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  "That's one of their strengths," Morse interjected.

  "Exactly, sir," Jeffrey said. "Right here right now he wouldn't be acoustically illuminated well enough for us to see."

  "And you're not worried," Wilson said, "with your helo torpedo bipolar thing, that this enemy boat could use active out-of-phase sound emissions to break up the echo, and active feed-through noise to plug the hole?"

  "Captain," Jeffrey said, "first of all, those subs don't have that capability like we do, at least not to the degree they'd fool our signal processing algorithms. At a fifth of our displacement they're too small to carry the computers and the power supply"

  "Okay," Wilson said.

  "Secondly, what makes it even harder for them is that the explosion would be gray noise, not a coherent active ping, with very complex reverb coming back from off those shoals … And thirdly, for hole-plugging emissions to do much good they need to know our relative bearing."

  "What if we go through this rigmarole and there's no contact?" Wilson said.

  "We should shift position and try again," Jeffrey said. "I'd head closer to the atoll, since in this case the greater risk exposure is underestimating target speed." Wilson gave Sessions a piercing look. "And what do you think, Sonar?"

  "Urn, I concur with Commander Fuller, sir. We can also use the wide-aperture arrays to do synthetic instant range gating. The data would be soft, but it would help us validate the firing solution."

  "Good," Wilson said. "That's what the ARCI's for."

  "And I think the helos should stay here for a while," Jeffrey said, "keep working this same area. Have them drop a few more 46s now and then."

  "Give the bad guys a false sense of security?" Wilson said.

  "Affirmative, sir," Jeffrey said, smiling. He glanced again at Ilse Reebeck, who seemed to be taking all this in while pretending not to. He made eye contact and she looked away.

  Jeffrey wondered if Morse and Wilson had intended this front/back bipolar trick themselves, all along. Maybe Wilson just wanted to see if Jeffrey could come up with it.

  "Sonar," Wilson said, "how's wave action topside?"

  "Long gentle swells now, Captain."

  "Coming way up from Antarctica," Jeffrey said, "as you'd expect this time of year."

  "Maybe three feet crest to trough," Sessions said.

  "Period?" Wilson said.

  "Four per minute, sir," Sessions said.

  "Any contacts overhead?"

  "Negative, Captain."

  "Very well, Sonar," Wilson said. "Fire Control, we'll use your plan." Wilson turned to Meltzer at the helm. "Bring the boat to periscope depth. Let's talk to those helos."

  "Chief of the Watch," Jeffrey said to COB, "stand by to raise the UHF antenna." Then Jeffrey grabbed a mike and punched in circuit 22. "Radio, Fire Control, prepare to send on short-range airborne tactical."

  * * *

  "Sir," Sessions said to Jeffrey, "the helo's parachute-retarded torpedo has hit the water. Unit has detonated, acoustic power spectrum confirms a Mark 46 conventional warhead."

  "Very well," Jeffrey said. Since the explosion was dead aft and relatively small, he didn't hear it through the hull.

  "Sir," Sessions said, "I have a submerged bipolar contact, bearings three two five and three two three on towed array and bow sphere! I designate the contact Master 1." Jeffrey watched the bearing lines appear and intersect on his target-motion analyzer window, all relayed to him by the combat control technicians sitting to his right. The range was over 30,000 yards. A 3-D representation of the tactical picture came up on the CACC's wide-screen situation display, with a multicolored halo around the contact — the probability envelope of uncertainty.

  "I have a second bipolar contact," Sessions said, "bearings three one nine and three two one! I merge and designate this Master 2!"

  "There's two of them?" Jeffrey said.

  "Affirmative!" Sessions shouted. "Shoal echoes put their speed at twenty knots!" The odds just lengthened badly, Jeffrey told himself.

  The enemy boats were at long range for him to make a good torpedo shot, and they were closer to Diego Garcia than Challenger was. They must be going all out now on batteries. Their endurance at that speed was short, clear indication they were up to something — something nasty — and it was coming very soon.

  "Flood tubes one through eight," Wilson said. "Equalize the pressure and open all muzzle doors."

  They were still well away from the targets, Ilse knew. Was this so they wouldn't hear us?

  "Sonar," Jeffrey said, "what's target depth?"

