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.
