For a system on the Moon, primary consideration was given to Cyclops-type arrays and to Arecibo-type arrays. These systems would be located on the far side of the Moon where they would be free from RFI; the systems would be constructed from locally available lunar materials. A lunar colony would have to be established to support the workers needed for construction and for operation and maintenance. The abundance of lunar craters of all sizes makes possible a significant improvement in the design of a lunar Arecibo-type system compared to an Earth-based system of this same type. A major portion of the cost of the actual Arecibo antenna is in the tall towers that support the feed. On the Moon it would be possible to reduce the cost of the antennas substantially by only partially filling a lunar crater with the reflector surface and by suspending both the feed and the dish on long cables extending from the crater rim. For this reason, Arecibo-type systems were found to be significantly cheaper than Cyclops-type systems on the Moon.

Overall cost was defined as the sum of the research and development (R&D) costs, procurement costs, and the operation and maintenance costs over the duration of the necessary search time. The overall cost was found to range from a few hundred million dollars to tens of billions of dollars for any particular system, depending on the values chosen for certain key parameters, particularly the assumed number of transmitting civilizations N, the desired probability of receiving a signal P, and the cost discount factor F. The results of the parametric analysis indicated that systems on the Moon are more expensive than systems on Earth or in space in all cases, even though it is assumed that the cost of transporting material from the Earth to the Moon is only $264/kg. The results also showed that if N is large enough so that the search need be extended only a few hundred light years from Earth, an array of conventional dishes on Earth may be the most cost-effective system (assuming effective protection from radio frequency interference can be obtained). However, for a search that has to extend out to 500 light years or more, for which a minimum of 250,000 stars would have to be examined one-by-one, it was found that there might be a substantial cost and search-time advantage in using a large spherical reflector in space with multiple feeds.

Thus, it was found that there is a reversal in relative cost effectiveness between low and high values of N. Space systems are more expensive than Earth-based systems for large N because of the large investment in R&D that is required before it would be possible to deploy even a moderately sized space system. However, the payoff from this investment increases if larger and larger systems are developed and deployed, so that, for the very large systems that would be required for small values of N, space systems may be cheaper than Earth-based systems. For the particular values of the various system parameters assumed in the SRI study, the overall cost of a system on Earth and of a three-feed system in space would be the same (~ $11.4 billion) for the case of N≈ 4×105. A Cyclops-type system for this case would have an antenna collecting area of about 7 km2, which corresponds to an array of about 890 dishes, each 100 m in diameter. This array could examine about 273,000 candidate stars out to a range of about 535 light years in a search time of about 18 years.

The SRI study was a very preliminary first attempt at comparing costs for alternate locations of an interstellar search system. As such, it should not be regarded as a definitive or in-depth

evaluation of interstellar search systems, but as a preliminary treatment in rough order-ofmagnitude terms. Many aspects of the cost estimates, especially for systems in space and on the Moon, are necessarily speculative at this time.

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Space SETI System: Artist's concept of an intermediate size (300-m) space SETI system antenna showing 2 feeds, a relay satellite, RFI shield and a Shuttle type vehicle. Located in geosynchronous orbit or beyond.