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A computer-generated diagram shows the projected orbital paths of SpaceX’s Starlink satellite and the European Space Agency’s Aeolus satellite. (ESA Graphic via Twitter)

The European Space Agency says it performed a collision avoidance maneuver over the Labor Day weekend to head off a potential crash between its Aeolus wind-measuring satellite and one of SpaceX’s Starlink broadband data satellites.

In a series of tweets, ESA said Monday’s event marked the first such maneuver taken to avoid an active satellite in what’s expected to become a “mega constellation” of thousands of satellites — and it warned that such maneuvers posed a grave challenge for future orbital traffic management.

“As the number of satellites in orbit increases … today’s ‘manual’ collision avoidance process will become impossible,” ESA tweeted.

The space agency said the maneuver was executed successfully about half an orbit before the close encounter.

One point of controversy relates to how satellite operators respond to potentially shifting assessments of orbital collision risks. For example, should SpaceX have maneuvered its satellite, which was descending through Aeolus’ altitude as part of a deorbit test?

In a statement sent via email today, SpaceX acknowledged that it would have taken more action if it weren’t for a “bug” in its on-call paging system:

“Our Starlink team last exchanged an email with the Aeolus operations team on August 28, when the probability of collision was only in the 2.2e-5 range (or 1 in 50k), well below the 1e-4 (or 1 in 10k) industry standard threshold and 75 times lower than the final estimate. At that point, both SpaceX and ESA determined a maneuver was not necessary.

“Then, the U.S. Air Force’s updates showed the probability increased to 1.69e-3 (or more than 1 in 10k) but a bug in our on-call paging system prevented the Starlink operator from seeing the follow-on correspondence on this probability increase – SpaceX is still investigating the issue and will implement corrective actions. However, had the Starlink operator seen the correspondence, we would have coordinated with ESA to determine best approach with their continuing with their maneuver or our performing a maneuver.”

Starlink satellites are designed to make global broadband access from low Earth orbit available to millions or perhaps even billions of people who are currently underserved. The satellites are designed and manufactured at SpaceX’s facility in Redmond, Wash.

When the first batch of 60 operational satellites were set for launch in May, SpaceX CEO Elon Musk said the spacecraft would automatically upload the coordinates for other orbiting objects from an Air Force database and adjust their own orbits accordingly. No reference was made to that capability after Monday’s event.

Several of SpaceX’s satellites failed to reach their intended 342-mile-high (550-kilometer-high) orbits, but the satellite at issue this week was apparently being used to test SpaceX’s system for deorbiting spacecraft at the end of their useful life.

In the process of lowering its altitude, the Starlink 44 spacecraft came into the vicinity of the Aeolus craft, which sticks to an altitude in the range of 200 miles (320 kilometers). ESA said its debris team determined that the safest option would be to increase Aeolus’ altitude and pass over the Starlink satellite for the projected close encounter at 11:02 GMT (4:02 a.m. PT) Monday.

Three thruster burns, ending less than an hour before the crucial time, were executed to increase Aeolus’ altitude by about 350 meters (1,150 feet), ESA said in a blog posting. After the projected encounter, Aeolus “called home as usual to send back its science data – proving the maneuver was successful and a collision was indeed avoided,” the operations team tweeted.

ESA said its satellite fleet performed 28 collision avoidance maneuvers during 2018, but most of those maneuvers were done to head off the risk of colliding with dead satellites or orbital debris. “It is very rare to perform collision avoidance maneuvers with active satellites,” ESA’s operation team said.

“These avoidance maneuvers take a lot of time to prepare – from determining the future orbital positions of all functioning spacecraft, to calculating the risk of collision and potential outcomes of different actions,” ESA said.

Authorities in Europe as well as the United States say the current system of space traffic management is likely to become overwhelmed if SpaceX and other ventures, including Amazon and OneWeb, execute their plans to put thousands of broadband data satellites into low Earth orbit over the next few years. That could raise the risk of cascading satellite collisions, with potentially catastrophic effects on global communication and navigation.

“No one was at fault here, but this example does show the urgent need for proper space traffic management, with clear communication protocols and more automation,” Holger Krag, head of space safety at ESA, was quoted as saying in today’s blog posting.

Krag noted that “in the absence of traffic rules and communication protocols, collision avoidance depends entirely on the pragmatism of the operators involved.”

“Today, this negotiation is done through exchanging emails an archaic process that is no longer viable as increasing numbers of satellites in space mean more space traffic,” he said.

ESA said it’s preparing to use artificial intelligence to automate the traffic management process. “From the initial assessment of a potential collision to a satellite moving out of the way, automated systems are becoming necessary to protect our space infrastructure,” the agency said.

The issue is likely to come up at a ministerial council meeting in November, ESA said.

Meanwhile, the U.S. Department of Commerce was tasked in April with developing a new system for managing space traffic and minimizing space junk. At last month’s meeting of the National Space Council, Commerce Secretary Wilbur Ross reported that his department is “actively pursuing” such efforts, and is working to develop an Open Architecture Data Repository for space debris information.

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