The planet was large. It might have enough mass to ignite thermonuclear fires in its core, but experience argued that its light was too weak. The computers pondered whether to classify the system as a binary star and eventually decided against it. Still, the waxing point of light ahead held promise.

The morning passed in puzzled argument.

Nigel wasn’t totally willing to abandon the hypothesis that Jupiter Monitor had malfunctioned. The flight engineers—a flinty crew, skeptical of nonspecialists, fond of jargon—thought otherwise. They gave ground grudgingly, pitting sweet cool reason against Nigel’s vague doubts. A complete run-through of J-Monitor’s error-detection modes, a new diagnostic analysis, a hand-check of transmissions—all showed nothing wrong. There was no mechanical flaw.

The quirky echo had faded away a little after 3 A.M.

The Monitor was no longer in its original ellipse around Jupiter; a month earlier its engines had stirred awake and fired, to nudge it into orbit around Callisto, fifth moon of Jupiter. Now it spun an elaborate orange-slice orbit, lacing over the icy glare of Callisto’s poles every eight hours.

Nigel snapped a cracker in half, swallowing it with some lukewarm tea, hardly noticing the mingling of sweet and tart. He closed his eyes to the ting and clatter of telemetry. The flight engineers had finally gone back to their burrows and he and Lubkin sat in the main control bay, at one of the semicircular tables; digital arrays ringed them.

“That puts paid to the simple ideas, then,” Nigel said. “I suppose we’d best have a glance at the Callisto orbit.”

“Don’t follow,” Lubkin said.

“If the signal came from a source outside J-Monitor, something cut it off. The echo must’ve faded because Callisto came between the source and J-Monitor.”

Lubkin nodded. “Reasonable. The same thing had occurred to me, but—” he looked at his watch. “It’s almost noon. Why didn’t the echo return around seven or so this morning, when J-Monitor came out from behind Callisto?”

Nigel had the uncomfortable feeling that he was playing the role of dull-witted graduate student to Lubkin’s learned professor. But then, he realized, that was precisely the impression a skillful administrator would try to create.

“Well … maybe the other source is occluded by Jupiter itself. Now it’s blotted out.”

Lubkin pursed his lips. “Maybe, maybe.”

“Can’t we rough out some sort of orbit for the source, given a triangulation with Callisto?”

Lubkin nodded.

Around every star stretches a spherical shell of space, and somewhere within the thickness of that shell, temperatures are mild. For an Earthlike world, given the right primordial nudge, water will be liquid on the planet’s surface.

One-third of a light-year from the burning nugget of the star, the craft surveyed this livable zone and found it good. There was no sign of a large planet like the yellow-brown gas giant circling further out. This was a crucial test, for a massive world, close in, would have made another stable orbit impossible within the life-giving volume. Had the ship found such a planet, it was under standing orders—encrusted, ingrained, so old they functioned as instincts—to accelerate through the system, gathering all possible data for the astrophysical index, and chart a course for the next in a lengthy record of candidate suns.

Instead, the ship quickened the rumble of deceleration. It uncapped its telescope more frequently and peered ahead for longer intervals. A blue-white splotch revolved into another gas giant planet, smaller than the first and further out. Its image resisted precise definition. The craft noted a blurred circlet of bluish light—the body was ringed, a not uncommon occurrence among heavy planets.

Another massive planet was found, thinly ringed, and then another, each further away from the star. The machines began lowering their estimates of the possibility of life in this system. Still, past experience held out a glimmer of hope. Small, dim worlds might lie further in, even if the weight of theory and observation made it seem unlikely. By a fluke, the ship could be approaching from the night side of a world and miss it entirely. The craft waited.

At one-sixth of a light-year out the computers found an ambiguous smear of blue and brown and white: a planet near the star. Reward circuits triggered. The machines felt a spasm of relief and joy, a seething electric surge within. They were sophisticated devices, webs of impulses programmed to want to succeed, yet buffered against severe disappointment if success eluded them.

For the moment they were content. The ship flew on.

Spherical trigonometry, the vectoring line of J-Monitor’s main dish, calculus, orbital parameters, estimates, angles. Check and recheck.

Slowly, the most probable answer emerged—3:30 P.M., an hour away. By then the source should arc into view of J-Monitor’s main dish. Nigel imagined it as a dot of light slowly separating from the churning brown bands of Jupiter, rising above the horizon. As it traced its own ellipse, J-Monitor would be surveying the snow fields of Callisto below with its own mechanical intensity; craters, wrinkled hill lines, fissures, glinting blue ice mountains.

“One hour,” Lubkin said.

“Can we realign the main dish that quickly, without disturbing the surveying routine?” Nigel asked.

“We’ll have to,” Lubkin replied firmly. He picked up the telephone and dialed Operations Control.

“Tell them to rotate the camera platform, too,” Nigel said quickly.

“You think there’ll be anything to see at that range?” Nigel shrugged. “Possibly.”

“The narrow-angle camera? We can’t move both in—” “Right. We should work out a set of shots. Use the filters, stepping down from ultraviolet to IR. They can sequence automatically.”

Lubkin began speaking rapidly and precisely into the telephone, smiling confidently now that there were orders to be given, men to be told.

The ship was still cruising in deep silence, far from the star’s warmth, when it began to discern radio waves. More of the higher functions of the craft came alive. The weak signals were weighed and sifted. Filtering away the usual sputtering star noise, they found a faint trace of emission localized to the planets.

The most powerful source was the innermost gas giant. This was an optimistic sign, for the world did orbit fairly near its star. If it had merely a transparent atmosphere it would be too cold, but analysis showed it to be cloaked in thick, deep clouds. Such planets could warm themselves, the ship knew, by gravitational contraction and by heat-trapping—the greenhouse effect. Life could well evolve in their skies and seas.

Still, such clotted blankets of gas and liquid meant awesome pressures. Life in similar worlds rarely developed skeletons and thus could not manipulate tools; the ship’s log carried many instances of this. Trapped in their deep bowl of ammonia and methane, free of technology’s snarls, such creatures could not communicate—and the ship could assuredly not fly into such pressures in search of them.

A smaller source of radio waves lay further inward. It was the third planet, blue and white. The signals wove complex overlapping patterns, faint tremors that could be atmospheric phenomena: thunderstorms, lightning flashes, perhaps radiation from a magnetosphere. Still, the world was wrapped in a clear gas, a hopeful sign. The craft flew sunward.

By 6 P.M. they became discouraged. The Monitor’s main dish was reprogrammed to carry out a methodical search pattern around the spot where the unknown radio source should appear.

It was functioning. The data were coming in. All operations were proceeding smoothly.

And there were absolutely no results.

The flight engineering staff was milling about, writing day summary reports, ready to go home. To them, the echo problem was a temporary aberration that cleared up of itself. Until it reappeared, no cause for alarm.