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Tons of Asteroids

Week of April 21, 1997
I have another surprising number this week. Ceres, the largest asteroid in the solar system is about 900 kilometers in diameter. The next three largest asteroids are all larger than about 400 kilometers across. How many asteroids do you think there are larger than 100 meters across (0.1 kilometers)?

I was recently researching some facts about asteroids for my Bad Astronomy web page, and I needed to know this number. It turns out it's not easy to find, so I emailed my old friend Dan Durda, who works at the Lunar and Planetary Laboratory in Arizona. He uses theoretical models of asteroids smashing into each other to see what comes out, like size distribution, orbit changes, things like that. Sure enough, he sent me a file that listed the number of asteroids versus the size of the asteroid. For example, there are 26 asteroids larger than 200 kilometers in the main asteroid belt.

Using the numbers he supplied, I calculated how many asteroids there are larger than 100 meters in diameter. To my shock I got the answer: there are about one billion asteroids bigger than 100 meters in the solar system! What a huge number! But keep this in mind: this is counting asteroids between Mars and Jupiter, and there is about one quintillion square kilometers of room out there for these rocks to roam in, or a billion square kilometers per asteroid. That number does not even take into account the fact that the orbits of the asteroids are tilted with respect to one another, so really, instead of an area, you need to think about the volume of space. Dr. Durda tells me that there are about two million kilometers on average between asteroids.

There may be a lot of asteroids out there, but space is BIG!

Surprisingly, the total mass of asteroids, even though there are so many of them, is quite small. All of the asteroids combined weigh less then our own Moon. A billion asteroids 100 meters or larger sounds like a lot, but that's misleading. The volume of an object goes as the cube of the radius. A billion asteroids 100 meters across would occupy the same space as a single asteroid with a radius of the cube root of one billion times 100 meters. This is equal to 100,000 meters, or 100 kilometers across. That's pretty small!

Want to know more about asteroids? If you're a regular reader you know what is coming next: go to Bill Arnett's The Nine Planets. His asteroid facts page has lots of info and further links to keep you busy for a while!

©2008 Phil Plait. All Rights Reserved.

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