Which of the following systems is in translational equilibrium?
a) A communication satellite moving at constant speed in a circular orbit around Earth
b) A pendulum bob at the highest point of its swing, momentarily at rest
c) A space probe far from all stars and planets, drifting at constant velocity with its engines off
d) A spacecraft drifting with its engines off while still within Earth’s gravitational field, moving in a straight line away from Earth