Solar electric propulsion (SEP) refers to the combination of solar cells and electric thrusters to propel a spacecraft through outer space. This technology has been exploited in a variety of spacecraft by the European Space Agency (ESA), the JAXA (Japanese Space Agency), Indian Space Research Organisation (ISRO) and NASA. SEP has a significantly higher specific impulse than normal chemical rockets, thus requiring less propellant mass to be launched with a spacecraft and it has been evaluated for missions to Mars.
Solar electric propulsion combines solar panels on spacecraft and one or more electric thrusters, used in tandem. There are many different types of electric thrusters, including a so-called ion thruster, a term that is often incorrectly used to describe all types of electric thrusters.
The NASA Solar Technology Application Readiness (NSTAR) ion engine has been used with photovoltaic solar panels, which was tested on the Deep Space 1 mission along with Solar Concentrator Arrays (Launched in 1998 as part of the New Millennium Program).
SEP has been studied as a technology for a mission to Mars. In particular the high specific impulse of the ion engines could lower overall mass and avoid having to use nuclear technology for power when coupled with solar panels. A 1998 study for SEP for a human mission suggest that a human-sized spacecraft would need 600 to 800 kilowatts of electrical power coupled with ion engines with a specific impulse of 2000 to 2500 seconds.