe-to-Earth communications antenna and—in a Bracewell probe—as a signal-monitoring and (if local technological life was found) local communications antenna. An E-sail, which uses positively-charged wires to repel the positively-charged solar or stellar wind ions, can use any number of such wires, from one up to dozens (in a “wagon wheel” configuration). [32] The wires (which are typically kilometers in length) are many wavelengths long at the frequencies used by spacecraft radio systems. Extremely long—in terms of multiple wavelengths—wire antennas are highly directional. As the ARRL Antenna Book says (the American Radio Relay League is the governing body for amateur radio in the United States), “The longer the antenna, the sharper the lobe becomes, and since it is really a hollow cone of radiation about the wire in free space, it becomes sharper in all planes. Also, the greater the length, the smaller the angle with the wire at which the maximum radiation occurs.” [33]

In other words, as the antenna is made longer (increasing the frequency of the radio transmitter and/or receiver that is connected to the wire causes the same effect), the radio energy is concentrated and emitted more and more off the end of the antenna (and the receive pattern is identical to the transmit pattern, so that the antenna is most sensitive to signals arriving at its end). In Section 2.1 of his online article, “Small Smart Interstellar Probes,” Allen Tough suggested that a probe could trail a very thin antenna. [34] A spinning “wagon wheel” E-sail, if its spin axis was precessed so that the Earth was in or near the sail’s plane of rotation, could communicate with Earth by electronically selecting each wire in turn (in a multiplexed way), as its far end was pointed at the Earth at some point during each rotation (multiplexed antenna systems have long been in use). This same arrangement could be used in the destination stellar system, for radio science data collection and (if the vehicle was a Bracewell probe) to listen for and communicate with any intelligent inhabitants in the system. Used with a variable matching network, the E-sail’s long wires could serve as signal monitoring and transmitting antennas over very wide frequency ranges.

An often-expressed objection to non-relativistic interstellar spacecraft (as generally expressed, space vehicles that travel at 10% or less of the speed of light) is, “It wouldn’t be worthwhile to send such slow interstellar probes because no one a century or more from now would be interested in data from them.” The systematic collection of other, often far older data in science does not support this contention. Astronomers directly image Jovian-type exoplanets that are hundreds of light-years away, and have even measured their wind velocities via transits, and that is—by definition—*very* old data. (Old astronomical photographs, and even hand sketches in notebooks, are prized because they enable more precise orbits of celestial objects to be computed, and because they record changes in such objects’ brightness and/or physical characteristics over time.) Slow starprobes would tell us a lot while they were en route to their target stars, and after arrival they would have far more detailed instrument and imager views than what we could ever perceive from the distant Earth. Another example that runs counter to this assumption involves Pioneer 10 and 11 and Voyager 1 and 2; no space scientists ever suggested that they be turned off “because they’re too old and obsolete.” On the contrary, they were (Pioneer 10 & 11) and are (Voyager 1 & 2) treasured for reporting on conditions in remote regions of space, where no other on-site instruments are available (New Horizons will also be so valued, after its last encounter).

Stellar system fly-through and rendezvous probes would carry out planet and satellite observations similar to those of planetary flyby and orbiter probes in our Solar System, and they would also examine the stars (and any companion suns) in their target stellar systems. Fly-through probes would seek out and target (for close flybys) any planets orbiting in their destination stars’ habitable zones, as well as observe other planets as closely as possible. Slower fly-throughs would enable more opportunities to observe more planets at closer range. Stellar rendezvous probes could, after braking into orbit around their assigned stars, change orbits to investigate the various planets; sail-equipped probes could conduct such “extrasolar Grand Tour missions” (including comet-like outer planet flybys to return to the near-star regions) without using any propellant.

Seeking Neighbors and Making New Friends

Bracewell interstellar messenger probes (of either the fly-through or rendezvous type) would, in addition to exploring their target stellar systems, listen for any local artificially-produced electromagnetic (radio or laser) signals, then attempt contact if any such signals were detected. Ronald Bracewell developed a complete, language-independent contact and communication plan, which the probes could implement if they heard intelligently-produced signals. By merely receiving such signals, a probe would know that on that frequency, the planet’s atmosphere was transparent to the signals. It would also know that somewhere on the planet and/or in its vicinity, someone (and likely many someones) would be operating a receiver capable of receiving that frequency. Armed with this knowledge, the probe—an electronic ambassador of the human race—could get to work. [35]

