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Orbits of Planets and Comets

10/01/2010 4:16 AM

Why the planetary orbits are elliptical in shape? Why orbits of comets are so much different from planets?

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#1

Re: Orbits of planets and comets

10/01/2010 4:43 AM

Play around with this applet (click here) - it should give you a "feel" for what happens.

There's some more about Kepler's laws on Wikipedia (click here).

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#2

Re: Orbits of panets and comets

10/01/2010 6:29 AM

Stable orbits tend to be circular (or very near so). Planets assume a stable orbit over millions and billions of years. The rouge guys get weeded out by perturbations from the gravity of other planets and the Sun. Generally, this happens early in the formation of the solar system. The rubble between Mars and Jupiter is thought by some to be the remnants of a collision between a planet and a rouge object. Our Moon is also thought to be the product of a collision between Earth and a rouge object.

After a period of time, things settle out pretty well and most of the rouge objects get cleared out. We are in a very stable period of our solar system's life.

However, deeper in space, in an area we call the Ort Cloud, still lies a lot of junk. This is where most comets come from. Gravitational interactions and perturbations between those objects (and lord knows what else out there) can send a comet out from the Ort Cloud and they assume a highly elliptical orbit that sends them close to the Sun.

Over time (hundreds or thousands of years), these objects either get consumed by the Sun or, as we saw with Shoemaker-Levy 9, impact a planet like Jupiter. Jupiter tends to act as a cosmic vacuum cleaner for us and keeps the number of Earth impacts low. Highly elliptical orbits tend to be relatively short lived on a cosmic timescale.

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#3

Re: Orbits of panets and comets

10/01/2010 6:52 AM

It is primarily related to orbital energy conservation. Here is a link to a site on orbital mechanics.

If the orbital energy of an object is <0 the orbit will be circular or elliptical, if it's equal to 0 it will be parabolic and if >0 it will be hyperbolic.

From Wikipedia here:

Under standard assumptions, specific orbital energy () of elliptic orbit is negative and the orbital energy conservation equation (the Vis-viva equation) for this orbit can take the form:

where:

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#7
In reply to #3

Re: Orbits of panets and comets

10/05/2010 9:01 AM

I just looked over my previous response and noticed some things did not copy/paste correctly. They were there when pasting initially, but disappear when selecting "preview comment".

So here's another try...from wikipedia on orbital mechanics ( link )

About half way down the page is this on energy. I have hand typed in the equation and variables.

Energy

Under standard assumptions, specific orbital energy (ε) of elliptic orbit is negative and the orbital energy conservation equation (the Vis-viva equation) for this orbit can take the form:

v2/2 - μ/r = μ/(2*a) = ε < 0

where:

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#8
In reply to #7

Re: Orbits of panets and comets

10/05/2010 9:20 AM

Thought it looked a bit strange .

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#9
In reply to #8

Re: Orbits of panets and comets

10/05/2010 9:37 AM

Yeah. What's odd is that it looked fine when I pasted it. Obviously I didn't really look at it when I went to the preview screen.

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#4

Re: Orbits of Planets and Comets

10/01/2010 7:40 AM

Why the planetary orbits are elliptical in shape?

1. They are almost solely under the influence of the Sun's gravity. 2. Gravity obeys an inverse-square law. 3. Angular momentum is conserved. From three 3 facts, Newton showed that elliptical orbits are a necessary consequence.

Why orbits of comets are so much different from planets?

At their aphelion distances they have much less angular momentum than planets do compared to when the planets are at their aphelion distances.

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#6
In reply to #4

Re: Orbits of Planets and Comets

10/05/2010 8:38 AM

Thanks all. My doubts are cleared up to a large extent.

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#5

Re: Orbits of Planets and Comets

10/04/2010 3:48 PM

Because if they were circular, that would be circular logic.

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