Death from the Skies!: These Are the Ways the World Will End... - Philip Plait (2008)

Chapter 1. Target Earth: Asteroid and Comet Impacts

THE ALARM WENT OFF AT 6:52 A.M. AS IT DID EVERY morning. Groggily, Mark slapped it off, then stumbled wearily to the bathroom. He splashed a bit of water on his face to accelerate the waking process, then began to brush his teeth.

Seeing that it was already a clear, warm day, he peered out the bathroom window to take in the scene as he brushed. The trees were covered in leaves, and flowers were in full bloom. The trees cast long shadows as the Sun slowly rose in the sky.

When he finished brushing, Mark noticed the odd silence. That’s funny, he thought. Why aren’t the birds chirping? From the corner of his eye he saw movement. Maybe it was an animal in the yard that had spooked the birds . . .

Stepping up to the window, he stood on tiptoe to look around the yard. What the—Every tree was casting two distinct shadows. His morning routine now forgotten, Mark watched in amazement as, for every tree, one of the shadows appeared to be moving, circling around the base of the tree like fast-motion video of a sundial. Nose pressed to the window, he looked up into the sky, straining to see what could be causing this strange display.

Suddenly, from under the eave, it appeared as if the Sun itself were streaking across the sky. Dazzled, Mark’s eyes took a moment to adjust, but it was still not clear what he was seeing. There was a disk of intense white light moving across the sky, faster than an airplane. Could it be a meteor?

It appeared to descend slowly to the horizon as he watched. Then, in the blink of an eye, there was a soundless but all-encompassing flash, so bright his eyes watered. He winced in pain. When he was able to look again, the small bright disk was gone, replaced by a much larger smear of light, fanning up from the horizon. The heat from the thing was palpable, even through the window. It was like standing near a fireplace. As the smudge in the sky expanded, Mark noticed something even odder: did the tops of the trees look funny? Was that smoke rising from them . . . ?

The heat became intense. It began to dawn on Mark that he might be in trouble. As he stood there wondering what to do, a sudden and sharp earthquake jolted the house, knocking him to the floor. It was over quickly, and as he stood up, dazed, he felt the heat more strongly than before as it poured through his now-broken bathroom window. He thought the worst was over, but what he didn’t know was that a wave of pure sound and fury tearing through the atmosphere was pounding toward him at 700 miles per hour.

Too late, he saw the face of the shock wave bearing down on him like a tsunami ten miles high. A mighty thunderclap swept over his burning house, pulverizing it to dust with Mark still inside, and the time for decisions was over.

Everything under this wave of sound was stomped flat. Trees that were ablaze a moment before from the heat of the explosion were snuffed out, then torn into millions of splinters. The expanding ring of pressure, already dozens of miles across, screamed past the location of Mark’s disintegrated house and continued moving, greedily consuming buildings, trees, cars, people.

Before it was over, the shock wave circled the Earth twice. Seismographs from around the globe registered the event as an earthquake of enormous scale, but no one paid attention to the scientific data for long. They were too busy struggling to survive.


The Earth sits in a cosmic shooting gallery, and the Universe has us dead in its crosshairs.

Consider this: the Earth is pummeled by twenty to forty tons of meteors every single day. Over the course of a year, that’s easily enough to fill a six-story office building with cosmic junk.

While that sounds like a lot, it’s really only a pittance compared to the size of the Earth, which is about a quintillion—a million million million—times bigger. But space is swarming with debris, and the Earth is constantly plowing through it.

The vast majority of this material is detritus, tiny bits of rock that burn up readily in our atmosphere. When you go out on a dark, clear night, you see these as “shooting stars,” what astronomers call meteors. You might be surprised to find out that even the brightest ones you’re likely to see are caused by tiny bits of fluff called meteoroids, no bigger than a grain of salt. Something as small as a pea would make a fantastically bright meteor—I once saw one that was so bright it lit up the sky and even left an afterimage on my eye. I stood transfixed for the two or three seconds it took to flash across the sky, but was just as shocked when I later calculated that the rock itself was probably no bigger than a grapefruit.

How can something so small get so bright? There are two factors to consider. You may be familiar with the first: compressing air heats it up. Think about how warm a bicycle pump gets after you use it—when the air is squeezed inside the pump, it gets hot and transfers that heat to the metal. You can actually burn yourself using a pump if you’re not careful. The more a gas is compressed, the hotter it gets. The second factor is the fantastic speed at which meteoroids travel. Most of them hit us at ten to twenty miles per second, and some come roaring in as fast as sixty miles per second! This is far, far faster than even a rifle bullet.

When something moving that rapidly enters our atmosphere, its velocity is translated into energy, which in turn is transferred to the air around it. As it screams through the upper atmosphere, a meteoroid rams the air violently—a rock moving at Mach 50 is going to compress the air a lot. The air gets squeezed so quickly and at such high pressure that it heats up thousands of degrees and starts to glow.

