The orbit of Halley's Comet around the Sun is a long thin ellipse. At its aphelion (point farthest from the Sun), the comet is 5.1 1012 m from the Sun and moves with a speed of 12.0 km/s. What is the comet's speed at its perihelion (closest approach to the Sun) where its distance from the Sun is 8.9 1010 m?
Km/s

If I were given this question, I'd probably try to set up an equation where gravitational energy and kinetic energy were equal at both sides. However, wikipedia is your friend, and has equations for orbital velocity:

Ta Dah (http://en.wikipedia.org/wiki/Orbital_speed)

Try using these formulas, and if you're still unsure, post your answer here for others to review.