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 1010 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.”
“But there were counterarguments,” Webb reminded him. “For example the lack of rocks spalling off along the trajectory, and the coincidence with the Beta Taurid comet swarm. And with a small change in the assumed trajectory they could accommodate a comet.”
Shafer asked, “What do the spectra say about the Earth-crossers?”
“There are hardly any available. They’re too faint.”
“Okay,” said Noordhof, resuming his place at the head of the conference table. “From what you people are saying it seems to me that I can come to an immediate decision. There’s too much at stake here to take chances. We have to invent something that will work whatever Nemesis is made of.”
“We could play with all sorts of deflection scenarios,” Judy said. “Solar sails, laser propulsion, kinetic energy impactors and so on. Either they take far too long to develop or they can’t deflect in the time available. Only nuclear weapons stand a chance, but as you’ve seen they could give us a cluster of debris or maybe even — if Nemesis turns out to be an old comet — a blanket of dust and a cosmic winter.”
“Hell,” said Sacheverell, “even a pure rock asteroid would give us that after it fried us.”
Noordhof put his hands on top of his head. “Am I going mad here? You are telling me the following: One, you will have to deflect Nemesis at less than a metre a second or it will break up and shower us with fragments. Two, you will have to deflect it at more than ten metres a second or it will hit us.”