As you can imagine, all that hot air is like a blast furnace. The meteoroid, traveling just a few inches behind that rammed air, feels that heat. It can’t last long in those conditions, and if it’s small it usually burns up in a matter of seconds. We see a bright glow, a streak across the sky that lasts for a moment or two, and then it’s gone, adding its nearly insignificant mass to the Earth’s.

To a stunned observer, a meteor looks like it’s traveling just over his head, but in reality the action is occurring fifty or more miles above the ground. At that height the air is very thin, yet still thick enough to stop small, dense particles. But what if the particle is bigger than a pea, or a grape, or a watermelon? What if it’s the size of, say, a couch, a car, a bus?

For a bigger object, things are very different. If it’s a few yards across, instead of simply burning up, that chunk of space debris gets squeezed by the air pressure as if it’s in a vise—the pressure can top out at over a thousand pounds per square inch at meteoric speeds. This pressure can flatten out the incoming object in a process called pancaking for obvious reasons. But a rock can only take so much of that before it crumbles and falls apart. Within seconds, instead of one big rock coming in, we now have hundreds or thousands of little ones, all still moving at velocities of several miles per second, and all dumping their energy into the air around them. They compress further, fracture, heat up, and so on . . . and within a fraction of a second we have a whole lot of rubble releasing a whole lot of heat all at once.

This is, by definition, an explosion.

So medium-sized meteoroids blow up in the atmosphere. Again, this usually happens fairly high up, depending on how tough the meteoroid is; ones made of metal can take more punishment and penetrate deeper into our atmosphere, but may still explode many miles above the Earth’s surface. The energy involved is impressive: a rock only a meter across can explode with the force of hundreds of tons of TNT. In fact, military records indicate that such an explosion from an incoming chunk of rock is seen on average once a month!

Since meteoroids explode so high up in the atmosphere, you’d expect we’d be safe from things that size.

Well, not exactly. Under some conditions, the incoming rock may break up, but some chunks can survive. If the main mass slows enough before it explodes, then smaller fragments can slow even more without totally disintegrating. These can make it all the way to the ground. Metallic meteoroids have even more structural strength and can remain intact all the way down as well. If they do survive and impact the ground, they’re called meteorites.1

Small meteoroids that make it down to the ground usually aren’t moving terribly fast when they impact. In fact, their initial velocity is completely nullified by our air, leaving them to fall at what is called terminal velocity. It’s as if they were dropped off a tall building or from a balloon; they wind up impacting at maybe one or two hundred miles per hour. Scary, sure, but not too scary.

Still, you wouldn’t want to get hit by a rock moving that fast. For comparison, they hit the ground faster than even a professional baseball player’s pitch. In November 1954, a woman named Ann Hodges from Sylacauga, Alabama, was actually hit by meteorite. It was fairly small, about the size of a brick and weighing just over eight pounds. It punched through her roof, bounced off a wooden radio cabinet, and smacked into her where she was lying on the couch, taking a nap that was rather rudely interrupted. Her hand and side were injured. She lived, but suffered one of the nastiest bruises in medical history.

This may be the earliest well-documented case of a meteorite damaging human property. But it wasn’t the last. With the advent of the video camera, it was inevitable that more and more spectacular meteors would be recorded.

On October 2, 1992, a meteoroid the size of a school bus entered the Earth’s atmosphere. It created a huge fireball as it traveled northeast across the United States, and was witnessed by thousands of people—by a happy coincidence, it was on a Friday night during football season, so many proud parents were already running their video cameras, yielding excellent footage of the meteor. The rock broke apart as it ripped its way across the sky, and one of the pieces, roughly the size of a football, fell onto the trunk of a young woman’s car in Peekskill, New York. It left a hole in the back end of the car that looked, not surprisingly, exactly as if it had been caused by a rock dropped from a great height. One can imagine the difficulty the owner had getting her insurance company to pay for the damage.

These and other stories notwithstanding, in the end the Earth’s surface is big, and most meteorites are small. The odds of anyone’s getting hit by one are really very small, and the odds of being killed by one are even smaller.


As it burned its way through the Earth’s atmosphere, the Peekskill meteor was captured on dozens of home movie cameras. It broke into smaller chunks, one of which hit a woman’s car.


Still, most meteorites are small. Some aren’t.


On June 30, 1908, the Earth and a smallish chunk of pretty weak rock found themselves at the same place at the same time.

The rock was probably seventy or so yards across. Its orbit intersected the Earth’s, and over time it was inevitable that the two objects would both be located at that intersection point simultaneously.

It came in over Siberia, in a remote region near the Podkamennaya Tunguska River. On that day, it entered the Earth’s atmosphere over Russia, traveling northwest. It plunged deeper into the air, and the increasing pressure put tremendous strain on the meteoroid. It broke apart, and each piece broke apart, and the cascade of rupture dumped a vast amount of energy into the air around it. The object exploded, releasing between three and twenty megatons of energy: the equivalent of three to twenty million tons of TNT, hundreds of times as much energy as the bomb dropped on Hiroshima thirty-seven years later.

