Midcourse correction: a small burn, usually only a few seconds long, that adjusts the trajectory to remove aiming errors accumulated since TLI.
- Small errors in the TLI burn, plus the uneven pull of Earth's gravity, mean the spacecraft will miss its intended aim point by some amount. A midcourse correction fixes that.
- It is a discrete event, not continuous steering. The spacecraft does not fly with its engine throttled up to hold a line; it coasts, drifts slightly off, then nudges back.
- Because the burn is tiny, its cost in propellant is tiny. Continuous steering would spend propellant every second of the trip.
Key point: corrections are rare and small by design, so they barely touch the fuel budget.