Why do spacecraft need to make course corrections during a journey through space?

Spacecraft require course corrections because travel in space is rarely a straight line or at a constant velocity due to the complex gravitational influences of celestial bodies and other external forces.1

Navigation systems, such as inertial guidance, are subject to tiny, cumulative errors over long distances that necessitate adjustments to maintain the intended trajectory. Furthermore, spacecraft are subject to perturbations—deviations from an ideal orbit caused by factors like the non-spherical shape of planets (such as Earth's equatorial bulge), gravitational interactions between multiple bodies (the three-body problem), and even radiation pressure.123

To correct these deviations, spacecraft use onboard thrust motors to adjust their velocity and attitude. These maneuvers are essential for tasks such as entering orbit around a target body, maintaining a specific position (like geostationary satellites), or performing rendezvous and docking operations. These corrections are guided by star trackers, GPS data, or commands from human flight controllers on Earth.123