The search for extraterrestrial artifacts (SETA)

Acta Astronautica Vol, 12. No. 12. pp. 1027-1034. 1985 Printed in Great Britain.

0094 5765/85 $3,00+ .00 4) 1985 Pergamon Press Ltd.

THE SEARCH FOR EXTRATERRESTRIAL ARTIFACTS (SETA)t ROBERT A. FREITAS JR. Xenology Research Institute, 8256 Scottsdale Drive, Sacramento. CA 95828, U.S.A. and FRANCISCO VALDES Kitt Peak National Observatory, National Optical Astronomy Observatories,* P.O. Box 26732, Tucson, AZ 85726, U.S.A. (Received 19 March 1985) A b s t r a c t - - T h e 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. THE CASE FOR INTERSTELLAR PROBES

Interstellar exploration to seek life and intelligence in the cosmos using automated messenger probes has been discussed by many authors[I-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 signals[8,9]. Numerous arguments disputing the viability of extraterrestrial artifacts as a means for interstellar communication have been critically reviewed in an earlier paper[ 10]. 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. 1.1 Energy considerations

To receive interstellar communications that arrive by radio or by messenger probe requires a search using either radiotelescopes or optical telescopes, either of which costs about the same. The costs to transmit observable radio signals or observable probes, over the duration of an entire exploration program covering, say, the nearest million stars, also are comparable. For example, to operate tPaper presented at the 35th Congress of the International Astronautical Federation, Lausanne, Switzerland, 8-13 October 1984. *Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 12:t2-E

a gigawatt beacon for 1 million years requires about the same energy as deploying a fleet of 100 kg 10%c probes to each of the million target stars at the rate of one per year. During this time the beacon may elicit no response to its call and thus yields zero knowledge. On the other hand, the probe fleet is certain to have returned comprehensive data on a million nearby solar systems even if no intelligent life is found. This seems more cost effective. A self-powered interstellar probe which first accelerates to o = 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 Purcell[ 11 ]. These are hardly excessive, considering the mass-ratio of about 22 for the Space Shuttle. The argument that R,, is enormous for v = 0.99c[11] 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, solar-electric propulsion) so massratio 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 Products[12] 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-

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way trip at l gee acceleration to a destination 100 light years away requires an equivalent relative energy expenditure for a Type lI 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 initiat device need be sent out. For a payload mass of 100 tonsil3] a single 10%c self-reproducing interstellar probe[ 14] can initiate a wave of galactic exploration for a transport-energy investment of only l02° J, an expenditure of 30 gW 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 planets[ 15] in the target system by remote command as a prelude to colonization by the senders.

1.2 Time considerations Electromagnetic signals do travel faster than probes, but upon arrival radio waves cannot seek out local civilization like a probe. If locals are not actively pursuing SETI, radio cannot detect them as an artifact can. The probe's enormous advantage in establishing a rapid information exchange with local communicants immediately upon arrival (or any time afterwards) more than offsets any slight disadvantage of slower initial transmission speed. Additionally, if there is no communicative life in the target system then radio waves cannot stimulate a response and the mission fails, whereas artifacts can still file a complete and informative report. 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 cxtrasolar planetary system. By this measure, probes actually save time because an intelligent artifact completes its mission in 102-10 ~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. 1.3 Technology considerations Although radio technology seems less complex, it is also less competent than probe technology for interstellar contact and communication--and technical competence and simplicity are at least of equal importance in engineering. From the viewpoint of the sending civilization, a " s i m p l e " technology likely to fail in meeting its objectives may not be considered more desirable than a " c o m p l e x " technology which almost guarantees mission success. Also, it is equally easy to receive probes or photons, reception being the only relevant parameter from the point of view of the recipient civilization.

Note too that on Earth the first gas reaction jet and first rockets were built 2200 years and 600 years ago. respectively, predating the invention of radio by many centuries. The results of the Project Daedalus starship study[ 16] suggest radio and interstellar flight tcchnolo gies probably should be regarded as virtually simultaneous technical developments in evolutionary and interstellar communication time frames. The transmi.s.~ion 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.

1.4 Advantages of interstellar probes Interstellar probes are the only known means to seek out either nontechnological intelligent life or nonintelligent life, neither of which beacon signals can detect. As for technological intelligent life, in the most optimistic case either a probe or a beacon may elicit some local response. But if the locals are uncooperative or are not listening, beaconers will hear nothing whereas probers still get a full report on the physical environment of another star system. The likelihood of gaining some useful information return is always greater for probes than for beacons. Also, a negative cannot be proven, so a beacon operated for, say, 106 years which receives no response permits few definitive conclusions regarding the galactic distribution of life and intelligence, Only probes can reliably and conclusively return information from every star system surveyed, sufficient to establish the truth or falsity of statistical assertions concerning ETI. 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-replicating[6,13,14] and could be capable of refueling or reproducing at every port of call. Launching a single self-replicating " s e e d " probe could establish a wave of exploration across the entire Galaxy in 1-10 million years, using just 1 I generations of daughters with 10 daughters per generation[17]. 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, selfrepairing 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 lor the transmitting society. Except in the rare case of

The Search for Extraterrestrial Artifacts 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 noisefree local communication channel on any frequency of the contactee's own choosing[2]. By comparison, all a remote beacon signal can do is keep signalling 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 interaction118]. 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 species[3]. Inter-

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stellar 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.

