Three Hours To Rescue Spacecraft From “Death”

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ESA Integral

The task of Integral, ESA’s International Gamma-Ray Astrophysics Laboratory, is to gather the most energetic radiation that comes from space. Credit: ESA. Illustration by D. Ducros

On September 22, around midday, ESA’s Integral spacecraft went into emergency Safe Mode. One of the spacecraft’s three active ‘reaction wheels’ had turned off without warning and stopped spinning, causing a ripple effect that meant the satellite itself began to rotate.

As a result of the spacecraft turning, data were only reaching ground control patchily and the batteries were quickly discharging. With just a few hours of power left, it seemed possible that the 19-year-old mission could be lost.

Integral Spacecraft

The task of Integral, ESA’s International Gamma-Ray Astrophysics Laboratory, is to detect and gather the most energetic radiation that comes from space. The spacecraft was launched in October 2002 and is helping solve some of the biggest mysteries in astronomy. Credit: ESA/D. Ducros

The Integral Flight Control Team, together with Flight Dynamics and Ground Station Teams at ESA’s ESOC mission control, teams at ESAC and Airbus Defence & Space, set to work. With quick thinking and ingenious solutions, they found the problem and rescued the mission.

What on Earth?

A Single Event Upset (SEU) occurs when a charged particle strikes a sensitive part of electrical equipment, causing a one-off ‘change of state’ that disrupts its functioning. These charged, ‘ionized’ particles often come from the Sun when it spews out matter and energy during solar flares or coronal mass ejections.

Rescuing Integral Infographic

Three hours to save Integral – what happened? Credit: ESA

“I don’t think that the SEU on this occasion was caused by our local, occasionally grumpy star. This strike happened on a day when no relevant space weather activity was observed,” explains 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.”

The Van Allen radiation belts are two doughnut-shaped regions encircling Earth, where energetic charged particles are trapped inside Earth’s magnetic field. Their properties vary according to solar activity and they represent a hazard to satellites and humans in space that pass through them. Because the lowest point of Integral’s orbit is now only 1500 km from Earth’s surface, the spacecraft passes through both radiation belts in 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 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 stabilize the mission.

Integral Flight Control Team

The Integral Flight Control Team set to work rescuing the mission. Credit: ESA

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 analyzed 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.

Integral’s ‘Apollo 13’ moment

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

“Most of the Control Team were working from home at this point – I was following operations from the train! –…



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