In what resembled a space “game of chicken”, two satellites heading towards each other had to decide which would move first. After a warning from the US Space Command’s tracking system, the European Space Agency (ESA) was forced to move its Aeolus wind study spacecraft on 2 September. This was in order to avoid a perceived chance of collision with a Starlink (“Starlink 44”) communications satellite after its owner, SpaceX, failed to move it and was apparently unresponsive to emails.
The collision avoidance manoeuvre happened after SpaceX failed to coordinate with ESA to move Starlink 44 (2019-029AV) – SpaceX noted that there had been an email failure within the company. Starlink 44 had lowered its orbit into a 345 x 311 km at 53 degrees inclination as part of a test of the Starlink constellation’s deorbiting techniques, which presented a collision risk as it crossed Aeolus’s orbit of 314 x 311 km at 96.7 degrees.
Holger Krag, who heads ESA’s space debris office, said the agency felt it had to make the move because the chance of collision was judged to be about 1 in 1,000. Not a high chance, but ten times higher than the ESA threshold to avoid a serious debris producing event.
Commentators have pointed out that Aeolus (aka ADM-Aeolus) has conventional chemical thrusters allowing fast moves, while the Starlink design has electric propulsion, which would not have allowed a manoeuvre to take place quickly enough. So, like the rules of the seas where motor powered craft have to give way to sail boats, even if coordination with SpaceX had happened, it was likely that Aeolus would have been moved anyway instead of Starlink 44. In the event, the manoeuvre was a success and no collision occurred. However, questions are now being asked about who had the legal “right of way”, especially given that Aeolus was placed into orbit before the Starlink experimental constellation.