Death from the Skies! (3 page)

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.

 

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 century
³
—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.