  "Best estimate is six five zero feet, sir," Sessions said. "Miss Reebeck helped make our ray traces more precise."

  "Good," Jeffrey said. "Very well, Sonar."

  Wilson looked at Ilse and nodded, then he glanced at Jeffrey. "Your thoughts on firing tactics, Fire Control?"

  "Sir, since we're in friendly waters, I'd recommend going shallow just before we fire, shooting from above the layer. That way if Master 1 and Master 2 stay deep, they're less likely to hear our ADCAPs till the terminal active-homing phase. I think we need to get much closer, though, maybe fifteen thousand yards, before we shoot."

  "Even though the ADCAPs go almost twice as fast as we can?"

  "Sir, the Mark 48s are fast but their propulsion system's loud. The longer the run, the more chance the targets have of a detection, and the more time the enemy has for using countertactics."

  "You're right," Wilson said. "So how do you propose to halve the range unnoticed, given the clock's still ticking?"

  "Well, if we go shallow and go fast, we'll cause a surface-wake anomaly and thermal scarring, which would be visible to long-range Axis airborne and spaceborne sensors, and any cavitation sounds would be worse from the lower water pressure. But if we stay deep and make a sprint, we'll be on the same side of the layer as Master 1 and Master 2. They're much more likely to hear us coming."

  "So which of these wonderful choices is it?" Wilson said.

  "Sir, my recommendation is to approach at top quiet speed at two thousand feet. Then we can come up to maybe one fifty carefully and open fire."

  "Why move in so deep?"

  "Sir, at the ranges involved, putting a wider chunk of the thermocline between own-ship and target depth might help shadow us acoustically. Because of temperature-induced sound wave diffraction, our radiated noise would bend down toward the bottom isothermal zone. Miss Reebeck's data suggest two thousand feet is optimum for us."

  "Very well, Fire Control," Wilson said. "Helm, make your course three four zero and make your depth two thousand feet. Ahead two thirds, make turns for twenty-six knots." Ilse listened to Meltzer repeat the orders, then she saw him working his controls. She was almost used to it by now. The deck tilted forward uncomfortably, but there was no other sense of motion, no vibration. A sound on the threshold of hearing grew slightly louder, a kind of whooshing with an underlying visceral rumble. It wasn't coming from the air-conditioning ducts.

  Ilse glanced aft. This is the closest I've ever been to a nuclear reactor, she told herself, and I can't jump in my car and flee if something goes wrong. She fidgeted with the dosimeter on her belt, but no one else seemed worried. Soon Ilse felt the boat leveling off, and she watched the digital depth gauge on her screen. It steadied at exactly 2,000 feet. Aren't there supposed to be popping, screeching noises this far down? Don't rivets start flying around the control room so close to crush depth?

  Suddenly the danger of their actions hit. If we make a peep, they could hear it. They'll shoot atomic bombs at us for sure. We're too far away for a good attack, the captain and Jeffrey said so. We have to get in closer, before they fire at Diego Garcia. And even if they fire prematurely and their cruise missiles fall short, they'll stir up a fearful shock wave and tsunami.

  Rut if we go in faster, they could hear us, and Captain Wilson said to go in fast. I don't want to die, Ilse told herself, not like this so soon. I have something to do first, dammit.

  "'Time's runn
ing out, Mr. Fuller," Wilson said. "What do you think?"

  "It would be ideal, sir," Jeffrey said, "if we could time our shots to hit them just as they come up through the layer to launch their missiles."

  "Devoutly to be wished," Wilson said, "but unrealistic. By the time we got in close enough for a good attack, they'd almost surely make a counterdetection and gel off a snap shot at us with a nuclear torpedo. So how could we tell they're going for the surface, assuming that's their plan, if we can't even hear them?"

  "We couldn't, sir," Jeffrey said. "But if they rise above the layer prematurely, they'll hear our own fish coming in plenty of time to evade and counterfire."

  "At close enough range they might even hear the ADCAPs through the layer," Wilson said. "You know acoustic shadow masking's always an iffy thing."

  "I'm starting to change my mind on something, sir," Jeffrey said.

  "Oh?" Wilson said.

  "I think maybe we should use a single weapon, a fission warhead. Set for highest yield, one-tenth KT. It's got a lethal radius big enough to catch them both, even if they run, even with some error in the firing solution." Relying on just two stale data points was not recommended practice, Jeffrey knew.