Ronald Bracewell divided the problem of contact with technological life in the Milky Way into three categories—abundant, sparse, and rare life—in which our nearest neighbors would be less than 30, 30 to 300, or from 300 light-years to the edge of the galaxy away. (He also listed two special, extreme cases, in which humanity is alone in the galaxy, or alone in the universe). Being moderately pessimistic (or moderately optimistic, depending on one’s point of view), he surmised that the nearest technological society was probably no closer than 100 light-years (within his sparse life category), meaning that our search would be in a spherical volume of space containing 1,000 stars likely to possess habitable planets. With such a large number of possible stars for us—and for the nearest extraterrestrial civilization—to each search via radio and/or optical SETI methods (and they wouldn’t all be the same stars), the odds of both societies happening to cross electronic paths would be significantly less than one in a million. These unpromising odds led him to develop the interstellar messenger probe concept, which overcomes the geometrical (statistical), fiscal (the cost in time and money), and political (funding and operation interruptions due to wars, revolutions, or economic depressions) problems that could derail long-term SETI and METI programs. No matter what happened at home (short of extinction or “being bombed back to the Stone Age” events), the probes would be transmitting their findings and would, if necessary, be waiting to re-establish contact.

Before arrival at its destination star, each probe would locate the star’s equatorial plane, in (or near) which its planets orbit. (Imaging the star’s starspots and tracking their motion, or viewing the star’s zodiacal light dust plane with an occulting disc coronagraph, would enable the star’s equatorial plane to be found.) Upon entering the system, the probe would observe the planets and their moons, paying particular attention to any planets orbiting in the star’s habitable zone. (Once it was close enough to the star, the probe could supplement its onboard power with stellar power collected via photovoltaic cells, thermocouples, or perhaps Stirling cycle generators.) If any such planets were present, the probe would enter orbit around the star in the habitable zone (or if it was a fly-through probe, it would adjust its velocity and course to ensure a slow pass through the system). If its monitoring revealed any artificial radio (or perhaps laser) signals, the probe would announce its presence by retransmitting portions of the signals back to their source, at the same frequency on which it received them. Anyone listening to (or watching, if the signals were video) the original signals would detect what seemed to be a strong echo, whose delay (of seconds to minutes, depending on the probe’s distance from the planet) would be twice the time required for the signal to go out to the probe. Such an odd effect would attract the attention of whoever was receiving the original signals, and radio direction-finding techniques would immediately show that the echo was coming from a point out in space. Not long after that, its orbit should be roughly known.

If such beings were intelligent enough to have radio, they should also be clever enough to signal to the probe that they know it is there, by changing their transmission to short phrases separated by quiet intervals in order to remove any overlap of each phrase and its echo. The probe would then detect that the character of the transmission had changed radically to one that was periodic, with a period which would indicate that the probe itself was influencing the distant transmitter. By promptly ceasing to echo, the probe could signal to its new neighbors that it knew that they knew it was there. In other words, both parties would then be aware of each other, and that each party was aware that the other knew of its presence. Once this milestone was reached, events could unfold in multiple ways.

The operator of the transmitter might be under some pressure to persuade the probe to change its frequency, so that the transmitter’s normal function could resume. But desiring to avoid losing this first precarious contact, the probe—which would have the initiative, yet would know very little about the capabilities of the aliens’ radio technology—might begin to test the capabilities of their radio equipment. Without understanding a word (or its equivalent) of their language (and vice-versa), the probe could discover technical parameters such as how sensitive their equipment was, its bandwidth capability (how fast they could receive), and whether another frequency would be more convenient (for technical or political reasons that the locals would know about, but which the probe wouldn’t). The probe would also need to ensure that its message wouldn’t be lost because it would sooner or later—due to the planet’s rotation—set below the horizon of its first contact. It might also have to be prepared for local phenomena (afternoon thunderstorms, sandstorms, starspots, etc.) that could interrupt radio communication.

To test the sensitivity of the aliens’ equipment it could simply weaken its echo. Each time they responded, the probe would weaken its reply, until its signal level dropped low enough that clear reception was difficult, after which the aliens—who could no longer “read” the probe’s signal—would cease to repeat. Having gathered this information, the probe would bring its transmission power back up to its normal level. It would then shift its frequency slightly, which the aliens (if that particular set of radio gear could do it) would follow, and then—if they were able to follow—it would shift its frequency a little more. (Ronald Bracewell noted that if an alien probe happened to first make contact with a commercial radio or television station on Earth, its operators would have great difficulty following the probe’s frequency shifts, but that as soon as a variable-frequency transmitter was brought into action, we could take the lead in changing the frequency slightly, to lead the probe off to another frequency that would be more convenient to us.) By slowly shifting its frequency (both up and down), the probe could also determine the frequency “windows” of the planet’s atmosphere (that is, at what frequencies the atmosphere was transparent to radio waves).

