"The State of The World" Number 25
Complete Index Of All Issues
When Space Rocks Collide With Earth
At the time of this writing in early September 2017, astronomers have announced that an asteroid "the size of a small house" is going to have a near miss with Earth on October 12. Present estimates state that it will pass by only about 35,000 kilometers / 23,000 miles - just one eighth of the distance between the Earth and the Moon.
How often does that happen? And what happens if and when asteroids don't miss?
First, what is an "asteroid"?
Asteroids, also known as "minor planets," are rocks (very much the same as rocks on Earth) of various sizes, from small, to hundreds of kilometers / miles in diameter, that orbit the Sun. As illustrated below, there are millions of them in the "asteroid belt" (the white dots in image), plus millions of others farther out, and closer in.
Most of the asteroids are harmless to Earth. It's only when their orbits cross the orbit of Earth that collisions are possible - and do happen. It's actually much more common than many people realize, as illustrated below (a "meteor" is the term for a small asteroid, less than a meter in diameter, while a "bolide" is the term for a very bright meteor).
Larger asteroids are too big to burn up in the atmosphere however. They then do one of two things - explode in the atmosphere, or collide with the Earth's surface.
The most famous example of an exploding asteroid happened in Russia in 1912. An asteroid that had been estimated to have been about 150 meters / 500 feet in diameter, exploded at an altitude of about 6 kilometers / 4 miles. The explosion was caused by superheating of the rock's exterior surface from friction as it entered the atmosphere at an estimated 25 kilometers / 15 miles per second. The "Tunguska event" destroyed an area of over 700 square miles, flattening millions of trees, with a force-of-nature that scientists have estimated to have been equal to a 20 megaton nuclear bomb.
There are much larger asteroids out there, some of which, sooner or later, could collide with Earth someday. Photographs of some the known asteroids are shown below. "Vesta," the largest shown below for scale, is about 570 kilometers / 350 miles in diameter.