The future of one of our greatest intellectual adventures is clouded in doubt.

Two decades ago, there was a new surge of interest in the idea of intelligent life beyond the Earth. In a famous paper published in 1959, physicists Giuseppe Cocconi and Philip Morrison pointed out that radio technology made it possible to search for signals from alien civilizations at interstellar distances.

During the same period, the reborn nebular hypothesis for the origin of the solar system suggested that planets might be common attendants of other stars. Laboratory experiments showed that the amino acids necessary for life as we know it could have evolved in the Earth's early atmosphere or in the "primordial soup" of its primitive ocean. If here, why not on the planets of other stars?

An optimistic school of thought began to emerge among scientists in the 1960s, gathering around the idea that life might be a common phenomenon throughout the universe. Since intelligence appears to be an evolutionary advantage, it seemed plausible that there could be other evolutions to civilization and technology.

Interstellar flight, the radio astronomers said, was too difficult to allow direct contact. But the great radio dishes that grew out of wartime radar would allow us to listen for intelligent whispers from space, and to reply. As early as 1960, radio astronomers began aiming their antennas at likely stars, searching for an alien beacon.

Advocates of the search tended to share liberal political views, hoping that the idea of contact would encourage us to think about our future as a species, and to rise above narrow concerns. Some foresaw receiving the accumulated wisdom of more ancient civilizations by radio, perhaps including lessons on how to survive our own technology. The whole concept of extraterrestrial intelligence was a kind of Copernican revolution in our thinking about ourselves, displacing us from a position of uniqueness.

There was growing international scientific interest, leading to the formation of a study group by the International Academy of Astronautics in 1965. U.S. and Soviet experts held a seminal meeting in Soviet Armenia in 1971. Beginning a year later, international scientific meetings on communication with extraterrestrial intelligence became a regular annual event.

The United States remained the primary center of activity. NASA produced a design for a giant array of radio telescopes, called Project Cyclops, that could be used in the Search for Extraterrestrial Intelligence (SETI). Numerous technical papers appeared in the journals. Experts concluded that a search for intelligently-produced radio signals was a valid scientific project, suitable for government funding. Meanwhile, Soviet scientists published a detailed plan for their own national SETI effort.

In 1978, NASA got up the political courage to request $2 million from Congress to begin work on a modest search program. While the space committees of both houses authorized this, the appropriations committees refused to provide the money. Sen. William Proxmire, who gave this proposal a "Golden Fleece" award, administered the coup de grace last September by getting Congress to cut out the $800,000 NASA had been using to keep research and development work alive.

In recent years, the optimistic school of thought also has encountered a growing intellectual challenge from scientists who say that extraterrestrial civilizations are extremely rare or do not exist at all. Meanwhile, the idea of traveling from one star to another gained new credibility as engineers studied the problem more closely. The British Interplanetary Society's Project Daedalus study, published in 1978, concluded that we could build starships with foreseeable technology. When put together with the notion that advanced societies might be able to create self-contained artificial worlds in space, this suggested that interstellar colonization would be a feasible project for a civilization more advanced than our own.

If we can travel to other stars some day, could not other civilizations have done this already? And if they have, why don't we see evidence of that travel, including artifacts in our solar system, or even on the Earth? Though some scientists have argued that the absence of extraterrestrials on Earth proves that there are no other advanced civilizations in our galaxy, others, including NASA's chief SETI expert, John Billingham, warn that we cannot draw any conclusions about the existence or non-existence of other civilizations without first searching for them. SETI pioneer Frank Drake of Cornell calls the whole interstellar colonization scenario "Star Trek-itis."

But there is another intellectual challenge, from the biologists. Many believe that the life we know on Earth today is the product of an extraordinarily complex sequence of events, which would never be reproduced anywhere else. They argue that life is a very rare phenomenon in the universe, and that we may be unique.

Attitudes toward extraterrestrial life and intelligence have gone through cycles dating as far back as the ancient Greeks. Sometimes the idea of many inhabited worlds was in favor; at other times it was not. Since World War II, we have been in an "up" phase.

Are we now entering a downswing? Twenty- three years after the Cocconi and Morrison paper, there still is no federal funding for an active SETI program. Despite widespread interest here and abroad, there is no sign of large-scale government support for a search in any other country. Most people seem content to look inward, and accept our cosmic loneliness.

Before jumping to any sweeping conclusion, we should remember that this cycle of interest in extraterrestrial life is different. It has coincided in time with our first journeys beyond the Earth. If we are the first technological civilization in the galaxy, we have the opportunity of exploring and even colonizing it ourselves.

That expansion should be guided by some ethical principles, and a sort of generalized strategy. A basic element must be a reconnaissance, a looking outward to see what we are likely to encounter. That survey already has begun, through our astronomical instruments and our planetary exploration spacecraft, like the Voyagers that swept by Saturn last year.

The search for extraterrestrial life and intelligence must be a part of that reconnaissance. We need to know if our outward reach will bring us into contact with other civilizations, before we venture into their domains.

Older arguments still apply. Finding other examples of life and other cultures would illuminate our own. The optimists may be right about a rewarding exchange of information. Above all, searching would help sustain a shared conception of ourselves.