Bracewell also considered the possible political implications of a probe making contact with an alien civilization, if the civilization of the planet in question had any degree of sociopolitical similarity to ours. Unless that world had global political, social, and/or cultural unity, any given “nation” (perhaps even of a different species or subspecies on the planet) might desire to enter into exclusive relations with a probe, for reasons of prestige or possible economic or military advantage (from using the probe’s knowledge and/or technology). The probe’s message (and its mere existence) would likely be disturbing to the planet’s inhabitants, so it would have to be resourceful in order to avoid being trapped into secrecy, to avoid exclusive relations with one power (which would invite an attack by a rival power), and to avoid having its signal jammed by a minor power. The very nature of the probe’s movements, however, would tend to encourage a degree of cooperation between even rival powers, because the probe would set below any station’s horizon within hours. Also, the probe could identify any planet-wide entity (if there was one), to which it could transmit its message. It could simply reduce its transmission power level until respondents lacking large antennas, sensitive receivers, and planet-wide interconnection dropped out; the probe would deal with any organization(s) (analogous to NASA and its Earth-wide communications capabilities) that remained. If some power insisted on communicating with the probe independently, the planet-wide entity would have to defer to that nation for part of each day, or invite jamming for refusing to defer.

Bracewell envisioned that the probe’s message would be in the form of a television broadcast, because TV is like sign language. Geometrical shapes provide a way for two people who don’t understand each other’s languages to learn them. If technological intelligent alien beings have sight similar to ours (this seems likely), they probably have some form of television, and it could be utilized to foster mutual understanding. He pointed out that the number of words in the dictionary that can be defined by drawings probably runs into the thousands, and that many more could be defined by animated drawings. Not only nouns, but many verbs, adjectives, and adverbs can be depicted via television. Other words are harder to define in this way, but given such a “vidioctionary” that defined a few thousand basic words using still and animated pictures, it would be possible to interpret at least some of the more difficult ones.

Until common linguistic understanding was established between the probe and its new neighbors, television would also enable them to learn the answers to basic questions of importance, such as where the probe came from. The probe would first reach a television format in common with that of the aliens. This could be done by repeating its message until they worked out the probe’s TV format and repeated it back to the probe, or the probe could simply adopt their format, transmitting images of simple mathematical shapes (circles, squares, etc.); they could repeat the signal back to tell the probe, “You’ve got our TV format right!” Once this was done, the probe’s message could be a “zoom movie” (using computer graphic imagery where necessary).

It could begin with a view of a constellation or a star field (calculated to appear as it would from their planet) that included our Sun, with the view quickly zooming in on the Sun. The view could then zoom in further until the Sun appeared as a substantial disc, with sunspots visible on it. From their motion (with other nearby—in the angular sense—stars visible in the frame), the Sun’s axis of rotation and the axis’ orientation in space would be shown. Closer zooming would show our Solar System, and at last the view would zoom in on the Earth. Supplementary animated plan views (which could come after a video tour of the Earth) could show the interrelationships between the rotational and orbital periods of the Earth, the Sun, the Moon, and the other planets. Such views would provide the relative rotational and orbital periods (but the aliens, once they knew which star the probe had come from, could determine the Sun’s “absolute” rotational period via spectroscopy, and—possibly—the Earth’s rotational period by monitoring terrestrial radio, TV, and radar emissions). A “zoom travelogue” of our home planet would show our newly-discovered neighbors our world, its natural beauty, its architecture, and views and voices of the beings—and their science, industry, and cultures—that created and launched the probe.

After this, the probe would have another, important piece of business to take care of—conveying the particulars (the frequency, listening schedule, etc.) of how the aliens could engage in direct radio (and/or laser) communication with the Earth. With multiple probes sent to as many stars, radio and/or laser communication telescopes on the Earth, the Moon, or in solar orbit would have to listen to the probes on a schedule, and blocks of time would also need to be allotted for receiving direct messages from any extrasolar civilizations discovered by the probes. (Any probe that found technological intelligent life could first send a report of the discovery to its Earthside controllers before attempting to contact the civilization—and probes that found no such life could also report their negative findings—as this would permit the most efficient use of the “listening time” blocks; such time periods need not be wastefully kept open for stars with no inhabited planets.) Bracewell believed—in the mid-1970s—that the direct interstellar communication information could be expressed by messenger probes using static and/or moving television images. With today’s vastly improved television and computer technologies, such information would likely be easier to convey effectively.

