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<\/p>\nThe only satellite to measure the globe\u2019s wind profile from space, Aeolus, has concluded its mission and returned to Earth through a first-of-its-kind assisted reentry on Friday, July 28. The satellite exceeded its mission by two years since its launch in 2018, and with limited propellant left over, ground teams were able to lower the spacecraft to an altitude of 120 kilometers before reentering the Earth\u2019s atmosphere on its own. The US Space Command confirmed the satellite\u2019s reentry occurred around 9:00 PM CEST (19:00 UTC) above Antarctica, exactly where ESA hoped Aeolus would be.<\/p>\n<\/p>\n
Aeolus is the fifth in the family of the European Space Agency\u2019s (ESA) Earth Explorer missions, which focus on observing the interactions between Earth systems, and was built by Airbus Defence and Space. The Aeolus return is part of a broader effort of reducing the already low risk of spacecraft reentry, and to pioneer a safer way of de-orbiting satellites nearing the end of their life.<\/p>\n
As the space agency commits to achieving an assisted reentry of a satellite for the first time, ESA\u2019s Aeolus operations director Isabel Rojo sat down with NSF to discuss why this mission is significant, and how accomplishing it will be a difficult feat.<\/p>\n
All about Aeolus<\/p>\n
While the spacecraft launched in 2018, the overall program was in the works for almost 20 years before flying into space. Aeolus was approved in 1999 and was scheduled to lift off in 2007 but was delayed for over a decade due to continuous technological hurdles. <\/p>\n
However, on Aug. 22, 2018, the $560-million satellite finally launched into space atop a Vega rocket from the Centre Spatial Guyanais (CSG) in Kourou, French Guiana. During its five-year mission, the satellite resided at an altitude of 320 kilometers above the Earth.<\/p>\n
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Aeolus lifts off from French Guiana atop a Vega rocket. (Credit: ESA)<\/p>\n
The main objective of the mission was to \u201caddress the lack of global wind profiles in the Global Observing System,\u201d according to ESA.<\/p>\n
\u201cDirect global profile measurements of wind fields are lacking, representing one of the largest deficiencies in the observing system and limiting improvements to numerical weather predictions and climate models,\u201d the agency said.<\/p>\n
Before Aeolus, the most direct observations of wind were from radiosondes that were launched from stations every day, mostly in the northern hemisphere. \u201cWind-field information in remote regions, over the oceans, in the tropics and Southern Hemisphere is largely indirect,\u201d ESA continued.<\/p>\n
But Aeolus was able to measure wind to an accuracy of just one meter per second in the planetary boundary layer, and two meters per second in the free troposphere. It was also able to determine the average wind velocity over 100-kilometer tracks and measure 100 wind profiles per hour.<\/p>\n
The satellite masses 1,360 kilograms and carries one instrument, called the Atmospheric Laser Doppler Instrument (ALADIN). According to ESA, \u201cALADIN fires short pulses of UV light towards the planet, which bounce off air molecules and other particles as they are blown through the atmosphere. By measuring the shift in frequency of the light that is scattered back to the satellite, Aeolus can determine the speed and direction of the wind in the lowermost 30 kilometers of the atmosphere\u201d.<\/p>\n
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ALADIN is equipped with two lasers to help observe the Earth\u2019s wind. (Credit: Airbus Defence and Space)<\/p>\n
The satellite has been deemed more than a success, after not only exceeding its expected lifetime but providing information almost impossible to measure from Earth. Its data has been used by meteorological organizations across the world to inform enhanced weather predictions. In April 2022, the British Met Office claimed Aeolus\u2019s data improved nearly all the organization\u2019s weather observations.<\/p>\n
Aeolus also became a prominent tool during the COVID-19 pandemic. The decline in commercial flights led to fewer measurements of weather forecasts, and Aeolus was able to pick up the slack and provide accurate data on the weather in the weeks ahead.<\/p>\n
First-of-its-kind assisted reentry<\/p>\n
After completing scientific observations for five years, the teams involved in Aeolus decided to use the remaining propellant for an assisted reentry into the Earth\u2019s atmosphere. Aeolus was built to burn up in the Earth\u2019s atmosphere naturally at the end of its life, but ESA found that it was possible to reduce the already minimal risk to life or infrastructure through assisted reentry.<\/p>\n
The task was unique because missions designed years ago didn\u2019t have to adhere to today\u2019s casualty risk regulations, but the satellites that are designed now are required to either completely burn up or undergo a controlled reentry. A controlled reentry wasn\u2019t possible for Aeolus by design, due to its lack of propellant and inadequate propulsion system among other reasons, as it would require ground teams to maneuver the satellite to an altitude of 50 kilometers.<\/p>\n
\u201cThis first attempt at an assisted reentry sets a new precedent for missions that didn\u2019t fall under such regulations when they were designed, but could be made to retroactively adhere to them,\u201d ESA said.<\/p>\n
But the mission would not go without its challenges.<\/p>\n
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ESA Main Control Room in Germany helping guide Aeolus. (Credit: ESA)<\/p>\n
\u201cWe\u2019re trying to push the limits of what the spacecraft can do and also what ground can do because the whole concept was built around assisted reentry, which for now was never proven that it can work,\u201d Isabel Rojo told NSF. \u201cSo, the difficulties are implementing the maneuvers, and acquiring the spacecraft between such large maneuvers, because if they substantially misperform, it could lead to needing to search for the spacecraft. So, there\u2019s a lot of technical difficulties inherent to the maneuvers themselves because they are quite large\u201d.<\/p>\n
To aid the spacecraft in its return, ESA\u2019s Main Control Room in Darmstadt, Germany, was tasked in completing several difficult maneuvers to lower its orbit, and ensuring that they found the way to configure the satellite such that it withstands the difficult conditions of flying at lower altitudes. The first was achieved on Monday, July 24, when Aeolus completed the largest firing of its thrusters, lowering it to an altitude of 250 kilometers. The burn lasted for 37 minutes and 24 seconds \u2013 more than three times the size of Aeolus\u2019 routine burns. According to ESA, the burn consumed six kilograms of propellant.<\/p>\n
\u201cIt was the first time we performed such a large maneuver in this direction,\u201d said Rojo. \u201cIt\u2019s pretty much the same kind of maneuver that we\u2019re going to be performing from now on in order to lower the perigee of the satellite. We needed that confidence to make sure that all the systems on board are responding properly to this sort of maneuver.<\/p>\n
\u201cThat\u2019s why this was a big first, and it\u2019s in such a way that the attitude of the spacecraft is in retrograde\u2026 due to the position of the thrusters on the spacecraft body, we need to turn the satellite around to be able to thrust it against the flight direction, which then reduces the speed and gets it lower,\u201d continued Rojo.<\/p>\n<\/p>\n