Gravitational capture
- Inside the Moon's sphere of influence, the Moon's gravity bends the spacecraft's path into a closed curve around the Moon.
- That bend is a change in direction, and it comes from gravity, not from propellant.
- The spacecraft arrives with a large velocity relative to the Moon, so the path is still an open hyperbola until something changes the energy.
Why the burn is small
- Gravity has already done most of the work: it slowed the spacecraft on the way out and now curves the path.
- The remaining job is to remove the excess speed that would otherwise carry the spacecraft back out. That is what lunar orbit insertion (LOI) does.
- Because the required velocity change is small compared with the outbound burn, LOI is a short burn and the fuel gauge barely moves.
- This is the payoff of the whole design: the large velocity changes are supplied by gravity, and propellant is spent only where gravity cannot finish the job.