JOHN BILLINGHAM says it's time to ask a basic question about life on other worlds.If the Search for Extraterrestrial Intelligence (SETI) program he heads for the National Aeronautics and Space Administration (NASA) - or any other SETI program - actually picks up a message should we reply? What should we say? Who is to decide? The fact that Dr. Billingham and his colleagues in the International Academy of Astronautics are seriously exploring this issue shows how far the quest to locate possible galactic neighbors has come. Peter B. Boyce, executive officer of the American Astronomical Society, says it now is established in the astronomical community as a worthwhile research field. Skepticism about its value has yielded to recognition that its detailed radio surveys of the sky will provide useful astronomical knowledge and edu cational opportunities. NASA will move that research to a new level when its SETI program goes on line next year. It plans to inaugurate the 10-year, $100-million effort on Oct. 12 - Columbus Day. Within a few minutes, the project should do more searching than all previous scans for alien signals put together, according to Billingham. The Planetary Society, which now sponsors the only observatories dedicated to SETI, does not expect to take a back seat. A few weeks ago, it announced plans to expand its own search capability in a way that will complement NASA's effort. Harvard University professor Paul Horowitz, designer of the Planetary Society equipment, remains enthusiastic for the quest even after eight years of essentially fruitless listening. "How kind the galaxy is to radio transmission [at the search frequencies]," he says, meaning that the interstellar medium in the galaxy doesn't dim or distort radio emissions at those frequencies much. Therefore, he says, "Radio looks like an awfully good way to communicate. If only those guys out there would do something." Actually, they may have been doing something. Dr. Horowitz's search isn't totally fruitless. He has picked up what he calls "curious hints" of signals of intelligent origin. "But these have not been repeated," he says. Nevertheless, Dr. Boyce says this has peaked the interest of many astronomers and helped SETI gain credibility. The Planetary Society is pursuing its search from the Harvard Smithsonian Oak Ridge Observatory west of Boston and Argentina's Institute for Radio Astronomy near Buenos Aires. It is following a strategy that Horowitz calls "guessable frequencies." This was first suggested by astrophysicists Philip Morrison and Giuseppe Cocconi, then at Cornell University, in a paper published in Nature in 1959 that has sparked sporadic SETI efforts worldwide over the past three decades. They noted that the 1,420-megahertz (MHz) emission of hydrogen, the most abundant interstellar material, is the most prominent natural radio emission in the cosmos. Any civilization technologically advanced enough to do radio astronomy would be scanning the sky at frequencies around 1,420 MHz. And any civilization broadcasting a "Hi there" signal would know this and so might transmit in the same frequency range. Horowitz's strategy is to scan across a frequency band based on this "magic" hydrogen emission and do it on many narrow frequency channels simultaneously. He began in 1983 with a 131,000-channel computer-controlled analyzer attached to Harvard's 84-foot-diameter radio telescope. He upgraded this in 1985 to the present 8.4 million channels covering a 350,000-MHz range with the help of a $100,000 grant from moviemaker Steven Spielberg. Last month, the Planetary Society announced plans to increase that sensitivity several hundred fold. With a $100,000 NASA grant and expected public contributions, the society plans within five years to have equipment listening on 6 billion frequencies at once. It expects to start in late 1993 with 160 million channels covering a range from hydrogen's 1,420 MHz to the 1,600-MHz to 1,700-Mhz emissions of what chemists call the hydroxyl radical, a combination of one oxygen atom and one hydrogen atom. Horowitz notes that hydrogen and hydroxyl are the constituents of water. By analogy to earthly animals meeting around a water hole, SETI researchers have dubbed this frequency range the cosmic "water hole" around which civilizations wishing to communicate might meet through radio transmissions. "Maybe that's pushing poetry too far," Horowitz says. But it does define a particular frequency range for searches. NASA, on the other hand, isn't relying on poetry. Its SETI program plans to cover as much as it can of the 1,000-MHz to 300,000-MHz frequency range over which the galaxy is "kind" to radio propagation. In practice, that means a cutoff at 10,000 MHz because of radio noise from oxygen and water in our own atmosphere. Billingham's program has two elements. A targeted search will use the largest available antennas around the world to conduct the most sensitive search yet made of stars within 80 light years of Earth. This will cover a frequency range of 1,000 to 3,000 MHz. A parallel survey will scan the whole sky over a range of 1,000 to 10,000 MHz, using NASA's Deep Space Network (DSN) tracking antennas. These are the searches that start next Columbus Day with the DSN site at Goldstone, Calif., beginning the sky surve y and the National Science Foundation's Arecibo Observatory in Puerto Rico launching the targeted search. Unlike the Planetary Society's dedicated SETI installations, NASA's radio equipment will piggyback on tele- scopes that are primarily dedicated to other work. Billingham advises us to "expect the unexpected." An alien signal may not conform to scientists' preconceived notions of its nature. Boyce, who is a NASA SETI investigator, also warns that a signal may not be a message. He notes that earthly radar astronomers have been scanning solar system planets for many years. These radar signals might, by chance, continue through space and wash over other star systems. Likewise, alien radar signals might reach Earth. "You could drive yourself crazy trying to find a message in that," Boyce says. Meanwhile, SETI researchers emphasize the here-and-now benefits of their work. Both the NASA and the Planetary Society programs are developing new sophisticated radio-listening equipment and techniques. Both offer students opportunities to work with this development. There are larger educational opportunities. With $700,000 from the National Science Foundation and $300,000 from NASA, the SETI Institute in Mountain View, Calif., is working with educators to develop materials for science teaching using SETI as an organizing theme. No one knows when an alien signal might actually be detected, if ever. But Billingham notes that it could come sooner rather than later. Thus, he says, we should think now whether and how we might respond. He adds that, if anyone has any reservations about revealing our existence to aliens, "they should join the debate now."