1. The problem statement, all variables and given/known data Halley’s Comet travels in a highly eccentric (non-circular) orbit. At its closest approach, it is about 9×10^7 km away from the sun and travels with a speed of 54.6 km/s. What is the comet’s speed when it crosses the orbit of Neptune, about 5 billion kilometres from the sun? 2. Relevant equations Conservation of Energy, Ei = Ef 3. The attempt at a solution I have made numerous attempts utilised conservation of energy because I don't see why we would need to bring angular momentum into it (given that we're not told that 5 billion km is the apopasis and therefore cant reduce it to mvr), why does my conservation of energy equation give me an answer of 8.3 km/s when the answer should be 0.8 km/s? Any help would be much appreciated. Whenever I insert the provided answer into the conversation of energy formula, it just doesnt work out.