Navigating space requires precise velocity measurements, a challenge absent on Earth with its readily available visual cues and GPS. Astronauts rely on physics principles, understanding that velocity is speed in a specific direction, crucial for missions like reaching Mars or lunar landings. Two primary methods are employed: Doppler effect and inertial measurement. The Doppler effect utilizes radio waves reflected off a spacecraft, measuring frequency shifts to determine speed relative to Earth. Inertial measurement relies on accelerometers and gyroscopes to track changes in velocity, which, when integrated with an initial velocity, can calculate current speed. However, inertial systems accumulate errors over time, necessitating recalibration using external references like stars, which provide positional data to infer velocity through repeated measurements.