In a sort of space “game of chicken” on 2 September 2019, 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) found that it was forced to move its Aeolus wind study spacecraft. This was in order to avoid a perceived chance of collision with a Starlink (“Starlink 44”) communications satellite after its owner SpaceX refused to move its satellite. ESA’s head of the space debris office, Holger Krag, said that the agency felt it had to make the move as the chance of collision was judged to be circa 1 in 1000. 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 in a quick enough time. So, like the rules of the seas where motor powered craft have to give way to sail boats, so some wondered if SpaceX had reasoned the same way. It had been originally alleged that SpaceX had declined to move their spacecraft but they denied this citing an internal communications error as the reason for their inaction.

Artist’s impression of ADM-Aeolus in orbit. Courtesy: ESA


In the event, the manoeuvre successfully took place 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, and that the “Starlink 44” (2019-029AV) had put itself “in the way” via a crossing orbit of Aeolus’s own orbit of 314 x 311 km at 96.7 degrees inclination after being lowered into a 345 x 311 km at inclination 53 degrees as part of a test of the Starlink constellation’s de-orbiting techniques.

Still from animation showing deployment of Starlink spacecraft’s solar array. Courtesy: SpaceX