After this task was accomplished, the probe would be dispensable, although the aliens could learn much more about us (and vice-versa) through continued interaction with the probe, which could be packed with enormous quantities of information (even using today’s technology). If the probe met an untimely end, the aliens would have to transmit to the Earth a pictorial dictionary followed by their text or pictographs, and hope for the best. But assuming that the probe didn’t fail, it and they could learn each other’s languages, which would be most helpful for their direct transmissions to us. (The probe would have an “alien/Terran” language translation program, which the probe could send them a copy of on request.) The probe could readily learn their language in printed form, by utilizing an animated pictorial dictionary that they could transmit to it. Its first attempts to “talk” to them in their language might be quaint to them, but they could televise corrected versions back to it.

To be sure that it was properly understood, the probe could do what human beings do—say it again in different words. If they didn’t understand, they could question. (This is a huge advantage of real-time, quick-feedback loop communication with a local “resident” Bracewell probe; it facilitates rapid learning and understanding by both parties, as compared with very long-delay direct interstellar transmissions that take years, decades, or centuries each way). “Probe-mediated initial contact” would enable the direct interstellar transmissions between civilizations to be in mutually-understandable forms from the start. Knowledge of aliens’ languages would enable probes to exchange scientific (including medical and astronomical), philosophical, and cultural knowledge with them, and their races’ knowledge would, of course, immeasurably enrich our own civilization.

If we found even one other technological civilization, the messenger probe concept (which is tolerant of diversion of resources to urgent priorities, because interruption of the launching program does not affect in-flight probes’ chances of success) would enable our societies’ combined efforts to greatly enlarge the volume of galactic space in which more such societies could be sought out for contact. Our space program and theirs could, without wasteful overlap, dispatch messenger probes into large “bubbles” of unexplored space centered on our respective suns. But even if we are the only technological society for thousands—or tens of thousands—of light-years around, none of the Bracewell probes would be wasted, because they would return scientific data and images from the stellar systems they explored. (Even probes that failed to reach their destinations in functional condition would, if they operated well for several years, gather useful information on the interstellar medium, the galactic magnetic field, and cosmic rays in their regions of space.)

Launching interstellar messenger probes would be a “no-lose” (“win-win”) situation. Current and/or soon-to-be-in-hand technologies would likely be sufficient to produce and launch Bracewell probes, particularly ones designed to travel at 1% of the speed of light. (“Sun-diver” solar sails and large ion-drive spacecraft—Soviet scientists advocated the latter as starprobes in 1973, and wrote that they were feasible with then-current technology—could attain that velocity, given some engineering development work and flight testing; no new principles need to be discovered.) The “brains and senses” of Bracewell probes are probably already within our technological reach, at least in “breadboard” prototype form. Computers and software of the necessary sophistication to discriminate between natural radio noise and artificial signals—and to conduct the contact activities that Ronald Bracewell envisioned—already exist, as does the high-density information memory storage that such “electronic ambassadors” would require. The variable matching network-equipped, wideband-tunable longwire antenna and antenna multiplexing are both decades-old technology (which could be utilized if a braking E-sail’s wires were also used as probe-to-Earth, radio science, and alien signal-monitoring antennas; slowly rotating ion-drive starprobes could also use such antenna technology).

Sending realistically-realizable probes to other stars, even the nearest ones, will require patience. But even the fastest-possible (with foreseeable technology) ones, even if launched today, would not reach the Alpha Centauri system, let alone the considerably more distant potentially habitable stellar systems, within the lifetimes of most living adult interstellar spaceflight advocates. That is all the more reason to begin work on such ventures as soon as possible, so that—like trees planted by old men who knew they would never get to enjoy their shade—the first pictures and data from other stellar systems can inform and inspire the immediate descendants of our generation.


[1] 1I/2017 U1 (‘Oumuamua) interstellar asteroid information, NASA Solar System Exploration website:

[2] The Galactic Club: Intelligent Life in Outer Space by Ronald N. Bracewell, page 83 (Published 1974 and 1975 by W. H. Freeman and Company, San Francisco, CA, ISBN: 0-7167-0353-X and 0-7167-0352-1 pbk.) link:

[3] “Meteorites and Cosmic Radiation” by I. R. Cameron (Scientific American, Vol. 229, No. 1, page 65, July 1973)

[4] “Self-Healing Transistors for Chip-Scale Starships” (IEEE Spectrum, January 30, 2017):

[5] Pioneer 6, 7, 8, and 9, Wikipedia article:,_7,_8,_and_9

[6] LES 1 satellite, Google website citations.

