Breakfast things were laid out on the kitchen table and Shafer was dithering around the microwave oven. Judy was in an easy chair; she was into a severe white blouse and black skirt, a plate on her lap, and she was using her sharp, red-painted nails to carefully peel a hard-boiled egg. Sacheverell sat next to her with a plate on his lap. He was also pouring her a coffee and she flashed him a smile. McNally, Leclerc and Kowalski were at the window, sipping coffee and looking out over an expanse of desert from which the occasional tree-covered mountain protruded like an island in the sea.
Noordhof was busy on a croissant. A row of cigars protruded from a shirt pocket. He made a show of looking at his watch as Webb entered.
Webb poured himself coffee from a big percolator, heaped a plate with sausage and scrambled egg, and settled down at the farmhouse table. “You’re giving me a hard stare, Colonel.”
“Please God, deliver this man unto my sergeant,” Noordhof prayed.
A screen on a tripod had been set up and an overhead projector on the end of the kitchen table was throwing white light at it. The soldier nodded to Sacheverell, who had a stubble and looked a bit ragged.
Sacheverell put fork and plate aside, wiped his fingers with a handkerchief, took a pile of transparent overlays to the projector and moved them on and off the machine as he spoke. The first one showed three teddy bears of different sizes, with bubble text coming from the mouth of each, like a comic. One bear was saying 104 Mt, another 105 Mt and the third 106 Mt. “I examined three scenarios which straddle the likely energy range. I’m calling them Baby Bear, Mummy Bear and Daddy Bear. As you see Baby Bear is ten thousand megatons, Mummy Bear a hundred thousand and Big Daddy is a million.
“First I had a look at Baby Bear, deep ocean impact. I had the idea that maybe the aggressors — the Russians? — might want to take out the UK or Japan while they were about it. Anyway, the Atlantic and the Pacific are big, easy targets. Okay. So half a minute into impact we have a ring of water three or four hundred metres high. Wave amplitude falls as it moves out but you’re still looking at a fifteen-metre wave a thousand kilometres from the impact site.”
“In the open sea?” Shafer asked.
“In the open sea. Tsunamis are long-range, because the ocean is a surface and specific energy drops linearly with distance rather than inverse square. An earthquake in Chile in 1960 created ocean waves which travelled over ten thousand miles and killed a lot of people in Japan.”
“What was its wave height?” McNally asked, coming back from the window.
“In the open sea, twenty centimetres. The wavelength is hundreds of kilometres.”
“An eight-inch wave killed people?” McNally asked, bewildered.
Sacheverell winced. “No. When the wave runs into shallow water the same amount of energy is being carried by less and less water. So when it approaches a shoreline it rears up. The twenty-centimetre wave became a metre or two high. Killed a couple of hundred people, if you count the ones that just went missing.”
“So what’s the run-up factor on your fifteen-metre wave, Herb?” Shafer asked.
“Ten to forty, depending on the shoreline. If we say twenty, the wave is three hundred metres high when it hits land, assuming the impact was a thousand kilometres offshore.”
“The height of the Eiffel Tower,” Leclerc said. “How far inland would a wave like that travel?”
“Again it depends. Topography, roughness of surface. Flat agricultural land would flood for ten or twenty kilometres inland. When I say flood, I mean the wave is still two hundred metres high maybe five kilometres inshore.”
Webb said: “An Atlantic splash of that order would take out nearly all the major cities in Britain.” Although he was actually trying to visualize a half-mile tsunami roaring up Glen Etive.
“I don’t believe these figures,” Shafer said, without bothering to explain why.
“Europe is protected by a steep continental shelf,” Sacheverell informed Webb. “It reflects about three quarters of the energy back into the ocean.”
“Great,” Webb said. “Really great. Now I know that when I turn into Piccadilly the wave coming at me is only a hundred metres high.”
Noordhof went to the percolator and came back with a refill. “And if Baby Bear hits land?”
“Blast, heat and earthquake. The blast is a pressure pulse followed by a hot wind. The nuclear weapons people use an overpressure of four psi to define total devastation although there’s huge loss of life even at two, mainly from blizzards of flying glass in urban areas. Hit L. A. and you’ll blow the roofs off houses in San Diego. A Baby Bear on Philadelphia would rip people up from Baltimore in the south to New York in the north.”
“You could take out England from London to Newcastle,” Webb interrupted, still doing his patriotic bit.
“Who would want to zap your feeble little island?” Sacheverell asked. “I’ve taken the threshold for fire ignition to be about a kilowatt applied to a square inch for a second. It turns out you ignite everything in sight — tyres, grass, everything flammable. A hundred miles away, it’s like standing four inches from an electric fire for ninety seconds.”
“That must depend on whether the asteroid hits the ground or breaks up in the air,” said McNally.
“No. The heat comes from the hot wake trailing the fireball. Lastly, earthquake. I’ve taken Gutenberg-Richter Nine as defining total devastation, and I’ve assumed five per cent of the kinetic energy goes into shaking the ground. We’re looking at Nine over a region about a thousand kilometres across.”
Noordhof took a sip at his coffee. “So Baby Bear takes out a few cities or floods one of our seaboards. But it doesn’t totally destroy the USA and it leaves our nuclear potential intact. So let’s turn the screw a bit. Herb, take us to Mummy.”
“Wave height scales as the square root of the impact energy, and the flood plane extends as the four thirds power of the run-up wave. These are approximations. They’re beginning to crumble when you get to the really big numbers. Mummy Bear makes an open ocean wave fifty metres high a thousand kilometres away. The run-up factor stays the same so you hit the coast with a wave a kilometre or two high. I guess the Rockies or Appalachians would protect the central USA. For an Atlantic impact, I don’t know how much of Europe would be left.”
Shafer said: “That’s just movie stuff. A wave that big would break up. The tsunami would only take out a few million people.”
Noordhof interrupted: “Our Kansas silos stay intact.”
Shafer said, “But you’re not expected to shoot back. This is just a great natural disaster, right?”
“And a land impact? Blast, heat, earthquake?” Noordhof’s voice had an edge to it.
“Ten times the impact energy gives you ten times everything else. And a sixteen-mile crater as a bonus.”
“I don’t like the sound of that,” Noordhof said. “You want to tell us about Big Daddy?”
“Give me an extra power of ten and I’ll shower the States with ballistic ejecta. At the impact site, everything as far as the horizon vaporizes. It gets thrown above the atmosphere, recondenses as sub-millimetre particles at a thousand degrees and falls back over an area equal to the USA. Allowing for heat lost to space etcetera I find that the thermal radiation at the surface is about ten kilowatts per square metre for an hour or more after impact. It’s like being inside a domestic oven. Try to breathe and your lungs fry. The whole of the United States turns into one big firestorm. I guess nothing would survive.”