Persistence pays as ESA lasers in on Aeolus launch

By measuring the Doppler shift of backscattered laser light, Aeolus will reveal wind speed and direction at various altitudes (ESA/ATG medialab)

7 February 2018

The laser instrument at the heart of the European Space Agency’s Aeolus wind monitoring satellite has passed what looks to be the final testing hurdle in what has been a long and challenging road to launch. But with the spacecraft having passed two months of thermal-vacuum chamber tests, it is now ready for “a few final tests” before being shipped across the Atlantic to Europe’s spaceport in French Guiana for launch on a Vega rocket in the autumn.

With perhaps elegant understatement, ESA describes Aeolus as having been “particularly tricky to build”, adding that “one of the main stumbling blocks has been getting its lasers to work in a vacuum”. Weather forecasters and climate scientists will be glad to hear that the spacecraft passed its vacuum chamber trial at the Centre Spatial de Liege, Belgium with what ESA’s project manager Anders Elfving describes as “flying colours” – because they’ve been waiting a very long time for this mission to fly; a launch was at one point penciled in for 2007, then 2011, and the Vega launch contract signed with Arianespace in 2016 initially looked to fly by end-2017.

Much of the delay can be attributed to the huge technical challenge of realising the mission’s sole instrument. Aladin – Atmospheric Laser Doppler Instrument – features two ultraviolet lasers and a telescope and sensors working as lidar – laser direction and ranging, a laser version of radar – to analyse backscattered light from air molecules, dust particles and water droplets to measure wind speed at various altitudes, particularly in the lower 30km of the atmosphere. Development of what will be the first wind lidar in space dates to 2000, but issues with the laser component of the instrument and with the optics, which have to survive exposure to the high-intensity laser pulses, meant the instrument was not ready until 2016.

But carrying what ESA describes as “one of the most sophisticated instruments ever to be put into orbit”, Aeolus – named after the “keeper of the winds” in Greek mythology – promises much. “This will be the first time that we will be able to directly measure profiles of the global wind field from space in cloud-free conditions. It has been a major challenge for us all – our ESA engineers, industry, our Member States – to overcome many technical and programmatic challenges.

“I am grateful to everyone for having gone through this and for having trust in ESA to finally make it happen. We are now very close to seeing the fruits of a long endeavour,” said ESA director of Earth observation Josef Aschbacher.

Aeolus has already logged plenty of miles. Aladin was built by Airbus in Toulouse, the satellite by Airbus in Stevenage, UK, and the laser transmitters by Leonardo in Florence and Pomezia. Before its critical thermal-vacuum test in Belgium, the satellite was shipped from Stevenage to France to test that it would survive the vibrations of launch; now it has been returned to France for final tests before being shipped for launch.

Aeolus will fly in a rather low Sun-synchronous orbit of 320km. ESA calls it a pre-operational mission that will pave the way for future meteorological satellites to measure wind. Aeolus is one of ESA’s Earth Explorers series of missions studying features including the planet’s magnetic and gravity fields, soil moisture ocean salinity, and ice coverage.

Aladin’s lasers generate ultraviolet light that bounces off air molecules and small particles; a small fraction is scattered back towards the satellite (ESA)