How Do You Calculate the Eccentricity of an Elliptical Orbit?

[Dustinsfl]

$\mu_{sun} = 132712000000$
$\mu_{earth} = 398600$
$\mu_{mars} = 42828$
$R_{earth} = 149.6\times 10^6$
$R_{mars} = 227.9\times 10^6$
$r_{earth} = 6378$
$r_{mars} = 3396$

The spacecraft will make 3 rev in 2 Earth years. I found the semi-major axis of the ellipse which is
$$
a = 1.14162979\times 10^8
$$
How can I determine the eccentricity of the ellipse with this information? I have been looking at every formula but can figure it out.

http://img20.imageshack.us/img20/182/orbit2.png

 

[gneill]

Please make use of the posting template when you start a thread. This is required.

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If you know the aphelion distance and the length of the major axis then you can determine the perihelion distance. Having both r_a and r_p you can determine eccentricity.

 

[Dustinsfl]

I wasn't thinking thanks.

 

[I like Serena]

From your drawing the aphelion of the spacecraft is the same as the radius of Earth's orbit.
And the sum of aphelion and perihelion is equal to the length of the major axis.