© 1965-2011 IEEE. Recently, a variety of small spacecraft have been launched and used for interplanetary missions. Conventionally, reaction wheels (RWs) and thrusters are used for these attitude control systems in almost all interplanetary spacecraft. While these actuators are promising for attitude control, the lifetime of the spacecraft mission is limited by the extra fuel needed for the thrusters. Moreover, it is difficult to install thrusters in all small spacecraft due to low reliability and strict limitations on mass and power consumption. To obtain both fuel-free and available attitude control for small spacecraft, this study proposes an interplanetary magnetic attitude control system including attitude stabilization and angular momentum unloading based on an interplanetary magnetic field (IMF) Kalman filter. In the proposed method, an electromagnetic coil interacting with the IMF is used for an attitude control system. To achieve the proposed method, the faint magnetic field must be detected. However, the IMF is too weak to sense using only on-board magnetic sensors. To deal with the technical issue, this study proposes a magnetic attitude control system with an unscented Kalman filter using gyro measurements and generated magnetic moment by the electromagnetic coil to estimate the weak magnetic field. With the estimated magnetic field vector, the spacecraft can achieve fuel-free attitude stabilization and RW unloading under the constraints. This proposed system does not require fuel for attitude control. Furthermore, the simple structure and electronic circuits of the electromagnetic coil allow the spacecraft to achieve a simple and reliable attitude control system. Numerical simulations demonstrate the effectiveness of the proposed attitude stabilization and RW unloading methods.