And that was another point-Hunt had spent a lot of time on Ganymede, and he still had many close friends among the UNSA personnel there with the Jupiter Four and Jupiter Five missions. Jupiter was a long, long way from the vicinity of Earth, which meant that no receivers anywhere near Earth would ever know anything about a beam aimed toward Jupiter from the fringe of the solar system, whether the beam diverged appreciably or not. And, of course, the J4 and J5 command ships were linked permanently to Earth by laser channels . . . which Caldwell and Navcomms just happened to control. It couldn’t possibly be all just a coincidence, he decided.

Hunt looked up at Caldwell, held his eye for a second, then turned his head to gaze at the two people from Washington. "You want to set up a private wire to Gistar via Jupiter to arrange a landing here, without any more messing around, before the Soviets get around to doing something," he told them. "And you want to know if I can come up with an idea for telling the people at Jupiter what we want them to do, without the risk of any Thuriens who might be bugging the laser link finding out about it. Is that right?" He turned his eyes back toward Caldwell and inclined his head. "What do I get, Gregg?"

Heller and Pacey exchanged glances that said they were impressed.

"Ten out of ten," Caldwell told him.

"Nine," Heller said. Hunt looked at her curiously. There was a hint of laughter in her expression. "If you can come up with something, we’ll need all the help we can get handling whatever comes afterward," she explained. "The UN might have decided to try going it alone without their Ganymean experts, but the U.S. hasn’t."

"In other words, welcome to the team," Norman Pacey completed.

Joseph B. Shannon, Mission Director of Jupiter Five , orbiting two thousand miles above the surface of Ganymede, stood in an instrumentation bay near one end of the mile-and-a-quarter-long ship’s command center. He was watching a large mural display screen from behind a knot of spellbound ship’s officers and UNSA scientists. The screen showed an undulating landscape of oranges, yellows, and browns as it lay cringing beneath a black sky made hazy by a steady incandescent drizzle falling from somewhere above, while in the far distance half the skyline was erupting in a boiling column of colors that exploded upward off the top of the picture.

It had been fifty-two years before-the year that Shannon was born-when other scientists at the Jet Propulsion Laboratory in Pasadena had marveled at the first close-ups of Io to be sent back by the Voyager I and II probes, and dubbed the extraordinary disk of mottled orange "the great pizza in the sky." But Shannon had never heard of any pizza being cooked in the way this one had.

Orbiting through a plasma flux of mean particle energies corresponding to 100,0000 Kelvin sustained by Jupiter’s magnetic field, the satellite acted as an enormous Faraday generator and supported internal circulating currents of five million amperes with a power dissipation of a thousand billion watts. And as much energy again was released inside it as heat from tidal friction, resulting from orbital perturbations induced as Europa and Ganymede lifted Io resonantly up and down through Jupiter’s gravity. This amount of electrically and gravitationally produced heat maintained large reservoirs of molten sulfur and sulfur compounds below the moon’s surface, which eventually penetrated upward through faults to explode into the virtually zero-pressure of the outside. The result was a regular succession of spectacular volcanoes of solidifying sulfur and sulfur-dioxide frost that ejected at velocities of up to a thousand meters per second, and sometimes reached heights of 300 kilometers or more.

Shannon was looking at a view of one of those volcanoes now, sent back from a probe on Io’s surface. It had taken the mission’s engineers and scientists more than a year of back-to-the-drawing-board experiences to devise an instrumentation package and shielding method that would function reliably under Jupiter’s incessant bombardment of radiation, electrons, and ions, and Shannon had felt an obligation to be present in person to observe the results of their eventual success. Far from being the chore he had expected, the occasion had turned out an exhilaration and served as a reminder of how easy it was for supreme commanders of anything to allow themselves to become remote and lose touch with what was happening in the trenches. In future, he thought to himself, he would make a point of keeping more up to date on the progress of the mission’s scientific projects.

He remained in the command center discussing details of the probe for a full hour after he was officially off duty, and then at last excused himself and retired to his private quarters. After a shower and a change of clothes he sat down at the desk in his stateroom and interrogated the terminal for a listing of the day’s mail. One item that had come in was qualified as a text message from Vic Hunt at Navcomms Headquarters. Shannon was both pleasantly surprised and intrigued. He had had many interesting talks with Hunt during the latter’s stay on Ganymede, and didn’t perceive him as being somebody with much time for idle socializing, which suggested that something interesting was afoot. Curious, he keyed in a command for Hunt’s message to be displayed. Five minutes later he was still sitting there staring at the message, his brows knitted in a mystified frown. It read:

Joe,

To avoid any further cross words on this subject, I looked for some clues in the book you mentioned and came across some references on pages 5, 24, and 10. When you get down to sections 11 and 20, it all makes more sense.

How they got 786 is still a puzzle.

Regards

Vic

Not a word of it meant anything to him. He knew Hunt well enough to be reasonably sure that something serious was behind the message, and all he could think of was that Hunt was trying to tell him something highly confidential. But why would Hunt go to this kind of trouble when UNSA possessed a perfectly adequate system of security codes? Surely it wasn’t possible that somebody could be eavesdropping on the UNSA net, somebody equipped with enough computer power to render its protective measures unreliable. On the other hand, Shannon reflected soberly, the Germans had thought exactly that in World War II, and the British, with their "Turing Engine" at Bletchley, had been able to read the complete radio traffic between Hitler and his generals, frequently even before the intended recipients. Certainly this message would mean nothing to any third party even though it had come through in plain English, which made it appear all the more innocuous. The problem was that it didn’t mean anything to Shannon, either.

Shannon was still brooding about the message early the next morning when he sat down for breakfast in the senior officers’ dining quarters. He liked to eat early, before the captain, the first navigation officer, and the others who were usually on early shift appeared. It gave him time to collect his thoughts for the day and keep up with events elsewhere by browsing through the Interplanetary Journal -a daily newspaper beamed out from Earth by UNSA to its various ships and installations all over the solar system. The other reason he liked to be early was that it gave him an opportunity to tackle the Journal’s crossword puzzle. He’d been an incurable addict for as long as he could remember, and rationalized his addiction by claiming that an early-morning puzzle sharpened the mental faculties in preparation for the demands of the day ahead. He wasn’t really sure if that were true, and didn’t care all that much either, but it was as good an excuse as any. There was nothing sensational in the news that morning, but he skimmed dutifully through the various items and arrived gratefully at the crossword page just as the steward was refilling his coffee cup. He folded the paper once, then again, and rested it against the edge of the table to scan through the clues casually while he felt inside his jacket for a pen. The heading at the top read: JOURNAL CROSSWORD PUZZLE NUMBER 786.