The SETI@home screensaver that crunches data in search of intelligent signals from space has produced a list of candidate radio sources that deserve a second look. After an equivalent to a million years of computation aided by more than 4 million computers worldwide, the researchers have highlighted where in the sky to find some of the most promising choices.
In March, for their project called 'Stellar Countdown', three members of the SETI@home team travelled to Puerto Rico. Their task was to point the Arecibo radio telescope at over 150 spots identified as the source of possible signals from intelligent civilizations. To determine if a strong radio signal is more than random noise, a glitch or a passing satellite, Arecibo's 1,000-foot diameter radio dish--the world's largest-- listened again to promising locations and frequency ranges.
The candidates for re-observation are particularly strong signals or ones that have been observed in the same spot more than once, some of them five or six times.
|Stellar Countdown locations. Skymap of Arecibo reobservations. Click image for larger view. SETI@home uses the idle time of over four million personal computers to sift through radio data for signals from extraterrestrial civilizations. The three main signal types of interest are:
Gaussians are the power curves produced when the Arecibo beam scans a steady celestial radio source. The signal is weak at first, strong when it is at the center of the beam, and then fades again. This produces a bell shaped power curve known as a gaussian.
Pulses represent any celestial radio signal of a fixed frequency that is distinguishable above the background noise.
Triplets are a sets of 3 equally spaced spikes. Whereas gaussians represent a constant signal from space, triplets may represent a series of pulses transmitted at fixed time intervals.
Their list of candidates was far larger than 150, but UC Berkeley physicist Dan Werthimer, SETI@home chief scientist, concluded that 150 is the maximum he and two colleagues could observe during the 24 hours available to them at Arecibo Observatory on March 18-20. Werthimer was assisted by graduate student Paul Demorest and project scientist Eric Korpela.
Their countdown list is a mere fraction of the database of anomalies. Since the project's launching in 1999, SETI@home users around the world have detected no less than 5 billion different signals called gaussians, triplets, and pulses, any one of which could potentially be that sought-for communication. This means that for every signal that was selected for reobservation at Arecibo, 25 million others were discarded, and most likely never be observed again.
Criteria for inclusion in the list included not only a strong radio signal and a signal observed more than once in the same spot and frequency range, but also the signal's proximity to a known star and whether that star is known to have planets.
"These factors let us estimate the probability that a candidate is noise," said UC Berkeley computer scientist David Anderson, director of SETI@home. "We're interested in the candidates that are least likely to be noise."
The early results from the Stellar Countdown are now being compiled. According to the project sponsors at The Planetary Society, Arecibo delivered '24 hours worth of very high quality data', and their reobservations tallied a total of 166 candidates.
Amir Alexander of the Society indicated that the scientists collected two types of data of different sensitivities from each signal candidate--the normal 2 bit recorder, which SETI@home uses throughout the year, and a highly sensitive 8 bit recorder.
Alexander pointed out that the task of treating the higher resolution signals is ongoing: "Because of their different format, the data collected by each of these instruments must be analyzed differently. Processing the data from the 8 bit data recorder is no simple matter. Because SETI@home had never before used this instrument, there are no computer programs in place ready to perform the necessary analysis."
But results of where in the star field these signals are coming from has been compiled already into a skymap.
These points represent a viable star system coupled to a persistent (repeated) signal which can initially be classified among the three types of communications: a pulse, three equally spaced spikes (triplets) or a steady source that rises and falls as Arecibo sweeps across its location. A further refinement on these choices is whether the signal appears to be corrected for its relative motion, or put another way, whether an intelligence is associated with how the frequency is maintained constant despite two planets moving relative to each other.
The candidate stars are located in the northern hemisphere because of Puerto Rican latitude, but the scientists plan to set up a southern hemisphere SETI program at Parkes Observatory in Australia, with that data to be fed into SETI@home.
For the higher resolution [8-bit] data, "the standard SETI@home program, which is installed on all users' computers, was not designed to process this type of information," said Alexander. "New processing programs for this data must be designed, and SETI@home Project Director David Anderson is working hard on the project. Along with other members of the SETI@home team, he is devising a new distributed computing system designed specifically to deal with the data from the 8 bit recorder. The system, however, will not be ready for several more months, and in the meantime Anderson, Werthimer and their crew are concentrating on the data from the less sensitive 2 bit recorder...the hard work of analyzing the results of the Stellar Countdown continues apace. It's taking somewhat longer than hoped."
One way to classify how worthwhile a particular signal may be to revisit is whether it has changed between its initial recording and the March reobservation. Alexander said, "The team is particularly interested to find out whether the score and ranking of any of the 166 candidate signals revisited during the Countdown had increased as a result of the reobservations. Such a candidate would certainly be the object of much further study."
A type of signal that is not closely examined yet, but in future will be part of a reobservation plan is short-pulsed radio beeps. A next generation project will be called, Astropulse, and will be designed to handle the beeps. In addition to processing SETI signals, Astropulse can detect pulsars, which blink on and off at periods up to nearly a millisecond and evaporating black holes, which should emit a brief pulse of radio waves as they blink out of existence. "Astropulse will be the first big test," which also provides enhanced, more realistic 3-D graphics, Anderson said. "If we get maybe 1,000 people participating in the first... trial, we could analyze the re-observation data in just a few days."
In addition to The Planetary Society, other major funders include Sun Microsystems, the University of California, Quantum Corp., Fujifilm Computer Products and Network Appliance.
Related Web Pages
Terrestrial Planet Finder Home Page
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Anybody Out There? Part I
Anybody Out There? Part II
Search for Life in the Universe: Neil deGrasse Tyson Interview
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Rare Earth? Are we so special?