Dr. Burke inside the Carnegie radio receiver truck that housed recording instruments at the discovery site.
Bernard Burke (right) and Charles Little (left) inside the receiver truck next to a Derwood radio telescope in 1964.
One chilly evening in the early months of 1955, scientists Bernard Burke and Kenneth Franklin found themselves standing in a rural field about 20 miles northwest of Washington, D.C. Every evening for the rest of the winter, they came to this field intending to map the northern sky—only to discover some out-of-this-world findings. Unlike their ancient counterparts who relied on telescopes and trigonometry to map the sky, Burke and Franklin used rather new instruments in the contemporary field of radio astronomy. [1] Radio astronomy had been around as a scientific field for only about 20 years; scientists began studying the radio frequencies emitted by celestial objects such as stars, galaxies and comets in the 1930s. Radio waves are electromagnetic radiation and invisible to the eye, but scientists can measure their wavelengths (also known as frequency) by receiving them through antennas. [2] Radio astronomy allows for more detailed observations of objects not easily visible in optical telescopes, such as gas planets and dying stars. By measuring an object’s naturally occurring radio waves, astronomers can predict temperature, distance and even elemental composition. [3]
Burke and Franklin set out that evening to observe the radio emissions from the Crab Nebula, an already well-known source of radio waves, and test out new equipment called the Mills Cross Array. Originally developed by astronomers B.Y. Mills and Martin Ryle, the Mills Cross Array was a formation of 66 dipole antennas connected by 5 miles of wire over 96 acres of land. Similar to the “rabbit ears” antennas used in older television sets, these dipole antennas were adjusted to intercept radio waves from a calculated position and transmit the collected frequencies over the connected wire to computing instruments. The Carnegie Institution of Washington, D.C., arranged the Mills Cross Array in the Maryland farmland and tasked Burke and Franklin to spend multiple nights running tests. [4]
Dr. Burke inside the Carnegie radio receiver truck that housed recording instruments at the discovery site.
After the first evening, the two noticed a few instances of interference—something was interrupting the radio waves coming from the Crab Nebula. Many terrestrial objects such as AM/FM car radios and power lines can emit interfering frequencies, so the scientists chalked it up to a farmer driving home late from a date. [5] Several evenings of testing passed, and the unpredicted interference began to show a pattern. At first, Burke and Franklin noted the emissions occurring around the same time each night. As months of data were collected and analyzed, they were able to track the radio wave interference. To their surprise, the interference would occur four minutes earlier each night, leading the scientists to believe that the unexpected radio waves were coming from a celestial object. After comparing the records, Burke and Franklin pinpointed Jupiter as being the culprit of the unidentified radio emissions. [6]
On April 6, 1955, Burke and Franklin presented their discovery of Jupiter’s radio emissions (also known as Jovian radio bursts) to the American Astronomical Society, becoming the first scientists to discover planetary radio signals other than Earth’s. This discovery prompted several other radio astronomers to review their own work, which led to nearly five years of data on Jovian radio bursts. [7] Now scientists knew Jupiter produced radio waves, but the question of where they were coming from remained. Scientists speculated that the source of Jovian radio bursts was Jupiter’s Great Red Spot—a century-old superstorm nearly twice the size of Earth. [8] However, the data couldn’t be matched with the gas planet’s cloud features, so the Great Red Spot was ruled out. The Jovian radio bursts followed a unique rotation cycle and had a polarized quality, indicating to scientists that the radio waves were being produced not by Jupiter’s gaseous surface but instead by its magnetic field! [9] By using radio astronomy, the largest planet in our solar system was found to have the largest magnetosphere as well, with strength nearly 20,000 times greater than Earth’s magnetic field. [10] In the decades following Burke and Franklin’s initial experiments, spacecraft such as Pioneers 10 and 11 and Voyagers 1 and 2 have flown past Jupiter to gather data to create more accurate models of the planet’s colossal magnetosphere. [11]
An aerial view of the Mills Cross Array’s 66-dipole antennas organized across the 96-acre Maryland farmland.
A view of one of the Mills Cross Array arms with a closer view of the dipole antennas and connective wire.
The 66 antennas of the Mills Cross Array no longer sprawl across the Maryland farmland where Burke and Franklin became the first to pick up radio signals from a planet other than Earth. However, visitors can stop at a historical marker near the site that commemorates their accidental scientific discovery!
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