Here's why:
* Inertia: An object at rest stays at rest, and an object in motion stays in motion at a constant speed and direction unless acted upon by a net force.
* Space is a vacuum: There's almost no friction or air resistance in space to slow the spaceship down.
However:
* Gravity: The spaceship is still affected by the gravity of nearby celestial bodies. This means it might be drawn towards a star, planet, or other object. This will cause the spaceship to change its velocity (speed and/or direction) over time.
* Other forces: Though minimal, other forces like solar wind pressure or collisions with micrometeoroids can also affect the spaceship's motion, but these are usually insignificant compared to the influence of gravity.
In short, a spaceship with its engines off will continue moving in a straight line at a constant speed unless acted upon by an external force, most notably gravity.
