Are there any alternative energy sources to power inter-stellar probes such as Voyager I and II developed or in development by any space agency?
Plutonium (238-Pu) has a half-life of about 87,7 years, which is a big limitation to the lifespan of such missions, even if we have a lot of spare energy at the start. Full nuclear reactor is too heavy.
Is there any concept of powering such probes with an energy source, that could work for hundreds of years? Otherwise, the only possibility to reach further is to accelerate probes to higher velocities.