Wait a minute!
Image credit: Barbara David

The European Space Agency performed an assisted/semi-controlled descent of its retired Aeolus, a wind profiling spacecraft lofted into Earth orbit in 2018.

Mission scientists and engineers took on the tricky task of targeting a remote stretch of the Atlantic Ocean for the plunge to Earth of Aeolus. A key aspect of assisted reentry is that for any spacecraft leftovers believed to survive the plunge, those bits and pieces would fall into a remote area.

Image credit: ESA

ESA confirmed that Aeolus reentered Earth’s atmosphere on July 28 above Antarctica, also verified by the U.S. Space Command.

The “adios to Aeolus” action underscores a novel approach for the safe return of active satellites that were never designed for controlled reentry.

Image credit: ESA

 

Windage

But “above Antarctica” versus the Atlantic Ocean caught my eye. Sounds like something didn’t go as planned?

“In an assisted re-entry you have to accept inaccuracies along the desired target coordinates and we required that it was maximum +\- half orbit centered in the middle of the Atlantic Ocean,” responded Tommaso Parrinello, ESA Aeolus mission manager, a target zone that was called a corridor.

Aeolus reentered over Antarctica on July 28. ESA’s Space Debris Office, based on U.S. Space Command tracking and ESA’s own data acquired during Aeolus’s last orbits, this map has been produced showing the assessed location of Aeolus’s disintegration in the atmosphere and where any surviving fragments may have fallen. Image credit: ESA

“We re-entered within less than a quarter of an orbit. Better than expected,” Parrinello told Inside Outer Space. It took almost a year to develop the assisted re-entry or semi-controlled concept, he said, designing the best corridor of re-entry, changing the satellite configuration, and designing the timeline, along with check and more checks via simulation.

Assisted living and reentry for Aeolus spacecraft. Image credit: ESA/J. Mai

As for the price tag of the Aeolus assisted re-entry, “the funds were within the foreseen operation costs…there is not a figure to give,” Parrinello said.

Minimize risk

“With the growing number of objects being launched into space, we certainly expect that many will re-enter over time, so I think ESA’s efforts to develop and test an assisted controlled re-entry is important to minimize the risk to human life on or near the surface,” said T.S. Kelso of CelesTrak, an analytical group that keeps a sharp eye on Earth-circling objects.

Similar in view is Darren McKnight, a senior technical fellow for LeoLabs.

“This is significant for several reasons,” McKnight said. “First, it is critical for everyone to note that sometimes the act of reducing orbital collision risk comes at the cost of risk to aviation and ground impacts. The U.S. has a self-imposed threshold of 1/10,000 chance of ground casualty from a reentry but that is not accepted worldwide.”

Taking the fall. Space hardware dives into Earth’s atmosphere with some fragments making their way to the ground.
Image credit: ESA/D.Ducros

McKnight said that it is laudable to see others minimizing this re-entry risk. He added that this spacecraft was not designed to do these maneuvers yet performed them admirably.

Continual innovation

“Much of the positive space safety behavior over the last few years has been by systems not designed to perform that way. The continual innovation by many has been impressive,” McKnight told Inside Outer Space.

Lastly, McKnight said that “active debris removal is a complex, but greatly needed operation to improve the state of the debris environment in low Earth orbit and it requires several steps: identify, rendezvous, grapple, de-tumble, and de-orbit safely. This exercise builds confidence in the ability to do the last critical stage of safe de-orbiting of large spacecraft.”

Tech. Sgt. Ronald Dunn, 729th Airlift Squadron loadmaster, guides a Mongolian driver in August 2011. Dunn was part of a crew from March Air Reserve Base, Calif., who were assigned to a mission to retrieve space debris that fell to Earth. The parts were identified as expended rocket parts from an Air Force rocket launched into space nearly a decade prior. Image credit: U.S. Air Force photo/Master Sgt. Linda Welz

Responsible behavior

Aeolus had a dry mass of 2,425 pounds (1,100 kilograms) and the most critical removal sequences, McKnight said, will be performed on objects over 1,000 kilograms.

“It should be noted that any spacecraft above 500 to 800 kilograms in mass is likely to have sufficient debris survive re-entry as to warrant controlled reentry to meet the 1/10,000 threshold for ground casualty,” said McKnight. There are currently nearly 800 rocket bodies and over 300 non-operational payloads in low Earth orbit with a mass over 1,000 kg, he pointed out.

