ASTRONOMERS listening for alien radio broadcasts have yet to tune in a signal. They have heard nothing from the three recently discovered planets circling stars similar to our own sun.
But they remain cautiously optimistic as the hunt for intelligent life enters what is being called a new and more effective era.
Advances in electronics and computer technology are giving scientists the ability to scan more of the heavens in less time. And they have the money to keep up the quest, despite government cutbacks.
The future of SETI (Search for Extraterrestrial Intelligence) ''is very bright,'' says Kent Cullers of the SETI Institute in Mountain View, Calif. ''The amazing thing is ... private funding is capable of carrying on this search,'' he says, adding, ''If this search persists, there's a high probability of success.''
But Dr. Cullers, and several other researchers at the annual meeting of the American Association for the Advancement of Science here, emphasized the difficulty of that search. They put a very low probability on detecting an alien signal in any given set of observations or in any given year. But as the search is extended systematically, they expect the probability of detecting at least one signal during the next few decades to rise significantly.
The ability of SETI equipment - mainly receivers and computer programs - to monitor candidate stars is increasing rapidly. Every year, astronomers are getting a doubling of technical capacity due to advances in electronics, instrument sensitivity, and computers.
Cullers expects his own institute to have peered at 1,000 stars by the year 2000, and a million stars by 2010. If the likelihood of detecting a signal is one in a thousand or even as low as one in a million, searchers should expect to have found something interesting by 2020 or so. If nothing turns up, he would rethink expectations about intelligent life in our galaxy.
Today's best SETI equipment could detect electromagnetic signals no stronger than Earth's most powerful radars from distances of hundreds of light years. After Congress cut funding, the SETI Institute's Project Phoenix picked up the former National Aeronautics and Space Administration program that had targeted sun-like stars out to about a hundred light years from Earth.
Meanwhile, Harvard University Prof. Paul Horowitz has begun a new phase of observations with equipment at Harvard, Mass. His system can listen to 250 million radio channels at a time. It's sensitive enough to pick up signals from 1,000 light years away. ''That's a million sun-like stars,'' he notes.
The Space Sciences Laboratory at the University of California at Berkeley is following a different strategy with its project SERENDIP. This mounts listening equipment on a working radio telescope and listens 24 hours a day in whatever direction the telescope happens to point in the course of its work.
Although two of these programs cooperate with government-funded telescopes, large and small private gifts support the work of all of them. The SETI Institute now has guaranteed private funding of some $15 million over the next five years.