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The Moon is generally thought to have formed from the debris ejected by theimpact of a planet-sized object with the proto-Earth towards the end ofplanetary accretion. Modeling of the impact process predicts that the lunarmaterial was disaggregated into a circumplanetary disk and that lunar accretionsubsequently placed the Moon in a near equatorial orbit. Forward integration ofthe lunar orbit from this initial state predicts a modern inclination at leastan order of magnitude smaller than the lunar value, a long-standing discrepancyknown as the lunar inclination problem. Here we show that the modern lunarorbit provides a sensitive record of gravitational interactions withEarth-crossing planetesimals not yet accreted at the time of the Moon-formingevent. The excited lunar orbit can naturally be reproduced via interaction witha small quantity of mass (corresponding to 0.0075-0.015 ME eventually accretedto the Earth) carried by a few bodies, consistent with constraints and modelsof late accretion. While the process has a stochastic element, the observedvalue of the lunar inclination is among the most likely outcomes for a widerange of parameters. The excitation of the lunar orbit is most readilyreproduced via collisionless encounters with strong tidal dissipation on theearly Earth. This mechanism obviates the necessity of previously proposed (butidealized) excitation mechanisms and can constrain the planet formation contextof the Moon-forming event and the pristineness of the dynamical state of theEarth-Moon system.

Collisionless Encounters and the Origin of the Lunar Inclination