Re: Mass of NEXT and power source ?
A quick calculation suggests that over 8kW of electrical power is needed...
From the NASA press release:
"The 7-kilowatt class thruster could be used in a wide range of science missions..."
So, same order of magnitude.
Also from the press release:
"...the engine consumed about 1,918 pounds (870 kilograms) of xenon propellant, providing an amount of total impulse that would take more than 22,000 (10,000 kilograms) of conventional rocket propellant..."
So, the engine + powerplant mass could be up to 9,130 kilograms (20,086 pounds) more than the mass of a comparable rocket thruster and the entire system would still be less massive for the 5.5 year mission.
According to http://esto.nasa.gov/conferences/nstc2007/papers/Patterson_Michael_D10P3_NSTC-07-0014.pdf (Section III table II), a single thruster string is about 56.3kg. Add in the High Pressure Assembly and you get a total of 58.2kg. Call it 60, and there's still 9,070kg left over for the solar array.
According to http://www.asertti.org/events/fall/2011/presentations-workshop/Landis.pdfthe ISS solar arrays have a total mass of ~1,000kg, and according to http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html, they produce 84 kW.
Assume we need this extra solar panel mass because the probe is heading to the outer solar system, and we still have over 8,000kg of mass savings or additional payload available.