## 1 Introduction  
In recent times there has been an increasing concern about debris in Low Earth Orbit (LEO); in fact it is more and more frequent to find in the space-related news episodes of collision avoidance or sometimes even real (deliberate or unwanted) collisions or fragmentations of space objects [1, 2]. This is due to the fact that the satellite population in LEO is growing exponentially as the number of satellites put into orbit is much greater than quantity of the ones that are removed. Moreover, actual predictions of future space trends show a further order of magnitude increase due to the rise of the so-called mega-constellations [3, 4]. Space experts, shareholders and institutions all around the world have often claimed that the current behavior cannot be tolerated as it is in the future, otherwise the possibility of Kessler syndrome with its consequent terrible effects could become real [5, 6]. Even without a catastrophic scenario, a crowded space environment could render it more difficult to operate in space, as, for example, a significant number of collision avoidance maneuvers could impact the delta-v budget of the mission and the cost of operations. Therefore, it is necessary to take actions in order to limit as much as possible the amount of inoperative material orbiting in LEO. At these altitudes, the common way to clean the orbits is trough deorbiting and re-entry/disintegration of the space object.  
It is worth noting that the largest portion (around 2/3) of orbital debris is concentrated in LEO, and that only 6% of Earth orbiting objects are operational payloads [7, 8]. Moreover, LEO altitude distribution shows a peak around 800 km, which is in fact one of the favorite orbital altitudes and the target of the new small-sat rideshare launch missions like the recent Transporter service by SpaceX [9].  
There are plenty of important aspects related with space debris like monitoring, prediction, traffic management, protection, policy, autonomous detection, operations and so forth [10; 11; 12; 13; 14]. This paper will focus only on a single, unavoidable aspect, the means for de-orbiting satellites at the end of life. This topic has been already discussed previously [15; 16; 17; 18], and many numerical methods for the solution of the perturbed two-body problem have been developed, e.g., [19; 20; 21]. However, here some new further-simplified analyses will be shown, with a specific focus on the system level impact of the added devices, in particular total mass and deorbiting time.  
Generally, the principal proposed ways to deorbit a satellite are the natural aerodynamic decay (sometimes with a drag-boosting device, i.e., a drag sail), propulsive de-orbit, electrodynamic tethers, solar sails.  
Compared to previous works [15; 16; 17; 18], the range of the initial conditions for the comparison (altitude, decay time, inclination, duty cycle, deorbiting profile etc.) has been extended. Moreover, the combinations between different technologies will be also discussed. Finally, the paradigm of drag compensation will be directly compared with conventional deorbit. Other aspects closely related with deorbiting will be also reviewed.  
The aim of this work is to better understand the different available options with their strength and weaknesses and provide simple tools, guidelines and warnings in order to support future selections or developments.