Collision avoidance
The growing density of human-made objects in space is resulting in an increase in conjunction events. Collision avoidance has become an essential component of satellite operations, with many organizations relying on on-call satellite operators to initiate avoidance maneuvers to mitigate collision risks. Given the expected proliferation of conjunctions in the coming decade, it will be essential to automate this process, thus reducing operator workload and enhancing scalability.
Overview about the amount of Space Debris and its impact on satellite operations
As of April 2026, the number of objects cataloged by space surveillance networks reached over 40,000. However, the catalogues mainly comprise of objects larger than 10 cm in size. Smaller but also dangerous objects are largely undetected and amount to more than 1 million space debris objects greater than 1 cm. More than 18,000 active satellites operate in this congested space comprising operational payloads and debris alike. Of these, over 12,000 satellites operate in Low Earth Orbit (LEO), making this regime a hot spot for critical conjunctions. Given the number of satellites in orbit is expected to exceed 100,000 by 2030, collision avoidance activities will be of utmost importance to minimize the risk of fragmentation events in an already crowded orbital environment. The United States Space Force 19th Space Defense Squadron (19 SDS), which is surveilling conjunction events in space, has generated approximately 600,000 Conjunction Data Messages (CDMs) per day. (as at 2023) Compared to 2020 the daily rate tripled.
The frequency of these critical warnings is expected to proliferate, imposing significant pressures on satellite operators who rely on manual, analyst-dependent collision avoidance procedures. Most of the collision avoidance process is automated – there are models in place for collision screening, calculation of critical parameters at the time of closest approach (TCA), propagation of the states and covariances of the collision pairs to the TCA, and avoidance maneuver optimization, if a maneuver is deemed necessary. A key aspect of the process is go/no-go decision-making – deciding for or against a maneuver to mitigate the collision risk. Starlink satellites alone performed 144,404 conjunction risk mitigation manoeuvres between November 2024 and May 2025. While some entities such as SpaceX automate decision-making using PoC thresholds, many organizations rely on analysts working on-call around the clock to make decisions when faced with a critical conjunction. The need for manual go/no-go decision-making stems from the high stakes involved – an incorrect decision could, at worst, result in the loss of a satellite. Conversely, excessive maneuvers deplete the fuel budget of the mission and disrupt planned satellite operations.
The three key stages of a Collision avoidance operation
Collision avoidance operations usually consist of three key stages: conjunction risk assessment, where the severity of a close approach is evaluated; maneuver execution, where a CAM is planned and uploaded to the spacecraft if needed; and post-maneuver recovery, where the spacecraft is restored to its mission configuration after temporarily deviating from its operational orbit. Focusing the attention on the first stage, assessing the criticality of a conjunction event follows a well-established procedure that calculates the Probability of Collision (PoC) and other key metrics of the encounter by considering the states and associated uncertainties of both primary and secondary object. At the German Space Operations Center (GSOC), the Flight Dynamics Services (FDS) team leverages its Collision Avoidance System (CAS) to carefully manage its missions. The CAS relies on Conjunction Data Messages (CDMs) issued by international space surveillance and tracking institutions, for example the European EUSST or the US 19th Space Defense Squadron (19th SDS). Upon receiving a new CDM, the CAS retrieves the latest orbit determination results for the primary object and extracts the secondary object’s state vector and covariance data from the CDM. These parameters are then propagated to the Time of Closest Approach (TCA), generating different products that support FD operators in their decision-making process.
However, this traditional 1-vs-1 risk assessment approach becomes inadequate in the case of in-orbit break-up events. In fact, immediately following a fragmentation, the encounter between the debris cloud and a target object cannot be treated as a series of individual encounters as the state estimation of each piece of debris may require time. This delay creates a “blackout” period during which satellite operators are unable to take mitigation actions to reduce collision risk. During this time, CDMs cannot be issued. Additionally, very small fragments often go untracked due to technological limitations of current sensors, potentially leading to a dangerous underestimationof the PoC. Within this context, the GSOC FDS team is developing a tool that provides an assessment of the collision risk posed by a fragmentation event to their assets, focusing on the first hours after the break-up.
Further information
Ramos C., McKissock D., Herzer N., Eppich K., Bourque B., Parris J., Lessons Learned on Mega-Constellation Deployments and Impact to Space Domain Awareness. https://ntrs.nasa.gov/api/citations/20230012741/downloads/AMOS_2023_Paper_Final.pdf
Ravi P., Frueh C., Chow P., Linares Miguel Á., Siminski J., Jung O., Wermuth M., Aida S., Kahle R., Fiedler H. (2025) Comparative Analysis of Collision Avoidance Decision-Making Across Organizations. 9th European Conference on Space Debris. ESA Space Debris Office. https://conference.sdo.esoc.esa.int/proceedings/sdc9/paper/112/SDC9-paper112.pdf
Trombetta A., Zollo A., Fasano G., Opromolla R. & Isoletta G.(2025) Debris-Cloud Collision Risk Assessment with GSOC Collision Avoidance System. 9th European Conference on Space Debris. 2025-03-31-2025-04-04. Bonn. Germany. https://elib.dlr.de/213606/
Lawrence A., Rawls M. L., Jah M., Boley A., Di Vruno F., Garrington S., et al. (2022). The case for space environmentalism. Nature Astronomy. 6(4), 428–435. https://doi.org/10.1038/s41550-022-01655-6
Virgili, B. B., Flohrer, T., Krag, H., Merz, K., Lemmens, S. (2019). CREAM - ESA’s Proposal for Collision Risk Estimation and Automated Mitigation. First International Orbital Debris Conference. December 9-12. Sugar Land. TX. USA. https://www.hou.usra.edu/meetings/orbitaldebris2019/orbital2019paper/pdf/6031.pdf
European Space Agency, (2025). Space debris by the numbers. https://www.esa.int/Space_Safety/Space_Debris/Space_debris_by_the_numbers
Orbiting Now, (2024). Active satellite orbit data. https://orbit.ing-now.com/
SPACE X Starlink ANNUAL Report 2025.