If They Are Here, Where Are They? Observational and Search Considerations

Introduction

Interstellar exploration to seek life and intelligence in the cosmos using automated messenger probes has been discussed by many authors (e.g., Bracewell, 1960, 1975; Freitas, 1980a, b, 1983; Tipler, 1981; Stephenson, 1982). This suggests an observational strategy in the search for extraterrestrial intelligence (SETI), using existing or foreseeable instrumentation, which optimizes the probability of detection of such artifacts in the solar system. A SETI program of this type complements searches for artificial radio signals, an approach which has already been extensively discussed in the literature (e.g., Morrison et al., 1977; Billingham and Pesek, 1979).

From an observational standpoint all extraterrestrial artifacts fall into one of three classes: (I) Objects intended to be found, (II) objects intended not to be found, and (III) objects for which detection by us is irrelevant or unimportant. The simplest assumption is that extraterrestrial technology is sufficient to guarantee the intended result. Thus class I objects cannot be present in the solar system because we have not observed their intentional manifestations. Class II objects may be present but it is impossible for us to observe them. Only class III objects may be observable.

The exploration goal restricts the search space for observable class III 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 the 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 y d m is required for unambiguous visual detection from orbit of intelligent activity on the surface of the Earth (Shklovskii and Sagan, 1966). To resolve objects of dimension y at wavelength » at a distance r from the extraterrestrial observer requires a diffraction-limited optical aperture of size ~(1.22)»r/y. From cislunar orbital space (70,000 < r < 384,000 km), this means a telescope size of ~3-30 m.

Second, an artifact must withstand catastrophic meteoroid impacts likely to occur during an on-orbit residence time t. The mass of a particle whose average flux across the surface of a cube of area 6L² is unity after Ä years in near-Earth space is approximated from the empirical data of Dohnanyi (1972, Fig. 1) as log (M_p) ~ log (ÄL²) - 6.7. A second empirical relation modified from Fujiwara (1980, Fig, 3) for the complete destruction of cubic rock targets by high-velocity frontal impact spallation is log (M_p) ~ log (2L^2.5/V²) + 7, where the smallest body which can survive an impact of energy (1/2)M_pV² is of size L. If artifact density and material strength are not grossly dissimilar from rock, then combining these empirical relations we have log (L) ~ log (V⁴) + log (IJ) - 28 and the minimum size for defenseless artifacts is >0.2-20 m for a >10⁵ to 10⁶-year residence time (~ speciation timescale on Earth; Rensch, 1960) with V = 22 km/sec (Volkoff, 1966) in the vicinity of Earth.

Third, numerical integrations by Katz (1975) of the motion of objects placed at the triangular lunar libration points show that escape time is not much affected when radiation pressure force/gravitational force < 10^-6. For radiation pressure in the geometrical optics case (Burns et al., 1979), this condition requires L > 0.2 m which also corresponds to an exponential decay time of >~10⁹ year for artifacts in planetocentric orbits due to Poynting-Robertson drag alone. Cislunar bodies larger than 20 mm with velocities 0.3-3 km/sec are not lost from circumplanetary orbit by radiation pressure-induced perturbations of eccentricity (Burns et al., 1979).

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 noise (e.g., Morrison et al., 1977; Billingham and Pesek, 1979). To achieve a gain of 1 < g < 10⁷ (~ Arecibo) requires a minimum waterhole antenna diameter ~ (»/À) g^1/2 = 0.01-30 m.

These considerations suggest that a reasonable search limit is an artifact of typical dimension ~1-10 m with a visual geometric albedo of ~-0. 1 (p_v = 0.02-0.35 for asteroids), termed the limiting artifact.

Artifact Search Space

The lunar/cislunar orbit assumption yields a probe SETI search space consisting of geocentric, selenocentric, Earth-Moon libration, and Earth-Moon halo orbits (see Fig. 1). Site properties, current observational status, and detection limits for limiting artifacts in each orbital region are discussed below.

