Radio Astronomy: Data from a Big Dish

The reflector of the world’s biggest radio telescope is nothing more than a dish of chicken wire lining a 1,000-ft.-wide hole in the ground. Above it, three tall thin towers poke toward the sky. From the towers’ tips, cables string out to suspend a tangle of girders over the center of the bowl. The complete contraption looks like the product of some errant giant playing with an outsize Erector set. But at its dedication in the hills south of Arecibo, Puerto Rico, last week, the great scope was tuned and ready—a sharp and farseeing eye focused on the farthest reaches of the universe.

Since the chicken-wire dish is firmly attached to the ground, it cannot be turned mechanically. But by changing the position of the dangling waveguide that feeds radio energy from above, it can be swung electronically 20° north or south of vertical. The rotation of the earth takes care of the east-west steering. Because the telescope’s position is only 18° north of the equator, it can reach all the planets, for at that latitude they pass high overhead.

The Arecibo telescope belongs to the Department of Defense’s Advanced Research Projects Agency and cost more than $8,000,000. It was originally conceived by Radio Engineer William E. Gordon of Cornell as a means of studying the electrified layers in the earth’s upper atmosphere by shooting enormously powerful radar pulses through them and listening for faint echoes. Since those electrified layers control long-distance radio communication and are involved in attempts to devise some defense against ballistic missiles, almost any information gathered by the scope promises to be worth the price.

More interesting, though, are the non-military possibilities. Used as a telescope that receives incoming waves from elsewhere in the universe, the Arecibo dish will be enormously sensitive, concentrating the signals that fall on more than 18 acres. It will work best with radio waves about two feet long, but Dr. Gordon believes it will also pick up the important data-rich 21-cm. waves that come from hydrogen in the distant parts of the galaxy.

Used as a radar, it has 40,000 times the power of the Air Force radar at Millstone Hill, Mass., which bounced electronic pulses off Venus. The mighty pulses from Arecibo will study Venus and the other planets more intimately. Turned on the moon, they will penetrate many feet below the surface. Their reflections received on earth will help predict what sort of rock or dust the first human explorers will find.