Lunar orbit insertion (LOI) is the burn that turns a lunar flyby into a lunar orbit.
- By the time the spacecraft reaches the Moon, the Moon's gravity has already bent its path into a bound trajectory. The spacecraft is falling toward the Moon, not racing past it under power.
- LOI is a retrograde burn — it fires opposite the direction of motion — to remove just enough speed that the path closes into an ellipse around the Moon instead of opening back out.
- Because gravity supplied the bending, LOI is small compared with the translunar injection burn. The engine only has to trim the orbit, not create the capture.
- The burn is timed near closest approach (periapsis), where the spacecraft moves fastest, so the Oberth effect gives the most velocity change per unit of propellant.
- A useful way to see it: gravity does the capture, the engine only tidies the orbit.
Key idea: LOI is small because the Moon's gravity has already done the hard part.