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.
SARAH EICHMILLER AND THE ALTOONA (PA) MIRROR
Still, most meteorites are small. Some aren’t.
SHALLOW IMPACT
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.
PAIN IN THE ASTEROID
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 unde
rneath 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.
PHOTO BY THE AUTHOR
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.
COSMIC WEAPONS DUMP
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.
Death From the Skies!: These Are the Ways the World Will End... Page 2