Here's why:

* Mechanical energy is the sum of an object's kinetic energy (energy of motion) and potential energy (energy due to its position in a gravitational field).

* Specific mechanical energy is the mechanical energy per unit mass.

In a stable orbit, the object is constantly exchanging kinetic and potential energy.

* As it moves closer to the Earth (lower potential energy), it speeds up (higher kinetic energy).

* As it moves further away from the Earth (higher potential energy), it slows down (lower kinetic energy).

The total mechanical energy, however, remains constant. This is due to the conservation of energy principle, which states that energy cannot be created or destroyed, only transformed from one form to another.

Therefore, the specific mechanical energy is a constant of motion for an object in orbit.