detailed analysis. At best, the restrictions assessed in 6(i), as applied to the smallest aperture telescope in the network would seem to be the limiting factor. 7. Conclusions The parameters for solar power satellites now under discussion would illuminate radio telescopes situated remotely from the SPS terrestrial col- lector at power levels at least 10 times greater than those received from the celestial sources now being investigated and several orders of magnitude greater than those from other man made sources. The extent of the inhibition to radio astronomical research would depend critically on the polar diagram of the space array, the harmonic content of the space transmitter and the number of such power systems operating in space. For a single space station with the best attainable polar diagram and negligible harmonic radiation it should be possible to filter the interfering signal at the radio telescope so that observations would be possible except on frequencies near the protected band of 2.45 GHz, although investigations of the celestial radiation within several degrees of the satellite would be severely restricted. On the other hand,operational systems requiring several satellites to supply significant amounts of energy to Earth would create serious hazards to radio astronomical research except in thinly populated areas of the Earth. For example, it is estimated that 8 space stations would be required to supply the needs of the U.K. by 2000 AD. If the terrestrial collectors were distributed over the U.K. then it is unlikely that any site would remain for the satisfactory operation of a radio telescope. Apart from these direct effects of the space power beam on radio astronomical research it may be noted that other possibilities are now under study which may influence such research. For example, measurements with the Arecibo radio telescope are in progress to determine the possible influence of the 2.45 GHz beam on the electron temperature in the ionosphere^); and studies at Harvard by Grossi and Colombo^) relate to the possibility of weather modification by using a space transmitter at 22.2 GHz. Should this be attempted then, at least,all research on the water vapour spectral line from celestial sources would be brought to a halt.
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