Space Trajectories. Low Thrust Orbital Transfer

Electric propulsion is becoming widespread on 21st-century satellites. Its principle is to accelerate ionized particles by subjecting them to an electrostatic or electromagnetic field, ejecting them at very high speeds. The specific pulses (1,000 s to 8,000 s) are far better than those of chemical propellants based on propellant combustion (300 s to 450 s), but the low mass flow rates mean that thrusts are generally below 1 N. As a result, electric propulsion is reserved for orbital maintenance or station-keeping phases. It offers significant weight savings, at the cost of more complex and time-consuming maneuvers. In contrast to a chemical thruster, the motor requires a high level of electrical power to operate. As this power is generated by solar panels, the transfer must take account of eclipse periods, during which propulsion is cut off.

This article deals with the launch of an electrically-powered satellite from an injection orbit reached by a launch vehicle. Two situations can be distinguished, depending on the orbit to be reached and the consumption and duration objectives.

The first situation concerns the positioning of satellites in high orbit. These satellites for television, meteorology and navigation are placed in circular orbits of the GEO (Geostationary Earth Orbit) equatorial type at an altitude of 35,786 km, or MEO (Medium Earth Orbit) inclined at around 55° and an altitude of 23,000 km. The launcher generally injects the satellite into a low-perigee elliptical orbit of the GTO (Geostationary Transfer Orbit) type, or a low circular orbit of the LEO (Low Earth Orbit) type. These satellites represent very costly investments, and must be made operational as soon as possible. As the transfer may take several weeks or months, the main objective is to minimize the transfer time, taking into account repeated eclipse passages. The first part of this article deals with the problem of transferring satellites in the shortest possible time.

The second situation concerns the positioning of satellite constellations in low earth orbit. These communications or Earth observation satellites are placed in circular LEO or SSO (Sun Synchronous Orbit) orbits, distributed over different planes covering the Earth's surface. The constellation is deployed by launching dozens or hundreds of satellites into a low initial orbit. Each satellite must then reach its final orbit, requiring a change of plane that is generally quite costly. The main objective is to minimize power consumption, in particular by taking advantage of the natural precession due to the Earth's flattening. The second part of this article deals with the problem of minimum power transfer.