Guiding Aeolus' safe reentry

ESA’s wind mission has been orbiting our planet for nearly five years, well outliving its design life. Its remarkable Aladin instrument has beamed down seven billion pulses of UV light to profile Earth’s wind.

Although Aeolus was designed as a research mission and to demonstrate novel technology, it has been so successful that for most of its life in orbit it provided data to Europe’s leading meteorology centres, significantly improving global weather forecasts.

Having surpassed all expectations, Aeolus’ fuel is now almost spent, and the mission is over.

Engineers kept Aladin on for as long as they could before reentry operations begin. Until last week, when it was finally turned off, the trailblazing wind-mapping laser had been as strong as ever. In fact, engineers ramped up the instrument to record energy levels in its last weeks of operation.

Now, gravity and the grasping wisps of Earth’s atmosphere as well as solar activity are dragging Aeolus down from its operational altitude of 320 km.

Aeolus was never designed for a controlled reentry, so under normal circumstances the satellite would naturally fall back to Earth within a few months. However, ESA is going above and beyond by attempting an assisted reentry – the first of its kind.

At ESA’s Space Operations Centre in Germany, mission control will use the remaining fuel to steer Aeolus during its return to Earth.

Most of the satellite will begin to burn up when it reaches an altitude of around 80 km. However, models show that several pieces of debris might reach Earth’s surface.

“This assisted reentry attempt goes above and beyond safety regulations for the mission, which was planned and designed in the late 1990s,” explains Tim Flohrer, Head of ESA’s Space Debris Office.

“Once ESA and industrial partners found that it might be possible to further reduce the already minimal risk to life or infrastructure even further, the wheels were set in motion. Should all go to plan, Aeolus would be in line with current safety regulations for missions being designed today.”

If this reentry attempt has to be aborted, which could still happen, Aeolus’ natural reentry continues.

Isabel Rojo, Flight Director for Aeolus, said, “Our teams of engineers and experts in debris, flight dynamics and ground systems, have designed a series of manoeuvres and operations to assist Aeolus’, and attempt to make its reentry even safer than was originally designed.”

Today, missions are designed according to regulations that require them to either burn up entirely or undergo a controlled reentry at the end of their lives in orbit.

This first attempt at an assisted reentry sets a new precedent for missions that didn’t fall under such regulations when they were designed, but could be made to retroactively adhere to them.

With Aeolus, ESA is paving the way for safe reentries and responsible space, which is particularly important considering the rapidly increasing amount of space traffic in orbit and problem of space debris.

When will Aeolus reenter?

Aeolus is currently falling at around one kilometre a day, and accelerating. Predictions become more accurate as the days go by, so it is still difficult to say exactly when Aeolus will reenter Earth’s atmosphere. A lot depends on solar activity.

Solar flares and coronal mass ejections might speed things up. Charged particles in space weather heat up Earth’s atmosphere. This causes denser air below to rise, replacing expanding layers higher up, which increases the drag of the atmosphere on Aeolus.

Conversely, a relatively quiet period of solar activity could mean that it takes a little longer for Aeolus to descend.

It is difficult to predict solar activity precisely, but ESA is confident that the reentry, if all manoeuvres are successful, will most likely happen at the end of July or early August.

Where will Aeolus reenter?

Most of the satellite will begin to burn up at an altitude of around 80 km. However, some pieces of debris might reach Earth.

Many months of expertise have gone into planning the optimal location for reentry, which minimises the already extremely remote possibility that falling debris would pose a risk to life or infrastructure.

The flight control team is aiming at a stretch of ocean beneath the satellite’s track – a long stretch of open water as far away from land as possible. 

What happens next?

For now, once Aeolus reaches an altitude of 280 km, a series of commands sent over six days will use the satellite’s remaining fuel to guide it towards the optimal position for reentry.

Then, a first manoeuvre will lower the satellite to an altitude of 250 km. This step will take several days, during which the teams will check on the health of the satellite and evaluate the next steps.

Four manoeuvres then usher Aeolus down to 150 km before 12 hours of final checks keep the satellite on track.

A final, critical manoeuvre at 150 km will direct Aeolus’ journey home. The satellite will return in a matter of hours, the vast majority of it burning up in Earth’s atmosphere.

“It has been incredible to see the skill and effort that has gone into preparing this ambitious reentry attempt,” said ESA’s Aeolus Mission Manager, Tommaso Parrinello. “We are confident we can succeed with this pioneering effort that will set a new standard for space safety and sustainability now and in the future.”