While testing antenna arrays for mapping galactic radio noise, Bernard Burke and Kenneth Franklin discovered unexpected radio bursts in the 17-meter band from the planet Jupiter, the first detection of natural radio signals emanating from a planet.

Although radio telescopes had been developed and employed in studies of galactic radiation since the mid-1930’s, specific radio investigations of the solar system began only after World War II. In 1945, Robert H. Dicke Dicke, Robert H. first detected radio emissions from the moon at a wavelength of 1.25 centimeters, shortly after the U.S. Army Signal Corps sent and received the first radar echoes from the moon. Despite technical advances in receiver electronics and antenna array design, however, as radio astronomy historians A. G. Smith and T. D. Carr noted in 1964, there was no science of planetary astronomy prior to 1955, since theoretical as well as technical concerns focused most astronomical efforts at stellar and galactic objects. Perhaps the chief driving force behind the development of radio astronomy for the solar system was the entirely accidental and serendipitous discovery by American astronomer Kenneth Franklin and physicist Bernard Burke of natural radio emissions from the planet Jupiter. Radio astronomy;Jupiter
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[kw]Franklin and Burke Discover Radio Emissions from Jupiter (Early 1955)
[kw]Burke Discover Radio Emissions from Jupiter, Franklin and (Early 1955)
[kw]Radio Emissions from Jupiter, Franklin and Burke Discover (Early 1955)
[kw]Jupiter, Franklin and Burke Discover Radio Emissions from (Early 1955)
Radio astronomy;Jupiter
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[g]North America;Early 1955: Franklin and Burke Discover Radio Emissions from Jupiter[04680]
[g]United States;Early 1955: Franklin and Burke Discover Radio Emissions from Jupiter[04680]
[c]Astronomy;Early 1955: Franklin and Burke Discover Radio Emissions from Jupiter[04680]
[c]Science and technology;Early 1955: Franklin and Burke Discover Radio Emissions from Jupiter[04680]
Burke, Bernard
Franklin, Kenneth

In mid-January, 1955, the two scientists were actively engaged in carrying out a variety of calibration and reception tests for a large (“Mills cross”) radio interferometer, which the Carnegie Institution’s Laboratory of Space and Terrestrial Sciences Carnegie Laboratory of Space and Terrestrial Sciences had installed near Washington, D.C. This array had been funded and designed specifically for mapping the general cosmic background of ambient and discrete radio sources at a frequency (wavelength) of 22.2 megacycles per second. The Mills cross design, in particular, is a symmetric square array with four equal arms centered on a fixed axis.

Previous radio astronomers had shown that this aperture configuration could be configured to yield a narrow search beam of about 2.5 degrees, which, then, could be optimally directed and focused to receive and track specific galactic regions. In a well-known account of these events, “An Account of the Discovery of Jupiter as a Radio Source” “Account of the Discovery of Jupiter as a Radio Source, An” (Franklin)[Account of the Discovery of Jupiter as a Radio Source] (1959), Franklin specifically recalled that during these calibration tests:

At times the records exhibited a feature characteristic of (irregular) interference (similar to those from lightning discharges). . . . I recall saying once that we would have to investigate the origin of that interference some day. We joked that it was probably due to the faulty ignition of some farm hand returning from a date.

Notwithstanding their initial attitudes, when Burke subsequently compared a number of test recordings from their array, he and Franklin were surprised to find that the “interference” always occurred as short, or transient, bursts at about the same sidereal time (time measured with respect to stellar and not solar positions). Although this suggested initially that their radio noise was likely celestial and not terrestrial in origin, a quick search through a star atlas failed to reveal any established or possible radio-star that would have been present in the antenna’s beam at the correct time.

Somewhat later, the seismologist Howard Tatel Tatel, Howard suggested rather facetiously to Burke and Franklin that the noise might very well be from the planet Jupiter. “We were amused at the preposterous nature of this remark, and for an argument against it, I [Franklin] looked up Jupiter’s position in the American Ephemeris & Nautical Almanac. I was surprised to find that Jupiter was just about in the place.” Shortly thereafter, Franklin hand-plotted on a graph of celestial time versus celestial coordinate (right ascension) the positions of all the anomalous radio interference events for a two-month period. He then plotted the celestial positions of Jupiter, Uranus, and two galactic nebula, NGC 2420 and 2392, which all were close to the apparent position of the radio bursts at the above observation times.

As each point was plotted, they appeared right between the boundary lines representing the beginning and end of each radio interference event. This meant that these events were recorded only when the planet Jupiter was in the confines of the narrow principal beam of the Mills cross. The source of the intermittent radiation was therefore determined to be associated with Jupiter.

After their initial publication, Burke and Franklin reviewed earlier array test records from 1956 and found that they had unknowingly missed a previous strong burst from Jupiter. Nevertheless, almost immediately after Burke and Franklin’s publication, Australian radio astronomer C. H. Shain Shain, C. H. , long active in lunar radar and galactic radiotelescope studies, reexamined an extensive collection of radio records from 1950 and 1951 recorded at a somewhat lower frequency of 18.3 megahertz per second. Shain independently confirmed and extended the results of Burke and Franklin, finding no less than sixty-one records having signals that could be attributed to Jupiter using their same tracking method.

