After almost five years of operation, the Doppler wind lidar instrument ALADIN (Atmospheric Laser Doppler Instrument) on board Aeolus was shut down on July 5, 2023. Having exceeded its planned three-year-lifetime by 18 months, the satellite of ESA's fifth Earth Explorer mission is running out of fuel, hence forcing the "God of the Winds" to return to Earth. Fuel consumption had risen sharply over the past year as a result of increasing solar activity, which is approaching the maximum of its approximately 11-year cycle, causing the satellite to experience increased drag from the Earth's atmosphere in its low orbit of 320 km. After a gradual descent and a few final maneuvers, the satellite is expected to burn up over the ocean in the coming weeks in a selected ocean corridor. It will be the first assisted re-entry of its kind and is expected to set a precedent for a responsible approach to reducing space debris.
Following the end of the nominal operations of Aeolus on April 30, 2023, a series of dedicated tests, so-called end-of-life activities, was carried out in May and June with the aim of gaining important scientific and technological knowledge for future lidar missions in space such as EarthCARE and Aeolus successors. In addition to sensitivity analyses of various system parameters, with a particular focus on the instrument alignment, the output power of the laser was maximized. As a result, the energy of the ultraviolet laser pulses, which ranged between 40 and 100 mJ during the operational phase of the mission, was increased to 182 mJ in the last days before the shutdown. This set a new record for a UV laser in space. The last wind observations show nearly clear atmosphere which allowed the final laser shots from a total of more than 7.3 billion since launch in August 2018 to shine into the Pacific Ocean just between North America and Hawaii, while providing full data coverage from ground up into the lower stratosphere (see Fig. 1).
Fig. 1: The last measurements of the Doppler wind lidar ALADIN onboard Aeolus on July 5, 2023. Before shutdown of the instrument over the Pacific Ocean at 14:30 UTC (see measurement track in the top panel), global profiles of the backscatter signal (bottom left) and the horizontal wind component along the laser line-of-sight (HLOS) from the ground to the lower stratosphere (bottom right) were measured for the last time at a record laser energy of 182 mJ. (Graphics: DLR / O. Lux, CC BY-ND-NC 3.0)
The Aeolus mission was a resounding success and has far exceeded expectations. Originally intended as an ESA technology demonstrator mission, the wind observations have proven to be of great value for the quality of numerical weather prediction so that the data was operationally used by numerous weather services for several years, including the German Weather Service (DWD) and the European Centre for Medium-Range Weather Forecasts (ECMWF). Despite the great importance of wind for weather and climate, the height-resolved, global acquisition of wind profiles only became possible with the development of ALADIN which was an enormous technological challenge. Thanks to successful Aeolus mission European research and industry have taken a global leadership role in lidar technology.
The Institute of Atmospheric Physics (IPA) has made a significant contribution to the success of the Aeolus mission. IPA scientists have been vital in developing and improving the ALADIN instrument and its data products, starting from the mission’s initial idea in the 1980s and 1990s up to the present day. The ALADIN Airborne Demonstrator (A2D) was built at DLR which had already been used many years before the satellite launch to gain valuable experience in the operation and data processing of the wind lidar technology. The three-dimensional wind profiles provided by the 2-µm Doppler wind lidar (DWL), also developed at the IPA, served as a high-accuracy reference. Together with partners from research and industry, the IPA used the data obtained with A2D and 2-µm DWL during numerous flight campaigns for the continuous further development of the Aeolus-L1B processor and for the validation of the Aeolus wind product.
In recent years, the improvement of the Aeolus data products has taken place within the Aeolus Data Innovation and Science Cluster (DISC). The Aeolus DISC is an international consortium of about 40 scientists and engineers from more than ten European institutes and companies (Fig. 2). On behalf of ESA and under the leadership of IPA, the DISC team has analyzed the Aeolus measurements, enhanced their quality and demonstrated their positive impact on numerical weather prediction.
Due to the great importance of global wind data for future weather forecasting and climate research, the ESA Ministerial Council Conference in November 2022 paved the way for an operational Aeolus successor mission as a cooperation between ESA and EUMETSAT with launch in the 2030s.
Fig. 2: The Aeolus DISC team on the Schneefernerhaus in March 2023. (Photo: DLR / O. Reitebuch, CC-BY-ND-NC 3.0)