Kitt Peak National Observatory
National Optical Astronomy Observatories*
P.O. Box 26732, Tucson, Arizona 85726, USA
Published in Acta Astronautica 12(1985):1027-1034, IAA-84-243 (A)
Note: This web version is derived from an earlier draft of the paper and may possibly differ in some substantial aspects from the final published paper.
The rationale for the use of interstellar artifacts by intelligent life in the universe is described. The advantages of using interstellar probes as a means of exploration and communication are presented and shown to be significant enough to counter the time, energy, and technology arguments generally raised against contact via extraterrestrial artifacts. Four classes of artifacts are defined: Those seeking contact, those seeking to avoid contact, those intended to provide a passive technological threshold for detection, and those for which detection is irrelevant. The Search for Extraterrestrial Artifacts (SETA) is based on the latter two classes. Under the assumption that an extraterrestrial probe will be interested in life in our solar system, a near-Earth search space is defined. This search space is accessible to us now with ground and satellite observing facilities. The current observational status of SETA is reviewed and contrasted with the achievable detection limits for the different parts of the search space.
1.0 THE CASE FOR INTERSTELLAR PROBES
Interstellar exploration to seek life and intelligence in the cosmos using automated messenger probes has been discussed by many authors1,2,3,4,5,6,7. This approach suggests a new observational strategy in the Search for Extraterrestrial Intelligence (SETI), using existing or foreseeable instrumentation, which optimizes the probability of detection of such artifacts in our solar system. We shall call this new strategy the Search for Extraterrestrial Artifacts, or SETA. A SETA program complements past and continuing SETI searches for artificial radio signals8-9.
Numerous arguments disputing the viability of extraterrestrial artifacts as a means for interstellar communication have been critically reviewed in an earlier paper10. The three principal objections are: (1) Interstellar probes require too much energy, compared to radio communication; (2) interstellar flight takes too long; and (3) radio technology is far simpler than probe technology.
A self-powered interstellar probe which first accelerates to v = 10%c, coasts to its destination, then decelerates to zero velocity, requires a mass-ratio (initial/final mass) Rm = [(c+v)(c-v)]c/V = 1.2 for a photon rocket (exhaust velocity v = c) or 5.0 for a fusion rocket (V = c/8), as calculated according to Purcell11. These are hardly excessive, considering the mass-ratio of about 22 for Space Shuttle. The argument" that Rm is enormous for v = 0.99c merely demonstrates that it is impractical and fortunately unnecessary -- to travel this fast. In addition, probes may be externally powered (e.g., solar-sails, ramjets, or solar-electric propulsion) so mass-ratio is not a threshold parameter for interstellar flight.
Calculations showing that the energy required to launch spacecraft to the stars is equal to many thousands of times the current total global power consumption or will cost 100 Gross World Products12 are irrelevant to SETI. This is because these computations chauvinistically presume current human society to be the standard of comparison for the entire universe. Almost certainly any race capable of transmitting either radio signals or interstellar probes for SETI purposes must be far in advance of ourselves, possibly on the level of a Kardashev Type II civilization (utilizing a major fraction of the energy output of their sun). To launch a 100,000-ton vehicle on a one-way trip at 1-gee acceleration to a destination 100 light-years away requires an equivalent relative energy expenditure for a Type II civilization as the launching of a few Saturn V moon rockets represented to human society more than a decade ago. Active interstellar exploration by advanced cultures will require commitment but hardly an outrageous sacrifice.
Further, if the probe is self-replicating only one initial device need be sent out. For a payload mass of 100 tons13 a single 10%c self-reproducing interstellar probe14 can initiate a wave of galactic exploration for a transport-energy investment of only 1020 J, an expenditure of 30 gigawatts over a period of 100 years (approximate travel time to the nearest stars) which could be entirely supplied by one 15-km-wide, 10% efficient solar power satellite in Earth orbit. Upon arrival, the self-replicating probe could be programmed to construct a gigawatt SETI radio beacon "for free" in the target solar system at a (militarily) safe distance from the home world, or to perform other operations upon planets15 in the target system by remote command as a prelude to colonization by the senders.