The blast itself was seen by hundreds of witnesses (the Soviet Union even created a stamp based on what was seen), and the explosion registered on seismometers designed to detect earthquakes. People were knocked off their feet hundreds of miles away.

Despite the incredible event and the excitement it generated, a scientific expedition took years to mount. The region is unbelievably difficult to reach; in winter it’s forbidding at best (we’re talking Siberia after all), and in the summer the Tunguska region is a swamp, infested with mosquitoes. But eventually the site was reached, and what greeted those weary travelers had never been seen before.

As they approached the area of the explosion, the expedition members were shocked to see trees flattened like toothpicks for hundreds of square miles. Moreover, the trees were lying in parallel formations. Following the trail, the scientists came to a spot where the trees were all knocked over radially, like spokes on a bicycle wheel. Even weirder, the trees at ground zero were still standing, though totally denuded of branches and leaves. It’s hard to imagine what they must have felt upon seeing such an eerie sight.

No blast crater was ever found, nor (yet) any definitive debris from the rock. It exploded several miles above the ground, and totally vaporized. The air blast created a shock wave that knocked down those trees. The trees at the center were still standing because the blast wave slammed straight down into them; it takes sideways force to knock trees down. Nuclear airburst blasts during weapons tests of the 1950s and 1960s replicated the same pattern.

While the remote location of the explosion made it hard to study, it also meant few people were killed. Had the explosion occurred over Moscow or London, millions would have died within minutes, making this a very serious threat indeed. Still, the immediate effect from the explosion was localized. Probably no one more than a few dozen miles away was hurt.

But then, not all impactors are only seventy yards across . . . and not all impacts are local.


Sixty-five million years ago, the dinosaurs had a really bad day.

Actually, recent findings show they were having a bad couple of million years. There are indications that the Earth’s climate had been changing, and many species were already in decline. However, there is overwhelming evidence that a great number of species indeed died practically overnight on a geological time scale. It’s now a matter of scientific fact that this event was triggered by the impact of a six-mile-wide asteroid—and at that size, the word “meteoroid” is seriously inadequate.

It was certainly large enough to do the trick. The mind boggles to think of the devastation wrought when a rock bigger than Mount Everest plummeted through the atmosphere and hit the Earth at ten miles per second. Imagine: when the surface of the asteroid contacted the ground, the far side was still sticking out above most of the Earth’s atmosphere.

The exact energy of the impact is difficult to know, but it would have been hundreds of millions of megatons—far, far larger than the heftiest nuclear bomb ever detonated. In fact, even if you detonated every single nuclear weapon on Earth simultaneously, the explosion generated by the impact of the dinosaur killer would have been a million times more powerful . . . all concentrated in one spot.

The dinosaurs had a really, really bad day.

That massive impact set off a terrifying series of events, each of which brought destruction on an unimaginable scale.

As the asteroid plunged through the air, it would have created a huge shock wave, superheating the atmosphere for miles around it. As bright as the Sun, it would have set everything underneath it aflame even before it hit. And if anything did manage to survive that terrible heat, it would then have to withstand the force of a giant shock wave slamming into it as the asteroid tore through the air during its supersonic travel.

Being so large, the asteroid would hardly have slowed its flight or lost any mass at all before it slammed into the ground. Scientists now know that the impact point was just off the Yucatán Peninsula in Mexico. It impacted water—which isn’t too surprising, as water covers 71 percent of the Earth’s surface. A huge section of the Gulf of Mexico would have exploded into steam as the ferocious energy of the asteroid’s motion was converted to heat upon impact. In the relatively shallow water there, the asteroid still would not have slowed much before hitting the continental shelf. Once it finally hit rock, the impacting mass would have stopped, and the remaining energy would have flash-converted to heat.


Meteor Crater, in Arizona, formed in an impact about 50,000 years ago. The iron asteroid that gouged this crater out of the desert was only 50 yards across. The far rim wall is almost a mile away; note the people in the lower right for scale.


At this point, what was moments before a horrifying scenario turns into complete apocalypse as several events occur at once. Slamming into the Earth’s crust, all those millions of megatons of energy exploded outward, sending molten rock and vaporized seawater upward and outward. The plume shot up miles into the sky, bright and hot as the Sun. The impact itself generated a huge ground shock wave, dwarfing any mere terrestrial seismic event and killing everything for hundreds of miles around the impact site.

Following the ground shock was an air shock, an epic sonic boom. Any creatures within a thousand miles that survived the initial impact were quite deaf once the thunderclap reached them.