  "Concerned that we're outnumbered?" Wilson said.

  "Respectfully, sir, yes. The fuel-cell Klasse 212s are vulnerable with all that liquid hydrogen on board, but they have twelve torpedo tubes between them, to our eight."

  "And taking potshots with our ADCAPs would just tip them off?"

  "Affirmative, sir."

  "Then what about going active, disguise our ping as biologic?"

  "You know how I feel about that, Captain. Once we start, we'd have to keep it up. Shrimp don't click real loud just once. Whale songs go on for minutes, even hours, so they can stay in touch moving in and out of each other's convergence zones. The enemy boats would track the source, watch our maneuvers. The water here's too quiet."

  "I concur, sir," Sessions said. "The range is a bit extreme for that tactic in any case."

  Wilson gave Jeffrey a hard look. "Still don't want to call for help?"

  "Sir," Jeffrey said, "our aircraft are heavily committed all along the front. If we suddenly vector in more stuff, including those two ASW helos we were working with, the 212s will know for sure we know they're here. We'll lose the element of surprise, and they'll fire off their missiles. The whole point of Challenger is we're invisible till we strike."

  "What's range and bearing to the helos?" Wilson said. "Twenty miles now, sir," Jeffrey said. "Bearing one seven five, crosswind from us."

  "All right, then," Wilson said. "Debate's over. Assistant Navigator, let the rough log show that at … zero five two three Zulu this day, CinCPAC Theater Nuclear Forces rules of engagement were satisfied for a tactical nuclear launch against submerged enemy contacts."

  "Assistant Navigator," Jeffrey said, "I concur." Jeffrey breathed a sigh of relief. Compared to this, Prospective Commanding Officers School would be a cinch.

  "Sonar," Wilson said, "where's the layer?"

  "One nine zero feet, sir," Sessions said.

  "Helm," Wilson said, "ahead one third, make turns for four knots."

  "Ahead one third," Meltzer said, "make turns for four knots, aye … Maneuvering acknowledges turns for four knots, sir."

  "Make your depth one five zero feet."

  "Make my depth one five zero feet, aye," Meltzer said.

  "Fire Control," Wilson said, "have Combat Systems warm up a nuclear Mark 88 torpedo. Same presets you put in the ADCAPs, to run above the layer. Load it in tube seven."

  "Aye aye, sir," Jeffrey said.

  "That way we'll slap them down real good if they stay deep," Wilson said, "and blow them to kingdom come if they've gone shallow."

  Jeffrey relayed the commands and took the electronic acknowledgments. He eyed his weapons status screen as tube seven's outer door rotated closed, sea pressure was relieved, the water drained, and the inner door swung open. The weapons autoloader shuffled the units around on the racks, then presented a wide-bodied Mark 88 at tube seven's breach. Lights in the CACC started flashing.

  "General security alarm," Jeffrey said. "A special weapon has been shifted."

  "Special weapon handling is authorized," Wilson said.

  "Very well," Jeffrey said. "Disregard the alarm." The lights stopped blinking. Next Jeffrey watched as the fire control technicians on the CACC starboard side worked their consoles, establishing the weapon presets — under current nuclear war-fighting protocols the combat systems officer himself, called "Weps" for short, manned a retrofitted station on a lower deck, for two-man positive control.

  Jeffrey knew that overall authorization for tactical nuclear weapons had been handed down by the President after the war broke out. Final decisions were made on the spot by Challenger's senior officers.

  "XO," Wilson said, "take the conn. Messenger of the Watch, have Weps meet me in my state-room. Assistant Navigator, accompany me with the electronic logbook. I'm going for the special weapons enabler tool."

  Ilse and Sessions spoke in undertones, their voices blending with the constant murmuring of CACC technicians.

  "Ever seen a nuclear torpedo detonate?" Sessions said. He kept his eyes glued to his sonar screens. Ilse shivered. "No. Have you?"

  "Just on film. It's awesome. Surface units caught some footage when things first got hot in the Atlantic."

  "What's it like?" Ilse said.

  "Depends on warhead yield and depth when it goes off."

  "How big do they get?"

  "U-235, using just one critical mass, you can go up to maybe twenty kilotons."

