What Is An Asteroid?

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What is an asteroid? That is a good question. The main exact definition is: “An asteroid is any of numerous small planetary bodies that revolve around the sun. Asteroids are also called minor planets or planetoids. Most of them are in the asteroid belt between the orbits of Mars and Jupiter.” That tells you what they are, but doesn’t really give any of the character that makes up an asteroid. From that definition it is hard to imagine the size of some asteroids or the danger that they can pose for those of us here on Earth. How about letting this article clear a little of the mud?

To help you understand what is an asteroid, let’s talk about their size. An asteroid can be any size from a dust particle up to the largest one, Ceres, which is 933 km in diameter and is considered to be a minor planet. Ceres accounts for as much as 1/3 of the mass of all of the asteroids in existence. Asteroid size is measured in one of three ways. In the first way scientists use telescopes to determine the asteroid’s distance from the sun, the amount of sunlight it reflects, and the amount of heat it gives off. The amount of sunlight or heat reaching the earth depends on the size of the asteroid and its distance from the sun. Therefore, calculations involving distance and either light or heat yield the size of the asteroid. The other two are a little less complex. In the second method, astronomers use a telescope to measure an asteroid during an occultation, when the asteroid passes in front of a star and is silhouetted against it. The third technique involves the use of radio telescopes to produce images of an asteroid. All render approximate sizes.

Scientists have tried studying the composition of asteroids to try and understand exactly what is an asteroid. These studies have brought astronomers to classify asteroids into two broad groups based on their composition. One group of asteroids dominates the outer part of the belt. These asteroids are rich in carbon. Asteroids in the second group, which are located in the inner part of the belt, are rich in minerals. These asteroids formed from melted materials.

Here is a great piece about asteroids written by NASA. Here on Universe Today we have several interesting articles that deal with asteroids. There is a great episode about them on Astronomy Cast.

References:
NASA Solar System Exploration Guide on Asteroids
NASA Asteroid Fact Sheet

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