SETI Program Search Statistics

An important aspect of a probe-SETI search program is the feasibility of detecting a limiting artifact using existing or foreseeable instrumentation. The most conservative assumption is that the artifact has a uniform distribution over the search space. If p is the probability of artifact detection per observation of duration ”t, then after n observations the probability of not having found the artifact is (1 - p)ⁿ ~ e^-np for p << 1, and the probability of having detected the object at least once is P_s = 1 - e^-np which requires a total observing time T_o = n”t = -”tp^-1 log_e(1- P_s). In an optical photographic survey using a telescope with search field f deg², N_f = S/f is the number of distinct fields and N_a = ”w/I is the number of distinguishable angular rate intervals for a maximum detectable angular rate interval I deg/see, so p = (N_fN_a²)^-1 for success per observation over a two-dimensional angular rate space. Given a plate resolution ”a deg, limiting visual magnitude V₁, and sky-limited exposure time ”t sec, an image which trails x”a causes a magnitude loss of 2.5 log x d V₁ - V_s in the limit for detection so the maximum detectable angular rate interval I = x”a/”t. Visual magnitude search limit is V_s = V_sun - 5 log (R) - 2.5 log (p_v) + 5 log (r) at 0° phase angle for artifact radius R, observer distance r, and solar magnitude V_sun at 1 AU.

The calendar search time is T_c = T_o/k, and we assume a reasonable search program is Tc d 2 years per orbital region. The wide-field Palomar Schmidt telescope has f = 44 deg², ”t = 900 sec, V₁ = +21 mag, ”a = 3 x 10^-4 deg (~ 1 arcsec seeing), and k = (8 hr/24 hr)(10%) = 0.03 is a plausible usage figure. For Palomar Schmidt searches with T_c = 2 years and P_s = 0.99, each orbital region except geocentric may be searched for 0.1-albedo objects to R ~ 3-5 m, and the geocentric region to ~40 m. These limits assume a random search pattern, hence are pessimistic since T_c down to a given R may be improved using optimal search techniques (Stone, 1975) by taking full account of orbital mechanics exclusions as each position-velocity element is examined. T_c may also be improved if part of the search is performed in parasitic or serendipity-data mode concurrent with other non-SETI programs which effectively increases the value of k. A space-based wide-field telescope with characteristics similar to the Palomar Schmidt but with V₁ = +24 could search each orbital region except geocentric to ~1 m, and geocentric to ~10 m, for k = 0.03, T_c = 2 years.

Radar and Infrared Detectability

Artifacts are detectable by radar and searches to ~1-10 m are feasible. Radar detection depends on the total reflection signal in the beamwidth, so p = N_f-1 and f ~ p(q1/2)². The total receiver integration time ”t is calculated after the method of Jurgens and Bender (1977) using their reference and asteroid default values (e.g., normalized radar cross section is so = 0.1) and an S/N = 5 dB detection limit. The Arecibo S-band radar cannot be pulsed gracefully, hence is unusable for cislunar asteroid searches. The Arecibo B-band radar (¸1/2 ~ 4 arcmin) can be adequately pulsed and gives lower search times than the Goldstone, Haystack, or Millstone facilities. Taking T_c, k, and P_s as before, the Arecibo system can detect limiting artifacts in each of the four orbital regions for values of the apparent rotation vector ©_p up to ~0.001-1 rad/sec, or ©_p up to ~0.1-100 rad/sec for Ã_o~1, as compared to the asteroid default value ©_p = 2.2 x 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 magnitude N = +11 at lunar distance and N = +7 to +10 in the geocentric region for R = 1 m at 10 mm (Morrison, 1977; Hansen, 1977). The AFCRL 11 mm sky survey achieved N = 0 to -1 (Lebofsky et al., 1978), a limit of ~100-300 m. and the IRAS orbiting telescope should reach N = +7 to +8 at 10 mm (Aumann kind Walker, 1977), a limit of ~3-6 m for all orbital regions after Tc = 0.25 year. However, these limits may not be secure. Selection criteria for the final AFGL catalog were chosen to exclude asteroids and satellites. IRAS has a mission lifetime of less than I year and the original IRAS software was designed to reject asteroids (Morrison and Niehoff, 1979), although now a specialty database for fast-moving asteroids evidently will be established (Thomas, 1983).

Acknowledgments

The author thanks Ronald N. Bracewell, Robert S. Harrington, Michael D. Papagiannis, Robert G. Roosen, Clyde W. Tombaugh, Francisco Valdes, George W. Wetherill, and an unnamed referee for their comments on an earlier version of this manuscript. This research was supported by the Xenology Research Institute.

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