“Realistically, how much warning do we get?”
Sacheverell put down his coffee. “If it approaches the night hemisphere you might see it in binoculars an hour before impact. Assuming you knew exactly where to look. It would be visible to the naked eye maybe fifteen minutes from impact.”
“Mister, what I want to know is, when do the phones start ringing?”
“When you see it as a bright moving star. Say twenty seconds from impact.”
Hooper sat down again, and stared into the middle distance for some moments. “Well it sure beats the hell out of Star Wars,” he said at last.
“Sam, it’s beautiful,” said Cannon. “The thing is undetectable, practically into our air space before we know what’s hitting us. When it does hit us, we’re obliterated. There’s no point in hitting back even if we could because, like you say, it’s just a freak natural disaster. Sam, you know I’m due to meet some senators from the Appropriations Committee in a couple of hours. I’m trying to get final approval for Bat-strike.”
“Which we’re selling on Middle East scenarios.”
“Fifty billion bucks down the tube, along with the new Grand Forks, our Navy, SAC, brilliant pebbles, C-cubed, all our surveillance systems, the whole BMDO. Everything we have, this thing beats it. And they don’t even have to worry about retaliation.”
“I don’t believe it,” repeated Hooper, grey-faced. “This is fantasy stuff.”
“And we reckon they set the whole thing up for a day’s defence budget.”
“What’s the timescale for this?” Hooper asked harshly.
“Heilbron thinks the Russians have pulled it off already,” said Cannon. “It’s somewhere out there now, on the way in.”
Downstairs, the shift was nearly over. The normally ebullient Pino had been unusually quiet, wrestling with some inner problem. Finally he said: “Colonel, do you know anything about astronomy?”
“Not much, Pino. What do you want to know?”
“Well, are we sure there are no men from Mars?”
“Relax, Pino, there are no Martians. Vince Spearman said it on TV.”
Pino seemed to be examining the arcane names on the screen in front of him. Then: “This guy Spearman—he’s okay?”
“AOK. He’s been checked out real good.”
The sergeant relaxed.
Webb said, “If Nemesis is more fragile than a rock . . .”
Sacheverell groaned. “You’re not still on that comet crap. Ever heard of the asteroid belt?”
“Which fails to give us the periodicity in the extinction and cratering records.”
“What periodicity?” Sacheverell sneered. “There is none. And how come Toutatis, Mathilde, Eros and Gaspra are rock?”
“Mathilde has the density of water. It’s a friable sponge.” Webb turned to Noordhof. “Colonel, Herb will tell you to plan on diverting a solid rock. But if it’s a degassed comet and McNally fires an H-bomb at it, we’ll end up with a dust ball heading for us. When the ball comes in it will incinerate the upper atmosphere as it slows from cosmic speed to zero. It’ll remove all the ozone. Then it’ll take a year to sink through the stratosphere and during that year the Earth will be wrapped in a highly reflective dust blanket. Down here we’ll be in twilight. We’ll have a major climatic upset. Freezing gales will blow from sea to land. The continents will end up looking like Siberia. We might cut the thermohaline circulation in the Atlantic and switch off the Gulf Stream. If we do, that will feed through to a permanent snowfall in Eurasia. That will switch off the monsoon. Nobody in Asia will eat for a year or so. You’ll shift masses of water to the poles as ice and change the spin rate of the Earth. Maybe you’ll flip the geomagnetic field, and set off seismic faults and vulcanisms
worldwide. Then when the dust clears you’ll be exposed to the full unshielded UV of the Sun, and you’ll have a global catastrophe on top of a global catastrophe. Herb’s certainties could do us in.”
“Maybe we want to get it right before we launch a bomb at it,” suggested Shafer.
Leclerc was looking puzzled. “I always thought comets had tails.”
“Not when they’ve degassed, André. They may crumble to dust but there are plenty of well-authenticated cases showing that they sometimes turn into asteroids.”
Leclerc said, “If we find Nemesis, could we tell its internal constitution by looking at it? Using Kenneth’s monster telescope? What do we actually know about the reflectivities of the Earth-crossing bodies?”
Sacheverell said, “We don’t. But the museums are stuffed with meteorites. They’re fragments of asteroids and they’re rocks.” He shot Webb a venomous look.
Webb said, “You don’t have comet debris in the museums because it breaks up in the atmosphere. Nemesis could be like Halley, with a crusty exterior and a fluffy inside.”
