Free Nemesis by Bill Napier

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.”
    Sacheverell tidied up his papers to show he was finished. There was a thoughtful silence. Webb broke it by saying, “These computations all have big uncertainties. My reading is you’d have less earthquake and more heat. You’d burn the States even with Mummy Bear. Partly I’m thinking of the Shoemaker-Levy 9 comet fragments which hit Jupiter in 1994. We had a coherent stream of material which gave us twenty impacts on to the planet. The heat flashes from the fallback of ejecta were a hundred times brighter than those from the fireballs themselves.”
    Shafer stirred his coffee. “Big Daddy is good news.” There was an astonished silence. Noordhof’s cup stayed poised at his lips.
    Webb nodded. “I believe so, Willy. There are maybe a couple of a million cometary asteroids out there, any one of which could give us a hydrogen-bomb sized impact. Theyprobably happen every century or two. This century we had Tunguska in the Central Siberian Plateau on June 30th 1908. It came in low from the sun at about 7:15 a.m. That was ten to thirty megatons. Hundred megatonners come in every few centuries. They’ve been recorded as celestial myths in Hesiod’s
Theogony
and the like. If you go to a few thousand megatons, you’re probably into the Bronze Age destructions: the climate downturn, Shaeffer’s mysterious earthquakes in the Near East.”
    “What has this guy been smoking?” Sacheverell asked.
    “Do you accept your own impact rates? The ones you keep re-publishing?”
    “What of it?”
    “With a decent chance of a thousand megatonner in the last five thousand years?”
    “Sure,” Sacheverell sneered. “Probably at the north pole.”
    “So we had ten megatons in Siberia in 1908, a megaton in the Amazon in 1930,