Proceedings Article | 21 May 2013
KEYWORDS: Space operations, Space telescopes, Sun, Telescopes, Contamination, Solar radiation, Control systems, Temperature metrology, Point spread functions, Mirrors
PICARD is a spacecraft dedicated to the simultaneous measurement of the absolute total and spectral solar
irradiance, the diameter, the solar shape, and to probing the Sun’s interior by the helioseismology method. The
mission has two scientific 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 design and testing of the TCS (Thermal Control System)
and in-orbit performance 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 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-more dimensionally stable structures. The
main scientific performance related difficulty was to ensure the thermal stability of the instruments. Space is a
harsh environment for optics with many physical interactions leading to potentially severe degradation of optical
performance. Thermal control surfaces, and payload optics are exposed to space environmental effects including
contamination, atomic oxygen, ultraviolet radiation, and vacuum temperature cycling. Environmental effects on
the performance of the payload will be discussed. Telescopes are placed on spacecraft to avoid the effects of the
Earth atmosphere on astronomical observations (turbulence, extinction, ...). Atmospheric effects, however, may
subsist when spacecraft are launched into low orbits, with mean altitudes of the order of 735 km.
