Remarkable ESA Aeolus will fall back to Earth, but with style

One of the European Space Agency’s most valuable science satellites is nearing its end. We’re talking about the ESA Aeolus probe, named after the Greek god of wind and part of the Earth Explorer program. Its purpose and innovative side was, as suggested by its name, the observation and the measurement of the Earth’s winds from orbit. Its laser-powered main instruments have now been shut off as the spacecraft slowly falls to burn in the atmosphere in the coming weeks.

Liftoff of ESA's Earth Explorer Aeolus from Europe's Spaceport in Kourou on the 22nd of August 2018. The ESA Vega integration building is visible on the left Aeolus is the first space mission to acquire profiles of the wind on a global scale. These near-realtime observations will improve the accuracy of numerical weather and climate prediction and advance our understanding of tropical dynamics and processes relevant to climate variability. — Liftoff of ESA’s Earth Explorer Aeolus from Europe’s Spaceport in Kourou
Credits: ESA / S. Corvaja

The Aeolus mission was launched on August 22, 2018, by a Vega rocket. The groundbreaking mission, despite various issues skillfully solved by ESA, has been working as planned every day for the last four years. It has enabled a whole new view of atmospheric phenomena, proving of great value for a wide spectrum of activities.

Weather prediction, aviation safety, and research on atmospheric dynamics have all benefited immensely. For the first time, the world had a global view of the Earth’s winds, instead of being reliant only on ground and airborne sensors and probes.

How did Aeolus collect data?

An illustration of the ALADIN LiDAR aboard ESA's Aeolus satellite, shooting a pulse of UV light to Earth and capturing its backscatter

Accurate wind measurements from space were enabled by a laser, precisely the Atmospheric LAser Doppler INstrument, ALADIN for short.

It transmitted a high-power UV light laser pulse towards Earth. A 355 nm wavelength was specifically chosen for its proneness to scatter back where it came from when it impacts particles of water or other molecules in the atmosphere. The backscattered light was collected by a 1.5 meter wide mirror, and directed to a specialized receiver.

The collected data can be interpreted as a “strip” of the atmosphere. Light coming back from lower layers of the atmosphere will reach the telescope later, providing data on distance and altitude (the basic functioning of a LiDAR). The velocity of the wind is found by analyzing the Doppler shift of the light. Since the reference is 355 nm, we can figure out how fast the wind is moving by comparing the backscattered light’s wavelength, whether it is shorter or longer.

ESA Aeolus' rappresentation of the Earth's atmosphere. A graph with altitude and latitude on its two axis, and different colors showing different wind velocities. — Rappresentation of Aeolus’ collected data. A “strip” of the Earth’s atmosphere between Turkey and the Antarctic Ocean
Credits: ESA

Assisted reentry

With a planned mission duration of 3 years, Aeolus has operated for over 18 months after its planned decommissioning date. According to ESA, even if the ALADIN instrument was working as good as ever in these last months, the remaining propellant needed to keep the satellite in orbit was almost depleted. Solar activity predicted to reach its peak in the coming months, has increased atmospheric drag and forced ESA to finally end the mission.

A render illustrating the Aeolus satellite orbiting the Earth and collecting wind data. The data is shown as a strip along the curvature of the Earth, with vectors indicating the direction of the wind passing through it. — *An illustration of Aeolus collecting wind data. Credits: ESA*

The plan for reentry, however, is being carefully studied. Usually, satellites are left burning in the atmosphere and the remaining pieces fall in a random location (hopefully) in the ocean. The ESA team in Darmstadt is instead preserving a small quantity of propellant to “steer” and guide the satellite during its fall. Deciding where their Aeolus satellite will fall in a specific reentry corridor, instead of predicting a general location. This first-of-a-kind attempt by Aeolus, being put to the test in these weeks, will be a great step in ensuring the safe use of space in the future.

A world record looking to the future

On April 30, 2023, Aeolus officially began its decommissioning phase, concluding its last observations. A series of End Of Life tests were performed to address the cause of some issues found and solved during operation. But the ESA team still wanted to test some unknowns. Specifically, the maximum power of the laser. On July 4, 2023, the Aeolus control team cranked up the laser’s power up to 182 mJ, more than double its operational power of 70 mJ.

The previous record for a UV laser in space was NASA's LITE, which had a maximum reported energy of 169mJ.

You can see in this graph how we've been ramping Aeolus up to its maximum energy this week 📈

Ingenious 🧞 from our Aladin instrument experts 🪔 pic.twitter.com/2EmPMujvbK
— esa aeolus mission (@esa_aeolus) July 4, 2023

It wasn’t just a record-seeking attempt. The test validated this technology to its highest limit, preparing for the future, as did the EOL tests. Aeolus was a complete success and had numerous impacts on weather predictions, but also with an estimated economic benefit for Europe reaching €3.5 billion. The European Space Agency has already confirmed that an Aeolus-2 satellite is in the works, furthering the use of satellites for advanced analysis of our planet’s climate, to our benefit.