Although missing the accolade of “discoverer,” Shain was able to demonstrate that the radio bursts came from a very localized area on Jupiter’s disk and not from the planet as a whole. Shain further showed that this localized radio source could be received only when it was near the center of the Jovian disk. He found that the burst occurrences encompassed one major intensity peak and two lesser flanking bursts, which although not associatable with any visible features such as Jupiter’s “red spot,” were highly directional in nature.

The discoveries of Burke and Franklin in 1955 and Shain in 1955 and 1956 quickly gave tremendous technical and competitive impetus to further radio studies of Jupiter and the other planets Radio astronomy;planets . In 1956, the first reported reception of what appeared to be similar sporadic radio signals from Venus at a somewhat shorter wavelength (11 meters) was found, although these were not confirmed by other researchers, and refocused many efforts to even shorter wavelengths. The first microwave (centimeter wavelength) radio emissions from Jupiter, Mars, and Venus were detected in 1956 by researchers at the U.S. Naval Research Laboratory in Washington, D.C. In contrast to recordings of Burke and Franklin, these decimetric radio emissions occurred as continuous radio noise.

Although from 1956 to 1958 microwave observations of Jupiter at 3-4 centimeters revealed no new findings about the planet, during the summer of 1958, radio measurements determined at 10.3 centimeters indicated a Jovian disk temperature of 640 Kelvins, unexpected in comparison to the ~140 Kelvins at 3.15 centimeters. It was then recognized that if, like Earth, the temperature of Jupiter’s atmosphere varied with depth, higher temperatures should be observed at longer radio wavelengths. Soon many observations at 21, 22, 31, and 68 centimeters, respectively, indicated temperatures on Jupiter’s disk of ~2,500 Kelvins, ~3,000 Kelvins, ~5,500 Kelvins, and more than 20,000 Kelvins. Limits on the Jovian atmosphere had been set previously only by observing pressure broadening of specific elemental lines in Jupiter’s visual spectra. Although the radiometric temperature of Jupiter’s outer layers agreed well with that estimated from assuming ice crystals of solidified ammonia, the longer-wavelength data caused notable rethinking of theories explaining planetary atmosphere formation and dynamics.

In June, 1959, a paper submitted to the Journal of Geophysical Research first proposed that synchrotron radiation Synchrotron radiation was responsible for the enhanced temperatures of Jupiter’s atmosphere. Synchrotron radiation is composed of electromagnetic waves emitted by charged particles moving in regular orbits in a magnetic field at relativistic speeds. The large magnetic fields of extended size and their associated strong polarization were confirmed subsequently by observations in 1961.

Another important discovery in 1964 demonstrated that the probability of receiving decimetric emissions from Jupiter was closely associated with the orbital position of the Jovian satellite Io. The satellite flybys of Pioneer Pioneer program
Space program, U.S.;Pioneer program 10 and 11 subsequently showed that Jupiter’s numerous cloud belts consist of different gases of differing temperatures at different heights. Additional irregular radio noise also has been observed, thought to arise from what are essentially lightning storms on Jupiter’s dark side. Both Saturn and Venus were discovered to be radiating in the microwave band, also.

Although intrinsically feeble in terms of absolute power, next to the sun, Jupiter has the solar system’s strongest natural radio source. Greater numbers of observations having improved accuracy, resolution, and spectral bandwidth have further refined, but not essentially altered, the findings fortuitously recognized by Franklin and Burke. Radio astronomy;Jupiter
Jupiter

• Bracewell, R. N., ed. Paris Symposium on Radio Astronomy. Palo Alto, Calif.: Stanford University Press, 1959. Gives a good collection of radio telescope designs and capabilities during the 1950’s.
• Burke, Bernard F., and Francis Graham-Smith. An Introduction to Radio Astronomy. 2d ed. New York: Cambridge University Press, 2002. Survey of the history, methodology, and discoveries of radio astronomy. Bibliographic References and index.
• Franklin, K. L. “An Account of the Discovery of Jupiter as Radio Source.” Astronomical Journal 64 (1959): 37-39. The principal source for the historical sequence of events of the radio source. Fascinating.
• Krüger, Albrecht, ed. Introduction to Solar Radio Astronomy and Radio Physics. Dordrecht, the Netherlands: Kluwer Academic Press, 1979. Presents an account of technology and methods in planetary radio astronomy.
• Rohlfs, Kristen. Tools of Radio Astronomy. New York: Springer-Verlag, 1986. A semitechnical account of radio telescope array, receiver, and signal processor principles.
• Smith, A. G., and T. D. Carr. Radio Exploration of the Planetary, System. Princeton, N.J.: D. Van Nostrand, 1964. The first detailed, nontechnical account of the rapid growth of solar system radio astronomy following Franklin and Burke’s discovery.
• Spilker, Thomas R. NH3, H2S, and the Radio Brightness Temperature Spectra of the Giant Planets. Washington, D.C.: NASA, 1998. Technical study of one aspect of the radio astronomy of Jupiter and the other large planets of the solar system.
• Wall, J. V., and A. Boksenberg, eds. Modern Technology and Its Influence on Astronomy. New York: Cambridge University Press, 1990. A collection of papers by historians of science examining the limits and opportunities on prior radio astronomy caused by hardware and software constraints.
• Washburn, Mark. Distant Encounters: The Exploration of Jupiter and Saturn. San Diego, Calif.: Harcourt Brace Jovanovich, 1983. A popular account of Pioneer flyby results for the middle planets.

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