I know perfectly well that at this moment the whole universe is listening to us.

--JEAN GIRAUDOUX, "The Madwoman of Chaillot" No wonder earthlings haven't heard from any aliens: we've been too busy gabbing instead of listening. As far back as 1899, J. P. Morgan bankrolled a scheme to beam radio signals into space from a huge coil of wire in Colorado; the transformer made the neighbors' hair stand on end, but ET didn't phone. Since 1939 the electromagnetic signals encoding "The Howdy Doody Show," "Mister Ed" and every other TV program ever broadcast have been hurtling toward the distant stars at the speed of light, and by now have arrived at 1,100 stars. Still, no one called in. And since 1974 a signal sent from the giant radio dish in Arecibo, Puerto Rico, describing our solar system, our DNA and our species has been speeding toward the constellation Hercules, dispatched in the hope that an alien hears it. But since Hercules is 25,000 light-years away, an answer would take 50,000 years to reach Earth, and then you'd have to hope your teenager wasn't on the phone when the call came.

Now scientists have decided that it's time to start listening. So on Oct. 12, Columbus Day, the 500th anniversary of the Old World finding the New, NASA launches its own age of discovery. At precisely 3 p.m. at Arecibo, astronomer Jill Tarter will switch on the biggest radio telescope on Earth, a 1,000-foot-across aluminum bowl suspended over a chasm in the Puerto Rican jungle. Across the continent in the Mojave Desert, NASA's Sam Gulkis will at the exact same moment turn on a Goldstone Tracking Station radio telescope. Together, the dishes will prick up their electronic ears for what is potentially man's most humbling quest: a search for advanced civilizations elsewhere in the Milky Way galaxy. In the first few hours of the 10-year, $100 million program, NASA will have scanned for more signals than in all the thousands of hours of every search that has come before. But is anyone dialing Earth's interstellar access code? Frank Drake of the University of California, Santa Cruz, the astronomer who in 1960 conducted the first modern radio search for ETs, has cast his bet: "I fully expect to witness [the detection of signals from an extraterrestrial] before the year 2000."

How will NASA catch the signal? It's as simple as the old 1930s vintage wooden radio that occupies a place of honor at Drake's SETI (Search for ExtraTerrestrial Intelligence) Institute, which he established in 1984 to research the possibility of ETs. Imagine a radio that catches the whole spectrum of frequencies, not just FM but microwave, shortwave, radar. Start at the bass end of the dial, and you catch the signal of exploding stars; twiddle it up a few channels and you hear the ping-ping of a spinning pulsar. But dial it to the microwave part of the spectrum, and you will be met with ... an almost cosmic silence. It is these quiet channels that NASA will tune to. Scientists whose jobs can be described as "psyching out ET" think that anything smart enough to launch signals into the vastness of space is also smart enough to know that the only way to be heard above the background cosmic din is to broadcast on a frequency that stars and other natural noisemakers don't. The quietest portion of the radio spectrum lies between 1,000 and 10,000 megahertz, or 1 billion to 10 billion cycles per second.

That's a lot of potential stations. But NASA's smart computers are set to channel-surf the skies:

The All-Sky Survey, centered at the Jet Propulsion Laboratory in Pasadena, Calif., will scan the entire celestial vault. It will use the 34-meter Goldstone telescope as well as a satellite-tracking antenna in Australia. The telescopes will pan across the heavens in figure eights, scanning 2 billion (at first) or 16 billion (once it's fully operational, in a few years) radio channels in the quiet frequencies. "Every second the computers will perform 100 billion operations to sort through the frequencies, identify any channel with an excess energy and throw out those from earthly sources," says SETI project manager Michael Klein of JPL. The sky survey can detect only strong signals, so it's best suited to hearing messages that still pack a punch even if they've come from far away.

