Fire in the Sky INTRODUCTION
Just before 4 a.m. on June 2, 2016, I was enjoying a pre-dawn cup of coffee on the back porch of my home in Arizona. And suddenly it was as if the sky was on fire.

It began with a glow that spread across the steep hillside behind my house, a kind of angry molten light, all red and orange and amber, the color of lava. The glow grew more and more intense, lighting up the mesquite trees and saguaro cactuses and casting their long, tortured shadows on the ground. It looked like something from the netherworld, like high noon in hell. Then, seconds later, way up in the sky off to the northeast, there was a blinding, thermonuclear-style flash, a burst of white light almost as bright as the sun, followed later by a sound like distant thunder that set my dogs to howling.

As the light faded and the night returned I stood there, transfixed, not quite believing it. As a soldier and journalist I’ve traveled the world for decades, and experienced all manner of cataclysmic natural and unnatural events—wars and riots and all manner of mayhem, typhoons, tornadoes, major earthquakes, even (from a distance) the deadly volcanic explosion of Mount St. Helens in 1980. But this was easily the most astonishing natural event I had ever seen. It occurred to me that this was how the world will end: with a flash of light, and a roar like God’s own artillery, and then—darkness.

Of course, the world didn’t end on June 2, 2016. And in the age of 24-hour news and social media, an explanation for this amazing phenomenon was soon forthcoming. Apparently a small asteroid, a rocky piece of space debris only about six feet across, had wandered into the Earth’s path and exploded in the sky some fifteen miles above Arizona’s White Mountains. The asteroid’s fiery passage through the atmosphere and subsequent brilliant explosion lighted up thousands of square miles of ground and startled observers as far away as Texas. Despite the early hour, the event had been captured on scores of dashcams and smart phones, so although no one was killed or even slightly injured, the Arizona “fireball” was a major story not only on local TV but on the national newscasts as well. And as I watched and read the news reports, a couple of things quickly caught my attention.

One of them was the almost unbelievable power of the explosion. Soon after the event, the National Aeronautics and Space Administration (NASA) reported that the little asteroid had burst apart in the atmosphere with the energy equivalent of half a kiloton of exploding TNT—that is, a million pounds of TNT. To put that into perspective, the U.S. military’s most powerful non-nuclear bomb is the GBU-43/B Massive Ordnance Air Blast (MOAB) bomb—the so-called Mother of All Bombs—which has a blast yield of a mere 22,000 pounds of TNT. That little asteroid made a MOAB look like a firecracker.

And there was another thing about that fireball in the Arizona sky that surprised me. That was the fact that no one had seen it coming. Sure, the streaking fireball and its subsequent explosion had been picked up by some U.S. spy satellites, by ground-based Doppler weather radars and by NASA’s All-sky Fireball Network, a national network of cameras set up to record events like this one. But that only happened after the asteroid entered the atmosphere, not before. That space rock hurtling out of the clear black sky was a complete surprise to everybody.

As often happens, the more I learned, the more questions I had. Such as, how in the world could a piece of rock the size of a La-Z-Boy recliner pack an explosive wallop almost fifty times greater than the most powerful conventional bomb in the U.S. military arsenal? Why in this era of satellites and space-mounted telescopes and world-spanning radar networks did no one spot the thing before it arrived? How often do events like this happen? What’s the history behind our understanding of Earth-colliding asteroids and comets?

Those are some of the questions this book will try to answer. And it will also take a look at the Big Questions: What are the chances that a much larger chunk of space rock, something hundreds of yards or even miles wide, will find itself on a collision course with Earth? And if that happens, what if anything will we be able to do about it?

Actually, I can answer that first Big Question right now. The chances that an asteroid or comet of potentially catastrophic size will come hurtling toward Earth are exactly 1-in-1. It’s 100 percent, a sure thing, a lead-pipe cinch. The only variable is when. It could be a hundred thousand years from now—or it could be next Tuesday. True, the U.S. government’s comforting official position is that for at least the next century or so, there is no significant risk of Earth being struck by any large asteroid or comet that we currently know about. But given the fact that our Solar System is home to billions of asteroids and comets that we don’t know about, that’s a pretty significant qualifying clause. It’s a loophole you could drive an asteroid through.

As for what we could do or would do if we spotted a potentially damaging space body headed our way—well, that remains to be seen.

