# Large Constellations of Small Satellites: A Survey of Near Future Challenges and Missions
## 3 Future Challenges
### Space Traffic Management  
Among the issues to be faced in the \"constellation race\" the space traffic management is probably the most critical, yet not directly faced. [7; 38] are the few works authors are aware of that discuss this topic, the first specifically within the large constellations framework and the second in general. Part of this topic is closely related to space debris. Debris are already a problem that is being faced actively with surveillance networks (e.g., the JSpOC, Joint Space Operation Center [39]) and avoidance maneuvers from the spacecraft operators. Will this network be able to withstand, i.e., generate alerts and provide further assistance, also for the future traffic? What happens if a constellation is very valuable but cannot embark a propulsion system for collision avoidance? First steps towards answering these questions are found in very recent studies: [7] discusses the changes in LEO population environment due to large constellations, while authors of [40] advocate the need for updating state-of-the art space debris modelling as a result of the evolving debris environment. A preventive approach is already taking place thanks to debris mitigation policies i.e., making the spacecraft reenter at the end of its life. [40] suggests that the constancy of the rate at which spacecraft fragmentations (historically the main cause of space debris) occurs, despite the drastic increase in the number of orbiting spacecraft [41], is an indication that mitigation efforts put in place are being successful. Whether or not such efforts will remain effective when hundreds of dismissed or failed constellation satellites will be deorbiting almost simultaneously, is still an open point. Alternative solutions to the debris problem include active removal [42] and space-based surveillance networks [43; 44]. On-Orbit servicing is a further option to decrease the amount of failed or dead satellites that become a debris. Its implementation, however, is still costly and technically challenging [45]. Regulatory aspects of space traffic management are instead poorly covered. Currently, once a free orbital slot has been identified, the common practice consists of seeking for a technically and economically viable solution to reach such a slot. Not much attention, instead, is payed to interferences affecting other operators, that might be caused while the spacecraft are reaching the target orbit or during de-orbiting at the end of life. The authors in [7] envisage an architecture similar to the air traffic management with traffic zones (orbital slots) and \"flight plans.\" Though up to now it has been safe to assume that space is so large that satellite operations do not interfere with each other, this may not be true in the near future. For instance, with thousands more spacecraft in orbit, an Earth observation satellite may find unexpectedly another one in its field of view, or a region of space may become so overcrowded as to impact the quality of space observations from ground. One such event was indeed experienced after the launch of the first Starlink satellites (Figure 5). Besides the regulatory part, there are also technical challenges to be overcome. In [46; 47] for instance, machine learning through support vector machines is used both to monitor satellite health and address management issues, highlighting a great potential of neural networks for enhancing space traffic management. Other approaches, such as space transponders and enhanced tracking with corner reflectors or onboard GPS receivers, are pushed forward in [7]. Novel concepts of conjunction assessment services are also on their way. In [38] a prototype of a ground-based service that can interface with all subscribed satellite operators (scalable solution) was presented. Besides integrating different object databases and giving alerts similarly to JSpOC, it can compute the most suitable avoidance maneuvers.