But if they were anywhere near the Gulf of Mexico, they wouldn’t have lasted long anyway. When the asteroid hit the water, it displaced vast amounts of the ocean, both because of the shock wave and through simple vaporization due to heat. What it created was a tsunami, but one on a huge scale.

In December 2004, an earthquake caused a tsunami a few yards high that moved slower than a car, yet killed a quarter of a million people. The tsunami generated by the asteroid impact was hundreds of yards high, and moved at 600 miles per hour.

Within minutes a roaring mountain of billions of tons of seawater slammed into the Texas coast, scouring it clean of any life. The tsunami marched inland for miles, killing everything in its path with a fierce devastation no tornado, hurricane, or earthquake could ever hope to match.

And yet this impact still had more death to deal. When the asteroid hit, it punched a hole in the Earth right through the crust. The energy of the impact sent molten rock hurtling into the air at speeds of several miles per second. At those speeds, the debris would actually go up and out of the atmosphere on ballistic trajectories, like intercontinental missiles. As they fell back down, these ejecta would heat up and burn, replicating the original event on a miniature scale, but billions of times over. Flaming rock would fall from the sky like a cloudburst for thousands of miles around the impact point, igniting forest fires across the globe that would rage out of control and fill the Earth’s atmosphere with thick black smoke.

Essentially, the whole planet caught fire.

Back at ground zero, the impact point itself would have been like nowhere else on Earth. A crater two hundred miles across and twenty miles deep was chewed into the crust, its temperature soaring to 6,000 degrees Fahrenheit. Inrushing water instantly vaporized, creating more devastation, if such a thing was even possible.

No place on Earth was left untouched. Fires blazed everywhere. As the world burned, the atmosphere darkened, letting very little sunlight through. Over time, the Earth cooled dramatically, eventually causing an ice age that would kill even the incredibly tough plants and animals that survived the initial onslaught.

Through sheer happenstance, the asteroid hit a spot on Earth that was rich in limestone and other minerals. The shock wave from the impact (and from ejected rock reentering the atmosphere) created nitrates from this material that then formed nitric acid in the air that rained down over the planet. Moreover, chlorine and other chemicals in the asteroid itself were released upon impact; catapulted into the upper atmosphere, they were sufficient to destroy the ozone layer thousands of times over. This killed not just plant life, but aquatic life as well. The food chain was disrupted at its most fundamental level on the whole planet, and when the fires finally died down, as much as 75 percent of all life on Earth was extinguished.

Eventually, the crater cooled, the fires went out, and the natural cycles of the Earth buried the evidence. Life remaining on the planet had it pretty tough for a long time, but with that much devastation there were many environmental niches to be filled. Life did as it always does—it found its path, and the Earth was repopulated.

Fast-forward sixty-five million years. Archaeologists digging through rock layers see a dramatic change in composition and color between two strata. Below this change are rocks and fossils from the Cretaceous period, and materials above it are from the Tertiary. This striking discontinuity, called the K-T boundary (unfortunately, the term C-T was already being used by archaeologists, so they had to settle for K-T), would be a mystery for decades, and not just among scientists: since it marked the end of the dinosaurs, it caught the public’s imagination as well.

After years of investigation, the smoking gun turned up: a layer of iridium was found in the rock at the K-T boundary—it’s an element rare on the surface of the Earth, but common in asteroids. Also, many areas on Earth have a layer of soot just above the K-T boundary, probably attesting to the global fires. Both pointed right to an impact from an asteroid. All that was needed to clinch the deal was the location of the crater.

It too was eventually found, centered just off the tip of the Yucatán Peninsula. You might think a huge crater would be easy to find, but in fact it’s difficult. Millions of years of erosion eradicated many crater features. Plus, the crater itself, called Chicxulub,2 is so big that it can only be seen easily from space. Amazingly, you could be standing in the middle of it and never know. It’s so large but so difficult to measure that scientists are still arguing over its size and depth.

After all this—the global destruction, the extinction of countless species (including, of course, the dinosaurs, which had previously enjoyed a pretty impressive two-hundred-million-year run), and an environmental impact that lasted for centuries—it might be worthwhile to note that the culprit, an asteroid six miles across, would be categorized by most astronomers as “small.”

Much, much larger asteroids exist. Most never get near the Earth. But there are several of similar size that not only approach us, but have orbits that actually cross that of the Earth. For them, an impact is not a matter of if. It’s a matter of when.

The dinosaurs had a very bad day, but our own day may yet come.


Where are all these rocks coming from?

The majority of asteroids in the solar system circle the Sun between the orbits of Mars and Jupiter in what’s called the asteroid belt, or the main belt. There may be billions of them there, occupying several quintillion cubic miles of space in a volume resembling a flattened doughnut. Most are tiny, grains of dust, or pea-sized. The largest, Ceres, is about six hundred miles across, and was the first to be discovered. On January 1, 1801—the first day of the new century3—the Italian astronomer Giuseppe Piazzi found it while scanning the heavens. Knowing that astronomers had supposed that the gap between Mars and Jupiter might hide a small planet, and seeing that his object moved from night to night, Piazzi thought he had finally found it. However, within a few years several more objects were found in the same region of space. As a group, they were named asteroids, meaning starlike objects; they were too small and too far away to be anything more than points of light to the telescopes of the time.

