## 4 Results
### Drag Compensation
The difference between the left figures and the right figures is the final altitude, which corresponds to the 1-month and 1-year limit, respectively. The difference between the three figures (Figures 45-47) is the satellite density (i.e., ballistic coefficient). The qualitative results are the same for all figures. Satellites flying at low altitude have a small deorbit need, but the propellant mass rapidly increases with satellite lifetime. On the contrary, spacecraft flying at higher altitudes have a significant deorbit need but the propellant mass varies little with the satellite lifetime as drag compensation requires little delta-v at those altitudes. Thus, short lifetimes favor low-flying satellites, while the opposite occurs for long lifetimes. Higher ballistic coefficients shift the optimal orbit to lower altitudes. Longer final decay times provide a higher minimum altitude and lower deorbit propellant mass needs. Another way to present the results is shown in Figures 48-50. The same cases have been considered. This time the propellant mass fraction is presented as a function of the flying altitude, parametric with the satellite lifetime. It is possible to see that an optimal altitude exists that minimizes the total propellant consumption. Above the optimal altitude, increased deorbiting needs cause a higher propellant mass, while below, drag compensation is to be blamed. The optimal altitude shifts to the right as the lifetime is extended because of the total impulse increase for drag compensation. The curves shift to the left as the ballistic coefficient is improved. As expected from the results highlighted in the previous figures, the results have very low sensitivity to the lifetime on the right of the graphs, where deorbiting  
Figure 50: Propellant mass fraction for station keeping and deorbiting (from flight altitude) with a chemical thruster (_Isp_ = 300 s, satellite mass 50 kg, density 800 kg/m\({}^{3}\)), parametric with satellite lifetime (years): (a) natural re-entry time of 1 month; (b) natural re-entry time of 1 year. need prevails, and a very high sensitivity on the left, where drag compensation prevails. Consequently, the optimal points for shorter lifetimes perform poorly for longer lifetimes, while the optimal points for longer lifetimes are not as bad for short lifetimes. Finally, the effects of a different re-entry time limit (in this case 1 month vs. 1 year) are only to decrease the propellant mass requirements and set a higher minimum altitude threshold, but the curves' behavior seems the same with the same optimal altitudes. Electric propulsion is particularly suited for drag compensation as the thrust (thus power) requirements are relatively low (without the thrust/time trade-off of deorbiting) and the high specific impulse limits the propellant mass burden [50]. It is important to highlight that flying at lower altitudes can have important side benefits other than the deorbiting aspects, particularly for Earth observation missions. In reality, these advantages are much more than simple side benefits, and they can already justify the choice of a lower-than-usual flying altitude by their own [51].