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Have you ever wondered, “Has the Earth ever been hit by an asteroid?” The question pops up in common conversation more often since the idea has been fodder for several blockbuster disaster movies. In reality it is a very important question since we know over the Earth’s long 4 billion year history that it has been hit by asteroids. When they do hit they can cause significant damage. One thing that is a saving grace for Earth is that it is very rare for truly large asteroids to hit Earth. This is due to many factors from the size of the solar system to the way its composition helps to defend the inner solar system.
Asteroids are left over pieces of space rock from the formation of the solar system. They vary in size from a couple of feet to several kilometers in size. Smaller size asteroids are more common but the larger an asteroid the smaller the chances of it hitting Earth. In fact according to the stats a large asteroid hits the Earth every 1 to 100 million years and a very large asteroid every 100 million years to every 1 billion years.
So what makes planet changing asteroid impacts rare? One reason is the setup of the planets. The majority of the asteroids in the solar system lie in the asteroid belt. The other part is that the mass of the asteroids affects its orbit. The larger the asteroid the more stationary its orbit is. The asteroids that pose the most threat to Earth are called NEA’s or Near Earth Asteroids. These asteroids have orbits that intersect with the Earth’s orbit. They can also have orbits that decay that cause asteroids to fall towards the sun. This is what makes asteroids likely to cause collisions.
One way scientists have learned about asteroid collisions in the past is to look at the geologic record. This includes studying impact craters, isotopes from the time of the impact, and other clues found in soil and rock that could point to an ancient asteroid impact. One significant fact is that while rare asteroid impacts on Earth have had major impacts on the planet. One such impact is thought to have ended the age of the Dinosaurs and made possible the dominance of mammals.
We’ve also recorded an entire episode of Astronomy Cast all about Asteroids. Listen here, Episode 29: Asteroids Make Bad Neighbors.