The Targeted Search focuses on the nearest 1,000 sunlike stars, within 80 light-years of Earth. This search will scan fewer channels (2 billion) over fewer frequencies (1,000 to 3,000 megahertz), but with much greater sensitivity. It will use the Arecibo dish, at which SETI will get about 5 percent of the observing time, and the 42-meter Green Bank National Telescope in West Virginia, which will be dedicated completely to SETI beginning in 1995. Computers will automatically search 15 million channels every second for signs of an intelligent signal.

It would sound (if amplified and run through a signal converter) like the pure tone of a tuning fork against a background hiss. Running about 50 million tests per second, the computers will compare any signal that doesn't look natural-that is, from a star-with a catalog of known terrestrial signals, such as military transmissions. That will leave a mere one in a trillion signals worth a further look. "If it's not in the catalog, we stop the star-scanning sequence and move the antenna off that star," says NASA's Kent Cullers, the country's only blind physics Ph.D. and the scientist who figured out how to detect intelligible whispers in the cosmic cacophony. "If we still hear the signal, that means it's leaking in from something on Earth. But if the signal goes away when we shift the telescope, it's coming from that star."

Is the signal changing frequency? Then it's probably not an intentional transmission: a smart ET would correct the signal so that it would stay on the same wavelength despite the rotation of his planet. Is it pulsing on and off? Then it begins to look more like ET: "Pulses can be seen at levels 10 times weaker than continuous signals," says Cullers. "If I were an ET I'd definitely consider pulses." Another attention-getter is a fan-beam, like a lighthouse's, sweeping across the galaxy. "You could sweep virtually the whole sky in just a few minutes," says JPL's Klein. If the signal passes all the tests, the telescope alerts a scientist. She comes running, looks at the signal, runs more tests and, if the signal still looks promising, asks a second site to check it. Unfortunately, this all takes enough time that "to verify it from a NASA site the signal would have to stay on for half an hour," says Cullers. "To alert a non-NASA telescope it would have to stick around for 24 hours." That's not likely, but at least the telescopes could keep their ears cocked for a similar signal.

While the Columbus quincentennial is a stroke of luck for the PR-conscious space agency, there are sound scientific reasons for listening to the seemingly empty, lifeless expanse of the light-years now. The first is the growing belief that we are not alone. It was Frank Drake's inspiration to quantify the probability of life elsewhere in the galaxy. It depends on the likelihood of there being other stars like the sun and other planets like Earth, of life arising, of life becoming intelligent, of intelligent life choosing to communicate and of chatty intelligent life avoiding blowing itself to oblivion with advanced technology. Multiply all the probabilities, and presto: the likelihood of ETS.

Except for the small problem of not knowing exactly which numbers to plug in. For instance, "the lifetime of civilizations is utterly unknown," says Kent Cullers. Should we figure on an ET Khrushchev who backs away from an alien missile crisis? Or an ET Saddam who brings Armageddon to his planet? Other elements of "the Drake equation" are easier to pin down. Calculating them has given ET studies some respect, unlike in the days when Drake was regarded as a bright young man who had gone over the edge. Recent discoveries on events from neta formation to the emergence of life have all "made the case [for ET] stronger," says Drake. Here's how it works:

Start with the 400 billion stars in the Milky Way.

Nearly 10 percent of them are like the sun in age and composition. That means 40 billion possible life-giving stars.

Based on observations by the infrared satellite IRAS, astronomers believe that planetary systems form just about whenever stars do, says JPL astronomer Richard Terrile. Maybe 10 percent have one Goldilocks planet-not too hot and not too cold for life. Down to 4 billion Earths.

Life may not be too hard to make. "There are a multitude of chemical pathways leading to life," says Drake, such as zapping the primeval atmosphere with lightning or cooking simple molecules in the heat of a volcano or deep-sea vent: "Where a planet is suitable, life will pop out all over." There is evidence that life has been cooking elsewhere. Complex organic molecules, such as formaldehyde, alcohol and the amino acids that form proteins, float in interstellar gas clouds and permeate meteorites as generously as mix-ins in a Ben & Jerry's pint.