Clearly, the proper temporal measure is not mere travel velocity but rather the total time required for successful completion of the mission objective -- to explore, possibly contact, an extrasolar planetary system. By this measure, probes actually save time because an intelligent artifact completes its mission in 102-l03 years after launch as compared to a potentially infinite mission completion time for radio signals which go undetected. If the average waiting time for communicative civilizations to emerge is of the same order as interstellar flight times (102-103 years) or longer, as seems likely, then even the small temporary speed advantage of radio waves evaporates.
Note too that on Earth the first gas reaction jet and first rockets were built 2200 years and 600 years ago, respectively, pre-dating the invention of radio by many centuries. The results of the Project Daedalus starship study16 suggest radio and interstellar flight technologies probably should be regarded as virtually simultaneous technical developments in evolutionary and interstellar communication time frames. The transmission technology a potential recipient species (e.g., humanity) happens to possess at a given instant in its historical development is largely irrelevant to the choice space of message senders.
Messenger probes become independent agents as soon as they are launched. If properly designed, there should be no further need for major energy expenditure by the originating civilization. Sophisticated artifacts may be self-repairing or self-replicating6,13,14 and could be capable of refueling or reproducing at every port of call. Launching a single self-replicating "seed" probe could establish a wave of exploration across the entire Galaxy in 1-10 million years, using just 11 generations of daughters with 10 daughters per generation17. Upon arrival, probes can employ local materials to construct and energize instruments of exploration and replication "for free." In theory, senders can install a permanent, self-repairing galactic communications network at the cost of one initial self-reproducing interstellar probe. Probes can park in orbit for thousands of years or longer, awaiting discovery by emerging communicative cultures on suitable planets in the star system. Alternatively, they may be programmed to execute multistar flyby trajectories until a communicative species is located, then enter into a dialogue with them at little further cost to the originators.
With beacons, huge transmitters must be constantly supervised, serviced, and fed energy that the sending society could probably better use elsewhere. Beacons may radiate energy and information for millennia or longer without gaining any response or any new information in return. This energy, since it was detected by no receiver, in essence was wasted and constitutes pure economic loss for the transmitting society. Except in the rare case of malfunction, no energy spent to build or to deploy a probe is ever wasted.
Another major advantage of probes during acquisition is their ability to place a high-strength "local beacon" signal in the immediate vicinity of the target planet, which will immediately attract attention as unmistakable evidence of the existence of ETI. The extraterrestrial origin of the probe could be quickly and accurately verified to the satisfaction of all skeptics. Much like the current "evidence" for UFOs, an interstellar beacon signal could all too easily be suspected as a hoax, military diversion, or be explained away as an unusual natural phenomenon.
Probes also have greater versatility of action and flexibility of response to unknown conditions at the target star system. Thus an intelligent artifact can select a course of action from a number of available alternatives including flyby, release of subprobes, entering orbit, refueling, contacting local civilization or remaining silent, or proceeding with self-replication. As an added benefit, an intelligent artifact could establish a high-bit-rate noise-free local communication channel on any frequency of the contactee's own choosing2. By comparison, all a remote beacon signal can do is keep signaling blindly. Remote beacons offer no possibility of modifying the signal until at least one light-speed round trip travel time has elapsed. Unlike probes, radio waves cannot modify themselves in response to unexpected target characteristics.
If acquisition occurs, interstellar probes are superior in terms of communications feedback. An intelligent artifact orbiting a garrulous inhabited world may engage in a true conversation with the indigenous civilization, an almost instantaneous, complex interaction between cultures. Responses to questions and answers are immediate, permitting real-time educational and linguistic exchanges with a precision and rapidity no remote (interstellar) radio signaling system could possibly match. Contact via probes provides a potentially richer, deeper interaction than via interstellar radio links. A probe's onboard memory elements may contain an appreciable fraction of the knowledge and culture of the sending civilization. By comparison, electromagnetic message scenarios appear little better than sterile data swaps or massive data dumps with extraordinarily poor bit rates.