2. CLASSES OF OBSERVABLE ARTIFACTS

From an observational standpoint all extraterrestrial artifacts fall into one of three major classes: (1) Objects seeking contact; (2) objects seeking to avoid contact; and (3) objects for which contact or detection by us is irrelevant or unimportant[4]. The simplest assumption is that extraterrestrial technology is sufficient to guarantee the intended result. Thus objects seeking contact cannot be present in our solar system because we have not observed their intentional manifestations. (The sole exception is an object seeking contact but intended to provide a passive technological threshold for detection. The observable members of this subset should be very few in number because the threshold must correspond almost precisely to current human capabilities to explain the lack of detection to date, and yet be observable to us now.) Objects seeking to avoid contact may be present but it is impossible for us to observe them. Thus, only class (3) objects are observable. 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

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SETA, a small fraction of the region has been partially observe the Earth from space impose size constraints on searched to + 19 mag[21,22]. Site properties, current possible messenger probes. First, an optical ground resolution of 10 meters is required for unambiguous visual observational status, and detection limits for limiting ardetection from orbit of intelligent activity on the surface tifacts in each principal orbital region are discussed below. Table 1 summarizes the SETA searches perfl)rmed of the Earth[l 9]. This requires an optical aperture of - 3 to date. 30 m for a 70,000-384,000 km cislunar orbit. Second, an artifact must withstand radiation pressure and cata3. I Geocentric orbits strophic meteoroid impacts likely to occur during the Cislunar geocentric orbits with apogees beyond the anticipated on-orbit residence time. Freitas[4] calculates mean distance of the interior collinear Earth-Moon Lathat the minimum size for defenseless artifacts is > 0 . 2 20 m for a > 106-106 yr residence time (~terrestrial spe- grangian point L1 (326,000 km) lack long-term stability. Szebehely[23] suggests a maximum stable radius for planar eiation timescale[20]) in the vicinity of Earth. Finally, exploration is useless unless the artifact can return in- circular orbits of 244,000 km, a more restrictive limit. formation to its creators. Radio transmissions at water- Orbits with perigees below 5500 km altitude have lifehole frequencies 1-10 GHz are optimum in terms of times less than 106 years due to atmospheric drag, whereas perigees above 8000 km yield lifetimes comparable to bandwidth, energy cost and background noise[8,9]. To achieve a gain of I -< g -< 107 (~Arecibo) requires a the age of the solar system[24]. Perigees between 300064,000 km altitude in the Van Allen radiation belts are minimumwaterholeantennadiameter~(h/Tr)g ~3 = 0.01unlikely because of the increased potential risk (c.g. to 30 m. electronic systems[25]) for long-duration missions. The These considerations suggest that a reasonable search limit is for an object of typical dimension ~ 1-10 m with . translunar Earth-Moon barycentric orbital volume lying between Earth-Moon L2 (449,000 km) and Sun-Earth a visual geometric albedo of ~ 0 . I (p. = 0 . 0 2 - 0 . 3 5 for LI (I .49 x 1@ km) while apparently quite largc appears asteroids), termed the limiting arti]act. stable only in a very restricted range of radii, if at a11123,26,27], and also is unattractive because of its greater 3. CURRENT SEARCH STATUS distance from Earth. Hence the most plausible search volume lies roughly between two geocentric spheres of The lunar/cislunar orbit assumption yields a SETA radii 70,000 and 326,000 kin, with Earth-relative vclncity search space consisting of geocentric, selenocentric, Earthlimits 1.1-3.4 km/sec, search area S = (360)~/rr dcg ~, Moon libration and Earth-Moon halo orbits (see Fig. 1). and total apparent angular rate range Ato = 5.2 x 10 ' The L4 branch of a fifth SETA region, the Sun-Earth deg /sec. Lagrangian tadpole orbits, also is shown in Fig. 1. Though Detection of a limiting artifact requires visual magthe Sun-Earth L4/L5 orbits are of marginal utility for

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Fig. 1. SETA orbital search regions for extraterrestrial artifacts, showing Sun-Earth Lagrangian L4 tadpole width at 10 x scale and Earth-Moon system at 200 x scale.

