Elon Musk's SpaceX has claimed that a mysterious comms "bug" was what stopped it from moving its satellites away from an orbital collision course.

The European Space Agency carried out a "collision avoidance manoeuvre" earlier this week when it realised one of its satellites was on course to potentially bump into a flock of SpaceX birds, as we reported.

SpaceX's Starlink constellation – specifically, Starlink44 – and the ESA's Aeolus Earth observation satellite were separated from their impending meeting around half an orbit before it was likely to take place.

What wasn't clear, however, was why SpaceX didn't take any action to shift its own satellites away from doom. And now they've piped up to say why.

"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," SpaceX told The Register. "At that point, both SpaceX and ESA determined a maneuver was not necessary."

So far, so good. But then... "the US 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."

Meanwhile, the increased probability of a space smash prompted the ESA to fire up Aeolus's boosters to be certain of passing 350 metres above Starlink44.

And that's what drove ESA to start issuing statements calling for "an automated risk estimation and mitigation initiative". Although SpaceX is "still investigating the issue and will implement corrective actions", it has claimed that if it saw ESA's (presumably increasingly frantic) messages, it would have acted on them.

Starlink is planned to grow to become a 12,000-strong fleet within the next nine months.

Terrestrial forms of transport have long dealt with the "what to do when you're about to crash into someone else" problem. At sea, the International Regulations for Preventing Collisions (COLREGs) say "turn right" if you're running head on into someone else. Aircraft must also turn right if they're in an imminent collision situation.

As for space, it'll evidently take a few years for people to agree on what to do when expensive and delicate items whizzing around in orbit start coming closer and closer together. ®