Should we be worried about this? Should we as individuals be concerned about the threat of cosmic impacts? Yes, but not to the point where we toss and turn all night over it—at least not yet. But we should understand that the threat is real; it’s science fact, not science fiction. And we should expect—even insist—that somebody pays attention to the problem. Fortunately, as we’ll see, a relative handful of men and women are dedicating their professional lives to assessing and planning for these threats from space. But one of the themes of this book is that there should be more resources committed to the impact problem. To do otherwise seems foolishly short-sighted.

I have to confess that when I began this book I knew next to nothing about asteroids or comets or other things astronomical. I don’t say that with any perverse, anti-brainiac sense of pride, the way some people boast about being terrible at math. It truly made me regret the shocking shortage of science classes in my college transcripts. Like most people, I had heard about the dinosaur-exterminating giant space body that hit Earth sixty-five million years ago, and I vaguely recalled the silly mnemonic for the order of the planets: My Very Energetic Mother Just Served Us Nine Pizzas, for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto—although even that was outdated, since poor little Pluto was kicked out of the planet community years ago and reduced to mere dwarf planet status. And of course I had seen some of the giant-space-rock-on-a-collision-course-with-Earth movies that Hollywood has been churning out for decades—although none of them offered much in the way of expanding my scientific knowledge, or anyone else’s.

So it was a little daunting to suddenly find myself immersed in a world of astronomical units and albedos and arguments of perihelion and the kinetic energy of hypervelocity impactors—which, as I’m sure everyone already knows, is equal to one-half the mass times the velocity squared, or Ek = 1/2mv2. I had a lot of catching up to do. There were hundreds of books and scholarly papers to read, countless hours spent perusing past and present articles in Sky & Telescope and Space.com and EarthSky.org and the Planetary Society website, and dozens of interviews with astronomers and planetary scientists and other people who are a lot smarter than I am.

On the other hand, starting out with a clean science slate did have its advantages. For one thing, even at a relatively advanced age there’s a certain child-like joy in learning something new, especially when that something new is—to use the scientific term—really cool. Also, not having any preconceived notions spares you from stubbornly holding on to them. As this book will show, when it comes to Earth-impacting asteroids and comets, stubbornly held notions set back the cause of scientific progress for generations.

I should note that while this book is about a complex scientific issue, it’s not really a science book. Instead, it’s a story—a very human story about our long struggle to understand Earth’s place in the Solar System, and the pivotal role of Earth-impacting asteroids and comets in shaping our world. Sure, there are scientists in this tale, men and women whose personal lives and characters are often just as fascinating as their scientific discoveries. But there are also cowboys and Indians and astronauts, Stone Age toolmakers and aerospace engineers, bold explorers and backyard amateur astronomers. King Tut and the mad teenaged Roman emperor Elagabalus make appearances in these pages; so do Teddy Roosevelt and the lead guitarist for the rock band Queen. On the technology side, picks and shovels and horse-driven machinery share the stage with space-deployed gravity tractors and laser ablation devices and ion beam shepherds.

In case you’re wondering, yes, there are UFOs in this book—millions of them in fact. But these unidentified flying objects are asteroids and comets, not spaceships from faraway galaxies. I’ll admit that given the breadth and scope of the universe, it seems likely that there is life somewhere beyond our own little planet. But with all due respect to the UFO belief community, when it comes to Earth visitations by vaguely humanoid space aliens, the science doesn’t seem to be in on that one quite yet. Besides, as far as I’m concerned, the proven science behind Earth-approaching comets and asteroids is already out-of-this-world astonishing; we don’t need to dress it up to make it more so. Same thing goes for astrology. I heartily agree that fate is written in the stars and planets. But the basic premise of this book is that the fate of Earth and our Solar System is determined by the collision of celestial bodies, not their alignment.

Finally, I should note that science, like the Earth itself, is a constantly moving target; sometimes it’s hard to get a bead on it. Theories that were once scientific heresy have become scientific dogma, and vice versa—with the asteroid-related extinction of the dinosaurs being a prime example. Over the past almost four decades, hundreds of scientists have written literally thousands of scientific papers on that subject; entire forests have perished. And although the asteroid- or comet-impact extinction theory has largely carried the day, some scientists are still arguing about the issue, often in the most bitter personal terms. In this book I will chronicle that and other scientific controversies concerning Earth-impacting asteroids and comets, but I don’t intend to litigate them. When there are competing theories on any given subject, I’ll say so—and then I’ll go with the one that currently seems to make the most sense.

With all that said, our story is waiting. It begins in the not-too-distant past, when another, much bigger asteroid came blazing through the Arizona skies. . . .