In free space, with no external forces acting on an object, it will continue to move at a constant speed in a straight line. This is due to Newton's First Law of Motion, also known as the Law of Inertia.

Here's why:

* Inertia: Every object has a natural tendency to resist changes in its motion. This resistance to change is called inertia.

* No Net Force: In free space, there's no air resistance, friction, or gravity from other celestial bodies. This means there's no net force acting on the object.

* Constant Velocity: When the net force on an object is zero, it remains at rest or continues to move at a constant velocity (constant speed and direction).

In essence, there's no force "keeping" the object moving at a constant speed; it's the absence of forces that allows it to maintain its motion.