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Author Topic: Why do comets and asteroids sometimes cross our path?  (Read 2079 times)

Offline John Chapman

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My understanding is that the universe is expanding at an ever increasing speed. This means that all the material should be spreading further and further apart (although I know that galaxies can be considered as closed systems with their own internal forces holding them together). My question is this:

If everything is travelling apart, all moving away from a single point, how can our paths ever cross with other bodies such as asteroids?
« Last Edit: 13/02/2009 18:45:56 by John Chapman »


 

Offline Vern

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Why do comets and asteroids sometimes cross our path?
« Reply #1 on: 13/02/2009 19:37:59 »
I think you just answered your question in the first paragraph. Galaxies hold themselves free of the great expansion via gravitational attraction. But, then we have rogue stars that are not part of any galaxy. So, I guess we have to exclude any gravitational system from the expansion since rogue stars don't seem to be expanding.

 

Offline John Chapman

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Why do comets and asteroids sometimes cross our path?
« Reply #2 on: 13/02/2009 21:43:34 »
Hi Vern

So are you saying that asteroids are travelling in orbits within the galaxies? That sounds pretty obvious, actually. Different size orbits centred on different objects are bound to collide.

The answer I was expecting to hear is that asteroids are travelling in a straight line through galaxies but the native bodies are all moving in circles (and circles within circles) and collide with the asteroids. Actually, now that I've written it down for the whole of Naked Science land to see I realise that is pretty stupid and I feel a bit embarrassed.



But.... I've just had a thought. If galaxies don't expand why do they form catherine wheel swirls which suggests material is subject to an outward force.

I could have it wrong. Is it just the smaller solar systems that don't expand but they drift apart in an expanding galaxy?
 

Offline Vern

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Why do comets and asteroids sometimes cross our path?
« Reply #3 on: 13/02/2009 22:11:08 »
Asteroids are mainly parts of the star systems and probably never make it between stars. Our own solar asteroids live mainly within the planetary orbits and comets reside a little further out in what's called the Ort Cloud, I think.

The Milky Way Galaxy is one big dude. It is about a hundred thousand light years across the plane of it and it is about twenty light years thick.
« Last Edit: 13/02/2009 22:12:39 by Vern »
 

Offline Vern

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Why do comets and asteroids sometimes cross our path?
« Reply #4 on: 13/02/2009 22:16:37 »
Quote from: John Chapman
But.... I've just had a thought. If galaxies don't expand why do they form catherine wheel swirls which suggests material is subject to an outward force.
I have read the dynamics of this but it escapes me at the moment. But whether or not the galaxy is expanding isn't part of the scheme.
 

Offline John Chapman

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Why do comets and asteroids sometimes cross our path?
« Reply #5 on: 13/02/2009 22:44:33 »
Thanks for that, Vern

It's always an education reading your replies. I shall google the Ort Cloud (and some other things in your replies this evening). Lots of interesting stuff there. ;)
 

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Why do comets and asteroids sometimes cross our path?
« Reply #5 on: 13/02/2009 22:44:33 »

 

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