If a spacecraft ventured beyond Earth's atmosphere and continued to move at a constant velocity in a frictionless environment, its speed would remain constant over time.
In the absence of external forces, such as friction or propulsion, an object will continue to move at a constant velocity according to Newton's first law of motion, also known as the law of inertia. This law states that an object will maintain its state of motion (including speed and direction) unless acted upon by an external force.
Once the spacecraft reaches its desired velocity and enters a frictionless environment, there are no forces acting upon it to change its speed. Therefore, the spacecraft would continue to move at a constant speed indefinitely, without any change over time.