The origin of the asteroids has been a mystery for a long time. At first, it was assumed that they were the rubble from a planet that existed between Mars and Jupiter that was somehow destroyed. Today, the weight of accumulated evidence indicates that the asteroids are actually leftover detritus from the formation of the solar system. These scraps were never able to accumulate to form a major planet because of the powerful gravitational influence of Jupiter; the gravity of the solar system’s largest planet accelerated the asteroids, increasing the speeds of their collisions. Instead of sticking together from low-speed collisions to form bigger objects, they hit at higher speeds, which shattered them.

Several hundred thousand asteroids are known today. Many have been discovered through dogged determination; astronomers huddled over their telescopes’ eyepieces, watching the sky, night after night. Today, there are automated telescopes—robots, in a sense—that use pre-programmed patterns to scan the sky. The vast amounts of data generated are then analyzed by computer to look for moving objects. It’s actually relatively rare these days for a human to find an asteroid.

While the majority of all known asteroids orbit the Sun in the main belt, not all of them do. Various processes, gravitational and otherwise, can change the shapes of the orbits of some main-belt asteroids over time. Their orbits can become more elliptical, dipping them closer in and farther out than the other asteroids in the main belt. Some cross Mars’s orbit, and some cross the Earth’s.

It’s the latter we need to be concerned about.

The search for these Earth-crossers (called Near Earth Objects, and dangerous ones tagged as Potentially Hazardous Objects) is a multinational effort, but it’s still somewhat small—fewer than two dozen astronomers work on it full-time, with the majority of the work being done in the United States. Even if we had more people looking, using better and bigger and more equipment, the smallish rocks a hundred or so yards across are still a threat: it’s very difficult to spot them with any reasonable lead time. Many this size are discovered just after they pass the Earth, in fact. It’s quite possible that the first warning we may get of a small Tunguska-level impact is the flash of light as it streaks across the sky.

So astronomers keep searching, and hoping they’ll catch the next impactor with plenty of time to do something about it. The goal is to find 90 percent of all Earth-approaching asteroids bigger than about a thousand yards across by the end of 2008. There are thousands upon thousands of such objects, so the astronomers have plenty of work to do. While the formal 2008 goal was not officially met (it will be eventually), the important thing to note is that, statistically speaking, a large number of asteroids with initially uncertain impact probabilities have been relegated to the “harmless” category.

We’ve known about asteroids for two hundred years, and it’s taken us this long to recognize their danger. The dinosaurs never had a chance.


Of course, the big difference between us and the dinosaurs is that they didn’t have a space program.

You’ve seen the movie a hundred times: an asteroid miles across is discovered and its orbit puts it on a direct collision course with Earth. If we don’t do something, it’ll wipe us out. Enter the team of brave hero astronauts/oil riggers/military men. Heroically they launch into space, heroically face down the monstrous rock, and heroically blow it to smithereens, which then rain down harmlessly as gawkers look on.

That sounds, well, heroic. There’s only one problem: it won’t work.

Actually, there are lots of problems with this scenario. For one, there’s no guarantee that nuking an asteroid will destroy it. A lot of asteroids are almost solid iron, so throwing a nuclear bomb at one might only warm it up a little.

Even if an asteroid is made of rock, there’s no guarantee a nuke will disintegrate it. First, if it’s really big, a nuclear weapon may not do all that much damage to it. But it also depends on the asteroid’s consistency.

Some asteroids have been found to have very low density, which was initially puzzling. Rock has a density of about two to three grams per cubic centimeter (roughly an ounce per cubic inch, or two to three times the density of water). But some asteroids have lower density than that. An asteroid called 253 Mathilde, for example, which orbits the Sun between Mars and Jupiter, has a density of about 1.3 grams per cc. It must have a texture like Styrofoam. How can that be?

When asteroids were finally observed up close by space probes, they were seen to be heavily cratered. Obviously, asteroids eat their own: they hit each other, leaving giant pits across their surfaces. If an asteroid gets hit hard enough, it’ll explode, breaking apart completely. But if it’s hit just softer than that critical speed, it won’t blow apart: the shock from the impact will shatter it in place, like a hammer tapping a crystal egg. The asteroid’s own gravity will still hold it together, but it’ll be riddled with crevices and cracks. In essence, it’s a floating rubble pile.

What would happen if you tried to nuke something like that?

Asteroid expert Dan Durda from the Southwest Research Institute in Boulder, Colorado, wanted to find out. He discovered that the scientific literature on asteroids didn’t have much information involving experiments on actual asteroidal material, so he set about to correct that. He obtained some meteorites known to have come from asteroids. One was dense, solid, and rocky, and the other was more porous, more like 253 Mathilde than a chunk of, say, quartz.

