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The Future of SETITargeting the Aliens
Frank Drake's first experiment was a "targeted" search, though that term wasn't in the SETI lexicon in 1960 (nor, for that matter, was the acronym SETI). Drake didn't slew his telescope across large tracts of celestial real estate in hope of a serendipitous hit from one of the distant millions of stars. Rather, he faced his antenna toward two Sun-like stars very nearby.
The only major targeted search under way now [as of 2001] is our Project Phoenix, a systematic scrutiny of about 1,000 nearby stars that are mostly similar to the Sun. The principal antenna we use is the giant Arecibo dish in Puerto Rico, which stares at the sky with an 18-acre eye 305 meters in diameter. But even with this prodigious collecting area, Phoenix is not sensitive enough to pick up the kind of radio leakage that Earth spews into space even if Earth were at the distance of Alpha Centauri, the closest star to the Sun. ("Leakage" includes TV carrier waves and other continuous transmissions. It does not include Earth's largest military radars, which would be visible to Phoenix from dozens of light-years away if they happened to be aimed just right at the right time.) This is discouraging, and more so when you consider that Phoenix is the most sensitive SETI experiment ever.
Moreover, radio leakage from a planet is only likely to get weaker as a civilization advances and its communications technology gets better. Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions. Therefore, what we're really looking for from the aliens is a beacon signal — a big, loud, continuous shout deliberately designed to attract the attention of interstellar listeners.
When it comes to sheer sensitivity, SETI's dream instrument is the proposed Square Kilometer Array, or SKA. In the years ahead radio astronomers hope to build this mother of all radio telescopes — a monstrous device with a total collecting area of one square kilometer, 14 times that of Arecibo. Although intended for conventional radio astronomy, it could easily have 50 to 100 times the sensitivity of Project Phoenix. Perhaps 10 percent of its synthesized beams could be dedicated to SETI research.
But the SKA is at least a decade or two away and could cost $500 million or more. So the STWG advocated building a more modest instrument that could be probing the universe within a few years: the One Hectare Telescope, now renamed the Allen Telescope Array (for its major benefactor, Microsoft cofounder Paul Allen). This instrument will be both a prototype for the SKA and a dedicated SETI machine — one that, unlike Arecibo, could be devoted to looking for alien signals 24 hours a day, 7 days a week.
The ATA will have as much collecting area as a single antenna 100 meters square (one hectare). But it will be made of 350 relatively small, cheap dishes. This way a large collecting area can be put together for a fraction of the usual cost of a big radio telescope. In addition, compact low-noise amplifiers are commercially available for these dishes at prices so low that they cause radio astronomers to salivate.
Of course off-the-shelf TV dishes must be modified to slew and track astronomical objects, and they will need to be tightly integrated by fiber optics to create a functioning aperture-synthesis array. Once again, the real cost is the huge computational load of synthesizing beams and splitting the radio spectrum into many channels. But the telescope can start small and grow as technology and money allow.
Even in its first incarnation, the ATA will have impressive specs. Its frequency coverage will stretch from 500 to 11,200 MHz, spanning the whole microwave window, with at least 100 MHz of this range being examined at any given moment. At least three synthesized beams would be computed to begin with, allowing for triple targets on the sky. The number of beams can be increased as computing costs ease, so the ATA might someday examine 100 spots simultaneously in a 3-degree patch of sky. Indeed, one of the principal attractions of an array is the ability to examine multiple targets.
The bottom line gets the heart pounding: while Project Phoenix will examine 1,000 nearby stars, the full ATA could check out 100,000 in the same time and with comparable sensitivity. Ultimately, it might survey as many as a million targets. The ATA is such an appealing idea that implementation was under way even before the STWG banged the gavel at its last meeting.
How will the antennas be arranged? There are tradeoffs. A compact pattern — cramming all the dishes into a small circle or oval — would be best for SETI, since this arrangement would create wide beams encompassing many stars at once. But conventional radio astronomers prefer the opposite: that at least some of the dishes be scattered far across the landscape, allowing the synthesized beams to be small and sharp for high resolution. The final design was a compromise between both.
One long-standing issue facing the SETI community is the relative merits of star-by-star targeting, as done by Phoenix, and wide-sky surveys, which cover a great deal more celestial real estate but with much poorer sensitivity. However, to some extent this issue is destined to go away — because there comes a point when any targeted search will grow into a wide-sky survey.
The synthesized beams of the ATA telescope will surely be no bigger than 10 arcminutes across. That's pretty tiny. It would take 2 million such beams to cover the entire celestial sphere. But if the eventual observing program is stretched to include the nearest 2 million good stars, there's not much difference between selecting targets and scanning the whole sky.
The reason for this is that each time a target star is examined, our instrument is also listening to the rest of the little 10-arcminute patch of sky around it. A patch this size will include an average of 100,000 distant background stars — and that's just in our own Milky Way. Ironically, our best targeted searches are destined to become sky surveys within a decade or so. [For the other side of this debate, see Smarter SETI Strategy].
There's no denying that SETI is an uncertain enterprise. No one can tell when or if success will ever come. But the next generation of instruments proposed by the STWG can assay our galactic environs star by star out to 1,000 light-years or more, sifting for signals from a million potential solar systems. That's an imposing number, and one that stirs the imagination of all who dare to look for company among the stars.
(The STWG's final report has been published as a 551-page book, SETI 2020, available from the SETI Institute.)
Seth Shostak is a radio astronomer at the SETI Institute in Mountain View, California. His current occupation is a consequence of a sudden realization (while still a graduate student) that the hardware he was using to study galaxies could also be used to prove the existence of cosmic company. His guess is that new instruments for SETI "will reveal extraterrestrial intelligence before two decades have passed." He is author of Sharing the Universe: Perspectives on Extraterrestrial Life.
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