Of course, life and intelligence are not synonymous, as a few minutes with C-Span make Clear. The fraction of planets with intelligent life, much less intelligent life that can communicate across the stars, is hard to quantify. But scientists doubt it's unique to Earth. So they fall back on oddsmaking. Say the chance of intelligent, radio-signaling life is one in a million. That means 4,000 advanced civilizations. Prefer odds of one in 10 million? That gives us 400 galactic neighbors.

Whatever the exact answer, the point is that it is probably more than one. "With apparently billions of opportunities for life to arise in our galaxy, it would be astounding if we turned out to be the sole example of intelligent life," says astronomer Paul Horowitz of Harvard University. "The galaxy may be teeming with life and technology." And our galaxy is only one of 100 billion.

So why haven't they called? Maybe the calculations are wrong--especially the part about intelligent life that doesn't blow itself up-and there is only one life form per galaxy. Us. Or maybe ET communicates not via radio waves but through fiber optics and cable TV; those technologies don't leak signals into space, and so wouldn't have registered as even a dial tone in the more than 50 ET searches since Drake's 200-hour hunt in 1960 at Green Bank. Maybe they are truly blessed and have not evolved beyond print. Or perhaps the aliens stay quiet out of fear of-or pity for-belligerent, bumbling earthlings. Who in his right mind would want to chat with the beings that beamed "My Mother the Car" out past Alpha Centauri?

Or maybe they have called. Most searches have found what astronomers call unexplained signals (table, page 68). None ever repeated, or stayed on the air long enough to be checked. But many are alluring enough to keep ET-hunters on the trail for years. Since an Ohio State team recorded a signal so intriguing a researcher scrawled "Wow!" in the margin of the printout (making it forever known as the "wow" signal), its search has run almost constantly for the past 15 years. But it's probably no closer to finding ET than to beating Michigan.

NASA has no generous Isabella, so SETI has had more trouble getting off the ground than the Nina did sailing out of Palos Harbor. In fact, killing the SETI appropriation has become an almost annual congressional blood sport. Former senator William Proxmire awarded the project a Golden Fleece Award, and the late representative Silvio Conte once said that rather than spending millions to find "rascally" aliens, all one need do is pick up a supermarket tabloid. Until NASA secured its $100 million, SETI's biggest patron was Steven Spielberg: in 1984 Carl Sagan persuaded the "E.T." creator to fund a SETI project at Harvard, sponsored by the Planetary Society, to the tune of $100,000. That project, run by Paul Horowitz, has been the most ambitious ET search going.

Horowitz tries to think on ET's wavelength. "It seems rational for the senders to choose a guessable frequency [to transmit on] rather than a random one," he says. That way, the intended recipients don't have to search the whole radio dial to find it. Horowitz calls them "magic frequencies." His money is on the song of hydrogen: this simplest and most abundant atom in the universe vibrates at a frequency of 1,420,405,751 cycles per second. This frequency, says Horowitz, "would make sense as a meeting place in the vast radio-frequency spectrum."

But ET's planet would move relative to Earth, and thus shift the magic frequency. Just as an ambulance's siren becomes deeper or whinier as it moves, so the pitch of a signal from space would change as its planet of origin moved. The pitch that started out as the hydrogen harmony would shift higher or lower, and no longer be "magic." But Horowitz figures that ETs are at least as smart as he is--smart enough to have figured out this "Doppler shift." So he has programmed his equipment to listen for signals he thinks ET would send: frequencies that are magic relative to, say, the center of the Milky Way, or any other reference point ETs would know about. Horowitz's META (Megachannel ExtraTerrestrial Assay) listens with an 84-foot telescope-no more than a concave tapestry of aluminum mesh-perched on a Massachusetts ridge where the only neighbors are apple and peach orchards. So far Horowitz doesn't think anyone's called, but in a paper awaiting publication he and Sagan list about 50 odd signals from META I and its twin outside Buenos Aires, META II. Some have characteristics that rule out their being messages from ET. But dozens remain, suspended forever in time like a ringing phone that you picked up a nanosecond too late.

