The payload adapter was the physical structure used to connect the orbiter to the launch vehicle.
MRO's Atlas V Launch Vehicle, Payload Adapter
Mars Reconnaissance Orbiter's payload adapter is the physical structure used to connect the orbiter to the launch vehicle.
The spacecraft was mounted on top of the Centaur using a 1,194-millimeter-diameter (47-inch) Payload Adapter. A clamp band secured the spacecraft to the launch vehicle. When the launch vehicle mission was complete, the Centaur released the clamp band and the spacecraft separated from the launch vehicle using "push-off" springs.
The payload interface provided the electrical interface between the orbiter and the launch vehicle.
During pre-launch and launch, the spacecraft communicated with the launch vehicle and the ground through wiring that disconnected when the spacecraft separated. This wiring permitted final checkout of the spacecraft before launch and provided telemetry data and other signals to the launch vehicle. The launch vehicle transmitted this information to engineers all across the country who were monitoring the launch.