The Historic Plunge of Aeolus: The Wind Satellite Setting New Benchmarks in Space

The Aeolus satellite of the European Space Agency (ESA), having exceeded its intended operational lifespan and significantly contributed to global weather predictions, is currently embarking on an unprecedented assisted reentry to Earth. This groundbreaking procedure is designed to improve space safety protocols and establish a new standard for upcoming space expeditions. Depending on solar activities, the reentry is anticipated to occur by early August, with the spacecraft expected to mostly disintegrate upon entering Earth’s atmosphere.

After surpassing its predicted orbital lifespan, ESA’s Aeolus wind satellite is in the process of returning to Earth. The satellite is presently descending about 1 km a day, a speed that is increasing. ESA’s spacecraft operators are poised to intervene and guide Aeolus in an unprecedented assisted reentry. But why is ESA undertaking this operation?

The wind mission of ESA has been orbiting Earth for approximately five years, well beyond its designed life. Its extraordinary Aladin instrument has transmitted seven billion pulses of UV light to profile Earth’s wind patterns.

Even though Aeolus was created as a research mission and to showcase innovative technology, its remarkable success led to it providing critical data to Europe’s foremost meteorological centers for most of its operational life, thereby enhancing global weather forecasting.

Now that Aeolus has exceeded all projections, its fuel reserves are nearly depleted, signaling the end of its mission.

Aeolus is the pioneer space mission to generate profiles of wind patterns on a global scale. These near-realtime observations will enhance the accuracy of weather and climate predictions, as well as advancing our comprehension of tropical dynamics and processes that impact climate variability. Credit: ESA/ATG medialab

Engineers kept Aladin operational as long as feasible before initiating reentry procedures. Up until last week, when it was finally deactivated, the trailblazing wind-mapping laser remained highly functional. In fact, engineers intensified the instrument to register energy levels in its final weeks of operation.

Presently, gravity and the faint tendrils of Earth’s atmosphere, along with solar activities, are pulling Aeolus down from its operational altitude of 320 km.

Aeolus wasn’t designed for a controlled reentry, hence, under normal circumstances, the satellite would naturally reenter Earth’s atmosphere within a few months. Nevertheless, ESA is going the extra mile by attempting an assisted reentry, a first of its kind.

At ESA’s Space Operations Centre in Germany, mission control will utilize the remaining fuel to guide Aeolus during its descent back to Earth.

Most of the satellite is projected to incinerate upon reaching an altitude of approximately 80 km (50 miles), but models suggest that some fragments might make it to Earth’s surface.

Key stages in Aeolus’ reentry. Credit: ESA

Tim Flohrer, Head of ESA’s Space Debris Office, elaborates, “This assisted reentry operation surpasses the safety requirements for the mission, which was conceptualized and designed in the late 1990s.”

“As soon as ESA and industrial partners realized that it might be feasible to further diminish the already minimal risk to life or infrastructure, action was taken. If everything goes as planned, Aeolus would comply with current safety regulations for missions being designed today.”

Should the reentry operation need to be called off, which remains a possibility, Aeolus will continue its natural descent.

Isabel Rojo, Flight Director for Aeolus, stated, “Our teams of engineers and experts in debris, flight dynamics, and ground systems have planned a series of maneuvers and operations to aid Aeolus’ reentry and attempt to make it safer than originally designed.”

Present-day missions are designed in compliance with regulations that mandate them to either completely burn up or undergo a controlled reentry at the end of their orbital lifespan.

This maiden attempt at an assisted reentry establishes a novel standard for missions that were not designed with such regulations, but could be retroactively made to comply with them.

With Aeolus, ESA is charting a path for secure reentries and responsible space operations, which is critical considering the escalating volume of space traffic and the issue of space debris.

When will Aeolus reenter?

Aeolus is currently descending at about one kilometer a day and accelerating. Predictions will become more precise as time progresses, making it challenging to pinpoint exactly when Aeolus will reenter Earth’s atmosphere. This largely depends on solar activity.

Solar flares and coronal mass ejections could hasten the process. Charged particles in space weather heat up Earth’s atmosphere, causing denser air below to rise and replace expanding upper layers, which in turn increases atmospheric drag on Aeolus.

On the other hand, a period of relatively low solar activity could potentially delay Aeolus’s descent.

While solar activity is challenging to predict accurately, ESA is confident that if all maneuvers are successful, reentry will likely occur at the end of July or early August.

Where will Aeolus reenter?

The majority of the satellites will start to burn up at an altitude of around 80 km (50 miles), but some debris might reach Earth.

A considerable amount of expertise has been invested in planning the ideal location for reentry to minimize the extremely remote risk of falling debris endangering life or infrastructure.

The flight control team is targeting a stretch of ocean beneath the satellite’s trajectory – an expansive stretch of open water as distant from land as feasible.

What happens next?

Once Aeolus descends to an altitude of 280 km (175 miles), a series of commands dispatched over six days will utilize the satellite’s remaining fuel to guide it to the optimal position for reentry.

Subsequently, a preliminary maneuver will reduce the satellite to an altitude of 250 km (155 miles). This phase will take several days, during which the teams will assess the satellite’s health and plan the next actions.

Four maneuvers then shepherd Aeolus down to 150 km (95 miles), followed by a 12-hour period of final checks to keep the satellite on course.

A decisive, final maneuver at 150 km (95 miles) will guide Aeolus’ homeward journey. The satellite will return within a few hours, with most of it incinerating in Earth’s atmosphere.

“It has been amazing to witness the skill and effort invested in preparing this ambitious reentry attempt,” said Tommaso Parrinello, ESA’s Aeolus Mission Manager. “We are confident in our ability to succeed with this pioneering endeavor that will set new standards for space safety and sustainability now and in the future.”

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