1. A fault detector for an attitude control system (ACS) of a spacecraft including a plurality of actuators, comprising: a first interacting multiple model (IMM) fault detection block for generating a normal model filter of the plurality of actuators and a plurality of upper level filters including fault model filters corresponding to the respective actuators, and detecting faults of the plurality of actuators using an IMM estimation technique from the plurality of upper level filters; and a second IMM fault detection block for generating a plurality of lower level filters each including a fault type model filter of the fault-detected actuator in the first IMM fault detection block, and detecting a fault type of the failed actuator using the IMM estimation technique.