EU Space Surveillance and Tracking (EU SST) confirmed that the reported potential collision between two inactive space objects – rocket body SL-8 R/B and meteorological satellite OPS 6182 – did not take place on Friday 9 April at 17:18 UTC. The EU SST network of sensors only detected a single echo for both objects after the Time of Close Approach (TCA). The collision could have generated a large number of fragments, creating serious risks for other objects in space.
In April 2021, the EU Space Surveillance and Tracking programme (EU SST) monitored what could have been one of the most impactful collisions between artificial space objects in history. In a successful cooperation between EU Member States, EU SST actively followed and communicated on the event as soon as it learned about the risky conjunction.
On 7 April, EU SST first reported on a close approach between two inactive objects located in Low Earth Orbit (LEO), a congested space region, with orbits around 780 km in altitude. The two defunct objects were SL-8 R/B (1981-041B), at 74 degrees of inclination, and OPS 6182 (1978-042A), located in a sun-synchronous orbit (SSO) at 98.5 degrees of inclination. The objects were expected to cross paths over the Bering Strait on 9 April at a relative velocity of 14.6 km/s. The first estimates pointed to a Probability of Collision (PoC) of 1.5% and a miss distance between the two objects of approximately 40 m. In LEO satellite operations, a PoC above 0.01% is usually already considered a high-level probability that prompts further analysis and potential mitigation measures if manoeuvrable satellites are involved.
Due to the high risk of the event, EU SST immediately activated a Taskforce led by SatCen created to deal with scenarios associated with critical operations and events of high public interest. This taskforce coordinated the monitoring and communication of the potential collision. The network of sensors contributing to EU SST was requested to observe the objects and provide additional measurements.
Over the course of the following 24 hours, the Operations Centres (OCs) in charge of collision avoidance (FR and ES) provided several refined estimates: the estimated miss distance seemed to decrease to below 10 m and the calculated Scaled PoC rose to over 20%. Simulations run by the OC responsible for EU SST’s fragmentation analysis service (IT) concluded that the collision would potentially generate more than 4 million fragments, out of which more than 400 would be larger than 20 cm. The plots below show the conjunction plane at TCA for one of the estimates, the Delta-V distribution of the whole cloud of fragments that would be generated, and a Gabbard diagram with the extent of orbital regimes that the simulated 400+ fragments larger than 20 cm would reach.
On 9 April, the last EU SST estimates before TCA (17:18 UTC) indicated that the close approach remained stable in geometry and in Probability of Collision, with a miss distance of ~21 m and a Scaled PoC still over 20%. EU SST informed the public about these findings on social media.
Within two hours after TCA, EU SST’s network of sensors detected objects SL-8 R/B and OPS 6182 as a single echo, respectively, at passes over three radars. The Operations Centres reported that, most likely, the collision did not take place. This assessment was confirmed one hour later with independent data from three additional EU SST radars, and from even more sensors since then.
After analysing all measurements in the days following the close approach, the two EU SST Operations Centres in charge of collision avoidance (ES and FR) each independently estimated that the objects passed at a mere distance of approximately 10 m of each other, with a radial separation of only 6.8 m in one estimate and 9 m in the second one. This was indeed a close call, as both objects are over 5 m in size.
In total, 17 sensors managed by six EU SST Operations Centres followed the objects over the course of the event. As the objects are in LEO orbits, radars are the most suitable sensors to observe these two objects. Two surveillance radars had regular measurements even before the event was detected, and started following it closely, with the help of three telescopes in the final hours prior to the TCA. After TCA, six radars (five of which are tracking radars) helped discard the collision within three hours, and telescopes provided additional confirmation.
During the whole event, the European Commission was permanently informed about new developments and the potential impact and risks of the event. Live updates were also provided on social media, raising high interest among users and specialised media outlets.
While this event ended successfully, it shows that continuous monitoring of the space environment is needed to ensure the safety and security of European economies, societies and citizens, as the number of close conjunctions is expected to grow given the increase in launches and the associated debris generated. To this end, close cooperation between all relevant actors in Europe will be essential, as proved by the coordinated action within EU SST in monitoring and communicating on this event.