What kind of signal do the astronomers hope to hear? A broad radio signal can pack the most information. But it's hard to distinguish from background noise-just as a broad hill r-an fade into the surrounding topography. But a skinny obelisk--a narrow signal-is tough to miss. A smart ET might combine the best features of each: send out a narrow beacon that says, "For more information, tune to channel . . ." That's the one that would tell us where they are, what they look like and, maybe, what they know.

Drake thinks he has a fair idea of what they'd look like. In his new book, "Is Anyone Out There?" (272 pages. Delacorte. $22), coauthored by Dava Sobel and published last week, he writes, "From a distance of a hundred yards at twilight, you might almost mistake them for human. They'll have their heads at the tops of their bodies, I suspect, and their eyes in their heads... I think they'll walk on two legs, too ... [but] intelligent extraterrestrials will have four arms instead of two ... Four make for a much better design." Moreover, he says, "I suspect that immortality may be quite common among extraterrestrials." Since "barely out of the Dark Ages" humans are close to fathoming the genetic and molecular bases for aging, he thinks more advanced civilizations may know how to make organisms live forever, or at least "transfer ... memories out of an old brain into a young brain." The only way to die would be by murder, suicide or accident-look for really good compliance with seat-belt laws on ET's planet. But dialogue between ET and earthlings might be impossible. Any other creatures are light-years away, so each message would take years to reach them. We could have "only lengthy monologues," says Drake, "crossing each other eternally in the interstellar mail."

The possibility of a space buzzing with messages from longtime members of the galactic club raises hope of eavesdropping on an encyclopedia galactica, on everything from how to beat mortality to how to defuse the waste from nuclear reactors. "I can envision a rich stream of information on the history of the transmitting civilization, and on others they might be in contact with," says John Billingham, who heads SETI at NASA's Ames Research Center. "There could be 100 different frequencies from 100 different civilizations out there, and all we'd need to know is how to tune in to them." Even a mere "We're here, too" would, by its very existence, offer hope that advanced civilizations need not self-destruct with their nuclear toys. If they made it, maybe we can, too. "It would tell us that it's possible to survive our technological infancy," says Jill Tarter.

And if NASA hears a cosmos echoing only with the sounds of silence? Perhaps intelligence, and the technologies it creates, does not help a species survive; perhaps civilizations smart enough to build phone booths to the stars are also smart enough to blow themselves up in a nuclear fire before they can send an interstellar hello. Or perhaps NASA's strategy is wrong. Perhaps no one is home on the nearest stars, and anyone broadcasting from afar isn't loud enough to hear. Just because we don't hear anyone doesn't mean no one's calling. But it might. Then it will look more and more as if intelligent life is an almost impossible thing to achieve, an unlikely coming together of organic molecules on an improbably hospitable planet. In short, a miracle. If no one else is here, the universe will seem much lonelier. And earthly life more precious.

Scientists have heard many inexplicable signals, coming from several constellations. But none has ever repeated:

Ohio State's radio telescope recorded a signal so intriguing a researcher scribbled "Wow!" on the data sheet. It was never heard again.

One of about 40 intriguing signals recorded by META (Megachannel Extra Terrestrial Assay), based at a radio telescope in Harvard, Mass.

Another signal caught by META, which is tuned to hear only "magic frequencies"--those ET might be expected to broadcast on.

This signal, also recorded by META, was on a magic frequency. By the time the astronomer arrived to check it, it had vanished.

This META signal looks man-or alien-made rather than from a star. It has not yet been traced to an earthly source.

Recorded by the Parkes radio telescope in Australia, and the signal Frank Drake finds the most like what an ET might send.