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The Search for Extraterrestrial Artifacts Table 1. Searches for extraterrestrial artifacts I Datn 1973-74

1979

1981-82

ObJe~lve Long-Delay

Instrument Helix Antenna Sbeppertoo UK

Tarllet Earth.Moon: L4/L5

Wavelenllth VHF

0.76-m Casnegraln Leuschner Obs. USA

Earth-Moon."

Optical

Orbiting Artifacts

50

Freitas and Valdes {~}

0.61-m Schmidt Kitt Peak Obs. USA

Earth-Moon: Lunffir orbits, LI/L2 vicinity, L3 vicinity, L4fl, S and assc¢, orbits Sun-Earth: L2 vi¢ioit?"

Optical

Orbiting Artifacts

56

Valdes and Freitas I$Sl

Echoes

L4/L5 and

Houris Observers ?? Lawton and Newton [631

~soc. orbits

nitude limits from + 15 (70,000 kin) 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 m objects, but none of these pencilbeam 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.[28] in a 5 yr 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 km 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. 3.2 Selenocentric orbits Stable selenocentric orbits cannot have apolunes greater than the mean distance between the Moon and EarthMoon L1 (58,100 km). Szebehely[23] and Markellos and Roy[27] give the maximum stable radii for planar circular stable orbits as 28,000 and 16,700 km for direct and retrograde satellites, respectively. Perilune is only weakly constrained by interaction with the present lunar atmosphere, though Chernyak's[29] suggestion of a possible ancient ~ 5 x 10 -7 bar lunar atmosphere could increase minimum altitude to several thousand kilometers. Hence the selenocentric orbital search volume lies between 300058,100 km, angular diameter 17° (S = 230 deg 2) viewed from Earth, with velocity limits 0.3-1.8 km/sec relative to the Moon (Ato = 5.4 x 10 4 deg/sec). Ground-based detection of a limiting artifact requires a visual magnitude limit of + 18 to + 19. Pickering[30] searched a 30 ° x 30 ° square swath to magnitude + 10 during lunar eclipse, which Bamard[31 ] extended to magnitude + 12. Tombaugh et a1.128] performed a multitelescopic survey during lunar eclipse of a 19° circular field

to magnitude + 13, save a small region near the EarthMoon axis where the limit was + 12. Zones of maximum elongation within 5 ° of the lunar orbital path were probed to magnitude + 14 to + 17. Approximately 50% of the selenocentric orbit space was examined by Valdes and Freitas[32] with limits from + 10 to + 16 magnitude. 3.3 Earth-Moon libration orbits In 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 analytically[33,34] and numerically[35] 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 180 ° 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 inclination[33,36-39]. Thus the three-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 deg 2 for both regions, and Ato = 1.1 x l0 -4 deg/sec. Detection of a limiting artifact requires a comprehensive search to magnitude + 18 to + 19. The regions near L4/L5 have been examined for luminous dust clouds following reports of their existence[40-46]. These investigations covered less than the full 450 deg z search space near each libration point to limiting magnitudes < + 10 and failed to confirm the discovery[47-52]. Kordylewski also searched the entire region for discrete objects to magnitude + 12, Bruman[53] examined a 44 deg z field near L4 during lunar eclipse using the 48 inch Palomar Schmidt to limiting magnitude + 15 to + 17, Gi-

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R, A. FREITASJR. and E VALDLS

clas[54] searched a 180 deg-' field near L5 to magnitude + 16, and Freitas and Valdes[55] performed a preliminary survey near the L4/L5 synodic libration orbits to magnitude + 14--all with negative results. An informal radar search near L4/L5 conducted at Arccibo found no targets >10 m~[56]. In the most thorough search to date, Valdes and Freitas[32] 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.

3.4 Earth-Moon halo orbits Periodic halo orbits near the collinear Lagrangian points L I and L2 have long been known to require stationkeeping due to their dynamic instability[57], but Breakwell and Brown[58] have demonstrated the theoretical existence of families of stable nonplanar orbits. The inclusion of lunar eccentricity ruins the stability, but Breakwell[59] believes there is a good chance real stable halo orbits exist. These families lie entirely within a rectangular hexahedral Moon-centered volume measuring 80,000 km × 80,000 km in the lunar orbital plane and 160,000 km in vertical extent. Typical halo object velocity is 0.8 km/sec relative to the Moon, with Aw = 2.8 x 10 4 deg/sec. The rectangular search space of S = 280 deg 2 overlaps considerably with the selenocentric space, representing an additional 90 deg 2. Pickering[30] surveyed this additional space to magnitude + 10, and Tombaugh et al. [28] 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 Freitas[32] searched a region 15° × 24 ° covering 100% of the predicted L 1/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 m 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.