“Fluffy snowballs, right?” Shafer asked, narrowing his eyes. “Dust and ice in equal proportions?”
Webb nodded. “Give or take. Try to nuke it and you end up with a billion tons of dust. Look, if we get this wrong we could reduce the species to foraging bands.”
McNally’s face was a caricature of dismay. He said, “I go for Sacheverell’s theory.”
Noordhof spoke, in a thoughtful tone. “Ollie. Do you realize what you’ve just said? That if Nemesis is a comet, the interests of the world at large are best served by letting the USA take it? Are you saying we’re on opposite sides, Ollie?” Noordhof asked softly, playing with another cigar. Suddenly the air was electrically charged. Judy, at a terminal, stopped typing and swivelled on her chair to face them.
McNally broke the stunned silence. He gulped, “Hey, if
the Russians changed its course without turning it to dust, so can we.”
Webb shook his head tensely, his eyes locked with Noordhof’s. “They probably had years of a start, letting them push it a few centimetres a second, without setting up big internal stresses.”
Judy Whaler turned back to the terminal. “A standoff burst! With neutron bombs!” she sang out over her shoulder, and carried on typing.
Webb blew out his cheeks with relief. “Thank you, Judy. Colonel, we just handle Nemesis with the utmost care. We use a standoff burst. Ablate a skin with neutrons.”
The relief was palpable. Shafer was scribbling. “Maybe yes, maybe no. Even a neutron bomb emits X-rays and they get to the asteroid first. If they create a sheath of plasma the neutrons might not get through. I don’t know that neutron bombs would help.”
She swivelled on her chair. “We do bombs at Sandia, Willy. We can handle the computational side. Neutron bombs are the ultimate capitalist weapon, remember? They’re designed to irradiate people, not destroy structures. Suppose instead of positioning a bomb on the surface of Nemesis we detonate it during a flyby, say a few hundred metres up. Instead of forming a crater, the top few centimetres are vaporized and blown off. The stresses are spread over a hemisphere instead of concentrating around a crater.”
Shafer said, “So we bathe the asteroid with neutrons and X-rays. And if it turns out to be a comet, we might still do it gently enough to preserve its structure.”
Noordhof asked, “Can we do it? Can we do it?”
Webb suggested, “Try a one-megaton burst at five hundred metres’ altitude and suppose the bomb energy is all in neutrons.”
Judy unconsciously swept her blonde hair back over her shoulder and said enthusiastically, “Neutrons get absorbed within twenty centimetres. If they just passed through you
they wouldn’t do damage. It’s because they get absorbed within your body that they make such brilliant weapons. The energy will be deposited in a top layer of Nemesis around the thickness of a human body.” She turned back to her terminal.
You’re a bundle of fun, lady, Webb found himself thinking.
Shafer drew a circle and a point some way off, and tangent lines from point to circle. Webb saw what was coming and tried to keep up on a sheet of paper. Shafer said, “Give Nemesis a radius
R
and put the bomb a distance
d
from its centre. We need to know how much of the bomb’s energy is intercepted by Nemesis.” He scribbled rapidly and said, “Seen from the bomb Nemesis fills π
R
2
/4π
d
2
or 1/4(
R/d
)
2
of the sky.”
McNally said, “If we explode the bomb five hundred metres up, like Ollie says . . .”
Shafer continued, “Then we have
d
=
R
and a quarter of the bomb energy is dumped on the facing hemisphere of Nemesis. That’s good. Now let’s skip the detailed trig and suppose the irradiation goes to a mean depth of five centimetres.”
Webb took up the story. “So we’re imagining that over the hemisphere facing the bomb, a surface skin maybe five centimetres thick takes twenty-five per cent of the blast. The concentration of energy will be prodigious.”
Shafer rapidly substituted numbers for symbols in a formula. “Okay so about half a million tons of surface regolith is exposed to a quarter of a megaton of neutron energy, coming in at a third the speed of light.”
Sacheverell asked, “Can we turn that into a speed?”
“Easy. Each exposed gramme gets a few times 10
10
ergs, about the same energy as dynamite. So the surface goes off like dynamite. It turns into a vapour expanding at five kilometres a second.”
“Now you’re talking,” said Noordhof. He was leaning forward intently, trying to follow the rapid exchanges between the scientists.
“It’s as if you’ve spread a three-inch sandwich of dynamite
over a hemisphere,” McNally repeated, his eyes gleaming. “A puff of vapour expanding at five kilometres a second from one side of Nemesis. It’s Christmas after all.”
