Space missions require fuel to be expended, and every kilogram of fuel on-board is a kilogram of scientific equipment sacrificed to preserve the mission feasibility. The ability to optimize the spacecraft trajectory by minimizing fuel is key to greater scientific return. In some cases, this becomes the only way for the desired mission to succeed. This thesis investigates the problem of trajectory optimization for the case of multiple Near-Earth Asteroids (NEAs) rendezvous missions using Electric Propulsion(EP). NEAs are of fundamental interest for the understanding of our solar system as well as for planetary defence and are stepping stones for interplanetary human flight.