1. A fault detector for an attitude control system (ACS) of a spacecraft including a plurality of actuators and a plurality of sensor modules, comprising: a federated Kalman filter (FKF) fault detection block including: a plurality of local filters for receiving data from the plurality of sensor modules to calculate an estimation value of an attitude model of the spacecraft, a prediction filter of an attitude of the spacecraft, a sensitivity factor calculation part for calculating sensitivity factors of the respective local filters using data from the plurality local filters and data from the prediction filter, and a comparison part for comparing the sensitivity factors of the respective local filters, wherein the FKF fault detection block transmits data of the sensor module used in the local filter having a smallest sensitivity factor to an upper level filter of a first interacting multiple model (IMM) fault detection block; the first interacting multiple model (IMM) fault detection block for generating a plurality of upper level filters each including normal model filters of the plurality of actuators and 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, wherein the IMM estimation technique uses a penalty upon calculation of reliability of each model filter.