Eighteen years earlier, two Cornell physicists, Philip Morrison and Giuseppe Cocconi, had tried to imagine how an intelligent alien civilization might try to signal Earth. We should look, they said, for a radio transmission. Radio waves are cheap to produce, don't require much energy and travel vast distances across space.

Cocconi and Morrison guessed that the aliens would choose a frequency that would mean something to creatures who know math and chemistry. Hydrogen is the most common element in the universe. Zap a hydrogen atom and it will resonate at a particular rate: 1420 megahertz (MHz). So look, they said, for a signal coming in at 1420 MHz. And look for something loud, something that would catch our attention.

And on Aug. 15, in it came, exactly as predicted.

What Jerry saw was, yes, a radio signal and, yes, a radio signal very, very close to 1420 MHz (it was 1420.4556, just a smidge from where it was expected). It lasted 2 to 2 1/2 minutes. It was loud. And the transmission had the shape that Cocconi and Morrison had predicted. If you look at this printout, you will see this sequence of letters and numbers: 6EQUJ5.

According to science writer Michael Brooks in his book 13 Things That Don't Make Sense, "The letters and numbers are, essentially, a measure of the intensity of the electromagnetic signal as it hit the receiver. Low power was recorded with numbers 0 to 9; as power got higher, the computer used letters: 10 was A, 11 was B and so on." So by the time you get to the last letters of the alphabet, you are getting a very powerful signal.

That's why when Jerry saw this letter U on his printout (U is the 21st letter of the alphabet) he knew something was up.

An illustration of detail from the "Wow!" document.

"I had never seen any signal that strong before," Jerry says. "U," in a logarithmic way, means about 30 times louder than the ordinary noise of deep space. That's kind of a "Hello!" level. And that explains Jerry's reaction.

"That's the nice thing about the word 'wow.' I was, uh ... I was astonished," he says.