The European Space Agency (ESA) is in the final stages of carrying out an “assisted return” of the Aeolus spacecraft, an earth science satellite. This effort aims to get the satellite to the Atlantic Ocean in just over a week, marking the end of a mission that exceeded all expectations. The Aeolus mission, launched in 2018was originally planned as a three-year mission to demonstrate a lidar’s ability to measure wind speed globally, however it exceeded its original goals, providing valuable data for operational weather forecasting and filling gaps in wind data during the pandemic.
The re-entry process involves a series of maneuvers designed to lower the perigee of the Aeolus spacecraft, allowing for a projected safe reentry over the Atlantic Ocean on July 28. These maneuvers are meticulously planned to minimize any chance of debris from the surviving re-entry spacecraft landing in populated areas. The first set of maneuvers is scheduled for July 24, which will lower the spacecraft’s perigee from 280 to 250 kilometers. A second series of maneuvers on 27 July will further lower the perigee to 150 kilometres. A final maneuver on July 28 will lower the perigee to 120 kilometers, after which the satellite should return within five hours.
The final maneuver is timed to cause reentry to occur during a track that the spacecraft’s sun-synchronous orbit takes over the Atlantic Ocean. However, due to spacecraft limitations, ESA cannot target a specific area for re-entry. This presents a unique challenge for the ESA team, as they have to ensure safe re-entry without the ability to precisely control the position.
“This spacecraft was designed and developed shortly before the guidelines” for controlled reentry came into effect, said Holger Krag, head of ESA’s Space Safety Office. This has led to what the agency calls an “assisted reentry” approach, which lacks the precision of a supervised reentry but avoids an unsupervised reentry.
“It will further reduce the risk, which is already small, on the ground posed by reentry”
Krag added.
ESA expects up to 20% of Aeolus, which weighs around 1,100 kilograms excluding propellant, to survive re-entry. The agency said the assisted reentry approach, if successful, would reduce the risk of debris hitting someone already extremely small by a factor of 42, a significant risk reduction, demonstrating the effectiveness of the assisted reentry approach.
What are the precedents related to operations similar to that of Aeolus?
This is the first time Krag has heard of a satellite operator attempting an assisted reentry. The closest comparison he has offered is the re-entry of NASA’s Skylab space station in 1979, where spaceflight controllers turned off the gyroscopes to bring the spacecraft down in an attempt to control the reentry position. This historical precedent provides some indications for the re-entry of Aeolus, but the ESA team is charting new territory with this operation.
ESA officials have billed the assisted reentry as part of a wider space safety effort by the agency. This commitment was underlined by an announcement during the Paris Air Show on 22 June that ESA would work with several European satellite manufacturers on a ‘Zero Debris Charter’. The signatories of this charter would commit, by 2030, to deorbit their satellites at the end of their life or to hire companies that provide active debris removal services to deorbit them.
“I think ESA has always been a responsible actor and, with that action on Aeolus, we are showing once again that we are willing to achieve anything, even with a space system that was not originally prepared for it”
Krag said. This declaration reflects the ethos of ESA, which is committed to pushing the boundaries of space exploration while maintaining a strong commitment to safety and accountability.
The reentry will mark the end of Aeolus, a mission that provided valuable data and demonstrated the potential of lidar technology to measure wind speed globally. Aeolus science operations formally ended in April, but the mission’s impact will continue to be felt for years to come.
“After almost five years it has exceeded all expectations and gone beyond the original goals”
said Tommaso Parrinello, mission manager of Aeolus. This included using Aeolus data in operational weather forecasting and filling gaps in wind data as commercial airline traffic, also used to collect wind data, dropped significantly during the onset of the pandemic.
At ESA’s ministerial meeting in November 2022, member states approved plans for a two-satellite follow-up mission, Aeolus 2, which is expected to be launched at the end of the decade in partnership with Eumetsat. “This decision made last year is the most tangible and solid demonstration of the value of the success of this mission, which perhaps was not obvious at the beginning,” said Parrinello.
Simonetta Cheli, director of ESA’s Earth Observation, said during the briefing that Aeolus has often been referred to as “mission impossible” due to the many technical challenges it faced during development. “It’s a real success story,” she concluded. The success of Aeolus is a testament to the dedication and ingenuity of the ESA team and sets a high bar for future missions. As the Aeolus mission draws to a close, the ESA team can look back on a job well done and look forward to the challenges and opportunities ahead.
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