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“There have been half a dozen observations since then, all of them by instruments that were measuring something else. No one’s looked for these waves directly; you’ll see why in a moment.

“The most impressive example was detected ten years ago, in ‘66, by a team doing a survey of Iapetus. They obtained quite a long recording, rather sharply tuned at nine kilohertz-that’s thirty-three kilometers wavelength.

I thought you might Eke to hear it….”

Duncan consulted a slip of paper and carefully tapped out a long sequence of numbers and letters on the Minisec. Into the anechoic stillness of that strange room, Karl spoke from the grave, in a brisk, businesslike voice.

“This is the complete recording, demodulated and speeded up sixty-four times, so that two hours is compressed into two minutes. Starting now.”

Across twenty years of time, a childhood memory suddenly came back to

Duncan. He recalled listening out into the Titanian night for that scream from the edge of space, wondering if it was indeed the voice of some monstrous beast, yet not really believing his own conjecture, even before

Karl had demolished it. Now that fantasy returned, more powerful than ever.

This sound—or, rather, infrasound, for the original modulation was far below the range of human hearing-was Eke the slow beating of a giant heart, or the tolling of a bell so huge that a cathedral could be placed inside it, rather than the reverse. Or perhaps the waves of the sea, rolling forever in unvarying rhythm against some desolate shore, on a world so old that though Time still existed, Change was dead…. The recording, as it always did, set Duncan’s skin crawling and sent shivers down his spine. And it brought back yet another memory-the image of that mightiest of all Earth’s creatures, leaping in power and

278 glory into the sky above Golden Reef. Could there be beasts among the stars, to whom men would be as insignificant as the lice upon the whale?

It was a relief when the playback came to an end, and Karl’s surprisingly unemotional voice commented: “Note the remarkably constant frequency the original period is 132 seconds, not varying by more than point one percent.

This implies a fairly high Q-say…”

“The rest is technical,” said Duncan, switching off the recording. “I merely wanted you to hear what the Iapetus survey team brought home with them. And it’s something that could never have been picked up inside the orbit of Saturn.”

A voice he had not heard before-young, rather selfassured—came out of the air behind him.

“But this is all old material, familiar to everyone in the field. Sandemann and Koralski showed that those signals were almost certainly relaxation oscillations, probably in a plasma cloud near one of Saturn’s Trojan points.”

Duncan felt his facade of instant expertise rapidly crumbling; he should have guessed that there would be someone in his audience who would know far more about this subject than he did-and possibly, for that matter, even than Karl.

“I’m not competent to discuss that,” he replied. -I’m only reporting Dr.

Helmer’s opinions. He believed that there was a whole new science here, waiting to be opened up. After all, every time we have explored some new region of the spectrum, it’s led to astonishing and totally unexpected discoveries. Helmer was convinced that this would happen again.

“But to study these gigantic waves-up to a million times longer than those observed in classical radio astronomy-we must use correspondingly gigantic antenna systems. Both to collect them-because they’re very weak-and to determine the directions from which they come.

“This was Karl Helmer’s Argus. His records and sketches contain quite detailed designs. I leave it to others to say how practical they are.

“Argus would look in all directions simultaneously like the great missile-tracking radars of the twentieth century. It would be the three-dimensional equivalent of cycLops-and several hundred times larger, because it would need to be at least a thousand kilometers in diameter. Preferably ten thousand, to get good resolving power at these ultralow frequencies.

“Yet it need contain much less material than cyCLOPS, because it would be built in Deep Space, under weightless conditions. Helmer chose as its location the satellite Mnemosyne, outermost of Saturn’s moons, and it seems a very logical choice. In fact the only choice…

“For Mnemosyne is twenty million kilometers from Saturn, well clear of the planet’s own feeble ionosphere, and also far enough out for its tidal forces to be negligible. But most important of all, it has almost zero rotation. Only a modest amount of rocket power would cancel its spin entirely. Mnemosyne would then be the only body in the universe with no rotation at all, and Helmer suggests that it might be an ideal laboratory for various cosmological experiments.”

“Such as a test of Mach’s principle,” interrupted that confident young voice.

“Yes,” agreed Duncan, now more than ever impressed by his unknown critic.