He took his sample to NASA’s Ames Research Center in California, which boasts the ownership of an unusual gun: it uses compressed air to shoot projectiles at several kilometers per second.

Durda set up his solid specimen in the gun’s sights, and slammed a BB into it at five kilometers per second. As expected, the meteorite exploded, disintegrating into hundreds of pieces.

He then put a porous piece of rocky material in the crosshairs. When the projectile hit it, the meteorite absorbed the projectile and didn’t shatter.

Durda asks, “What if an object like that were coming toward the Earth and you were trying to stop it? . . . How would it respond if we were to throw some sort of a projectile into it at a very high speed to try to break it up? What if you were to try to put a small nuclear device next to it to try to break it up? Would it actually respond in the way you normally think of a solid hunk of rock?

“You put a brick next to you, and take a hammer, and you slam the brick, and it goes flying into pieces . . . that’s what you think of when you talk about breaking up an asteroid.

“But you take a sandbag, and you whack it with that same hammer, and nothing happens. It just kind of goes thud, and that’s the end of it.”

That’s bad news for us. A rubble pile is pretty good at absorbing damage, and so a nuke won’t destroy it. If we see one headed our way, we can bomb the heck out of it, and it’ll laugh all the way down to impact.

Actually, many scientists are rethinking the idea of nuking an asteroid. A huge disadvantage of blowing up an incoming threat, even if we could, is that it would create thousands of small potential impactors out of one big one. That may sound better than the alternative (a giant one hitting intact), but even a rock a hundred yards across could easily take out a city. A dozen of those impacting at the same time would be disastrous no matter where they hit. While the explosions would be smaller, they’d be more spread out, scattering damage across the globe instead of confining it to one place.

Durda adds another danger of blowing up small asteroids. “If you take the cosmic composition of a typical asteroid of that size . . . there’s enough chlorine and bromine in that object to destroy the ozone layer. So it doesn’t matter whether or not that object hits all at once, in one big piece, or if all of that small debris left over from breaking it up into a million pieces still comes raining through and vaporizing into our atmosphere. You’re still depositing all of that very harmful stuff into our fragile atmosphere.”

It might be better, then, if there’s no way to stop it, to just let a smaller asteroid hit us.

That’s unsatisfactory, of course, especially if you’re sitting in the bull’s-eye.

But there may yet be another solution.

One idea is to drop a bomb not on the asteroid, but near it. Blowing up a bomb next to an asteroid, say, a few hundred yards away, would generate a huge amount of heat and vaporize part of its surface. The solid rock or metal would turn into a gas and expand rapidly, acting like a rocket. It would push a little bit on the asteroid, moving it.

It wouldn’t be much, but in space you don’t need much: every little push adds up. If you blow up several bombs, you can actually generate enough thrust to move the rock significantly. If you move it enough, it’ll miss the Earth entirely.

And a big advantage of this technique is that it would work on rubble piles too, though it’s not clear exactly how well.

There are some disadvantages to this method. You need a lot of lead time, for one thing. The farther away an asteroid is from impact, the less you have to change its orbit to make it miss us. Most experts think ten years’ warning is enough, though they’d be happier with twenty. A century would be just fine. This technique would work best on smaller asteroids since they’re easier to move, but a smaller asteroid is also fainter, and thus harder to find. Lead times would be shorter so there would be no room for mistakes. And getting one bomb to an asteroid is hard; getting twenty or more is a lot harder.

Another problem is that it’s nearly impossible to know how an explosion would affect the orbit of the asteroid. It might be enough to have the rock miss us, or it might nudge it into an orbit that will hit us on the next pass around the Sun.

For example, look at asteroid 99942 Apophis. It’s an Earth-crossing chunk of rock about 250 meters across, and is a potential impactor. At that size and mass, it would do considerable damage should it hit, exploding with the force of 900 megatons (more than a dozen times the yield of the largest nuclear weapon ever detonated). Apophis will pass by the Earth on April 13, 2029; there is no risk of impact at that time, but it will pass so close that it will actually be closer to the surface of the Earth than many weather and communication satellites.

The asteroid will approach so close to us, in fact, that its orbit will be seriously affected by Earth’s gravity, and just how much its orbit is changed depends on just how close it gets to Earth in 2029. In fact, there is a region of space called the keyhole such that if Apophis passes through it, the orbit will be changed precisely enough that on its next return in 2036, Apophis will impact the Earth.

This sweet spot is not terribly big, but our knowledge of Apophis’s exact trajectory isn’t good enough to completely preclude the asteroid’s passing through it. The odds are incredibly low, maybe less than 1 in 45,000, but it’s worth investigating.

