The radio emission of a planet was first detected in 1955, when Burke and Franklin (1955) identified the origin of interference like radio noise on their records at about 15 meters wave length as emission from Jupiter. This sporadic type of planetary radiation is discussed by Burke (chap. 13) and Gallet (chap. 14). Steady radiation which was presumably of thermal origin was observed from Venus at 3.15 and 9.4 cm, and from Mars and Jupiter at 3.15 cm in 1956 (Mayer, McCullough, and Sloanaker, 1958 a, b, c), and from Saturn at 3.75 cm in 1957 (Drake and Ewen, 1958). In the relatively short time since these early observations, Venus has been observed at additional wave lengths in the range from 0.8 to 10.2 cm, and Jupiter has been observed over the wave-length range from 3.03 to 68 cm.

The observable characteristics of planetary radio radiation are the intensity, the polarization, and the direction of arrival of the waves. The maximum angular diameter of any planetary disk as observed from the earth is about 1 minute of arc. This is much smaller than the highest resolution of even the very large reflectors now under construction, and consequently the radio emission of different regions of the disk cannot be resolved. It should be possible, however, to put useful limits on the diameters of the radio sources by observing with large reflectors or with interferometers. Measurements of polarization are presently limited by apparatus sensitivity and will remain difficult because of the low intensity of the planetary radiation at the earth. There have been few measurements specifically for the determination of the polarization of planetary radiation. The measurements made with the NRL 50 -- foot reflector, which is altitude azimuth mounted, would have shown a systematic change with local hour angle in the measured intensities of Venus and Jupiter if a substantial part of the radiation had been linearly polarized. Recent interferometer measurements (Radhakrishnan and Roberts, 1960) have shown the 960 -- Mc emission of Jupiter to be partially polarized and to originate in a region of larger diameter than the visible disk. Other than this very significant result, most of the information now available about the radio emission of the planets is restricted to the intensity of the radiation.