The European Space Agency (ESA) revealed on Monday that its 19-year-old International Gamma-Ray Astrophysics Laboratory (INTEGRAL) had a near-death experience last month when failure of a small yet significant part caused it to spin uncontrollably and prevented its solar panels from generating power.
According to ESA's blog, one of the scope's three active 'reaction wheels' – flywheels that help to stabilise attitude – turned off without warning. Absent the reaction wheel's energy, INTEGRAL rotated dangerously.
The ESA activated Emergency Safe Attitude Mode, but that was ineffective because a July 2020 failure had left the geriatric satellite's thrusters inoperable.
ESA boffins were therefore presented with the challenge of sorting things out despite a patchy data connection from the satellite to ground control, and batteries with just three hours of charge remaining.
The team was able to reactivate the malfunctioning wheel, but the spacecraft just kept wobbling about its axis. The groundside crew deftly turned off non-critical components to buy more time and sent commands to the reaction wheels they hoped would stop the spin.
The team eventually regained control.
Then they lost control, again. The team speculates the second loss was due to the Earth blocking INTEGRAL’s view of the stars that it uses to orient itself. Thankfully, a repeat of the corrective actions repaired the spin again.
To make matters more fascinating, the rescue happened while most of the control team were working from home – thereby presenting the best argument for ongoing remote work, for every job, forever.
Erik Kuulkers, ESA's project scientist for INTEGRAL said the satellite has returned to studying "unexpected explosive events in the Universe".
ESA boffins have fingered a random charged particle hitting INTEGRAL's electricals and sparking a changed state as the possible cause of the malfunction.
"It looks like that the anomaly was triggered by charged particles trapped in the radiation belts around Earth," said Juha-Pekka Luntama, ESA's Head of Space Weather.
Integral's highly elliptical orbit puts its perigee smack dab in the middle of the Van Allen radiation belt and its charged particles held in place by Earth's magnetic field.
The continual belting INTEGRAL takes from such particles, over the years, caused degradation of the telescope's solar arrays.
Keeping a satellite going for two decades is quite a feat. In June of 2020, missions operations manager Richard Southworth detailed a laundry list of challenges the team faces keeping the spacecraft functioning to The Register, including degrading power output and many one-off failures over the years.
But for a spacecraft that has lasted decades beyond its original five-year mission, it's doing very well indeed – living long, and even prospering. ®