An interesting alternative suggestion is that the radio message may consist of instructions to build a probe (or computer) which can then achieve the desired swift interaction". Unfortunately, there are numerous difficulties with this approach: (a) It assumes the technical competence of the recipients is sufficient to correctly construct a device of alien design; (b) it assumes all necessary materials and tools are available to the recipients; (c) it assumes recipients will be brave (foolhardy?) enough to assemble and activate a device whose purpose and operation they cannot possibly fully comprehend; (d) it requires the entire information bank of the probe to be downloaded by interstellar link, plus the general-purpose instructions on building a probe, which together must be a larger data set than the information bank alone.
The use of artifacts gives the overwhelming advantage of military security to the transmitting species3. Interstellar radio beacons are an invitation to disaster at the hands of unknown predatory alien civilizations. In any situation involving contact via electromagnetic signals, the sending society must give away the position of its home star system and its self-encyclopedia at great risk for mere speculative benefits. On the other hand, if local technological activity is detected by an intelligent messenger probe, the device may initiate surveillance or contact without ever having to disclose the identity or whereabouts of its creators. The probe is also free to withhold any sensitive information concerning weaknesses or strengths of the senders. (It could even supply military misinformation, making the senders appear more formidable than they really are and thus discourage aggression.) If the probe must make periodic reports home, this may be accomplished in a manner virtually impossible to trace or to decipher. Probes permit senders to retain control of the interaction; beacons donate that control to unknown, possibly unfriendly, recipients.
Finally, transmitted radio energy rapidly dissipates with time. On the contrary, the databanks of a self-repairing artifact may preserve for geological timescales the knowledge and cultural heritage of the transmitting civilization. This record of successful and failed survival strategies would be an archeological find of incalculable value to an expanding technological species.
The exploration goal restricts the search space for observable class (3) artifacts. These objects must elect to reside in the best possible location from which to monitor phenomena relevant to their mission. Earth is the only known site of life and intelligence in the solar system, so the simplest assumption is that the probe will recognize this fact and take up residence nearby, stationing itself in lunar or cislunar orbit. The Earth's surface is an unlikely site because of increased risk in the active and degradative terrestrial environment, more restricted access to solar energy, and the probe's impaired ability to continuously monitor the entire terrestrial and interplanetary environment during long-term studies.
The goal of long-duration exploration and the need to observe the Earth from space impose size constraints on possible messenger probes. First, an optical ground resolution of 10 meters is required for unambiguous visual detection from orbit of intelligent activity on the surface of the Earth". This requires an optical aperture of ~3-30 meters for a 70,000-384,000 km cislunar orbit. Second, an artifact , must withstand radiation pressure and catastrophic meteoroid impacts likely to occur during the anticipated on-orbit residence time. Freitas4 calculates that the minimum size for defenseless artifacts is >0.2-20 m for a >105-106 yr residence time (~ terrestrial speciation timescale22) in the vicinity of Earth. Finally, exploration is useless unless the artifact can return information to its creators. Radio transmissions at waterhole frequencies 1-10 GHz are optimum in terms of bandwidth, energy cost, and background noise8,9. To achieve a gain of 1 < g < 107 (~ Arecibo) requires a minimum waterhole antenna diameter ~(l/p)gl/2 = 0.01-30 m.