  "In this war that's pretty much a strategic weapon," Ilse said.

  "Yup," Sessions said. "For comparison Hiroshima was roughly twelve. Nagasaki, they used a plutonium bomb, maybe twenty-two KT. Warsaw, when this war broke out, they think was ten."

  "So what happens when one goes off underwater?"

  "There's two things — really just like a regular depth charge, only bigger. Step one, warhead blows. That immediately lifts the surface of the sea, 'cause water's incompressible, and sends a suction wave back down."

  "You get a big white fountain?"

  "With a dot one KT explosion, could be a hundred yards across. Step two, blast of dirty water hits the surface from below, bigger than the first spout. That's the warhead burst itself. It pulsates as it rises."

  "The bubble energy fights back and forth with the water pressure?" Ilse realized now that everyone said "dot" instead of "point" for decimals — less ambiguous?

  "In this case that's the fireball," Sessions said. "It's buoyant, hot as hell, so it comes up really fast. There's a nasty airborne shock wave when it breaks the surface."

  "How hot is it?"

  "Try ten million centigrade."

  "Ouch."

  "It dissipates, cooling on the way, but being underwater doesn't help."

  "How come?"

  "Compared to air, the hydrostatic pressure confines the blast, concentrates the fireball. The water boils, of course, but that won't carry off much heat. Seawater's got poor transparency too, from all the stuff that's floating in it—"

  "Suspended particulates, organic matter. "

  "Yeah, Ilse, you would know. The whole photon flash on detonation, the gamma rays and X rays, ultraviolet, visible and infrared, not to mention all the neutrons, they get held in close, strengthening the fireball. On the other hand, seawater does suppress the EMP, the electromagnetic pulse that fries unshielded circuits … Anyway, first you have this giant burst of water, then you get the fireball. Timing between the two depends how deep the thing went off. There'll also be what's called the base surge, a kind of ground fog that spreads out like a fluid and evaporates, ocean surface atomized by the vicious shock wave through the water. You know an underwater blast's much more destructive to naval vessels than an airburst at a given distance. Water's much more dense and rigid, and sound travels five times as fast."

  "How much
fallout is there with a nuclear torpedo or, or a depth charge?"

  "Nothing like an H-bomb used on land."

  "But how bad is it?"

  "Not counting any from the ships they hit?" Sessions said. "There're the weapon parts, of course, vaporized. Fish and plankton, what's left of 'em. And loads of radioactive steam, from carbon, sodium, trace metals in the seawater, by neutron activation."

  "Tidal waves?"

  "At least two surges," Sessions said, still watching his displays. "They subside eventually with the kind of warhead yield we're using."

  "So they aren't like the big ones from an earthquake?" Ilse said. "Like that one in the Caribbean that wrecked your cruiser Memphis years ago?"

  Sessions shook his head. "Then the thing's a meter high way out at sea, but moving literally fast as a jumbo jet, piling up murderously onshore. Nuclear tsunamis act more like ripples in a pond, except they're fifty, sixty feet from crest to trough. They die off, mostly, in a matter of miles."

  "Can't big surface ships just ride that out in open water? I got caught on the edge of a typhoon once, on a research trip."

  "Yeah," Sessions said, "assuming the enemy doesn't ripple fire or hit from several bearings at once. A ship's hydrophones will give some warning, if you don't receive it on the data links or see the fireball glow, and you can turn bows-on. Still one hell of a ride."

  "I can imagine."

  Sessions turned to face Ilse. "The worst is if you're close, caught by the blast itself, or lose propulsion from the shock and take a wave from off the beam that's any higher than you're wide. Busted open, capsized, either way you've had it."

  "Warhead locks are bypassed," Jeffrey said. "Special weapon loaded in tube seven."

  "Very well, Fire Control," Wilson said. "I am relaying the permissive action link code."

  "Green light in the torpedo room," Jeffrey said. "PAL code is accepted. Weapon is enabled, armed guard has withdrawn."

  "Very well, Fire Control," Wilson said.

  From here Jeffrey knew the procedures were the same as were used with a conventional fish. To be fully armed, the warhead had to first be surrounded by seawater, then get an electronic signal, then feel the g-force of launch, then run out for a preset safety distance.