[7] LES 8, 9, Gunter’s Space Page article:

[8] Lincoln Experimental Satellite Turns 40, MIT Lincoln Laboratory website article:

[9] AMSAT-OSCAR 7, Wikipedia article:

[10] International Cometary Explorer, Wikipedia article:

[11] Morphic Resonance: The Nature of Formative Causation by Rupert Sheldrake, page 247 (4th Revision, Published 2009 by Park Street Press, Rochester, VT, ISBN: 978-1594773174 and 1594773173) link:

[12] “Redefining Robots” by P. Trachtman (Smithsonian Magazine, February 2000, pages 97 – 112)

[13] Robot Explorers by Kenneth Gatland, pages 239 – 244 (Published 1972 by Blandford Press, London, ISBN: 0-7137-0573-6) Amazon link:

[14] Planetary Exploration: Space in the Seventies by William R. Corliss (NASA Publication EP-82, June 1971):

[15] Computers in Spaceflight: The NASA Experience by James E. Tomayko, pages 149 – 153 (NASA Contractor Report 182505, March 1988):

[16] Is Anyone Out There? by Jack Stoneley with Anthony T. Lawton, pages 15, 17, and 174 (Published 1974 by Warner Paperback Library, New York, NY) Amazon link:

[17] Nuclear pulse propulsion, Wikipedia article:

[18] “Sprites: A Chip-Sized Spacecraft Solution” (Centauri Dreams, July 17, 2014):

[19] Breakthrough Starshot, Breakthrough Initiatives website:

[20] NASA 2069 Alpha Centauri solar sail probe, Google website citations.

[21] “Interstellar Vehicle Propelled by Terrestrial Laser Beam” by G. Marx (Nature, Vol. 211, July 1966, pages 22 – 24)

[22] “Interstellar Vehicle Propelled by Terrestrial Laser Beam” by J. L. Redding (Nature, Vol. 213, February 1967, pages 588 – 589)

[23] “Was Marx right? Or how efficient are laser driven interstellar spacecraft?” by J. F. L. Simmons and Colin R. McInnes (American Journal of Physics, Vol. 61 (1993), pages 205 – 207)

[24] Interstellar Communication: Scientific Perspectives by Cyril Ponnamperuma and A. G. W. Cameron, page 100 (Published 1974 by Houghton Mifflin Company, Boston, MA, Library of Congress Catalog Card Number: 73-11945, ISBN: 0-395-17809-6) Amazon link:

[25] “Transcription of the Lectures of Ronald Bracewell on the Studies of Extraterrestrial Life” (Stanford Alumni Conference Lecture, delivered on June 1, 1974, pages 16 and 17 [this transcript is available by e-mail from the author at:]).

[26] Life in Other Solar Systems by Frederick I. Ordway, III, pages 78 and 79 (Published 1965 by E. P. Dutton & Co., Inc., New York, NY, Library of Congress Catalog Card Number: 65-12184) AbeBooks link:

[27] “Interstellar Probes: A New Approach to SETI” by Robert A. Freitas, Jr. (Journal of the British Interplanetary Society, Vol. 33, pp. 95-100, 1980):

[28] “The Case for Interstellar Probes” by Robert A. Freitas, Jr. (Journal of the British Interplanetary Society 36:490-495, November, 1983):

[29] Self-replicating spacecraft, Wikipedia article:

[30] Bracewell probes, The Worlds of David Darling website article:

[31] The Fountains of Paradise by Arthur C. Clarke (Various hardcover, paperback, audio, and e-book editions, first published 1978) Amazon link:

[32] Electric sail, Wikipedia article:

[33] ARRL Antenna Book, pages 13-1 and 13-2 in Chapter 13, “Long Wire and Traveling Wave Antennas” (Published at intervals by the American Radio Relay League, Newington, CT):

[34] Small Smart Interstellar Probes (Section 2.1), Professor Allen Tough’s website:

[35] The Galactic Club: Intelligent Life in Outer Space by Ronald N. Bracewell, pages 59 – 83 (Published 1974 and 1975 by W. H. Freeman and Company, San Francisco, CA, ISBN: 0-7167-0353-X and 0-7167-0352-1 pbk.) link: