Cranking Up the Allen Telescope Array

In the patchwork of dry, cow-fouled ranch lands 250 miles northeast of San Francisco, an unusual crop is poking above the dusty shrubbery. Three dozen metal mushrooms have sprouted near the modest village of Hat Creek, and are turning their aluminum eyes skyward. These antennas, 20 feet in diameter and the height of a football goal post, are the vanguard of an eventual herd of 350 dishes, sprinkled over more than a half-mile of dirt and lava. They are the first installment of the Allen Telescope Array.

By spring, 42 of these antennas will be working, and while this is scarcely more than 10% of the ATA's final tally, even this partial sub-array can do interesting science. Beginning a few months from now, it will.

The young ATA's first foray into SETI will be known by the straightforward (if not overly galvanic) name of Inner Galactic Plane Survey. The word "survey" may surprise many who are familiar with this telescope's design. After all, it's being finely tuned to speedily examine large numbers of star systems in a so-called "targeted search". The completed array will be exceptionally nimble at such individual scrutiny, and will leave previous targeted searches in the data dust.

But the promise of future speed, and future sensitivity, makes a targeted search now--using only the sub-array--a less-than-exciting idea. It's akin to riding a burro to California a year before they finished the transcontinental railroad. You might as well wait and get a better ride.

On the other hand, while the array is being built--and consequently while the sensitivity and resolution are still on the upswing--there are opportunities to use it for short SETI projects that take a different tack than systematically searching one life-friendly star after another. So beginning this spring, the ATA will embark on a six-month effort to scan the Milky Way's innermost sanctum: the region near the galactic center.

Now many readers will drop their mandibles at this, since they've been told that the galactic center is a ruinous place for life, intelligent or otherwise. After all, it's a bad neighborhood, riddled with harsh radiation, and the locus of a massive, malevolent black hole. All of that is true, and the central hundred light-years or so of the galaxy probably is a "no-aliens land." But when you point a telescope at the Milky Way's nasty nub, you have the chance of picking up signals from all the civilizations between you and it, and all those beyond to the galaxy's far, fanning fringe. By sighting through the galaxy's most heavily populated neighborhoods, you guarantee that star counts are high: roughly tens of millions per square degree. This is the Milky Way's urban core, as thickly clotted as Devon cream.

The Inner Galactic Plane Survey will check out this teeming celestial real estate in a ribbon 2 degrees high and 10 degrees wide. Two or three pinpoint "beams", each about 1/30th the apparent area of the full moon, will slowly scan this strip, stopping at every position long enough (a half-minute for each frequency band) to build up sensitivity to weak signals. The digital receivers that sift through the incoming static will be tuned from 1390 to 1720 MHz (if you're SETI savvy, you may recognize this as more or less the "water hole"), and that wide spectral range will be segmented into 450 million, ultra-narrow 1 Hz channels. As this SETI sweep progresses, researchers interested in the physics of the galactic center will be doing their astronomy thing by making pictures of this unusual region with their own thousand-channel receiver.

There's a problem with training a telescope on the galaxy's inner hub: it's a southern hemisphere object, passing overhead in places like, say, Paraguay. But fortunately, the galactic center is not so far south as to be invisible in California, and Earth's rotation heaves it into view at Hat Creek for 5 hours per day. So it will take seven months to complete this survey, which--in keeping with today's trends--is somewhat of a hybrid: partly targeted search and partly sky survey.

It won't survey all of the sky, and it won't zero in on individual star systems. But roughly a half-billion stars will come under the Inner Galactic Plane Survey's radio gaze. Nearly all of these stellar realms are far away--thousands, and tens of thousands of light-years distant--which means that a detectable signal will require a powerful alien transmitting effort. But in little more than a century, humans have gone from building pipsqueak one-watt radio transmitters to megawatt monsters. What would a society thousands of years beyond ours be able to bolt to its antennas?

We can speculate, but we don't know. And that's a good reason for putting that first crop of metal mushrooms to work.

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Seth Shostak
Senior Astronomer, SETI Institute

Seth Shostak is an astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, who places a high priority on communicating science to the public. In addition to his many academic papers, Seth has published hundreds of popular science articles, and not just for; he makes regular contributions to NBC News MACH, for example. Seth has also co-authored a college textbook on astrobiology and written three popular science books on SETI, including "Confessions of an Alien Hunter" (National Geographic, 2009). In addition, Seth ahosts the SETI Institute's weekly radio show, "Big Picture Science."