PICARD Payload Thermal Control System: From the design to the flight performances

Abstract

PICARD is a spacecraft dedicated to the simultaneous measurement of the absolute total and spectral solar irradiance, the diameter, the solar shape, and to the Sun’s interior probing by the helioseismology method. The mission has two scientic objectives, which are the study of the origin of the solar variability, and the study of the relations between the Sun and the Earth’s climate. The spacecraft was successfully launched, on June 15, 2010 on a DNEPR-1 launcher. PICARD spacecraft uses the MYRIADE family platform, developed by CNES to use as much as possible common equipment units. This platform was designed for a total mass of about 130 kg at launch. This paper focuses on the TCS (Thermal Control System) design, testing, and rst in-orbit performances of the payload, which mainly consists in two absolute radiometers measuring the total solar irradiance, a photometer measuring the spectral solar irradiance, a bolometer, and an imaging telescope to determine the solar diameter and asphericity. Thermal control of the payload is fundamental. The telescope of the PICARD mission is the most critical instrument. To provide a stable measurement of the solar diameter to few milliarcseconds over three years duration of mission, telescope mechanical stability has to be excellent intrinsically, and thermally controlled. Current and future space telescope missions require ever-higher dimensionally stable structures. Main diculty was with respect to scientic performances to ensure the thermal stability of the instruments. Thermal control surfaces, and optics of the payload are exposed to space environmental eects including contamination, atomic oxygen, ultraviolet radiation, and vacuum temperature cycling. Environmental eects on the performance of the payload will be discussed.