MadSci Network: Astronomy

Re: Why do asteroids stray from their orbits while the planets stay put?

Date: Mon Oct 23 18:25:26 2000
Posted By: Robert Macke, Instructor, Astronomy
Area of science: Astronomy
ID: 970071198.As

Jr. Redulla,

It's all about the mass.  To think about how mass comes into play when two 
objects interact, let's start by analogy.  Imagine a train is barreling 
down the tracks.  Now imagine a compact car is racing down the tracks 
directly toward it.  They have a head-on collision.  The train may slow 
down only a tiny bit, but the car is completely stopped and then reverses 
direction as the train shoves it out of the way.  What's the difference 
between the two?  Their masses.

Now imagine you have a planet encounter an asteroid.  They don't have to 
actually collide like in the analogy, but simply interact gravitationally.  
This can be done at quite a distance.  Both will exert exactly the same 
gravitational force on each other, but the planet will have a much larger 
mass, which will help it resist changes to its motion.  It will not be so 
easily moved from its regular orbit, but the small asteroid will be thrown 
around like the compact car.

---Bob Macke

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