MOUNTAIN VIEW, Calif. For the tiny cadre of scientists probing the cosmos for signs of alien life, the most difficult question isn't always, ''Are we alone?''
Sometimes it's the shopworn, ''What do you do?'' from a fellow airline passenger.
Jill Tarter generally doesn't like to answer that question when she first meets someone. She's director of the Center for SETI Research, as in Search for Extraterrestrial Intell-igence.
But after four decades of frequent ridicule, the astronomers seeking signs of life in the heavens are gaining some respect. Since 1960, when Tarter's colleague Frank Drake first pointed a radio telescope at a pair of nearby stars in hopes of dialing in an alien broadcast, there have been about 100 searches for ET signals.
No space aliens have been found.
But new planets have. Astron-omers have located more than 100 outside our solar system since the first was discovered in 1995.
Whether those distant worlds teem with life, much less intelligent life, remains unknown. But each new discovery further energizes the search for ET. Even NASA shaken from its second disaster in manned space flight is getting back into the act. It's been a full decade since it cut off money for SETI amid cries in the halls of Congress that it was bankrolling a hunt for ''little green men.''
Some scientists are beginning to talk in terms of when, not if, they'll be able to answer a question that's vexed humankind probably since we first peered upward at the stars that pierce the dark. For Martin Rees, Britain's astronomer royal, finding that answer represents the main exploratory challenge of the next half-century.
''Our cosmic importance depends on whether we are alone or not,'' Rees said in a telephone interview from Cambridge, England.
It's also worth answering simply because it is such a big question, Rees added: ''The main aim of science is to take steps toward answering the big questions.''
It's that sentiment that keeps the hunt going, even without results.
''It's a great goal. That is a lot of what sustains you the payoff,'' said Kent Cullers, director of research and development for the Center for SETI Research. ''That's why we keep the champagne on ice.''
Even SETI skeptics, like University of California, Los Angeles astronomer Ben Zucker-man, concede it's three things cosmology, black holes and the search for life in the universe that drive public support for his field.
''And a substantial fraction of the funding comes from the third of those,'' Zuckerman said in a telephone interview from atop Mauna Kea, where he was using the Keck Observatory to investigate the formation of planets around young stars in the Milky Way.
So how do you carry out such a search? Directly listening for a signal by far has been the most popular approach, at least judging from its appeal to the general public.
As of early August, 4.6 million people have signed on to one SETI project alone, run by the University of California, Berkeley. The Web-based project enlists idle computer time and processing power to scan packets of data collected by the mammoth radio dish at Arecibo, Puerto Rico, for traces of alien signals. That level of interest is not matched by employment in the field or the modest private funding, merely in the millions of dollars.
The SETI Institute's Drake estimates that just 20 people work full-time on the hunt for extraterrestrial intelligence, with maybe 40 more working part-time.
''It's tiny,'' Drake said, quickly adding that the field has never been larger.
SETI's not likely to grow much bigger anytime soon, but it is about to get a whole lot stronger, faster and cheaper. When he started out in 1960, Drake used an 85-foot diameter dish in West Virginia to check out just two stars. This fall and coming spring, SETI hopes to wrap up a survey of 1,000 nearby stars similar to our sun, listening to each for five minutes for a trace of an alien signal. The survey's completion would mark the finale of a project initially funded by NASA but dropped after a year.
The SETI Institute picked up the search. It uses equipment at Arecibo and Britain that is 1 quadrillion times more powerful than the operation Drake cobbled together 43 years ago.
Further advances in computing power and a drop in price should allow for searching even more stars over a greater number of frequencies and with vastly improved sensitivity, Drake said. He estimates SETI's power doubles every 250 days.
To capitalize on that, the SETI Institute and Berkeley together are building an array of 20-foot diameter radio telescopes in northern California that should be capable of surveying 1 million stars over a decade. Three of the dishes are in place now; it should bristle with 350 by 2006. Microsoft Corp. co-founder Paul Allen is bankrolling much of the cost. And a slew of teams around the world are proposing building an array of telescopes 100 times larger. Tarter predicts that such advances will allow SETI to canvass much, if not all, the Milky Way and its 100 billion-plus stars within several decades' time.
Drake, using an equation he devised and which now bears his name, estimates there could be 10,000 technologically sophisticated civilizations in the galaxy. The implications of contact with just one are hard, if not impossible, to gauge.
''I think science fiction is our best guide,'' Rees says.
To think that Hollywood's take on the issue from ''The Day the Earth Stood Still'' to ''Mars Attacks!'' might serve as a roadmap can be chilling. In fact some feared that a 1974 effort, the first of its kind, to deliberately broadcast a radio message to a cluster of 300,000 stars would do little more than alert hostile aliens to our presence.
However, there really isn't too much to fear about physical contact, much less dialogue, with an alien civilization should a signal be detected. Bridging interstellar space takes lots of time. Even a simple conversation, across distances measured in light years, would be tedious at best.
Answers to questions could come thousands of years after they were posed. The 1974 message, sent from Arecibo, still has 25,000 years to go before it hits anyone's inbox.
''It's more like the dialogue you have right now with Shakespeare or the ancient Romans,'' Tarter said of the possibility of receiving an alien signal. ''You can't necessarily ask them questions. You have to infer the knowledge that you want to gain from what it is they have told you.''
The odds of making contact remain incredibly slim, even SETI boosters acknowledge.
For UCLA's Zuckerman the fact that no civilizations have come calling during the several billion years since life arose on Earth is strong evidence that there are no such creatures beyond Earth.
Any technologically sophisticated civilization, he reasons, would have surveyed the galaxy and spotted the telltale chemical signatures of life, including oxygen. Such aliens would be hard-pressed not to come visit, Zuckerman believes.
Critics of such views say that merely anthropomorphizes the issue: What if aliens are nothing like, say, curious astronomers, but more like sullen teens or meditative swamis?
''You could have intelligent life living a contemplative life on the bottom of the ocean and not wanting to communicate,'' Rees suggested.
For those undaunted by the quest, like Jill Tarter, there is no time like the present to keep asking the fundamental question of our cosmic uniqueness.
''I realized I lived in the very first generation of all human beings that had ever lived that could try and answer this question that humans have been asking themselves forever and by doing an experiment, rather than asking the priests and the philosophers and having a belief system give you an answer,'' Tarter said. ''There's only one time in history that that's going to happen, and I happen to be living at that time.''
That sort of reasoning has drawn NASA back into the game. The types of searches SETI is undertaking and those NASA is about to undertake are different, yet complementary and are likely to grow even more so, said Charles Beichman, chief scientist for astronomy and physics at NASA's Jet Propulsion Laboratory.
The space agency has a trio of space telescopes on the drawing board that would seek out planets like our own. Such planets presumably would become SETI's No. 1 targets, drawing the programs even closer. The first of the NASA missions is expected to launch in 2006.
SETI efforts also could be bolstered by the discovery of life even microbial life much closer to home. Scientists believe Mars and Europa, a moon of Jupiter, could harbor life deep in the dust and ice, respectively, covering each. A British mission is also en route to the Red Planet, where beginning in December it will scratch the surface and sniff the air for evidence.
While such microbes are unlikely to be proficient radio operators, their discovery inside the bounds of our solar system would open up the hunt for intelligent life forms within the greater galaxy, said Rees, the British astronomer.
''If you could say life arose twice, independently, in one solar system,'' he said, ''that would tell you straight away the origin of life didn't involve a rare fluke and there must be some sort of life on millions of other planets.''
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