Question

which statement describes what objects in space that are moving at a constant velocity in a straight line do?
they keep moving at a constant velocity until some force acts on them.
they require constant energy to keep moving
they stop on their own when they run out of energy.
they always move in an orbit.

Explanation:

According to Newton's first law of motion (the law of inertia), an object in motion will stay in motion with a constant velocity (in a straight line) unless acted upon by an external force. Objects moving at a constant velocity in a straight line do not require constant energy to keep moving (as per the law of inertia, no force is needed to maintain constant - velocity motion in the absence of external forces), they don't stop on their own when they run out of energy (because there is no such "energy running out" in the context of inertia without external forces), and they don't always move in an orbit (orbital motion is a result of a centripetal force, which is not relevant to straight - line constant - velocity motion).

Answer:

They keep moving at a constant velocity until some force acts on them.