Mother Earth, What Big Ears! Better to Hear You, Cosmos Dear : Radio astronomy: A satellite dish as big as two football fields is being built to eavesdrop on the universe. It could provide clues to the origin of our own Milky Way galaxy.

As the world’s most sensitive electronic eardrum, the Green Bank Telescope is expected to pick up signals from outer space that none other has ever heard.

The radio telescope, under construction at the National Radio Astronomy Observatory, will help map the universe by receiving radio waves more clearly than any other instrument on Earth, said site director Jay Lockman.

It will be the largest fully steerable radio telescope when finished in December, 1996. Its collecting bowl, resembling a satellite dish, will be 330 feet in diameter, as big as two football fields.

Radio telescopes collect signals that help fill in gaps left by optical instruments. They read radio waves emitted by most gases and objects in space, including those that may not radiate or reflect visible light.

“One without another doesn’t give you the whole picture,” said Martha Haynes, a Cornell University astronomer who draws pictures of galaxies by studying clouds of gas between them.

“If you tune your FM car radio in between stations, you’ll hear a hiss,” Lockman said. “Some of that radio signal is coming from noise in your receiver, but other parts are coming from high up in the atmosphere, the ionosphere or even out in the Milky Way.”

To read those signals now, a radio telescope with a 140-foot diameter dish and four smaller radio telescopes tower above the clumps of trees around the observatory. All five are aging, built between 1958--when the observatory opened--and 1965.

A 300-foot diameter radio telescope also was used for decades until it collapsed suddenly in 1988. An investigation found that several supporting pieces in the 26-year-old structure had simply worn out. Nobody was hurt.

Haynes and a graduate student were on a waiting list to use that radio telescope at the time. They’ve had to delay their research since then because none other would suffice.

“I’ve had to become an optical astronomer in order to do interesting stuff,” she said.

The collapse was a surprise, Lockman says, but it gave astronomers an opportunity to push for a new radio telescope that would incorporate updated technology.

The $75-million replacement won’t be much larger, but it’s designed to be at least twice as sensitive, work more than 30 times faster and pick up much higher frequencies, Lockman said.

Another radio telescope in Puerto Rico is evidence that bigger isn’t necessarily better. That dish is 1,000 feet in diameter, the biggest in the world, so it can pick up very distant signals.

But it is too big to be steered much, so it can only read signals from limited parts of the universe, Lockman said.

The fully steerable Green Bank radio telescope will cover 85% of the sky.

It will also be innovative because it will be shaped like an oyster shell.

Other radio telescope dishes have a familiar circular, satellite-dish shape. They bounce signals into a focusing eye centered above by tepee-like support legs. But the legs often block the incoming signals, Lockman said.

The new shape means the focusing eye can be held by one leg off to the side, where the joint of an oyster shell would be, leaving the collection dish clear, Lockman said.

“The accuracy of other telescopes is limited by the scattered radiation of the feed legs. This will be a revolution, I think,” said Carl Heiles, an astronomer who studies interstellar gas at the University of California in Berkeley.

Also, the radio telescope will use lasers to keep the 2,204 collection panels on the dish aligned with each other to at least a tenth, possibly a hundredth, of a millimeter, Lockman said. That will allow signals to be collected evenly so that the radio telescope can read even very narrow frequencies.

“It’s like building a battleship with the precision of a watch,” Lockman said.

But none of these features would matter much if the radio telescope were not being built in the Deer Creek Valley, the center of the world’s only “radio quiet zone.”

Within the zone, which covers roughly a 100-mile radius into much of eastern West Virginia and western Virginia, radio, television and cellular-telephone transmitters are required to point away from the valley to avoid causing interference. The observatory is about 220 miles west of Washington, D.C.

The radio telescopes are also protected from interference by the surrounding mountains of the Monongahela National Forest.

Because of the zone, Green Bank is the only radio telescope location that is almost free of interference from earthly sources. Besides Puerto Rico, radio telescopes in the United States are also set up in Arizona and New Mexico.

“If I’m trying to pick up a signal in a really distant galaxy, and there’s somebody driving around with a cellular telephone right next to me, that cellular telephone signal is a lot stronger than the signal I’m trying to pick up,” Haynes said.

So the location, along with the innovative dish design and laser technology, will make the new radio telescope “the only game in town” for radio astronomers, Haynes said.

It should be able to detect, for instance, molecules of water halfway across the known universe, Lockman said.

And because it will listen with such detail, it should be able to show clearly how other galaxies are forming, Haynes said. That could provide clues to the origin of our own galaxy, the Milky Way, she said.

The observatory gets requests for about twice as much time as it can grant even for the existing radio telescopes, Lockman said. Researchers from all over the world must draw up detailed proposals to compete for the 140-foot instrument, the main research radio telescope at Green Bank now.

If a request is accepted, the National Science Foundation, a federal agency, pays the researcher’s expenses.

Besides the astronomers, about 20,000 visitors tour the observatory at Green Bank every year, even though the nearest cities are several hours away.

“We don’t really advertise and you know it’s not so easy to get here,” Lockman said. “But there are people who come here from all over the country because they’re just interested in what’s ‘out there.’ ”