Emailer Ken Talton asks:

I’m curious about the VASIMR engine, its performance and maturity. I understand you are working on Prometheus via JIMO and was wondering if the VASIMR technology is going to be used on that mission. For that matter how close to an actual space engine is this? What I’ve seen in print indicates that it has performance throttleable between ion engine efficiency and thrust like a chemical rocket. How does this compare to, say, a Spaceshuttle engine? Could it be used in a launch vehicle for instance? How big a breakthrough is it really?

JIMO is planned to use ion propulsion, a technology that is currently in use (in communications satellites and in Deep Space 1), and only requires scaling up. VASIMR is an entirely different kind of electric propulsion. Both types work by accelerating charged particles with electromagnetic fields, but ion propulsion is driven by electrostatic forces, whereas VASIMR accelerates a plasma using electromagnetic forces. It has the potential for much higher thrust (though lower specific impulse, so the fuel efficiency isn’t as good), but it’s only in the preliminary development stages. Neither type of engine would have high enough thrust/weight ratio to be used as an engine on a launch system–they’re only useful in space. There’s a good tutorial on the subject here.