As surprised as he was, Jerry was also puzzled. Where did the signal come from? SETI scientists traced it back to the constellation Sagittarius, just to the northwest of the globular cluster M55. But when they looked for the source, there was nothing there, no planet, no star. Still, the shape of the signal, its narrow AM/FM-like focus, not to mention its surprisingly tantalizing frequency suggested intentionality. Maybe whoever or whatever sent the signal had moved on?

 
B.J. Mochejska (CfA), J. Kaluzny (CAMK), 1m Swope Telescope
SETI scientists traced the Wow signal to an area just northwest of M55, a cluster of 100,000 stars inside the constellation Sagittarius.

Or maybe not. Maybe there was no signal. Jerry and his colleagues checked all the alternative explanations they could think of. Was it a satellite transmission (no), a military signal (no), an aircraft signal (no), a broadcast beam (no), an accidental beam ricocheting off space debris (no)? Could it have been something natural? A pulsating star? That's where things began to unravel.

Astronomers keep discovering new noisemakers in space: colliding black holes, glitching pulsars, gamma ray bursts. Columbia University astronomer Caleb Scharf says it is very hard to exhaust the possibilities when we are learning more every year about the universe.

For Jerry Ehman, the big puzzle is: Why only one signal? If an alien intelligence is trying to send a message somewhere, wouldn't it make sense to send the message a few times? The signal landed once on Aug. 15, 1977. It never repeated.

"That's key in the scientific method," says Columbia's Scharf. "You want to see a repeat." That way, other scientists can confirm the finding.

And yet, the "Wow!" signal is the only reliably recorded sound apparently received from deep space that has the quality of an intentional signal. Jerry Ehman has never claimed he'd heard from E.T. Where the signal came from is still an open question for him. All Jerry Ehman will say is that having eliminated (as best he can) every other explanation, a message from E.T. is one possibility he can't dismiss.