The SETI League, Inc., a membership-supported, non-profit {501(c)(3)}, educational and scientific organization Searching for Extra-Terrestrial Intelligence

Ask Dr. SETI ®

Chapter 1: Astrophysics

How Much of the Sky Have You Covered?

Dear Dr. SETI:
I am preparing an article for my newsletter (The New England Journal of Skepticism) on the scientific legitimacy of SETI (I will be defending). One question that has arisen in the debate which I cannot find a reliable source on is: What percentage of the sky has been searched so far by SETI efforts? I have heard one estimate of 2%, but the derivation of this number was not available. I understand this estimate will vary based upon how it is calculated. Does the SETI League have an official estimate, and if so how was it derived?

Thank you,
Steven, MD (a skeptic)

The Doctor Responds:
Dear Dr. Skeptic,

That's a complicated question, to which I'll try to give you a simple answer. The search space includes five degrees of freedom: the traditional x, y, and z coordinates (that is, azimuth, elevation and distance) plus frequency and time. The question you framed deals with the first three parameters. Although the world's collective SETI programs have by now probably looked in most of the possible directions at some frequency for some period of time, we've so far only looked reliably out to a distance of perhaps 200 light years. But our galaxy is 100,000 LY in diameter. So in terms of the volume of interstellar space, we've seen less than one fifteen millionth of our own Milky Way galaxy.

But it's worse than that. Up until now (that is, until The SETI League launched its Project Argus search two years ago), SETI was done with giant radio telescopes, very sensitive instruments which look at extremely narrow slivers of sky. One such instrument (and there are only handful of them in the world) can see one part in a million of the sky. So if you happen to be looking on exactly the right frequency, at exactly the instant the Call comes in, there's still a 99.9999% chance you'll be pointing the wrong way.

Now let's talk about those frequencies. ETI communication could conceivably occur anywhere on the radio dial. But our searches to date have concentrated primarily on a handful of "magic frequencies" where we hypothesize other races may be trying to communicate. If we guess wrong, then it doesn't matter how much of the sky we cover -- we'll never hear anything. So recent SETI projects have tried to increase the frequency dimension of the search. The best (so far) in this regard is The SETI Institute's Project Phoenix search, which covers all frequencies between 1.2 and 3 GHz. Still a rather small fraction of the total radio spectrum.

Now comes the time dimension. The late NASA SETI program (launched in October of 1992, and cancelled by Congress in October 1993) included an all-sky survey component, which amassed all of 1000 hours of observing time before the plug was pulled. What fraction is that of all time, or even of all human technological history?

So the official SETI League answer is not expressed in percentages. We can't count that low. Your question relates closely to one addressed in an earlier column, "Aren't you becoming discouraged?" As we said there, not only have we not yet scratched the surface, we haven't even felt the itch.

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