"At our level of sensitivity, there was nothing we could uniquely identify as an extraterrestrial signal," said project leader Stuart Bowyer, a professor in the graduate school at UC Berkeley and an astronomer at the campus's Space Sciences Laboratory.
Nevertheless, the SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) team already has embarked on a new generation search using an improved instrument mounted a year ago on the Arecibo radio telescope.
"Our negative results don't rule out the possibility of civilizations out there because we are still only covering a small part of the radio spectrum," said Dan Werthimer, co-director of SERENDIP. "We are continually getting better. Since we started the project 20 years ago, our ability to survey the sky has grown by a factor of a million. Our newest instrument, SERENDIP IV, will give us 40 times more coverage than SERENDIP III."
Sabine Airieau, a junior member of the SERENDIP team who graduated from UC Berkeley last year, presented a status report on SERENDIP III at the June 7-11 meeting of the American Astronomical Society in San Diego, Calif.
During their sky survey, the team also looked for signals from six recently discovered extrasolar planets: 51 Pegasi, 70 Virginis, 55 Cancri, Tau Bootes, HD114762 and Rho Corona Borealis. No signals were detected.
SERENDIP is one of the world's longest running searches for extraterrestrial intelligence, known as SETI projects. SERENDIP III, which collected data from 1992 until this year, employed a third-generation instrument piggybacking on the large radio dish at Arecibo.
The SERENDIP III instrument scanned about a third of the sky every six months, looking in a radio band centered around a wavelength of 70 centimeters -- a radio region typically used for communications, and which includes UHF and mobile phone channels.
A fourth-generation instrument, SERENDIP IV, was installed on the Arecibo telescope in May of 1997, designed to scan the same region of the sky but in a frequency band centered on a wavelength of 21 centimeters. That wavelength is considered by many the most likely at which a civilization would broadcast its presence.
When a new receiver comes on line at Arecibo later this year, the SERENDIP IV instrument should be able to analyze 40 times more signals than SERENDIP III.
The search is conducted by looking for repeating signals. Thus, the more often they look at a given area of sky, the greater the chance of detecting an extraterrestrial signal -- if there is one. With specially designed computer circuitry and software, SERENDIP IV will simultaneously examine 168 million frequency channels every 1.7 seconds. The 168 million signals are analyzed immediately for radio intensities above background levels. Those found are immediately transmitted to UC Berkeley, where they are analyzed to eliminate the ones caused by interference from Earth-based or near-space radio sources.
"Ninety-nine percent of recorded signals are rejected at this point," said SERENDIP software director Jeff Cobb. "Those that remain will be studied closely for patterns consistent with an artificial signal from space."
"When and if we find something that is compelling, we will call up the telescope director and ask for time to take a longer look at that area of the sky," Werthimer said.
Despite piggybacking on the world's most sensitive radio telescope, he said, the instrument could not detect random radio noise emanating from a civilization like ours, which has been leaking radio and TV signals for less than 100 years. For SERENDIP and most other SETI projects to detect a signal from an extraterrestrial civilization, the civilization would have to be beaming a powerful signal directly at us.
"With available instruments we are unlikely to detect Earthlike planets or civilizations," Airieau said. "This sort of detection will not come within our realm for another few decades."
Werthimer added, "The first civilization Earthlings detect is likely to be more advanced than ours -- perhaps 10,000 to billions of years old."
SERENDIP was started in 1978 by Bowyer and astronomer Michael Lampton on a UC Berkeley radio telescope located in Hat Creek, Calif. SERENDIP II followed with two years of observations (1986-88) at the National Radio Astronomy Observatory in Green Bank, W. Va., using a 300-foot telescope that collapsed several years ago. SERENDIP III was mounted at the Arecibo Observatory in 1992.
SERENDIP is supported by the Planetary Society and the SETI Institute of Mountain View, Calif., with major donations from numerous companies and from the Friends of SERENDIP, a group led by novelist Arthur C. Clarke.