What is the Difference Between Asteroids and Comets

by Jerry Coffey on June 20, 2009

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The solar system

The difference between asteroid and comet. Image credit: NASA


The main difference between an asteroid and a comet is what they are made of. Asteroids are made up of metals and rocky material, while comets are made up of ice, dust and rocky material. Both of these space objects were formed during the earliest times of the solar system, around 4.5 billion years ago. Asteroids formed much closer to the Sun, where it was too warm for ices to remain solid. Comets formed farther from the sun where ices would not melt. Comets, which approach the Sun, lose material with each orbit because some of their ice melts and vaporizes to form a tail.

A few other important differences between an asteroid and a comet exist. Obviously, comets have tails and asteroids do not. The heat from the Sun causes ice and other materials on a comet’s surface to heat up until they vaporize. That vapor is what is seen as the comets tail. Another difference is in their orbital patterns. Comets tend to have very extended and elongated orbits, many times going more than 50,000 AU from the Sun (1 AU, or astronomical unit, equals the distance from the Earth to the Sun).

Asteroids tend to have shorter, more circular orbits and they seem to want to group together in belts.

Another difference between an asteroid and a comet is in the numbers of each. There are only 3,572 known comets, but there are many millions of asteroids. Some as small as dust particles.

While the main difference between an asteroid and a comet is what they are made up of, there are a few interesting differences. Asteroids may not have the flash of a tail, they do have many scientists studying them for many purposes, including the possibility of them hitting the Earth. The mining of asteroids is becoming another venture that scientists and industrialists are keeping their eyes on.

We have written many articles about both asteroids and comets for Universe Today. Here’s an article about how we might keep asteroids and comets away from the Earth. And here’s an article about the strange asteroid BD 2009.

Want more resources on asteroids? Here’s NASA’s Lunar and Planetary Science Page on asteroids. And here’s Hubblesite’s News Releases about Asteroids.

We have recorded two episodes of Astronomy Cast about asteroids. There’s Episode 55: The Asteroid Belt, and here’s Episode 29: Asteroids Make Bad Neighbors.

References:
NASA: Asteroid Fast Facts
IPAC Cool Cosmos

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