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NASA Gives Solar Ionic Propulsion A Monster Boost (networkworld.com)
coondoggie quotes a report from Network World: NASA this week took a giant step toward using solar electric power for future space missions by awarding a $67 million contract to Aerojet Rocketdyne to develop an advanced electric propulsion system. Network World writes, "Specifically, Aerojet Rocketdyne will develop and deliver an integrated electric propulsion system -- known as the Advanced Electric Propulsion System (AEPS) -- consisting of a thruster, power processing unit (PPU), low-pressure xenon flow controller, and electrical harness. Such a system would deploy large solar arrays that can be used to convert sunlight into electrical power that ionizes atoms of xenon which is the propellant for the spacecraftâ(TM)s thrusters. In addition, such a power plant could potentially increase spaceflight fuel efficiency by 10 times over current chemical propulsion technology and more than double thrust capability compared to current electric propulsion systems, NASA said." NASA's plan is to use this propulsion system on its future Asteroid Redirect Mission, as well as on its mission to Mars.
I had the same question, and found the answer readily enough in layman's terms on wikipedia: https://en.wikipedia.org/wiki/....
It seems like there are three key reasons (which I am listing also in layman's terms):
1) As a "noble gas" xenon is typically inert, which reduces corrosion in storage and long-term usage as a thruster fuel
2) The gas can be stored in liquid form (more dense) at room temperature, unlike liquid oxygen or hydrogen, which makes it easier to transport and handle
3) It is far up enough on the periodic table that its electron shells can be excited with less energy input than other inert gases, making it more ideal for low-power
systems such as the solar panels that power the ion engine.
Bear in mind that I may be interpreting those wrong, but I do only have two years of university chemistry.