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Radar Echoes from the Moon


ACCORDING to recent announcements in the daily Press, scientists of the U. S. Army Signal Corps have succeeded in detecting, with the aid of specially designed radar equipment, echoes of radio waves reflected from the moon. The possibility of such an achievement was foreshadowed by Sir Edward Appleton in his Kelvin Lecture to the Institution of Electrical Engineers delivered, in April 1945. A similar prediction was made independently by Pietro Lombardini in a paper entitled “The Possibilities of Astronomical Radio Soundings with Metre Waves” (Commentationes-Pont. Acad. Sci., 8, No. 2, 13 ; 1944). In the latter paper, the author estimates that by using a wave-length of 1.5 m. and a large aerial array for both transmission and reception, it should be just practicable to detect a radio echo from the moon, although it would appear to be problematical whether this technique could be extended to the planets. In Sir Edward Appleton's lecture (Inst. Elect. Eng., 92, Pt. 1, 340 ; Sept. 1945), the relevant formula was given for the energy we might expect to get back from the moon if a beam of radio waves was directed on to it. It was calculated that, with a powerful sender, and with sending and receiving aerials of high gain, it should be possible to get back detectable radio echoes after their 2½ sec. journey to the moon and back. The success of the experiments just announced from America would appear to afford a confirmation of the accuracy of this prediction. These experiments are to be distinguished from those made a few years ago in various countries, during which a radio wave component in the solar radiation spectrum was detected on a very sensitive receiver of the type used for radiolocation (see, for example: “Radio Waves from the Sun”, Nature, Sept. 1, 1945, p. 273).

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Radar Echoes from the Moon. Nature 157, 129 (1946).

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