April 13, 2024

Solar Orbiter prepared for the ‘worst case scenario’

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Tracking sunspots up close. Credit: European Space Agency

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Tracking sunspots up close. Credit: European Space Agency

The ESA/NASA Solar Orbiter is approaching the closest point to the Sun in its current orbit. It is an important time for the mission’s scientific activities and ESA’s mission control team is constantly preparing for any possible problems the spacecraft may face as it passes by our active and unpredictable star.

Mission control for Solar Orbiter. Enter Solar Orbiter…

“It’s our worst-case scenario,” says flight controller Daniel Lakey. “If Solar Orbiter were to experience any serious problems on board, we would not be able to restore communications.”

Solar Orbiter’s approach to the Sun (“perihelion”) is a period of peak scientific activity.

It requires flight control teams and flight dynamics experts at ESA’s ESOC mission control center to carry out a series of highly complex operations.

If something goes wrong during these activities, the spacecraft can automatically restart in “safe mode.”

In safe mode, the spacecraft’s software is restarted and only its most basic functions are reactivated. Teams on Earth then figure out what triggered safe mode, fix the problem, and restart more advanced systems like scientific instruments.

A safe mode during perihelion would be particularly bad given the severe impact on scientific operations during this busy period.

The Solar Orbiter also has less energy available during perihelion, as the intense heat requires it to angle its solar panels away from the sun in order to avoid damage.

The spacecraft must be recovered as quickly as possible before the science is lost or, worse, runs out of power.

The stars guide the way

“The sun is so bright that even a basic solar sensor is enough to ensure that Solar Orbiter always knows where the sun is and can always point its heat shield at it. This sensor activates during safe mode and keeps the systems interiors of the spacecraft protected from the radiation emanating from our star,” says Lakey.

“So we know that Solar Orbiter will always point its ‘front’ towards the Sun. But to find out which way it is ‘up’, we rely on star trackers.”

The top priority for a spacecraft in safe mode is to point its communications antenna at Earth and reestablish contact as quickly as possible.

Star trackers are automatically activated during safe mode and the spacecraft uses them to recognize certain star patterns. It can then determine its orientation and in which direction it should point its antenna to communicate with Earth.

“But if the star trackers can’t locate the right stars, or if the recovery sequence is interrupted before they can be turned on, Solar Orbiter will have no way of knowing where Earth is.”

The Solar Orbiter spacecraft during tests carried out in December 2018 in the thermal vacuum chamber at the IABG facility in Ottobrunn, Germany. Powerful lamps simulate solar radiation to demonstrate that the spacecraft can sustain the extreme temperatures it will encounter near the sun. Credit: European Space Agency

Spinning on control

To make the situation even more challenging, in safe mode, the Solar Orbiter can only use its backup communications antenna.

The backup antenna can move “up and down” on one axis, but not “left and right” on the other. This avoids a number of potential complications, but it also means that the entire spacecraft must rotate to point the antenna in certain directions.

The solution is “strobe” – if Solar Orbiter ever finds itself in safe mode and unable to locate Earth, it will begin to rotate on an axis while keeping its heat shield pointed safely at the sun.

“In strobe mode, Solar Orbiter emits a signal with a special ‘tone’ – a beacon in the darkness of space,” says Lakey.

“Eventually, this signal will sweep across Earth. Once we detect it at one of our ground stations, we will be able to assess the situation, figure out what caused the safe mode, and perform our troubleshooting and recovery operations.”

That’s the theory, anyway. During Solar Orbiter’s four years in space, it never needed to rely on stroboscopic recovery – and it was never tested in flight.

Until now.

ESOC teams took advantage of a recent period of low communication delay with Solar Orbiter to test whether they are ready to handle a real stroboscopic recovery.

“We started rotating the Solar Orbiter and seeing if we could detect the backup antenna beacon,” says Lakey. “We preloaded commands to return to normal operations if we were unable to detect it, so there was never any risk to the spacecraft.”

Recovery tests were a success. The teams confirmed that they could detect Solar Orbiter’s emergency beacon and identify the spacecraft’s status in the event of a safe mode with faulty star trackers.

These are vital first steps towards regaining control of the spacecraft and demonstrate the team’s readiness for this critical but unlikely scenario.

“We have also successfully tested our ability to communicate with the satellite in particularly complicated situations, such as when its own heat shield partially obscures the antenna’s view of Earth.”

This is just one of hundreds of potential problems that our teams dream about and plan for every day. ESA missions are unique spacecraft: they can face problems that no other spacecraft has ever faced.

There are few similar examples from which we can learn and few established procedures to follow. It is essential to test our spacecraft recovery operations in space and for teams on Earth to practice them when they have a good opportunity.

“We will never stop thinking about the new challenges our missions may face,” says Lakey. “Or about how we were going to overcome them.”

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