Three hours to save Integral – what happened?

Integral uses ‘reaction wheels’ – wheels that store energy as they spin – to subtly control the direction the spacecraft points in without the need of thrusters.

Suddenly, one of these reaction wheels stopped and, because of the law of conservation of energy, that turning force previously in the wheel had to go somewhere else – the entire spacecraft. The spacecraft began to spin, triggering an Emergency Safe Attitude Mode which unfortunately, due to a previous failure, was no longer reliable and did not manage to stabilise the mission.

The reaction wheel was reactivated by teams on the ground, but the spacecraft kept spinning at an average rate of about 17 degrees per minute (roughly one rotation every 21 minutes), as well as wobbling unpredictably about its axes. This may not sound like much, but the spacecraft was rotating at five times its maximum when under control.

“The data coming down from Integral was choppy, coming in for short periods due to it spinning. This made analysis even harder,” explains Richard Southworth, Operations Manager for the mission.

“The batteries were discharging, as there were only short charging periods when the panels briefly faced the Sun.”

The first challenge was to decrease Integral’s energy consumption to buy more time. First estimates of the charge remaining before blackout and the loss of the satellite was just three hours. Step by step, by turning off various instruments and non-critical components, this increased to more than six hours. Next step – stop the spinning.

With support from industry experts, the team at ESOC analysed the state of the reaction wheels, coming up with a series of commands to change their speed and brake the spinning satellite. By late afternoon, the commands were sent and immediately showed success, but another three long hours passed before the satellite was fully under control and out of immediate danger.