1. The problem statement, all variables and given/known data A spacecraft is in circular orbit around the earth. with respect to a point below it on earth, it is travelling towards the east with speed v. Specify the reference state for potential energy. 2. Relevant equations 3. The attempt at a solution the total energy is E=K+U = 1/2mv2-GmM/r but the reference state for potential energy i don't understand?