ELEVEN YEARS OF AGEING OF SSM TEFLON ON THE SUNSYNCHRONOUS ORBIT - SPOT

摘要

The SPOT 1 spacecraft has been launched in February 1986 and is still fully operationnal after eleven years of life. SPOT 1 and its brothers SPOT 2 (6 years old), SPOT 3 (3 years old) and its cousins ERS1, ERS2, HELIOS I have allowed us to acquire a good knowledge of SSM (aluminised Teflon) behaviour in such sunsynchronous orbit at 800 km and for solar hour close to noon. SSM is used as thermal coating for radiators. The technological experiment THERME, on SPOT 2 and SPOT 3 board is specificaly devoted to fine measurement of Teflon-SSM and Kapton ageing. Here are briefly described the spacecrafts, the associated thermal control system and the THERME experiment. The variations of the radiators temperatures since the beginning of life are presented and the variation of the thermo-optical characteristics is deduced from them The observed behaviour is not as assumed before the launch of SPOT 1. The slow rising of the temperature shows the ageing of the materials i. e. the increase of the solar absorptivity of the coatings. This ageing is very smooth but not regular. For some years we see a measurable degradation, and in some others critical periods the coatings seems to rejuvenate and the thermal control is looking younger. It is shown that this evolution is very dependant of the atomic oxygen effects. Indeed we notice the effect on this behaviour of the position of the radiator versus the orbital movement of the spacecraft. On the other hand we point the strongly dependency of the ageing of the eleven years solar cycle which modulate the oxygen density at the orbit level. In spite of the very low atomic oxygen density on such orbits, the effect on the materials and coatings is very sensitive and must be taken into account forthe thermal control design. On the leading edge of the spacecraft the degradation of the Teflon-SSM is twice lower than on the trailing edge and in the maximum solar activity periods this coating is turned new. The erosion of the kapton films of the MLI blankets is an important phenomenum, because it could be, for a long time, an important and very dangerous source of contamination of the orbits by millions of small space debris. Globaly, the ageing of the SSM coating is very slow and moderate after ten years it provides a very good thermal behaviour of all the spacecrafts of the big SPOT family on heliosynchronous orbits.