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How an ESA Team Saved a Satellite From Disaster
How an ESA Team Saved a Satellite From Disaster

The flight control team for the European Space Agency’s (ESA) Integral satellite did not expect a crisis. Around midday on September 22, one of the satellite’s three ‘reaction wheels,’ devices that subtly control its attitude without the need for thrusters, inexplicably ceased spinning, causing the entire spacecraft to begin rotating out of control.

The first safeguard, an Emergency Safe Attitude Mode that was triggered automatically, failed to stabilize Integral because of a dysfunction the prior year. As it kept turning, the satellite only transmitted patchy sets of data back to Earth and its solar panels caught brief glimpses of sunlight. Power drained rapidly, the batteries had three hours left to live and it looked to ground control as if the 19-year-long mission might come to an end.

In 2002, the ESA launched Integral principally to study violent explosions known as gamma-ray bursts, powerful phenomena such as supernova explosions and regions in the Universe thought to contain black holes.

When the COVID-19 pandemic struck Earth, Integral’s flight control team retreated to their homes and began working remotely. In spite of this, they leaped into action on September 22. First, they wrenched back control of the reaction wheel. The satellite, however, continued to rotate. Next, they shut off non-critical components and other instruments to save energy and buy time. On the countdown clock to mission blackout, three hours thankfully became six. They now had time to breathe, and more importantly, to think.

Through the late afternoon, Integral’s team consulted industry experts and analyzed the reaction wheels, devising a series of commands to change their speed and stop the spinning. It took three more hours to bring the satellite fully under control and out of immediate danger.

How an ESA Team Saved a Satellite From Disaster
Andreas Rudolph

“Everyone breathed a huge sigh of relief,” said Andreas Rudolph, Head of the Astronomy Missions Division in ESOC’s Mission Operations Department. “This was very close, and we were immensely relieved to get the spacecraft out of this ‘near-death’ experience. Most of the Control Team were working from home at this point – I was following operations from the train! – and worked until four in the morning to get the spacecraft fully stable, back into position and facing the Sun to recharge its batteries.”

As the team regrouped a few hours later, the mysterious rotation returned from the grave. All three reaction wheels cranked into high gear. By this time, it was the next day, September 23, but the team quickly repeated the previous routine, using the lessons they had learned. Integral was back under control in only a couple of hours.

Since the crisis, Integral has functioned normally, and from September 27 all systems were back online. On October 1, it resumed its exploration of the universe’s black holes, bursts and explosions.

How an ESA Team Saved a Satellite From Disaster
Three hours to save Integral – what happened?/ESA

Integral’s orbit passes through the Van Allen radiation belts, two doughnut-shaped regions encircling Earth, where energetic charged particles are trapped inside Earth’s magnetic field. ESA believes the initial anomaly on September 22 was caused when one of these particles struck a sensitive piece of the satellite’s electrical equipment.

How an ESA Team Saved a Satellite From Disaster
Juha-Pekka Luntama

“This strike happened on a day when no relevant space weather activity was observed,” said Juha-Pekka Luntama, ESA’s Head of Space Weather. “Based on a discussion with our colleagues in the Flight Control Team, it looks like that the anomaly was triggered by charged particles trapped in the radiation belts around Earth.”

As for the second anomaly on September 23, ESA is not certain of the cause, but suspects that Integral was effectively “blinded” by the Earth, as the planet blocked its view of the stars it uses to orient itself.

How an ESA Team Saved a Satellite From Disaster
Richard Southworth

“Thanks to our quick-witted team and the help of experts from across the industry, Integral lives on,” said Richard Southworth, Operations Manager for the mission. “Almost two decades old, it is far outliving expectations for what was meant to be a five-year mission.”

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