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SETI Searches Today
SETI researchers have always had trouble getting valuable radio-telescope time for their admittedly chancy pursuits. Yet telescope time, vast quantities of it, is what the search demands. Project SERENDIP neatly sidesteps this problem. The idea is to "piggyback" an extra receiver onto a radio telescope without getting in the way of the telescope's other work. This idea was originally conceived by SETI researchers at the University of California at Berkeley. Since 1978 they have carried SERENDIP through five big stages of evolution.
SERENDIP (then in version IV) gave birth to the famous SETI@home. The receiver is suspended high above the Arecibo dish, and scans the sky for narrowband signals wherever it happens to get pointed. The aiming of Arecibo is controlled by other radio astronomers working on their own projects. Although SERENDIP can't choose where it looks, it can run essentially around the clock all year. (In practice, however, data can be collected less than half the time.)
Starting in the fall of 1998, SERENDIP IV listened to 168 million radio channels simultaneously, each 0.6 hertz wide, comprising a band 100 MHz wide centered on the hydrogen emission frequency of 1,420 MHz (21 centimeters wavelength).
First, it is an important frequency in radio astronomy (since hydrogen is the most common element in the universe), so maybe alien engineers would choose it as a logical hailing frequency where other astronomically minded civilizations would think to listen. But that argument is less compelling now than when it was first proposed in 1959, during radio astronomy's early days. Since then radio astronomers have found many other important cosmic emissions and have proposed countless other "magic frequencies" where extraterrestrials might logically try hailing instead.
The second reason for choosing the hydrogen line is more down-to-Earth. Frequencies around it are supposed to be protected for radio astronomy, so Earthly interference problems are less severe than elsewhere in the radio spectrum.
SERENDIP IV repeatedly scanned most points on the sky between about declination +2° and +35°. That's about 30 percent of the entire sky. (Arecibo can't see any farther north or south because the dish points straight up; its view is limited to the band of sky that passes near Puerto Rico's zenith.) Potentially interesting signals that survived false-alarm tests were kept on file. The most important test is whether a signal repeats when the same point on the sky gets scanned again. Dedicated follow-up observations at the locations of the best candidate signals have been part of the plan.
Project scientist Dan Werthimer says that first looks at the SERENDIP IV data have turned up nothing obviously exciting. Full analysis of the data will take more time.
SERENDIP IV went idle in 2007. The new, much improved generation is SERENDIP V. It uses Arecibo's same new ALFA multibeam receiver as the current version of SETI@home, "SETI@home II." SERENDIP version Vb was installed at Arecibo in June 2009. From the SERENDIP website: "SERENDIP Vb is the most powerful spectrometer yet. . . . When complete, it will channelize over 2 GHz of instantaneous bandwidth (7 beams x 300 MHz) at less than 2 Hz resolution."
This version brings several major improvements over SERENDIP IV. It is not only five times more sensitive, it listens to 300 megahertz of radio channels at once instead of 100 MHz. It also listens at two polarizations instead of one, and will eventually monitor all seven beams of the ALFA multibeam receiver.
In addition, the seven independent beams (pointing directions) can reduce false alarms and help provide a fast first cut at confirming any real signal.
Strengths: SERENDIP uses the world's largest radio telescope to scan a fair fraction of the celestial sphere. This means it samples many billions of Milky Way stars and many thousands of background galaxies. No one star gets as deep a scrutiny as Project Phoenix provided, but the number of stars being scanned is immense.
Weaknesses: No real-time followup yet. This is a problem for piggyback SETI, partly because weak signals from beyond several hundred light-years should fade in and out of audibility, on a timescale of minutes, due to "interstellar scintillation" caused by the thin gas between the stars. Therefore several repeat observations of each point on the sky will probably be needed to catch a single repeat of a continuous weak signal. And, of course, if the aliens turn their transmitter elsewhere (or off) before a dedicated followup is scheduled, the chance to confirm a signal disappears.
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