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. This is due to the absence of external forces acting on the spacecraft to change its velocity. According to Newton's first law of motion (the law of inertia), an object will continue to move at a constant velocity unless acted upon by an external force. In this case, since there is no friction or other forces present, the spacecraft would maintain its initial speed indefinitely, assuming no propulsion or external influence is applied.