Simultaneous data returned by the Apollo Suprathermal Ion Detector Experiments are combined to yield information on the properties of plasmas in the magnetosheath at lunar orbit. An interface between the magnetosheat and the plasma sheet has been identified. This layer is characterized by a systematic change in the magnetosheath flow properties in the vicinity of the magnetopause. The flow is observed to turn inward toward the plasma sheet and become thermalized, indicating that the magnetosheath particles can gain access to the plasma sheet at lunar distances. this layer, which we term the plasma sheet boundary layer (PSBL), is believed to be the extension of the magnetospheric boundary layer (also called plasma boundary layer) observed on the dayside. Rough order of magnitude calculation shows that the flux crossing the PSBL is capable of supplying the roughly 5¿1025 particles/s necessary for maintenance of the plasma sheet. Measurements of the thickness of the plasma sheet boundary layer, the magnetic field, and the plasma flow velocity allow an estimate of the potential drop across this layer of 4.2 kV. Since this is much less than the potential drop of opposite polarity across the balance of the tail, we conclude that the magnetosphere is open.