“This demonstration showed how individual responsible behavior can contribute to the growing space safety expertise highlighting that space safety does not have to be debilitating for space operators,” McKnight concluded.

Detrimental effects

On the other hand, there remains the issue of rubbish from spacecraft falling out of orbit having harmful effects on global atmospheric chemistry.

The atmospheric layers from the ground up to the boundary with space, showing natural phenomena, human inputs and resultant impacts. These human inputs impact the troposphere (by enhancing climate change), the stratosphere (through ozone loss from multiple causes), the mesosphere (by influencing metal chemistry and accumulation and increasing noctilucent clouds), and the thermosphere (by likely causing contraction which will impact orbiting satellites).
Image credit: Jamie D. Shutler, et al.

Some experts are concerned that the growing scale and pace of space activities may lead to new unforeseen impacts on the environment and climate. Furthermore, what appears required is improved monitoring of the situation, as well as regulation to create an environmentally sustainable space industry.

These are observations from recent research on atmospheric impacts of the space industry led by Jamie Shutler, associate professor of Earth observation in the Center for Geography and Environmental Science, College of Life and Environmental Sciences at the University of Exeter, Cornwall.

Full impact, not known

As for the outcome from assisted re-entry, like that done with ESA’s Aeolus satellite, it’s a tough call, Shutler told Inside Outer Space.

“It’s good that agencies are now starting to consider the environmental aspects of these technologies and how previous standard approaches are not sustainable,” Shutler said, such as shifting satellites to a graveyard orbit or just leaving the satellite in its original orbit to slowly de-orbit.

“But equally, de-orbiting for burn-up in the atmosphere and with roughly 20 percent of the satellite landing in the ocean is not sustainable or environmentally good,” Shutler said. “The satellite components don’t just vanish, they instead get re-distributed throughout the atmosphere and the full impacts are not known.”

Ozone loss

For example, Shutler added, satellites are mainly made up of aluminum and we know that aluminum in the upper atmosphere can promote ozone loss. “Whilst being claimed to be ‘harmlessly falling in the ocean’, it’s still littering in the ocean on which we rely for food, and for regulating our weather and climate.”

Earth orbit is a junkyard of human-made space clutter.
Credit: Space Junk 3D, LLC. Melrae Pictures

Shutler said the ESA effort is a step in the right direction, “but only if this is the start of greater efforts by all agencies and private organizations to question and reduce the environmental impact of space activities.”

Sustainable space?

In Shutler’s view, greater efforts need to be made, for example, in controlling the overall quantity of satellites in orbit, like sharing resources efficiently, rather than large scale duplication, as we see now with commercial activities. Also needed is reducing the quantity of aluminum within those satellites, “and showing greater consideration for the atmosphere, the ocean and whole of Earth’s environment, instead of just protecting the land, whilst simultaneously considering everywhere else as being acceptable for littering.”

Shutler’s bottom line: “Overall I would hope that the approach of simply de-orbiting all satellites is not the future. Much more needs to be done to address the problem, and recognizing this wider issue, as it seems is being done by ESA…a good first step. But despite the social media tagging, we have a long way to go before we have anything close to ‘sustainable space.’”

A main propellant tank of the second stage of a Delta 2 launch vehicle landed near Georgetown, Texas in January 1997.
Image credit: NASA Orbital Debris Program Office

Re-entry regime

Also noting the Aeolus outcome is Aaron Boley, an associate professor of physics and astronomy at the University of British Columbia in Canada.

“It is positive to see the ESA using available spacecraft capabilities to reduce the risks of lethal re-entry debris, instead of leaving the re-entry outcome entirely to chance,” said 

For large satellites in orbit that were never designed to conduct controlled re-entries, assisted/semi-controlled re-entries are a step in the right direction, Boley told Inside Outer Space.

“It should nonetheless be recognized, while things went well in this case, that such maneuvers are not a controlled re-entry and still carry substantial re-entry time uncertainties. Not all large spacecraft in orbit will be capable of such a semi-controlled re-entry, either,” Boley pointed out.

Image credit: Johan Swanepoel/Adobe Stock via RAND

Moving forward, Boley said that states and operators need to work together to develop a controlled re-entry regime for new satellites and launch vehicles in an effort to limit the risks of re-entry debris to people on the ground, at sea, and in airplanes in flight.

“Such a regime would include requirements for controlled re-entries, and in circumstances where a controlled re-entry is not feasible, the requirements would include risk reduction measures as part of the satellite design,” said Boley.

Leave a Reply