Radar Astronomy: Closeup of Venus
Friday, Aug. 09, 1968
Though it is the brightest planet in the heavens, Venus has always been less than clear to astronomers. Wrapped in dense clouds of gases, the Venusian surface remains hidden even to the most powerful optical telescopes. Now, scientists are employing electronic means to explore the mysteries of the earth's sister planet. Using radio beams, Radar Astronomers Richard M. Goldstein and Shalhav Zohar of Caltech's Jet Pro pulsion Laboratory reported last week that they have mapped 160,000 sq. mi. of Venus, an area about equal to the size of the entire U.S. Northeast.
Their rough but unique closeup of Venus stems from 17 radar probes with NASA's 210-ft. dish antenna at Goldstone, Calif., last summer. At that time Venus was only 26 million miles from the earth. Since then, the scientists have been "drawing" a map by feeding their electronic findings into a computer. The result shows three blotches of extremely rough terrain, which Goldstein presumes are mountains, moonlike craters or fields of boulders.
Clocking the Signal. Celestial radar mapping is based on the same radio-echo techniques used in plane spotting and ship navigation. But bouncing radar waves off planets requires far more power and precision. For the Venus experiment, the Goldstone installation operated at 100,000 watts, twice the power of the largest U.S. commercial radio stations. When the signals came back 41 minutes later, they measured just a tiny fraction of a watt.
Yet even that faint feedback carries a definite message. If the signals bounce back polarized?in other words, with their electric fields reversed?they indicate rough terrain. Unpolarized echoes, on the other hand, mean smooth surfaces. In either case, the target areas are pinpointed by a system of coordinates similar to latitude and longitude. One coordinate is located simply by clocking the signal: the quicker it bounces back, the closer the bounce-back point is to that part of Venus nearest to Earth.
Determining the other coordinate is a more complicated matter of listening for a so-called Doppler shift in frequency. If the echo comes from the side of the planet spinning toward Earth, it will rise in frequency, just as the whistle of an approaching train seems to move up in pitch. If the reflection comes from the side rotating away from Earth, it will go down in frequency.
While their map is the clearest view yet of Venus, the Caltech researchers are not the only radar astronomers mapping that planet. Similar surveying is being carried out by Cornell scientists using the 1,000-ft. dish telescope at Arecibo, Puerto Rico, and by MIT astronomers at two sites in Massachusetts. In March, Venus will again approach Earth. By boosting their radar signal to 450,000 watts, Caltech's electronic cartographers expect to make even more detailed maps.