Find eccentricity of orbit knowing comet speed and position

jimz

1. The problem statement, all variables and given/known data
A comet has speed v when it is a distance r from the Sun. Its direction of motion makes an angle φ with the radius vector from the Sun. Find the eccentricity of the comet’s orbit.


2. Relevant equations
[tex]v^2 = G M \left( \frac{2}{r} - \frac{1}{a} \right) [/tex]


3. The attempt at a solution
Not really sure where to start... help!
Thanks.