These considerations suggest that a reasonable search limit is for an object of typical dimension ~1-10 m with a visual geometric albedo of ~0.1 (pv = 0.02-0.35 for asteroids), termed the limiting artifact.
||Lawton and Newton |
Leuschner Obs., USA
L4/L5 and assoc. orbits
||Freitas and Valdes |
Kitt Peak Obs., USA
L4/L5 vicinity and assoc. orbits
Sun-Earth: L2 vicinity
||Valdes and Freitas |
Detection of a limiting artifact requires visual magnitude limits from +15 (70,000 km) to +18 (326,000 km). Existing military radars such as the Altair radar at Kwajalein Missile Range monitor the geocentric volume to 40,000 km for 5-meter objects, but none of these pencil-beam radars is effective for search. The new GEODSS (Ground-based Electro-Optical Deep Space Surveillance) system which replaces the SAO Baker-Nunn network can detect +16.5 mag near-Earth satellites but has not been employed for significant asteroid searches and cannot perform well at the required limiting sensitivity. The most comprehensive work is by Tombaugh et al.31 in a 5-year program during which 15,567 photographs were taken in an attempt to discover small natural Earth satellites. The search was confined entirely to the ecliptic and equatorial planes, with excellent coverage below 10,000 km altitude to magnitude +15. However, in the cislunar SETA region Tombaugh searched only three concentric circular orbital zones along the ecliptic — 85,000 km retrograde (173° coverage), 88,000 km direct (342° coverage), and 121,000 kin retrograde (158° coverage). Even if the search could be deemed complete out to Earth-Moon L1 within ± 5° (Tombaugh's Schmidt field) of the ecliptic, 90% of the search volume would remain unexamined to any serious magnitude limit.
Ground-based detection of a limiting artifact requires a visual magnitude limit of +18 to +19. Pickering33 searched a 300 x 300 square swath to magnitude +10 during lunar eclipse, which Barnard34 extended to magnitude +12. Tombaugh et al.31 performed a multitelescopic survey during lunar eclipse of a 190 circular field to magnitude +13, save a small region near the Earth-Moon axis where the limit was +12. Zones of maximum elongation within 50 of the lunar orbital path were probed to magnitude +14 to +17. Approximately 50% of the selenocentric orbit space was examined by Valdes and Freitas58 with limits from +10 to +16 magnitude.
3.3 Earth-Moon Libration OrbitIn the classical three-body problem, small objects placed at one of the five Lagrangian points in the plane of revolution are in dynamical equilibrium, and two of these, the triangular points L4 and L5, are stable. The true Earth-Moon system represents a four-body problem because of the significant gravitational influence of the Sun. L4 and L5 themselves are unstable, but large, stable libration orbits around them, synchronized with the synodic month, have been shown analytically35-36 and numerically37 to exist. Thus the positions of objects in the stable libration orbits can be computed and unique time-variable ephemerides determined. Each synodic libration orbit has two stable phases 1800 apart with semimajor axis 150,000 km and semiminor axis 75,000 km. Oscillations in the Earth-Moon plane are likely for objects parked for very long periods of time, but deviations from the solar-synchronized positions should be minimal for powered artifacts inserted on precision trajectories. Out-of-plane motion is not seriously excited by the Sun and is almost certainly less than the mean lunar/ecliptic inclination24,35,38-40. Thus the 3-dimensional search volume near L4/L5 is a squat, elliptical cylinder, measuring 300,000 km x 150,000 km in the lunar orbital plane and extending at most 35,000 km above and below the plane, with velocity limits 0.7-1.3 km/sec relative to the Earth, S = 900 deg2 for both regions, and Dw = 1.1 x 10-4 deg/sec.
Detection of a limiting artifact requires a comprehensive search to magnitude +18 to +19. The regions near IAIL5 have been examined for luminous dust clouds following reports of their existance41-47. These investigations covered less than the full 450 deg2 search space near each libration point to limiting magnitudes < +10 and failed to confirm the discovery48-53. Kordylewski also searched the entire region for' discrete objects to magnitude +12, Bruman54 examined a 44 deg2 field near L4 during lunar eclipse using the 48-inch Palomar Schmidt to limiting magnitude +15 to +17. Giclas55 searched a 180 deg2 field near L5 to magnitude +16, and Freitas and Valdes56 performed a preliminary survey near the L4/L5 synodic libration orbits to magnitude +14 - all with negative results. An informal radar search57 near L4/L5 conducted at Arecibo found no targets >10 m2.
