INTEGRAL Revisits Earth - Low Perigee Effects on Spacecraft Components

Since its launch in 2002, ESA's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) has been observing astronomical objects simultaneously in gamma-rays, Xrays and visible light. Using its four instruments, it provides insight into the nature of sources of extremely high-energy radiation, and 9 years after its launch the scientific interest is still very high. The performance of INTEGRAL has far exceeded design specifications. To date, it is still the most sensitive gamma ray observatory in space, all prime units are still in use, no major failures have occurred and the degradation of spacecraft components is minimal. Last but not least, the nominal lifetime of two years has already been far exceeded, and the remaining fuel on board still allows for another decade of operations. Major challenges the mission faces are the orbital evolution and the aging of spacecraft components: on October 25th 2011, INTEGRAL reached the lowest perigee altitude since its launch. This resulted in both increased exposure to proton radiation during perigee passage and an increased heating of the spectrometer’s cryocooler due to the Earth albedo, leading to a small reduction in payload performance. Furthermore, all electrical components will have reached their total dose qualification limit soon. This paper summarizes the effects of the period of low perigee altitude on the degradation of the materials and spacecraft components. Furthermore, it presents the associated countermeasures, operational strategies as well as the impact on the fuel consumption and the science. Finally, the prospects for future INTEGRAL operations are summarized along with the lessons learned from the period of low perigee.