On September 22, at around noon, ESA’s Integral probe went into emergency safe mode. One of the spacecraft’s three active “reaction wheels” extinguished without warning and stopped spinning, causing a ripple effect that meant the satellite itself began to spin.

As a result of the spacecraft spinning, the data only reached ground control unevenly, and the batteries quickly drained. With only a few hours of battery life remaining, it seemed possible that the 19-year-old mission could be lost.

The full flight control team, as well as the flight dynamics and ground station teams of ESOC mission control from ESA, the teams from ESAC and from Airbus Defense & Space, focused on job. With quick thinking and ingenious solutions, they found the problem and saved the mission.

What on Earth?

A Single Event Upset (SEU) occurs when a charged particle hits a sensitive part of electrical equipment, causing a one-time “change of state” that disrupts its operation. These charged and “ionized” particles often originate from the Sun when it spits out matter and energy during solar flares or coronal mass ejections.

“I don’t think the SEU on this occasion was caused by our sometimes cranky local star. This strike occurred on a day when no relevant space weather activity was observed,” said Juha-Pekka Luntama, manager. of ESA space weather.

“From a discussion with our colleagues on the flight control team, it appears the anomaly was triggered by charged particles trapped in radiation belts around Earth.”

Van Allen’s radiation belts are two donut-shaped regions surrounding the Earth, where energetic charged particles are trapped inside the Earth’s magnetic field. Their properties vary according to solar activity and they represent a danger to satellites and humans in space that pass through them. Since the lowest point of Integral’s orbit is only 1,500 km from the Earth’s surface, the spacecraft passes through both radiation belts of its orbit.

“Darmstadt, we have a problem”

Integral uses “reaction wheels” – wheels that store energy as they spin – to subtly control the direction the spacecraft points without the need for thrusters.

Suddenly one of those reaction wheels stopped, and due to the law of conservation of energy, that rotational force previously in the wheel had to go somewhere else – the whole spacecraft. The spacecraft began to spin, triggering an Emergency Safety Attitude Mode which, unfortunately, due to a previous failure, was no longer reliable and failed to stabilize the mission.

The reaction wheel was reactivated by ground crews, but the spacecraft continued to spin at an average speed of about 17 degrees per minute (about one spin every 21 minutes), while oscillating unpredictably around its axes. It might not seem like much, but the spaceship was spinning at five times its maximum when under control.

“The data coming from Integral was choppy, arriving for short periods of time due to its rotation. This made the analysis even more difficult, ”explains Richard Southworth, Mission Director of Operations.

“The batteries were discharging, as there were only short periods of charge when the panels briefly faced the sun.”

The first challenge was to reduce Integral’s energy consumption to save more time. The first estimates of the charge remaining before the failure and loss of the satellite were only three hours. Gradually, turning off various non-critical instruments and components, it grew to over six hours. Next step: stop spinning.

With the support of industry experts, the ESOC team analyzed the condition of the reaction wheels, proposing a series of controls to change their speed and brake the spinning satellite. By late afternoon the commands were sent and immediately succeeded, but another three long hours passed before the satellite was fully under control and out of immediate danger.

Integral’s Apollo 13 Moment

“Everyone breathed a huge sigh of relief. It was very close, and we were immensely relieved to get the spacecraft out of this ‘near-death’ experience, ”recalls Andreas Rudolph, head of the Astronomical Missions Division of the Mission Operations Department. ESOC.

“Most of the control team were working from home at this point – I was following operations from the train!

Unfortunately, a few hours later, as the team reconvened to discuss next steps, the spacecraft began to spin again, its reaction wheels spinning again at high speed. The reason for this is still not fully understood, but it is believed to be associated with “star tracker occultation” or “blindness” that was not handled properly by the control systems of the. satellite – effectively when the Earth interferes with the spacecraft’s view of the stars, which it uses to orient itself.

The team repeated the steps from the previous days to stabilize the spacecraft and return to a solar pointing position, this time without hindering the star trackers. The recovery only took a few hours, putting into practice the lessons learned from the first time.

Integral has since remained under control and as of September 27 all systems are back online. Since October 1, after a long check, its instruments are back to observe the high energy universe.

One of Integral’s first goals will be to observe massive stars in the Orion region and study the impact on their environment when they become supernovae.

“We have also reverted to ‘target of opportunity’ observations, which means Integral is reacting quickly again to study unexpected explosive events in the Universe,” said Erik Kuulkers, ESA project scientist for Integral.

A pushing problem

This is not the first time that this nearly 20-year-old mission has scared the control team at ESA’s ESOC operations center. Last year, Integral fired its thrusters for the last possibly scheduled time, after a failure in its propulsion system.

It was this deficient propulsion system that meant that a normally rectifying Safe Mode was ineffective on this occasion. With the mode now deactivated, the control team is working on a new automatic rescue sequence that should mimic many operations performed after this anomaly, but much faster.

When the propulsion system failed, the team realized that they would have to learn how to maneuver the four-ton satellite using only its highly sensitive reaction wheels, to discharge energy at regular times and counter forces on the spacecraft, including the slight push of sunlight. . It was a solution that had never been tried before.

“I didn’t believe it was possible at first. We checked with our colleagues the flight dynamics and the theory said it would work. After doing a simulation, we tested it on the spacecraft. worked, ”says Richard.

“Thanks to our quick-witted team and the help of experts from all industries, Integral lives on. Almost two decades old, he far exceeds expectations for what was meant to be a mission to five years.”