For more than twenty years, Dr. Fred Bortz worked as a scientist, researcher, and teacher. Now he spends most of his time writing books and articles for young readers like you.
He enjoys both science and writing for the same reason: HE LOVES QUESTIONS. He writes for people your age because he knows you love questions, too.
To find hot-links to other "Ask Dr. Fred" questions and learn how to send Dr. Fred your favorite question, go to the main "Ask Dr. Fred" page.
For many millions of years, dinosaurs ruled the earth. Then, about sixty-five million years ago, every dinosaur on earth died. Every species of dinosaur, and more than half of all the other species on earth, became extinct in a very short period of time.
For many years, scientists had many theories about what happened and how long it took for all the dinos to die. Most thought the earth's climate changed, but they couldn't agree on what caused the change or how many thousands of years it took until the last dinosaur disappeared. Almost no one thought it happened suddenly, in no more than a few years -- not even University of California Geology Professor Walter Alvarez, who found an interesting piece of limestone in Gubbio, Italy, in 1977 and brought it home to Berkeley as a gift for his father, Nobel Prize-winning Physics Professor Luis Alvarez.
The limestone included a thin layer of clay that marked a time in our planet's history known as the Cretaceous-Tertiary (K-T) Boundary. Below the clay layer were fossils of tiny animals from the Cretaceous Period, the last time period when dinosaurs roamed the earth. Above the clay were even tinier fossils from animals that lived during the beginning of the Tertiary Period, when mammals began to dominate the planet. The Alvarezes wondered if they could measure how long it took to deposit that clay layer. If so, they would know how many years it took for the great K-T extinction to take place.
Luis Alvarez had an idea. Every year, space dust from millions of tiny meteorites drifts down onto our planet. That dust contains much more of some rare elements, such as platinum and iridium, than would normally be found in the rocks. He knew about a technique called neutron activation analysis that might detect the iridium in the clay. He and Walter took the rock to Frank Asaro and Helen Michel, two of the most careful chemists they knew of at the Lawrence Berkeley Laboratory, and asked them to analyze the clay layer.
Asaro and Michel didn't expect to find anything. Even ten thousand years worth of space dust would barely have enough iridium to measure. Still, they took the job. Much to everyone's surprise, they detected far more iridium than they would have ever expected. They were certain they had made a mistake.
Further testing confirmed their result. What did it mean? After trying and rejecting lots of ideas, Luis Alvarez finally came up with one that seemed to fit. As outrageous as it seemed, he theorized that an asteroid or comet about ten kilometers (six miles) in diameter must have hit the planet. It would have sent up a huge cloud of dust, and it would have ignited fires around the world, causing a thick blanket of smoke to blot out the sun. The planet would get very cold, and, without sunlight, the tiny plants of the ocean would die, along with the animals that ate them, and the animals that ate those animals, all the way up the food chain to the dinosaurs. It couldn't have taken more than a few years!
Soon, scientists all over the world were testing K-T boundary clay for iridium and searching for signs of an ancient crater. In the late 1980s, the finally found a crater, now known as the Chicxulub crater, just the right size and age, at the edge of Mexico's Yucatan Peninsula. One of the greatest questions of science had a very surprising answer. An asteroid impact had ended the reign of the dinosaurs and transformed life on earth forever.
To find out more about this great story told in their own words by Frank Asaro and Helen Michel, read Dr. Fred's book 
To the Young Scientist, Chapter 2 (Franklin Watts, 1997).
Another of Dr. Fred's books 
Collision Course! Cosmic Impacts and life on Earth discusses this and a number of other great cosmic collisions and near misses, and what we might do in the future. The Collision Course! web page has links to the latest news on the subject. That book opens with the story of how people reacted in March 1998 when the first orbital calculations of the newly discovered mile-wide Asteroid 1997 XF11 indicated that it would come very close to -- and might even collide with -- our planet in 2028. Fortunately, old photographs of the asteroid were located, and a new orbital calculation revealed better news. It will probably miss us by 600,000 miles (2.5 times as far away as the moon).
It also includes the story of On June 30, 1908, when a much smaller rock exploded over the skies of Tunguska, Siberia, set 400 square miles of forest on fire, and knocked down trees over twice that area. You can read more about that event and the possibility of asteroid impacts in the future in The Day the Sky Split Apart by Roy A. Gallant.
More advanced books on the topic of asteroid and comet impacts include Impact Jupiter: The Crash of Comet Shoemaker-Levy 9 by David Levy, co-discoverer of the Comet, The Three Big Bangs by Philip M. Dauber and Richard A. Mueller, and another book by David Levy, Comets: Creators and Destroyers.
Visit the website of the discoverer of Asteroid XF11, James V. Scotti and see his planetary artwork as well as learn about him as a planetary scientist.
Dr. Fred's favorite piece of Scotti's art shows the newly formed Moon, about a year after a giant collision between the young Earth and another planet about as large as Mars. That illustration is reproduced in Collision Course!.
Dr. Fred logo and art may not be reproduced in any form for commercial or educational use without the written permission of its owner, Alfred B. Bortz.