Death from the Skies! Read Online Free Page A

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 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