INTEGRAL Picture Of the Month
November 2021

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Approaching 19 years old, INTEGRAL got dizzy and entered its 9th emergency safe attitude mode

On 22 September, one of the reaction wheels onboard INTEGRAL, used for attitude control, switched off due to a so-called "Single Event Upset" (SEU). Galactic cosmic rays, solar particules and highly energetic protons captured in the Earth vicinity can affect electronic components operating in space and disturb their functioning, and cause these SEUs. Out of the 17 safe modes experienced during the almost 40 years of combined operations of INTEGRAL and XMM-Newton, eight were due to SEUs, among which four affected a reaction wheel.

Now that INTEGRAL's fuel-based propulsion system has been declared unstable in 2020 (see INTEGRAL POM June and October 2020 ), the response to expect from the thrusters was unpredictable. A marginal firing of the thrusters was observed in the telemetry, but not enough to maintain the Sun-pointing attitude desired during such an emergency. Following the laws of physics, the angular momentum normally absorbed through the three active reaction wheels was redistributed and brought the satellite into rotation. As seen on the top subplot (showing the spacecraft spin rate for the three axes of the satellite), INTEGRAL was tumbling on each of the three axes, with up to 1000 degrees per hour in roll. Every hour, during three windows of about 10 minutes, the spacecraft had its back turned to the Sun. During these so-called blackouts, batteries took over to provide the power required to keep the essential electronics alive. At the time of the SEU, the satellite could survive on batteries only for 3 more hours; after that one would have lost the satelliute.

To save any minute of battery lifetime, all the instrument computers were switched off. After desaturating the wheels and cancelling the spin rates, a Sun-facing attitude was eventually acquired and the batteries could charge properly. After 17 hours of intense battling by the engineers at ESOC and industry experts, INTEGRAL was again stable. The recovery and the health checks to ensure platform and instrument health took a few more days to be completed. Actions are now being taken to ensure that any future loss of control can be recovered as well, despite the absence of a reliable propulsion system.

Image credit: Thomas Godard, XMM-Newton/INTEGRAL Spacecraft Operation Engineer, RHEA System GmbH, ESA/ESOC.

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