The European Space Agency (ESA) is urging development of automated collision avoidance systems to replace “archaic” email exchanges between satellite operators to prevent satellites from running into each other in Earth orbit. Yesterday ESA had to maneuver its Aeolus earth science satellite to ensure it did not collide with SpaceX’s Starlink 44. SpaceX acknowledged today that a “bug” in its on-call paging system prevented its Starlink operator from realizing the risk of collision had increased.

ESA announced yesterday that it had moved Aeolus to a higher orbit to avoid Starlink 44, the 44th of SpaceX’s first set of 60 Starlink satellites launched in May. Three of the 60 did not work properly. SpaceX said they will “passively deorbit.” Another two are being intentionally deorbited to simulate end-of-life disposal. The satellite in question, Starlink 44, reportedly is one of the latter.

Today, ESA expanded on the story, explaining how its space debris office calculated the growing probability of a collision over the past week. Last Wednesday, it contacted Starlink personnel and was told the company had no plans to move its satellite. ESA’s threshold for conducting an avoidance maneuver is a more than 1 in 10,000 chance of collision. That was reached on Thursday so it began making preparations to raise Aeolus’s altitude by 350 meters. By Sunday, the chance was “about 1 in 1,000, 10 times higher than the threshold.” On Monday, ESA executed the maneuver.

Some media reports were critical of what appeared to be SpaceX’s refusal to move Starlink, but ESA painted it in a different light.

Contact with Starlink early in the process allowed ESA to take conflict-free action later, knowing the second spacecraft would remain where models expected it to be. — ESA

One of the problems with moving a satellite to avoid a collision is making sure it does not put the satellite in a different perilous situation. It also expends fuel, which can shorten the satellite’s operational lifetime.

SpaceX issued its own statement today.

“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.” — SpaceX

Whatever the specifics of this particular instance, ESA’s message is that this type of collision avoidance system — emailing between satellite operators — is not going to work as more and more satellites are launched.

Holger Krag, ESA’s Head of Space Safety, said that no one did anything wrong, but it shows the “urgent need for proper space traffic management, with clear communication protocols and more automation.”

“This example shows 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.” — Holger Krag, ESA

For its part, ESA is getting ready to automate the process of calculating the possibility of a collision — “conjunction analysis” — through to the actual step of maneuvering a satellite out of harm’s way. It wants to rely on space-based communications links to “save precious time.”

The amount of time will shrink as mega-constellations, each comprising hundreds or thousands of small satellites, begin populating Earth orbit. SpaceX’s Starlink is just one example. OneWeb, Telesat, Kepler and LeoSat are others.

Avoiding collisions between a burgeoning number of active satellites in addition to tens of thousands of pieces of uncontrollable space debris is a major concern for all users of Earth orbit today and in the future. The Air Force’s 18th Space Control Squadron at Vandenberg Air Force Base collects data on objects in orbit and shares most of that data with others. It wants to be relieved of the responsibility for interfacing with civil and commercial operators, however, so it can focus on military requirements.

President Trump’s Space Policy Directive-3 (SPD-3) assigns that interface responsibility to the Department of Commerce, but Congress has not passed legislation approving it. Opponents argue that the FAA should be in charge because of its expertise in air traffic management. Secretary of Commerce Wilbur Ross told the National Space Council on August 20 that the Department is moving ahead nonetheless and creating an Open Architecture Data Repository that will allow it to provide conjunction analyses in the future.