Spacecraft designed for NASA and industry are increasingly using ion propulsion (a type of electrical propulsion) as their main method of propulsion.  They produce a much lower amount of thrust than traditional chemical propulsion, thus the trajectories they follow are known as low-thrust trajectories.  There are many reasons ion engines are appealing.  One reason is their highly efficient use of fuel and electrical power.  Another is they are capable of greater Dv’s than conventional rockets.  For earth satellites, these advantages mean reduced launch costs, increased payloads, and/or longer lifetimes.  For interplanetary missions the benefits are even greater in that many long missions that were unaffordable or impossible with conventional engines are suddenly viable with ion propulsion.  Two NASA spacecraft have been launched with ion engines as the main method of propulsion and a third one is in development.  Additionally, many industrial earth bound satellites are currently in orbit about the earth.
            In support of this technology, much research is required in the areas of low-thrust trajectory design and control.  This effort is necessary because ion engines operate differently from conventional rocket engines.  Ion engines produce a very small amount of thrust about 0.5 newtons, and they are capable of operating for many months at a time.  On the other hand, conventional rocket engines produce much higher thrust, on the order of 10 to 100 kilonewtons.  However, it only lasts a few minutes at most.  It is this difference in thrust and duration that necessitates low-thrust trajectory research.  It is a difficult, highly nonlinear problem with complex dynamical models so it requires much effort.  Research topics in this area include:  Optimal Low-Thrust Trajectory Design Methods and Closed-Loop Trajectory Control of Low-Thrust Spacecraft.

 

Figure An Optimal Low-Thrust Trajectory