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The SMOS Satellite was launched on 2nd November 2009, being the second Earth Explorer Mission of the European Space Agency. The SMOS Payload is a single instrument, the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), which is basically composed of 69 antennas distributed in a Y-shape configuration (3 deployable Arms with 3 individual Segments each plus a central body which is called Hub).To ensure the proper operation of the Payload, specific thermal requirements were set for the receivers of the 69 antennas in order to guarantee the validity of the scientific data. In particular, the target temperature of the receivers is 22°C with a maximum spatial gradient of 6°C among all of them and a maximum orbital excursion of 4°C. The SMOS Payload Thermal Control Subsystem (TCS) is based on a passive design supported by heaters. The TCS, as well as the rest of the Payload design, has a distributed architecture. The central computer of the Payload controls in closed loop remotely distributed units (12 in total) named Control and Monitoring Node (CMN) units. Each CMN unit acquires the telemetry of the temperature sensors (6 per heater line) for the heater control lines distributed in the Arms and in the Hub. The Thermal Control is enabled during all payload operational modes including measurement and calibration.We present in this paper some features of the TCS observed during more than four years in orbit: fulfillment of target temperature requirements, seasonal effects, anomalies seen in-flight and recovery from failure states. In particular, a failure due to an anomalous temperature reading in one of the CMN units that happened in flight and produced an abnormal heating in one of the Arm Segments is analyzed in detail. This failure may eventually lead to the in-operability of the Payload. However, it is shown that it is possible to implement an "Alternative Thermal Control" for one sector of the instrument. The Alternative Thermal Control produces only minimal changes for the thermal control of the Segment and it is acceptable for the quality of the scientific data.

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