In the most thorough search to date. Valdes and Freitas58 photographed the entire 45° libration orbit region within +2.5 deg of the lunar orbital plane near L4/L5 to limiting magnitude +17 to +19 for a limited range of artifact velocities using the 24-inch Warner and Swasey Schmidt telescope at KPNO, but found nothing.
The rectangular search space of S = 280 deg2 overlaps considerably with the selenocentric space, representing an additional 90 deg2. Pickering31 surveyed this additional space to magnitude +10, and Tombaugh et al.31 examined about half to magnitude +13. This search is similar to the selenocentric survey except for the anticipated slower motion of halo objects as compared to lunar satellites.
Valdes and Freitas58 searched a region 15? x 24? covering 100% of the predicted Ll/L2 halo orbital space. The magnitude limit for objects moving near the sidereal rate ranged from 12-15th magnitude within 2-4? of the moon to 17-18th magnitude beyond about 8? from the lunar disk, corresponding roughly to objects 3-30 meters in size having lunar surface albedo. For objects moving closer to the lunar rate, 1-2 magnitudes would be lost to image trailing, giving a range of 10-16th magnitude, approximately doubling the minimum sizes of detectable objects.
Until recently, no searches for discrete objects fainter than 14th magnitude near Sun-Earth L2 had been reported in the literature. Valdes and Freitas58 surveyed Sun-Earth L2 because its distance from Earth is only four times that of the Moon, and thus was the only remaining reasonable target for terrestrial-based Schmidt telescope searches for small libration objects. The position of L2 was taken as collinear with the Sun and the Earth-Moon barycenter78. The Sun-Earth L2-centered survey covered a 14? x 14? field to a limit of 17-19th magnitude, which corresponds, depending on the amount of image trailing which might be expected, to an object size of about 4-10 meters at lunar albedo.
Because of the large spatial and angular rate search space, deeper searches must rely on digital technology which relieves the human observer of the task of visually inspecting images. This requires two major pieces of technology -automated survey telescopes of moderate fields with large format digital detectors, and automated image analyzers. Progress is being made on both fronts. Valdes and Freitas58 attempted to use automated detection techniques with limited success. The ultimate limit with foreseeable ground-based technology is 23-24 mag.
Searches for the smallest likely extraterrestrial artifact in all potential SETA orbital regions may require limits of +27 to +28 magnitude in the visual. This necessitates the use of wide-field, large-aperture, space-based telescopes. The Space Telescope can achieve this limit but. even if devoted full time to a SETA search, would need many decades because of the small field and low data rates to the ground. Selenocentric artifacts could more easily be detected using a lunar-based (surface or orbiting) telescope facility because proximity to the target reduces the required magnitude limits to +17 to +23 for an exhaustive search for the smallest likely artifact.
Artifacts plausibly might also be detectable in the infrared. Cislunar objects in equilibrium with the insolation65,66 having a bolometric emissivity ~ 0.1 and a Bond albedo ~ 0.1 have a temperature 390? K and infrared magnitude N = +11 at lunar distance and N = +7 to +10 in the geocentric region for R = 1 m at 10 gm. The AFCRL 11 mm sky survey67 achieved N = 0 to -1, a limit of ~100-300 m, and IRAS68 reached N = +7 to +8 at 10 mm. This gives a theoretical limit of ~3-6 m for all orbital regions after Tc = 0.25 yr, but these limits may not be secure. Selection criteria for the final AFGL catalog were chosen to exclude asteroids and satellites. The original IRAS software was designed to reject asteroids, although a specialty database established for fast-moving objects is being compiled.