end of her first year on that job.
The chairman of the JCS was nodding, but President Flaherty was giving him a hard, squinting look, and not just because of the light coming in the window from the skies over Camp David. She thought he was up to something. Trying to shift blame. Trying to push some kind of new agenda. “Go on,” she said. Then, remembering her manners, “Dr. Harris.”
“Four days ago I watched Kidney Bean break in half,” Doob said. “The Seven Sisters became eight. Since then, we’ve seen a near miss that could have fractured Mr. Spinny.”
“I would almost welcome it,” said the president, “if we could get rid of those ridiculous names.”
“It’ll happen,” Doob said. “The question is, how long does Mr. Spinny have to live? And what does that tell us?” He clicked a small remote in his hand and brought up a slide on the big screen. Heads turned toward it and he felt a mild sense of relief at not being stared at anymore by the president. The slide was a montage of a snowball rolling down a hill, a fuzzy bacterial culture growing in a petri dish, a mushroom cloud, and other seemingly unrelated phenomena. “What do these all have in common? They are exponential,” he said. “The word gets tossed around a lot by people who use it to mean anything that’s getting big fast. But it has a specific mathematical meaning. It means any process where the more it happens, the more it happens. The population explosion. A nuclear chain reaction. A snowball rolling down a hill, whose speed of growth is pegged to how much it’s grown.” He clicked through another slide showing plots of exponential curves on a graph, then to an image of the moon’s eight pieces. “When the moon had only one piece, the probability of a collision was zero,” he said.
“Because there was nothing to collide with,” Pete Starling, the president’s science advisor, explained. The president nodded.
“Thank you, Dr. Starling. When you have two pieces, why then, yes, they can collide. The more pieces you get, the higher the chances of any two pieces banging into each other. But what happens when they bang into each other?” He clicked the control again and showed a little movie of Kidney Bean’s breakup. “Well, sometimes, but not always, they break in half. Which means you have more pieces. Eight instead of seven. Nine instead of eight. And that increase in number means an increase in the odds of further collisions.”
“It’s an exponential,” said the chairman.
“It occurred to me four days ago that it did have all the earmarks of an exponential process,” Doob allowed. “And we know what happens to those.”
President Flaherty had been watching him intently but she now flicked her eyes over at Pete Starling, who made a dramatic upward zooming gesture with one hand, tracing the profile of a hockey stick.
“When an exponential hits the bend in the hockey stick curve,” Doob said, “the result can be indistinguishable from a detonation. Or it can look like a slow, steady increase. It all depends on the time constant, the inherent speed with which the exponential thing happens. And on how we perceive it as humans.”
“So it might be nothing,” said the chairman.
“It could be that a hundred years will pass before we go from eight chunks to nine chunks,” Doob said, nodding at him, “but four days ago I got worried that it might be one of those things that looks more like an explosion. So my grad students and I have been crunching some numbers. Building a mathematical model of the process that we can use to get a handle on the time scale.”
“And what are your results, Dr. Harris? I assume you have some, or else you wouldn’t be here.”
“The good news is that the Earth is one day going to have a beautiful system of rings, just like Saturn. The bad news is that it’s going to be messy.”
“In other words,” said Pete Starling, “the chunks of the moon are going to keep banging into each other indefinitely and breaking up into smaller and smaller pieces, spreading out into a system of rings. But some rocks are going to fall on the ground and break things.”
“And can you tell me, Dr. Harris, when this is going to happen? Over what period of time?” the president asked.
“We’re still gathering data, tuning the model’s parameters,” Doob said. “So my estimates could all be off by a factor of two, maybe three. Exponentials are tricky that way. But what it looks like to me is this.”
He clicked through to a new graph: a blue curve showing a slow, steady climb over time. “The time scale at the bottom is something like one to three years. During that time, the number of collisions and the number of new fragments are going to grow steadily.”
“What is BFR?” asked Pete Starling. For the graph’s vertical scale was labeled thus.
“Bolide Fragmentation Rate,” Doob said. “The rate at which new rocks are being produced.”
“Is that a standard term?” Pete wanted to know. His tone was not so much hostile as unnerved.
“No,” Doob said, “I made it up. Yesterday. On the plane.” He was tempted to add something like I am allowed to coin terms but didn’t want things to get snarky this early in the meeting.
Seeing that Pete had been silenced, at least for a moment, Doob tried to get back into his rhythm. “We’ll see an increasing number of meteorite impacts. Some will cause great damage. But overall, life is not going to change that much. But then”—he clicked again, and the plot bent sharply upward, turning white—“we are going to witness an event that I am calling the White Sky. It’ll happen over hours, or days. The system of discrete planetoids that we can see up there now is going to grind itself up into a vast number of much smaller fragments. They are going to turn into a white cloud in the sky, and that cloud is going to spread out.”
Click. The graph continued shooting upward, rocketing up into a new domain and turning red.
“A day or two after the White Sky event will begin a thing I am calling the Hard Rain. Because not all of those rocks are going to stay up there. Some of them are going to fall into the Earth’s atmosphere.”
He turned the projector off. This was an unusual move, but it snapped them all out of PowerPoint hypnosis and forced them to look at him. The aides in the back of the room were still thumbing their phones, but they didn’t matter.
“By ‘some,’” Doob said, “I mean trillions.”
The room remained silent.
“It is going to be a meteorite bombardment such as the Earth has not seen since the primordial age, when the solar system was formed,” Doob said. “Those fiery trails we’ve been seeing in the sky lately, as the meteorites come in and burn up? There will be so many of those that they will merge into a dome of fire that will set aflame anything that can see it. The entire surface of the Earth is going to be sterilized. Glaciers will boil. The only way to survive is to get away from the atmosphere. Go underground, or go into space.”
“Well, obviously that is very hard news if it is true,” the president said.
They all sat and thought about it silently for a period of time that might have been one minute or five.
“We will have to do both,” the president said. “Go into space, and underground. Obviously the latter is easier.”
“Yes.”
“We can get to work building underground bunkers for …” and she caught herself before saying something impolitic. “For people to take refuge in.”
Doob didn’t say anything.
The chairman of the Joint Chiefs of Staff said, “Dr. Harris, I’m an old logistics guy. I deal in stuff. How much stuff do we need to get underground? How many sacks of potatoes and rolls of toilet paper per occupant? I guess what I’m asking is, just how long is the Hard Rain going to last?”
Doob said, “My best estimate is that it will last somewhere between five thousand and ten thousand years.”