The gravitational potential energy of a satellite–planet system is defined to be zero when the objects are separated by an infinite distance. A probe is launched vertically upward from the surface of a moon with exactly the escape speed. Which statement best describes the total mechanical energy of the probe–moon system as the probe travels away from the moon?
(a) It increases from negative to zero as the probe moves farther away.
(b) It is a negative constant because gravitational potential energy is always negative.
(c) It remains zero at all distances because the probe was launched with the escape speed.
(d) It is a positive constant because kinetic energy is always positive.