Webb said, “Hey, imagine exploding like dynamite when one of Judy’s bombs goes off.” Noordhof shot him a cold glance.
McNally returned to an earlier formula. “On the week-before-impact scenario, that would shift Nemesis at two metres a second. We’re still well short.”
“But we’ve gained a power of ten,” Shafer said. “Maybe we could even use a bigger bomb.”
“No,” said Webb. “You’d bust it up.”
“I agree with Ollie,” said Judy, turning back again from the terminal. “Hit the asteroid and it rings like a bell. If you hit it so hard that its velocity change is more than its escape velocity then you’ll break it up.”
“Now you’ve lost me,” said Noordhof.
“Imagine the asteroid as a fragile bell, made of glass or something. The Russians tapped it with a pencil years ago and made it ring. But now it’s rushing at us and we’re having to shift it with a hammer.”
Shafer said, “You’re forgetting that it might have internal strength. Jim, if you can rendezvous with Nemesis a hundred days before impact we might be able to deflect it in principle, maybe even if it’s one of Ollie’s degassed comets. Fifty days, maybe. Ten days or less and we’re in trouble whatever it’s made of.”
McNally sounded as if he was in pain. “A hundred days? Willy, can we get back to the real world here?”
Sacheverell said, “Cracks and fissures in rock could change the whole story.”
Judy turned to them, a satisfied smile on her face. “We’re in.”
“Where?” Webb asked, startled.
“Welcome to the wonderful world of teraflops, Ollie. While you people are handwaving I log on to God. I fix up
a simulation algorithm using Sandia’s own shock physics hydrocode, and they run it for me.” McNally pulled the curtains half shut to cut down on stray light, and they clustered round her terminal. She logged in through a series of gateways, each one with a different password. “I’m using fifty million finite elements and all nine thousand processors. Give me the internal constitution of your comet or asteroid, cracks, fissures and warts, and I’ll tell you what happens when you neutron it. Look on my works, ye Mighty, and despair!” She paused, a finger over the keyboard.
“Hey, you read Chaucer?” McNally asked.
Judy raised her eyes to heaven and then pressed the carriage return button with a flourish. “Even the Teraflop will take some minutes.”
Noordhof, looking at the blank screen, said, “It seems to me, people, that the critical thing is the internal constitution of this asteroid. Is it rock, iron, ice or what?”
“That’s the sharp end of the debate,” Webb agreed.
Noordhof said, “I get the impression you guys don’t know a lot about what’s out there.”
Webb agreed. “Here be dragons. But it’s vital.”
The screen came to life. Judy said, “I want to show you three simulations. Here’s number one. I’m exploding a megaton four hundred metres above a one-kilometre rock with the tensile strength of a carbonaceous chondrite.” On the little screen, a potato-shaped, black mass appeared. It rotated slowly for a few seconds, as if being viewed from an exploring spacecraft, and then froze in position. A brief flash filled the screen. The rock shuddered. Black fingers spread out in a cone. When the debris had left the screen, a sizeable hole had appeared in the side of the rock, which was drifting slowly off to the left. “This is very satisfactory. The nuclear deflection has worked.”
A second potato appeared on the screen, identical to the first. “Okay, this one is stony, silicon oxide. I’ve made it a fragment of a large asteroid which has been pounded over
geological timescales: it’s been weakened. It has internal fissures. It’s just a rubble pile.” This time, when the debris had cleared, the rock had fragmented. Half a dozen large fragments, and dozens of smaller ones, were drifting slowly apart.
Noordhof said, “That looks like trouble.”
Judy nodded. “Deep trouble. It depends how early we could deflect Nemesis.”
“If we ever find it,” said Leclerc.
A third potato materialized. “Now the last one, this is a comet with the tensile strength of the Kreutz sun-grazer. Let’s see what happens.” The bomb flashed briefly. Instantaneously, the potato disintegrated. But there were no fragments to fly apart. Instead, a white amorphous mass gradually filled the screen, apparently growing white hairs as it approached. “All we’ve done is generate a dust ball.” The simulation ended. McNally opened the curtains and bright daylight streamed into the room.
Shafer moved over to the window and looked out. “I seem to remember the Sandia people carried out Tunguska fireball simulations some years ago,” he said over his shoulder. “And the 1908 data were best fit by a rocky asteroid.”