Figure above (left plot) depicts the dependency of the power generated by solar arrays to the distance from the Sun. There are also other factors that affect the net power such as degradation in the efficiency of solar panels due to radiation. Part of the power is used for energizing several sub-systems of a spacecraft. Some portion of the power is used to energize the power processing unit (PPU), which is responsible for modulating the power sent to thrusters. The net power has to be distributed between engines in an "optimal" manner. The right plot depicts the changes in performance of a number of electric thrusters versus the engine input power. Figure below (the left plot) depicts the the trajectory of a spacecraft from earth to comet 67P Churyumov–Gerasimenko (it was the destination of the European Space Agency's Rosetta mission). Here, in our model, the spacecraft is equipped with four BPT-4000 Extra High-Isp engines. The right plot (lower subplot) depicts time history of thrust generated by these four engines, and in the upper sub-plot, the distance from the Sun is shown. This minimum-fuel trajectory consists of three revolutions around the Sun en-route to the comet, whereas the optimal thrust profile has a complex structure. The change in the magnitude of the thrust is an indication that during some portions of flight multiple engines have been used. For instance, during the early phases of flight all four engines are switched on since power input is enough, whereas the thrust value changes as the distance from the Sun increases.