NASA is asking American industry to help develop solar electric propulsion engine powerful enough to meet the goals of its Asteroid Redirect Robotic Mission (ARRM).
ARRM’s goal is to land on an asteroid, retrieve a boulder of approximately twenty tons and then haul the boulder to a position close to the moon. Once there, astronauts aboard an Orion spacecraft will rendezvous with the boulder and take samples for analysis on earth.
One great benefit of this mission will be the human-robot interaction, a core skill for a mission to Mars. Solar electric propulsion (SEP) is considered essential to making this mission successful.
A SEP engine uses solar power to electrically charge xenon atoms creating xenon ions. The SEP then electrifies a pair of grids which accelerate the xenon ions through a nozzle creating thrust.
NASA has had great success with smaller scale Ion engines used on its Dawn and Deep Space 1 probes. These engines were nowhere powerful enough to accomplish the goals of ARRM.
And so, NASA turns to American industry to find a solution.
Efficient, powerful, proven, ion engines are the key to long range missions in the solar system, electric power is seen as the key to making them run and for now the plan is to use solar power to provide the electricity.