The Aeolus (Atmospheric Dynamics Mission Aeolus) Earth observation satellite of the European Space Agency (ESA) narrowly avoided a collision in low Earth orbit (LEO) with one of the Starlink internet satellites operated by SpaceX identified as Starlink 44.

ESA said it was forced to take the evasive maneuver on Sept. 2 that prevented the destruction of both satellites because SpaceX either wouldn't or couldn't do so. ESA talked to SpaceX, which declined to move Starlink 44, said Holger Krag, head of the ESA Space Debris Office.

"Based on this, we informed SpaceX, who replied and said that they do not plan to take action," Krag told Forbes. "It was at least clear who had to react."

ESA commanded Aeolus to fire its thrusters on the morning of Sept. 2. The maneuver increased Aeolus' orbit to pass safely over that of Starlink 44. The Starlink bird is one of the first 60 satellites in SpaceX's internet mega-constellation that will ultimately number close to 12,000 small satellites.

"The manoeuvre took place about 1/2 an orbit before the potential collision. Not long after the collision was expected, #Aeolus called home as usual to send back its science data — proving the manoeuvre was successful and a collision was indeed avoided," tweeted ESA.

"It is very rare to perform collision-avoidance manoeuvres with active satellites. The vast majority of ESA avoidance manoeuvres are the result of dead satellites or fragments from previous collisions. #SpaceDebris," said another tweet.

ESA took action after learning from the U.S. military the probability of a collision was 1 in 1,000. This probability is 10 times higher than ESA's threshold for conducting a collision-avoidance maneuver.

The maneuver, however, burned-off more of Aeolus' precious fuel, thereby shortening the life-span of this satellite. Aeolus' orbit at 320 km means it has to fire its thrusters every week to maintain this altitude, using up more of its remaining propellant.

SpaceX said its inaction was caused by a glitch in the Starlink paging system. It said "a bug in our on-call paging system prevented the Starlink operator from seeing the follow-on correspondence on this probability increase."

SpaceX said it's still investigating the issue and will implement corrective actions. It noted that had the Starlink operator seen the ESA correspondence, "we would have coordinated with ESA to determine best approach with their continuing with their maneuver or our performing a maneuver."

SpaceX said its Starlink team last exchanged emails with the Aeolus team on Aug. 28 when the collision risk was 1-in-50,000 range.

Launched in August 2018, Aeolus is the first satellite capable of observing what winds are doing on Earth and into the stratosphere 30 km high. Starlink 44 was among 60 Starlink satellites orbited last May.