3.5 Earth-Moon L3 and Sun-Earth L2 orbits The possibility of halo-like stable nonplanar orbits in the vicinity of Earth-Moon L3 is unlikely but cannot yet be ruled out on theoretical grounds[59]. The only published search for objects at this position is by Valdes and Freitas[32], who surveyed a roughly L3-centered field measuring 25 ° × 13 ° to 17-19th magnitude, corresponding to a hypothetical object size of 1-3 m at lunar albedo. Until recently, no searches for discrete objects fainter than 14th magnitude near Sun-Earth L2 had been reported in the literature. Valdes and Freitas[32] surveyed Sun-Earth L2 because its distance from Earth is only four times that of the Moon and thus was the only re-

maining reasonable target for terrestrial-based Schmidt telescope searches for small libration objects. The po sition of L2 was taken as co]linear with the Sun and thc Earth-Moon barycenter[60]. The Sun Earth L2-ccntered survey covered a 14° × 14° field to a limit of 17-19th magnitude, which corresponds, depending on thc amount of" image trailing which might be expected, to an object size of about 4-10 m at lunar albedo.

4. FUTURE PROSPECTS 4.1 Optical searches Though the work of Valdes and Freitas[32] has covered a significant portion of the near-Earth SETA search spacc, this work is not complete and does not approach the ultimate limits on optical magnitudes achievable in the foreseeable future. Continued ground-based optical searches with established technology can complete the coverage of the potential sites and improve the magnitude limits of the surveyed sites to approximately 20-21 mag. This could be done with a large Schmidt photographic camera in 1-2 yrs on a part-time basis[4l. 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 Freitas[321 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. 4.2 Radar and inJ)'ared searches Radar and infrared observations offer few significant improvements over visual searches. Artifacts are detectable by radar and searches to - 1 - 1 0 m are feasible. The total receiver integration time At is calculated after the method of Jurgens and Bender[61 ] using their reference and asteroid default values (e.g. normalized radar cross section is o'o = 0.1) and a S/N = 5 dB detection limit. The Arecibo S-band radar cannot be pulsed gracefully, hence is unusable for cislunar asteroid searches. The Arccibo B-band radar (O~.: - 4 arcmin) can be adequately pulsed and gives lower search times than the Goldstonc, Haystack or Millstone facilities. For a 1-2 yr part-time effort, the Arecibo system can detect limiting artifacts in each of the four orbital regions fnr values of the apparent rotation vector D,~, up to ~-0.0()1-1 tad/see, or ~, up

The Search for Extraterrestrial Artifacts to --0.1-100 rad/sec for t r 0 - 1, as compared to the asteroid default value l)p = 2.2 × 10 4 rad/sec. Artifacts plausibly might also be detectable in the infrared. Cislunar objects in equilibrium with the insolation having a bolometric emissivity ~0.1 and a Bond albedo ~0.1 have a temperature ~ 390 K and infrared magnitudeN = + l l a t l u n a r d i s t a n c e a n d N = + 7 t o + 1 0 in the geocentric region for R = 1 m at 10 Ixm[62,63]. The AFCRL 11 Ixm sky survey achieved N = 0 to - 1[64], a limit of ~ 100-300 m, and the IRAS reached N = + 7 to + 8 at 10 txm[65]. This gives a theoretical limit of - 3 - 6 m for all orbital regions after a search time of 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 for fast-moving objects has been compiled. 4.3 Other SETA proposals A few past proposals have emphasized a search for artificial emissions rather than the probe itself. Bracewell[l] suggested that the well-known long-delay echo (LDE) phenomenon[66,67] was of the type which might be expected as a call sign from an extraterrestrial artifact parked in Earth orbit and desiring to communicate. Lawton and Newton[68] performed a series of LDE experiments and concluded the reflection signals were of a pure physical nature, though they later proposed[69] that radio call signals should be transmitted to likely probe positions in an attempt to stimulate a response. Kardashev reported "coded signals" from within the solar system of possibly alien origin[70], but Western experts believe the signals came from U.S. military communications satellites or from magnetospheric energy discharges[71]. Kuiper and Morris[72] proposed intercepting radio communications between alien probes in the solar system and their extrasolar senders, but admitted that the bandwidth of alien signals may be too wide to easily detect using a modest antenna. Targeted radio listening searches could also be conducted of likely probe residence orbits in an eavesdropping mode to detect unshielded electromagnetic emissions. And searches for radio beacons could establish indirect limits on the existence of probes in the solar s y s t e m - - t h e all-sky survey outlined in [73] would provide observational limits on the minimum size of a solarpowered Earth-Moon orbiting artifact maintaining its own local acquisition beacon.

Acknowledgments--SETA observation facilities were kindly made available by Kitt Peak National Observatory and by Leuschner Observatory of the University of California, Berkeley, CA. Travel funds to present this paper at the 35th International Astronautical Congress in Lausanne, Switzerland, were provided by Kitt Peak National Observatory, the American Astronomical Society Travel Grant Program and the Xenology Research Institute.

REFERENCES

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