Collision Probability Was 10 Times Higher Than ESA Threshold
In a series of tweets, ESA revealed it had to fire the thrusters on the satellite to move it and avoid colliding with Starlink 44 as the orbital paths of the two vehicles intercepted each other 320 kilometers above Earth.
ESA's head of Space Debris Office Holger Krag said that the agency's conjunction assessment team noticed the potential close approach about five days earlier using data from the U.S. Air Force's 18th Space Control Squadron, which monitors space traffic.
Collision probability increased over the following days prompting ESA to start the maneuver preparation and share the plans with SpaceX. Elon Musk's company, however, refused to move its satellite even after learning of the potential collision.
"Based on this we informed SpaceX, who replied and said that they do not plan to take action," Krag said.
"It was at least clear who had to react. So we decided to react because the collision was close to 1 in 1,000, which was ten times higher than our threshold."
In a statement, SpaceX said it missed the update that the probability of the two satellites colliding has increased to more than 1 in 10,000 because a bug in its on-call paging system prevented the Starlink operator from seeing the follow on correspondence.
"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," the statement said, as cited by CNET.
ESA said it has 28 collision avoidance maneuvers in 2018, but most of these were to avoid dead satellites or space debris. Maneuvers to avoid collision with active satellites are very rare, but this could become more frequent with the arrival of mega constellations like Starlink.
ESA said that as the number of Starlink and other satellite constellations increase to hundreds or thousands of satellites, manually avoiding collision will become impossible. It is now working on automating the process of collision avoidance using artificial intelligence.