Byline: Tom Webber
I don't want to cause a panic, so let's all remain calm as we talk about rocks that can fall from space anywhere, anytime and without warning.
2013 has been a quite a year for space rocks. There was all the hubbub about asteroid 2012 DA14 that passed within 17,200 miles of Earth in February. Then the world was shocked when an undetected asteroid entered the atmosphere over Russia that very same day and exploded, injuring more than 1,100 people and causing $33 million in damage.
These two events cast the spotlight back on Apophis, a large asteroid that will pass near to Earth in 2029 and 2036.
And, of course, our home planet is constantly being pummeled with cosmic debris in the form of meteors and meteorites. It is estimated that the mass of Earth increases by thousands of tons a year by being sprinkled with cosmic dust and debris. (Note: It could be argued that, because of the launches of satellites and rockets, Earth also loses mass. But that amount is negligible by comparison.)
Asteroids and meteors and meteorites - oh my! It should be emphasized, however, that these three words are not interchangeable. Asteroids are minor rocky planets in the solar system that can be as large as 700 miles across. They are irregular in shape and mostly lie in a belt between Mars and Jupiter.
A meteor, on the other hand, hits a little closer to home. It is a piece of rock or dust that has actually entered Earth's atmosphere and is in the process of burning up, creating a bright flash in the sky. Because of this beautiful light trail, meteors are sometimes called "shooting stars."
Finally, a meteorite is a meteor that survived its journey through our atmosphere and has struck the surface. Meteorites leave an impression on our planet. Perhaps the most famous is the Barringer Crater in Arizona, formed when a meteorite measuring 50 meters across struck Earth 50,000 years ago. This crater is 1.2 kilometers across and 200 meters deep.
Some collisions are more drastic. In the 1970s, geological dating determined that iridium-rich clay found in a layer between limestone strata all over the world was 65 million years old. That date coincides with a time when more than two-thirds of the species, including the dinosaurs, suddenly became extinct. The connection? At the end of the Mesozoic Era, a meteorite, or possibly a comet, approximately 10 kilometers in diameter, struck Earth, ejecting enough dust (including iridium) into the atmosphere to block the Sun for several years, causing decreasing temperatures and mass extinctions.
Without the dinosaurs the stage was set for mammals to develop and thrive in the Cenozoic Era.
Evidence now shows that our world has experienced several large impacts in the past that have severely altered the biological landscape of this planet.
The odds of the planet being hit by something that massive are very low, and the number of collisions has decreased dramatically over Earth's 5 billion-year history. …