[fremont_steve]

From “Wikipedia”

“A radioisotope thermoelectric generator (RTG, RITEG) is a nuclear reactor technology electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.”

I believe these are the power sources used on the Voyageur space-craft for instance. And we KNOW that it’s going to last till the 23rd century. You DID see Star Trek the Movie didn’t you? :-)

[thackney]

Those are the power source for the electronics. But propulsion was by the ejection of mass.

[thackney]

If you (or anyone else) is interested:

Voyager 1 and Voyager 2 were launched in 1977, and are the oldest operational spacecraft. At launch, each spacecraft carried two propulsion systems, a Delta-V system, including four 100 lbf and four 5 lbf monopropellant hydrazine thrusters made by Aerojet, and an attitude control system including 16 0.2 lbf monopropellant hydrazine thrusters. The Delta-V systems have long since been jettisoned, but the attitude control systems remain operational today. The 100 lbf thrusters are the original version of the thrusters intended for Orion’s crew module and the 0.2 lbf thrusters are the original version of the thrusters currently in use for the Global Positioning System Block IIR, and are similar to those newly in service for GPS Block IIF.

http://spacefellowship.com/news/art24543/aerojet-propulsion-remains-operational-as-voyager-1-approaches-interstellar-space.html

[Svartalfiar]

Converted into electricity for power for on-board systems. Not for propulsion. And nope, sorry, no Star Trek for me yet :p