THE LAST DAY
Advanced Concepts
“We have nearly ten thousand strategic nukes. Seven thousand active, and another two on the reserve list.” Judy was wearing large gypsy earrings, a white T-shirt, classic Levi 501’s and Nike trainers. Dark sunglasses protected her eyes from the strong sunlight which streamed in through the cockpit window. Incongruously, she was wearing a pearl necklace.
McNally’s tone revealed his surprise. “The USA still has seven thousand bombs?”
“But they’re mostly the W-series, just a fraction of a megaton each. Great for knocking off cities and the like but no way do they have the punch to deflect a small asteroid. Not on our hundred-day guideline. No, Jim, if you’re looking for real action you have to go for the old B-53s. And we only have fifty of these.”
“One will do,” McNally declared.
“I don’t believe so. They’re not neutron bombs.”
“Let’s run with your B-53s for a moment anyway.” The NASA Administrator glanced at the compass and made a tiny adjustment on the joystick. Desert drifted below them. He had flown straight from Toulouse to Tucson where Judy and the jet had been waiting. He was now en route to the Johnson Space Center at Houston, first dropping Judy off at the Sandia National Laboratories, twelve square miles of nuclear wisdom tucked securely inside Kirtland Air Force Base near Albuquerque. Judy was briefing him as they flew.
She produced a bar of dark chocolate, broke off a couple of squares and offered them to the NASA chief, who accepted happily. “Okay,” she said. “They’re the oldest nukes still in service. They’ve been operational since 1962. But they’re also the largest and they’re pretty lightweight for their power. That’s one of the nice things about nuclear weapons: the yield to weight ratio increases with power. The bigger the bomb the more punch per pound.”
“These B-53s — just how much punch are we talking about, Judy?”
“Nine megatons. Now that is destructive enough for any conceivable military target, but the bomb itself weighs only four tons. It’s a three-stage weapon. That’s classified information, by the way, but in the circumstances…”
“Don’t you people have anything bigger? I seem to recall the Russians exploded a fifty-megatonner once.”
“The Tsar Bomba. A wonderful thing,” Judy smiled. “It was really a hundred-megatonner but they configured it for fifty when they exploded it in Novaya Zemlya. Even then you could pick up its pressure wave on ordinary domestic barometers anywhere in Europe. We think it weighed thirty tons.”
“So, what have we got to match it?”
“Zilch. Our military asked permission to develop sixty megatonners way back in the fifties, but this was denied. We’ve always gone for precision targeting rather than massive zaps.”
McNally slid his sunglasses down his nose and looked over them at Judy. “We lack the nuclear punch to deflect Nemesis? Are you serious?”
“If it needs more than nine megatons.”
McNally took a few seconds to absorb this startling new information. “Tell me about your neutron bombs.” A small town was drifting about twenty thousand feet below them, narrow white roads radiating from it through the desert. A plume of smoke rose from a farmhouse some miles to their left.
“Jim, they’re just tactical tank-busters. Artillery shells with no more energy than a Hiroshima. Armoured personnel are hard to kill, but neutrons penetrate armour. Some tanks, like our M-1, are reinforced with depleted uranium, which is very dense and hard to penetrate with explosives. But listen, this is really smart. If you set off a neutron bomb you activate the depleted uranium so the soldiers find themselves cocooned in a radioactive tank at the same time as the neutrons from the bomb are penetrating it. At a few miles’ range their blood drains out from every orifice in a few minutes. Closer up and they just dissolve into a hot ooze. Closer still and they explode. More chocolate?”
McNally declined. He loosened his tie.
“But as a Nemesis killer, they’re far too small. They’re made that way so you don’t have military commanders wiping out too many towns at a time when they’re hitting Russian tank brigades in Europe. I don’t believe our stockpiles include neutron bombs in the multi-megaton range.”
The NASA Administrator responded to some chatter on the radio. “By the way, we’re now in New Mexico. What’s a three-stage weapon, Judy?”
She hesitated. “I guess I can say. Start simple, with a gun firing two sub-critical masses of uranium together. That’s fission for you, a straight one-stage atom bomb. The trouble is, it has limited power. The fission reaction is slow to develop and the bomb blows itself apart before all the fissile material is used. The Hiroshima bomb was only 1.4 per cent efficient, for example. You can’t get much more than a critical mass to explode. But fission bombs do give you a plasma a metre or less across with a temperature of about fifty million degrees, and that’s hot enough to start you on the fusion route, transmuting four hydrogen atoms into one helium one with the mass deficit emerging as energy through E = mc2.”
“I’ve never been clear what form of hydrogen you use,” McNally said.
“That’s classified too, but what the hell. It varies. Liquid hydrogen is best but you can use compressed gas and we’ve even used a hydrogen-impregnated solid. Anyway, more than eighty per cent of the energy from a simple fission bomb comes out as X-rays. Teller and Ulam got the bright idea that, because the X-rays are moving at the speed of light — they are light — maybe you could use them to compress a large capsule of hydrogen at very high speed, before the assembly got disrupted. The fastest reaction at fission temperatures is between the heavy hydrogen isotopes, deuterium and tritium. So you stir these isotopes into the brew, light the touchpaper and retire a long way back. Four hydrogens fuse to give you one helium, as per undergraduate physics courses, but this leaves surplus mass in the form of a 14 MeV neutron and an eighteen MeV photon which is an impressive quantum of energy.”
“That’s a two-stage weapon, the touchpaper being an A-bomb.”
Judy finished the chocolate bar with a satisfied smile. “Correct. Not only Teller and Ulam, but also Sakharov in Russia got the radiation implosion idea. So let us give thanks unto these gentlemen for the hydrogen bomb. But why stop at two stages? If you want a bigger bomb, use the fusion explosion to compress and explode a third, fission stage. It makes for a dirty bomb but a powerful one, and no new scientific principles are involved. Each stage can be ten or a hundred times more powerful than the one before. No question, Tsar Bomba—King of Bombs — must have been a three-stager. There was even a Soviet design for a layer cake at one stage.”
“The mind boggles,” said McNally, his mind boggling.
More radio chatter. McNally explained, “We’re now entering restricted airspace. Let’s hope Noordhof fixed it like what he said he would.” He spoke into his mouthpiece and trimmed the aircraft. Far above them, two Tomcats passed swiftly across their bows, right to left. A third fighter appeared from nowhere and started to probe inquisitively, looking at them from all directions and keeping a safe twenty metres away. They flew on for some minutes. Then the pilot waved, and the jet tipped its wings and hurtled into the sky above.
“Judy, it seems to me you’re going to have to tart up a B-53, turn it into a neutron bomb.”
She brushed little flakes of chocolate off her white sweater. “But Jim, the way a neutron bomb works is that you let the neutrons escape during fission instead of absorbing them to create more energy. That means a neutron bomb will always be a low-energy device. If we’re going on a last-minute deflection, meaning we need energies in the megaton range, the neutron bombs we need don’t exist.”