How does Newton’s first law affect a rocket?

Hint: A rocket will only travel in a straight line at a constant velocity if there are no outside, unbalanced forces acting on it.
Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.

Complete step by step answer:
Newton's first law says that an object if it is travelling in a straight line will travel in a straight line at a constant velocity unless it is acted upon by an outside, unbalanced force. This force can have different effects on the body; it can accelerate, decelerate, stop it and even can change its direction.
There is a special case when the velocity is zero, that is the object is at rest, then the object will remain at the same position with zero velocity unless acted upon by an outside, unbalanced force.
As we know there is absolute vacuum ion space i.e., no friction, no air drags no external force.
For a rocket, this means that if it is far from any star or planet and has its engines off, it will travel in a straight line at a constant velocity forever.
If the engines are on or if the rocket feels the force of gravity from a planet or if there is drag from the air, the rocket will no longer have a constant speed and direction. This means that if we want to slow your rocket down, we will also need to apply a force opposite to our direction of motion.

Note:
• If the rocket wants to fly forever with its engines off it should be away from stars and planets because the gravitational; forces of these can act as unbalanced force ad can affect the flight
• On earth when we roll a ball and even if we don’t try to stop it but it automatically stops because the force of friction is acting against the motion
• If there is absolute vacuum and no external unbalanced force then the ball could have moved on forever.