For the past three months, Dr. Paul Horowitz of Harvard has been systematically scanning the northern skies--the first scientist in the world to have his own radiotelescope dedicated to nothing more than listening for intelligent signals from an alien civilization.

But he will have plenty of company before long. In the next 10 years, a serious and ambitious new worldwide program called SETI--the Search for Extra-Terrestrial Intelligence--could tell us whether anybody is out there.

The search soon will be under way in at least three places in the United States. Canada is planning to start its search at a radiotelescope in northern Ontario. The Soviet Union already has begun a search, using an array of radiotelescopes in and around the Caucasus Mountains. Japan is building a radiotelescope that should be ready by 1984. The Netherlands, West Germany and France all have similar plans.

"The 1980s will see a more thorough search of the sky for artificial signals in our galaxy than has ever been done before," said Dr. Thomas McDonough, SETI coordinator for The Planetary Society and a lecturer in engineering at the California Institute of Technology. "The likelihood of success of discovering an alien civilization is greater than it has ever been."

Why look for aliens in space? Because if there are aliens they may have lived so much longer than we have that they could tell us how to survive or how to improve ourselves, or teach us what they have become and what we are likely to become.

Direct contact by spaceship is out of the question. A round trip to the nearest star at even 70 percent of the speed of light would require all the electric power the United States would consume in half a million years and would cost as much as $100 billion.

Instead, SETI will use radiotelescopes and the newest and fastest computers. Why radio? As Dr. Philip Morrison of the Massachusetts Institute of Technology explains: "God knows how to make light but He doesn't know how to make radio microwaves. They're made on Earth only by people."

The telescopes will scan the sky for signals beamed randomly into space. The computers will sort out the signals at an astonishing 10 million per second, separating the celestial wheat from the chaff.

The chaff could be transmissions from an orbiting satellite, routine radio noise from a commonplace star, vibrations of interstellar gas, the man-made signal of a military radar, and sounds of a passing truck or even a nearby microwave oven.

The wheat would be a steady microwave radio beacon, far removed from the frequencies of orbiting satellites, whose message could have been sent only by aliens from a distant planet.

"Using current technology, we could communicate with another civilization near any of the several million nearest stars," Harvard's Horowitz said. "An interstellar telegram to one of the farthest of these stars a distance of 1,000 light-years would cost about a dollar per word."

Horowitz has been using an 84-foot-wide telescope, operated by the Smithsonian Observatory on land owned by Harvard, that the Smithsonian was about to shut down because of a shortage of funds.

Originally, Horowitz wanted to build a $25,000 "suitcase SETI," a portable, advanced microcomputer that could be used with existing radiotelescopes around the country. Using it, he would be able to "listen" simultaneously to as many as 250,000 celestial radio signals on a single frequency and sort out any that seemed to indicate an intelligent microwave beacon from deep space.

But Sen. William Proxmire (D-Wis.) had put an amendment in the fiscal 1982 NASA budget banning it from spending any money on SETI.

Undeterred, Horowitz approached the Planetary Society, an independent body in Pasadena, Calif., that thrives on generous outside contributions and the $15 annual dues of more than 120,000 members. Its board of directors comprises Cornell University's noted astronomer Carl Sagan, former Jet Propulsion Laboratory director Bruce Murray and the society's own Louis Friedman.

"He Horowitz came to us a year ago with his plan for the suitcase SETI, which he said he would make portable so anybody in the country could use it," Friedman said. "Two weeks later, we sent him a check, which is an advantage of not being a bureaucracy."

By the time Horowitz built his device and successfully tested it, he heard of the Smithsonian's intention to shut down the Harvard telescope, lay off its caretaker and board up the building.

Back went Horowitz to the Planetary Society with a fresh scheme. If he could get $4,000 to fix the roof before winter, $13,000 for devices to mesh frequencies on the telescope and $25,000 to retain the caretaker and operate the telescope, Horowitz said, he wouldn't have to carry his SETI around the country begging for telescope time. He would have his own telescope he could use for his search 24 hours a day.

"This time we had to go to the members with his proposal, but they responded very strongly, much stronger than we thought they would," Friedman said. "They gave Horowitz a five-year commitment at $25,000 to $30,000 a year. It was a mandate to spend our money on the search for extra-terrestrial intelligence."

While all this was going on, NASA was quietly lobbying Proxmire to have him withdraw his anti-SETI amendment for the fiscal 1983 budget. Proxmire's main objection was that SETI would require large sums of money for new telescopes to send signals to millions of distant stars and to listen for the return signals, a process that might take 25,000 years.

The space lobbyists told Proxmire they were planning a program "on the cheap" that would use only existing telescopes, listen only for signals already reaching Earth from ancient aliens and never cost more than $2 million a year for the next five years.

Proxmire withdrew his amendment and NASA put $1.5 million into its fiscal 1983 budget to start the search. The money will go toward developing frequency analyzers and signal processors, electronic devices that can discriminate across a broad band of radio frequencies and sort out signals at a speed of 10 million a second.

Frequency discrimination is the toughest technical problem to solve. There is so much radio noise from military radar and orbiting satellites that it is getting harder every year to get through radio interference.

"This is not a trivial question," the Jet Propulsion Laboratory's Michael Kline said. "The radio skies are becoming cloudy, which is one reason we want to start the search right now."

NASA's approach to SETI is two-pronged. One aspect is to do an all-sky survey, with a number of existing radiotelescopes listening for signals at one of the so-called "magic" frequencies: 22,000 megahertz, at which water can be made to vibrate and transmit a radio signal. Most astronomers believe that if an alien civilization wanted to send a stellargram, it might use the "water line."

NASA's other approach is to lock onto 773 nearby stars (within 80 light-years of Earth) that are about the same size and energy as our sun.

"You have to define some boundaries," Kline said. "Otherwise, you can be looking and listening for the rest of your life."

The plan now is to place small electronic devices later this year at two NASA radiotelescopes at Goldstone in California's Mojave Desert, where both types of NASA searches will be conducted.

The prime candidate for the 10-million channel device, to be ready in 1986, is the 1,000-foot-wide antenna at Arecibo in Puerto Rico. Though it cannot swivel and turn as most radiotelescopes do, the Arecibo telescope is so powerful it can hear signals from one-third of all stars in the Milky Way.

Commenting on the sudden urge to search for alien civilizations, MIT's Phil Morrison said: "Even if there's nobody else out there, that's important to know. I don't want to sit here and speculate on haw likely alien life is. All I know is we have to look.