[180] blank page
This study was carried out during 1975 and 1976 by the Stanford Research Institute (SRI), Palo Alto, California. The study was concerned with evaluating the comparative cost effectiveness of several different microwave receiving systems that might be used to search for signals from extraterrestrial intelligence. Specific design concepts for such interstellar search systems were analyzed in a parametric fashion to determine whether the optimum location is on Earth, in space, or on the Moon.
System evaluations were performed in terms of a great number of parameters, including the hypothesized number of transmitting civilizations in the Galaxy (N), the number of stars that must be searched to give any desired probability P of receiving one of these signals, the required antenna collecting area, the necessary search time, the maximum search range, and the overall cost. The underlying principle of the method of parametric analysis was that systems are compared only after performance is normalized by requiring that all systems have the same probability of detecting a given signal for any postulated value of N. In practice, this performance normalization was accomplished by requiring that each system examine the same number of stars for any given case (as defined by the values assumed for P and N). Systems with limited sky coverage thus need to search to greater ranges than do systems with more complete sky coverage to examine any given number of stars.
For systems on Earth, primary consideration was given to a Cyclops-type array of conventional parabolic dishes and to an Arecibo-type array of fixed spherical dishes. Preliminary consideration was also given to a Cyclops-type array with a solar power capability. This system has the advantage that costs can be offset by the sale of electricity, but its design must be worked out in detail in a future study. Arecibo-type arrays were found to be slightly more cost effective than Cyclops-type arrays under the assumption that sinkholes are freely available for many dishes to be constructed in the same manner (and for a comparable cost) as the existing dish. This assumption clearly breaks down for large numbers of dishes, and in a future study the possible additional cost of excavating should be evaluated.
For a system in space, a concept was developed that consists of a spherical primary reflector made of lightweight mesh, with three Gregorian subreflectors and feed assemblies that permit three different stars to be searched simultaneously. A disk-shaped shield placed between the antenna and the Earth protects against radio frequency interference (RFI); a series of relay satellites permits the IF signal to be beamed to Earth for processing and to transmit instructions from Earth for station-keeping and attitude control. A number of orbital locations are possible, but for the SRI analysis it was assumed that the system would be at the L3 libration point of the Earth-Moon system, a point at lunar distance but on the opposite side of the Earth from the Moon. Among several desirable features of an orbiting antenna system are the complete sky coverage, the ability to track a given star continuously if a signal is detected, the reduced system temperature, and the potential structural simplicity and longevity made possible by the weightless and benign environment.
[182] 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 N4 x
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 1 8 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 [183] evaluation of interstellar search systems, but as a preliminary treatment in rough order-of-magnitude terms. Many aspects of the cost estimates, especially for systems in space and on the Moon, are necessarily speculative at this time.
