The reactors in Voyager 1 rely on the heat produced by decaying plutonium, which decays into uranium. We could’ve powered it with the uranium, But, uranium decays INCREDIBLY slowly. You’d have needed like, some 70,000 tons of it to power the thing, But, it would’ve been powered for hundreds of thousands of years. Radium would’ve been the better choice. You’d only need about 600 pounds of it, And it would keep it running for thousands of years. But, a radium system would weigh over five times the current spacecraft, And would’ve had to be assembled in space, because it never could’ve left the ground. Honestly, we should’ve done that. </3 The reason it would weigh so much, is that the insulators needed to contain it would be immense. Due to decaying radium having the likelihood of screwing with electronics, you’d have to contain it. The plutonium is contained with a little bit of lead, But, radium would need a LOT of insulation. The most likely insulator would’ve been tungsten. Decayed uranium would insulate better, but also cause a couple of other problems, and use more fuel. So, tungsten would’ve been used. And this all would’ve been very completable in 1977.

But, wouldn’t it have been worth it to keep this thing running thousands of years longer? Instead of just one light-day, we could’ve reached multiple light-weeks!

(This was written specifically for Voyager 1, but it may be applicable to Voyager 2)