And what if it turns out that Apophis will glide right through the keyhole? We’ll have just seven years to move it enough to miss us. A better idea is to prevent it from passing through the keyhole in the first place. If we get to Apophis before 2029, then we hardly have to nudge it at all; calculations show that changing its velocity by even a few thousandths of an inch per hour would work. So you might think that a well-placed nuclear weapon would do the trick.

Unfortunately, it won’t. That keyhole isn’t alone: there are dozens of keyholes, thousands. That first keyhole is just for a return of Apophis in seven years, but other keyholes will bring it back in ten years, twelve, twenty . . . instead of saving us, a detonation just buys a little bit of time, and there’s no guarantee that we can move it away from some other keyhole—or knock a chunk or ten of it into another keyhole.

Controlling the resulting orbit is a key issue, and blowing up a nuclear weapon is not exactly subtle.4 We need more fine-tuning on asteroid steering.


By now it may have occurred to you that maybe we don’t need a bomb. The impact of an asteroid on the Earth releases energy like a bomb, so why not try impacting the asteroid itself? If we hit it hard enough with some sort of impactor, we won’t need a nuke.

There is a very big advantage of this method: we’ve done it before. On, appropriately enough, July 4, 2005, NASA’s Deep Impact probe slammed into the comet Tempel 1, creating a flash seen by hundreds of scientific instruments across the world. The impactor was an 800 pound block of copper, which was steered into the comet at over six miles per second. The resulting explosion was the equivalent of about five tons of TNT detonating. The size of the resulting crater is unknown; the flash and debris hid the impact from the spacecraft’s camera.

Steering a probe into an object moving at several miles per second is an engineering triumph. There were no second chances, and even the exact shape of the comet nucleus was unknown until the probe got there.

On the other hand, the comet itself was three by five miles in size, which is pretty big. Had it been a small asteroid, it’s unclear whether the NASA engineers would have been able to hit it. Still, it was a first shot, and a successful one. Much was learned from the attempt that can be applied to ramming a potentially dangerous asteroid.

But it must be stressed that the impacting scenario suffers from most of the same issues as bombing an asteroid: it might shatter the asteroid, producing many smaller impactors; if the asteroid is porous it will simply absorb the impactor; and again we cannot control the resulting orbit, so we might just be pushing it into some other future impact event. While this might change the orbit enough to miss the Earth, it can’t be known in advance just how much, and in this game inches matter.


Still, there may be other ways to rid ourselves of a potential planet-buster. Perhaps, instead of blowing one up, we can instead gently persuade the asteroid to change its trajectory.

The B612 Foundation—named after the asteroid home of Antoine de Saint-Exupéry’s titular Little Prince—is, for lack of a better description, a kind of doomsday think tank, consisting of dozens of scientists, engineers, and astronauts whose express purpose is to figure out a way to save humanity from the threat of giant impacts. The foundation has held meetings, written papers, and had members (such as Apollo 9 astronaut Rusty Schweickart) testify before Congress about doomsday rocks.

Their Web site reads like a science-fiction novel, full of ways to stop an asteroid from hitting us. However, the emphasis is on the science. While many of the methods would be difficult to execute and are clearly only in the very early stages, others involve mature technology or adapting what we already have.

For example, one method is to physically land a rocket on an asteroid, secure it in place upside down, and then start firing it. Over time, the thrust will push the asteroid into a new orbit, making the rock miss us.

This may be the safest method, and it certainly makes sense, but in reality it would be pretty hard to do. For one thing, it’s not entirely clear how you would secure the rocket to the surface of the asteroid. What if the surface is powdery, or it’s a rubble pile, or it’s metal? For another, every asteroid spins, which means you can only fire the rocket for short periods of time when it’s pointed in the right direction. That means you need more lead time, and in many cases time is precious. Worse, some asteroids tumble chaotically, and for those a rocket would be nearly useless.

These problems kept the B612 Foundation members thinking . . . and they came up with an answer that is really quite surprising. What if you don’t land the rocket at all?

Asteroids are small compared to planets, but they still have mass. And any object with mass, said Isaac Newton, has gravity. The rocket itself has mass, and therefore gravity as well. So imagine this: a rocket is placed in a parking orbit near the asteroid, but not physically in contact with it. The asteroid’s gravity will pull on the rocket, making it fall toward the asteroid. In the same way, the rocket’s mass will pull on the asteroid. Now the rocket is fired, but very, very gently, just enough to counteract the fall toward the surface. The result is an asteroid tug. But unlike a barge or a tugboat on Earth that uses ropes to haul other boats, the rocket is virtually connected to the asteroid by gravity. Over time, the gravity from the rocket pulls the asteroid into a safe orbit.


An innovative idea in asteroid impact mitigation is to use the gravity of a small spacecraft to move a hazardous asteroid out of harm’s way. Given enough lead time, this is a very precise method of altering an asteroid’s orbit.


