A payload of mass \(m\) is released from rest and falls vertically through the atmosphere. The magnitude of the resistive force exerted on the payload by the air is modeled by \(F_D = bv^2\), where \(b\) is a positive constant and \(v\) is the payload's instantaneous speed. After falling for a sufficient time, the payload reaches a constant terminal speed \(v_t\). Which of the following statements correctly describes the total mechanical energy of the payload-Earth system while the payload falls at this terminal speed, and provides the correct justification?