- Nov 29, 2023
With a New Deorbiting Method, ESA’s Aeolus Offers New Space Junk Strategy
During its five-year mission, the European Space Agency’s Aeolus satellite improved weather forecasts and climate models of a rapidly changing Earth. And when it came time to end the mission, flight controllers boldly chose to crash the satellite, which was never designed for a controlled reentry, back to Earth in a way never tried.
Aeolus, launched in 2018, was the only satellite to measure Earth’s wind profile from space. The small satellite, built by Airbus Defence and Space for about $560 million, was fifth in the family of ESA’s Earth Explorer missions.
ESA deemed the satellite a huge success, not only exceeding its expected lifetime but also providing data that would have been almost impossible to measure and gather from Earth, according to a statement. After completing its mission, the Aeolus team decided to use the remaining propellant for a safe, assisted reentry into Earth’s atmosphere. In doing so, a member of the Aeolus team told SpaceRef, they developed a new way of decommissioning older satellites to prevent them from adding to the increasingly-hazardous cloud of space debris orbiting Earth.
A new deorbiting strategy
“My personal view is that we have proven that a satellite can now be reentered using another approach, a kind of semi-controlled reentry in addition to the uncontrolled and the controlled one,” Tommaso Parrinello, ESA’s Aeolus Mission Manager, told SpaceRef. “The important thing is that space is a precious environment for all humanity, and we must do the utmost to make it sustainable, improving the way we launch, operate, and remove our hardware at the end of its operational life. Aeolus’s assisted reentry is a clear example of how this can be done in full alignment with ESA’s approach, which is to make space a safer environment.”
According to a recent Pew Research study, 69 percent of Americans believed that space debris from rockets and satellites will pose a major problem in the coming years. The ability to safely guide spacecraft out of orbit reduces risk to astronauts and infrastructure in orbit.
Most satellites reenter in an uncontrolled manner. Small spacecraft launched at about 400 km, or 250 miles (the altitude of the International Space Station) will naturally decay in under five years thanks to the destructive friction of dragging against Earth’s atmosphere. And during years of increased solar activity, such as is happening now, Earth’s atmosphere expands, creating even more drag on satellites in low Earth orbit.
However, satellites at orbital altitudes beyond 500 km, or 310 miles, are farther away from the atmosphere and therefore there is no guarantee that they will naturally decay in the 25-year suggestion made by the Inter-Agency Space Debris Coordination Committee. The IADC is an international forum of space agencies, including NASA and ESA, for the discussion of space debris. A satellite built today is required either to completely burn up or undergo a controlled reentry. ESA designed Aeolus in the 1990s when no such regulations were in place. It lacked propellant and carried an inadequate propulsion system, making a controlled reentry according to the new suggestions impossible. So, the ESA took a more proactive approach by using the remaining fuel aboard the satellite to lower its orbit.
ESA mission control commanded the first step of the process on July 24th by lowering Aeolus’ orbit to 250 km (or 155 miles).
“During the five days of reentry operations, we executed seven maneuvers overall, interleaved by three check points with go/no-go criteria, and two assessment periods which proved to be strategically crucial for the whole venture,” Parrinello told SpaceRef.
Some days, Parrinello said, were full of critical and stressful decisions including situations considered highly unlikely such as moving the satellite into an attitude that resembled a crab landing technique used by pilots when landing in high crosswinds.
“We did this to reduce the high fuel consumption caused by very high level of atmospheric drag experienced by the satellite when flying at low altitude,” he said. “In our case, the wind was the flow of the atmospheric drag over the satellite. This attitude, which had never been used in such conditions before in space, was verified beforehand on the simulator but considered unlikely to be used. It was considered a bit exotic, coming out from some theoretical analysis.”
However, Parrinello said, the decision to trust the simulator and attempt the maneuver “saved the assisted reentry,” as the satellite was running out of fuel and wouldn’t have been able to complete the last planned maneuver. Had the last maneuver failed, the satellite would have reentered on its own.
“So, we can also say that we have validated a crab flying attitude. Another unexpected achievement for the ’impossible mission,’” he added.
By July 28th, the orbit reached about 150 km, or about 93 miles. At this point, controllers let gravity and atmospheric drag take over. The US Space Command confirmed the satellite’s reentry at 19:00 UTC July 28th over Antarctica; exactly where ESA hoped it would be.
The endeavor, Parrinello said, illustrated some crucial takeaways for anyone else attempting a high-stakes deorbiting.
“The first important lesson to be learned,” said he told SpaceRef, “is that you must be ready for the worst-case scenario and be ready to react. The second important lesson is clearly to have all the experts — who designed the reentry -— in the room with the flight control team, readily available to help solve any foreseen and unforeseen anomalies. This might seem like an obvious one, but it is not. Contrary to a satellite launch, in a reentry, time plays against you as the satellite is quickly coming down and the window of reaction and decision making is very tight. The risk of losing control of the satellite is very high, which impacts the remaining sequence of events, despite all the preparation.”
Lessons for reducing space debris
According to a NASA website on space debris, 60 percent of the 11,370 satellites placed in orbit are still there but only 35 percent are still active. The US tracks more than 500,000 objects with a diameter between one and ten centimeters. More than 25,000 pieces larger than 10 cm are in orbit between geostationary orbit and low-Earth orbit.
For Washington, DC-based LEOLabs, a partner in bringing Aeolus back to Earth, the lessons learned included successfully tracking a satellite in very low Earth orbit, or VLEO. All together, these new techniques could help space agencies and companies bring other aging satellites that, like Aeolus were launched without reentry in mind, back out of orbit.
“Not only do we hope to continue contributing to these types of exercises and missions through our tracking capabilities, but also we aim to foster a more data-backed conversation regarding debris remediation and mitigation,” a spokesperson for LEOLabs told SpaceRef. “This includes applying analytics to our orbit catalog to understand the most at-risk orbits in LEO in which remediation efforts should be targeted, as well as identify what derelict objects should be prioritized for active-debris removal missions. We look forward to continuing to work with commercial companies and space agencies, like ESA, who are actively contributing to these sustainability efforts.”