The law of physics in control of a spacecraft cruising through space at constant speed without using any fuel is Newton's First Law of Motion, also known as the law of inertia.

Here's why:

* Newton's First Law: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

In this scenario:

* The spacecraft is already in motion.

* There are no significant external forces acting on it (like friction from air resistance or gravity from a nearby celestial body).

* Therefore, the spacecraft continues moving at a constant speed in a straight line.

Important Note: While Newton's First Law explains the constant speed, it doesn't explain how the spacecraft got to that speed in the first place. Getting the spacecraft moving initially required the use of fuel and propulsion.