The comment chain on What is the feasibility of launching a probe to Sedna? indicates RTG thermocouples decay. With the passage of time the thermocouple may even have the ability to transduce an infinitesimal fraction of the power it could originally even when the RTG pile itself is still good.
At the other end this news-report writes to say
The cause of the so-called "Pioneer Anomaly," it turns out, is heat coming from the electrical current flowing through the probes' instrument and power systems. This heat pushed back on the spacecraft, causing them to decelerate slightly, according to a new study
The Pioneer Anomaly was/is an inadvertent side-effect. It may be possible to improve on it by designing to use heat as a propellant.
For instance: Surface asymmetry may cause a surface to heat in a dis-uniform fashion.
Assuming the report is in order
- Are there any studies on radiating heat to propel space-craft in outer space?
- How close are these studies (if any) to experimental confirmation?
- How much heat would be necessary to accelerate a 1 kg body through 0.01G in outer-space?