But of course there are technical difficulties with this method too; there always are. The rocket cannot fire straight down, toward the asteroid, because it will push the asteroid back, negating the effect of the tug. So the rocket will have to be tilted outward, firing at an angle away from the asteroid. That means that pairs of rockets are needed to balance each other and keep the tug from spinning out of control.

The amazing thing about this method is that in some cases, the mass of the tug need not be all that much. For 99942 Apophis, for example, a tug massing only a single ton can be effective in moving the asteroid away from the keyhole even if it gets to the rock only two years in advance. To be fair, in general it will take longer to move an asteroid away from an impact trajectory; for Apophis we need only move it so it misses a small region of space, but for a direct-impact trajectory the asteroid needs to miss a whole planet. That means moving the orbit thousands of miles, which in turn means a longer lead time (or a more massive tug). One current thought, developed by Schweickart, is a hybrid solution: using a kinetic impactor (literally whacking it with another rock) or nuke to move the asteroid out of the immediate threat zone, then using the gravity tug to fine-tune the orbit so that we don’t get a surprise a few orbits later.

Still, as promising as these technologies sound, we need to be honest with ourselves. We currently don’t have the technology to implement any of these methods. We’re close—maybe only a few years from developing the gravity tug—but even lobbing a nuke at an asteroid is pretty difficult. A report written in 2007 from NASA to Congress suggests that sending an impactor to an asteroid is currently our only workable option.

But that is due to current knowledge and current technology. The B612 Foundation is hoping to incrementally test technology that will prevent an asteroid from hitting us. Even better, some of their ideas, like the gravity tug, allow us to manipulate the orbit of an asteroid any way we want. We might even be able to nudge one into a safe orbit around the Earth. It would be far too small for its gravitational pull to affect us, but close enough that we might be able to set up mining operations on it. That might sound far-fetched, but some estimates show that the metals in even a small asteroid could be worth trillions of dollars. That would make a mighty tempting target for industry.

Instead of its aiming at us, we would be well suited to aim for it.


There is still another lurking problem about which we need to be aware. Asteroids tend to have nice, predictable orbits. They are dead hunks of rock and/or metal, so once we observe them for a while, we can predict their orbits for decades.


In this artist’s work, Space Shuttle astronauts see a comet nucleus dozens of miles across impact the Earth. Orbiting astronauts may be the only survivors; the Earth they would eventually return to would be devastated by such an event.


But asteroids aren’t the only threat. Comets are lovely, wondrous specters in the sky. Unlike asteroids, comets are like dirty snowballs: rock, gravel, and dust mixed in with ice holding it all together. When they get near the Sun, the ice melts.5 Many comets have pockets of ice under the surface, and when those sublimate the gas vents out in a jet. This acts like a rocket, pushing the comet around. If the comet is spinning—and most are—this means the comet will get pushed around randomly. That makes it extremely hard to accurately predict their orbits, and that much harder to land a rocket in them, or to use a gravity tug.

And it gets worse. The solar system looks something like a DVD seen edge-on: the planets orbit the Sun in the same plane. Asteroids too tend to stick to that plane. That means looking for them is a lot easier; we only need to keep checking the same parts of the sky.

But comets are wild cards. They aren’t confined to the solar system plane, and can come literally from any part of the sky. This can significantly cut into the lead time we have to do something about a killer comet approaching Earth. While we might have decades of notice for an asteroid impact, we might only have a few years for a comet. Even comet Hale-Bopp, which was one of the brightest ever seen, and which delighted hundreds of millions of people, was only discovered about two years in advance of its passage of Earth. Had it been aimed at us, there wouldn’t have been a damn thing we could have done about it. Hale-Bopp’s nucleus—the solid part of the comet—was twenty-five miles across. Had it hit, it would have made the asteroid impact that wiped out the dinosaurs look like a wet firecracker.

But even a small comet could have a disastrous, well, impact. Assuming it wasn’t confused for a sneak attack of some kind, the direct consequence of a small impact or Tunguska-like airburst over a city could lead to thousands of deaths and billions of dollars of damage. If it happened over a major city or economic landmark—New York City, California’s Central Valley (where much of the nation’s fruits and vegetables are grown), Tokyo—the results could be far worse. The good news is that long-period comets like Hale-Bopp represent less than a few percent of the overall impact hazard, and most short-period comets are easy to spot.


So how big a danger are asteroid and comet impacts?

Statistically speaking, you’re not going to like the answer: the odds of getting hit are 100 percent. Yes, really. Given enough time, and if we do nothing about it, there will be impacts, and one will be big.

But the key part of that sentence is the “if we do nothing” part. The point is, we can do something. While the techniques described here sound like something out of a movie, they are all possible. Technically they’ll be tough, and they’ll be expensive. But the stakes are pretty high: global survival versus utter annihilation.

I think that given this, it’s about time we took these science-fiction ideas and made them science fact.