## 6 Conclusions
Above around 60\({}^{\circ}\) parity is achieved, while near a polar orbit the thrust drops to near zero. They are also the less mature technology for the moment. The most interesting combination is chemical propulsion with a drag sail, which are the most different between each other but both relatively simple. In fact, the first is fast and effective at any altitude (included higher ones) but becomes heavy for large displacements, while the second is slow but passive, lightweight and much more effective at low altitudes. A proper combination of the two technologies can provide a minimum mass solution that rapidly take out the spacecraft from the original (crowded) orbit and let it decay slowly from a lower altitude. For very short times the total mass approaches the one of a chemical-only system, while for very long times the total mass approaches the one of a sail-only system. For intermediate deorbit times on the order of 1 year savings around 30% are potentially possible. The deorbit device can be integrated or independent. The first solution is generally the simplest, cheapest and lightweight but requires the satellite to stay alive up to disposal. An independent system is more challenging (and probably heavier and expensive in total) but could provide deorbit after satellite death, giving the possibility to exploit the full satellite lifetime. This is particularly interesting for large platforms with very long lifetimes. Health monitoring can help the integrated solution to adapt to the single satellite history. Active debris removal is a possibly interesting future option both to retrieve current space junk but also for new spacecrafts that fail to deorbit or that do not carry a disposal system. This technology could go hand in hand with life extension solutions and the general new paradigm of commercial in-orbit servicing. Re-entry can be controlled or uncontrolled. The first solution requires a high thrust solution, at least for the terminal part, and it is necessary for recoverable/reusable systems or spacecrafts that produce dangerous debris upon re-entry. The uncontrolled re-entry is a simpler and more flexible option but requires compliance with design for demise practices. A completely different paradigm is drag compensation. In this case no deorbit device is necessary and re-entry is guaranteed at the end of life as the satellite flies at very low altitudes. This solution requires a propulsion system for station keeping. Drag compensation is favored by shorter lifetimes (few years), higher specific impulses (e.g., electric propulsion) and high ballistic coefficients (i.e., slender design) and is particularly interesting for Earth observation missions where a lower altitude can improve the resolution by a factor of 2. Plenty of options are available for deorbiting satellites in LEO, and this paper can be used as a starting point for mission/system design, trade-offs and preliminary selection. The most important step forward currently seems to be the regulatory framework, imposing stricter rules and active mandatory sanctions. This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. Data Availability Statement: Not applicable. **Acknowledgments:** The author wants to thank [PERSON] (now at Technology for Propulsion and Innovation, T4i) for helping with the reference material. A thank you also to [PERSON] (University of Padova and T4i CEO) for useful ideas. **Conflicts of Interest:** The author declares no conflict of interest.