Calculating Comet Halley's Speed Across Neptune's Orbit

[musicmar]

Homework Statement

At its perihelion in February 1986, Comet Halley was 8.79x10^7 km form the Sun and was moving at 54.6 km/s. What was its speed when it crossed Neptune's orbit in 2006?

Homework Equations

K0+U0=K+U

The Attempt at a Solution

I know I need to do this using conservation of energy. I have v-initial and r-initial, and presumably I can look up r-final. The mass of the comet will cancel, and big M will be the mass of the sun(also can look up). I am just confused about the r's, though. I was wondering why it is necessary to know the dates or that it was at its perihelion. How do I find the distance it was from the sun when it crossed Neptune's orbit?

 

[musicmar]

I figured it out. You actually just have to use the mean distance of Neptune from the Sun, and then you have all of the necessary information to use conservation of energy.