An apparatus and method for penetrating the bone marrow is provided. The apparatus includes a housing, a penetrator assembly, operable to penetrate the bone marrow, a connector operable to releasably attach the penetrator assembly to a drill shaft, the drill shaft operable to connect the penetrator assembly to a gear assembly, a gear assembly operable to engage and rotate the drill shaft, a motor operable to engage the reduction gear assembly and drive the penetrator into the bone marrow by rotation of the drill shaft, and a power supply and associated circuitry operable to power the motor. The apparatus and method may be adapted to insert a probe through the skull and into the brain.