Since we're talking in present tense that's the remaining 1%.

Moon base can be fine with power beamed from a satellite or plain mirrors in orbit, no atmosphere in the way. Might end up being still cheaper than hauling nuclear reactor there plus all the infra to reliably dump waste heat from it.

"Solar and wind power are not sufficient for spacecraft"

I guess you're right that solar is useful for 99% of spacecraft -- in that they use it currently. Not a very useful observation

"and many other applications"

Power in polar regions. How well does Solar work in Antarctica? Or Alaska for that matter?

It works super-great, collected in the tropics and shipped in chemical form. Before you object to depending on imported liquid fuel, consider that most of the world does already.

The main difference is that literally anybody can make it, not just "oil exporting countries" and "fuel refiners". And, will. And export excess production when local tankage is full.

Last I looked, round-tripping solar via liquid fuel and back to electricity was under 2% before transport costs

Maybe look again without assuming hydrocarbon. Ammonia is a good transport medium, liquid at room temperature under low compression.

> How do you power a moon base with solar? 14 days of battery?

Either that or a place near one of the poles where you get water and lots of sunshine. A small fission reactor is a handy thing to have, however.

> Fine. How do you power a Europa base with Solar?

A lot more solar panels, or wire loops harnessing Jupiter's magnetic field and Europa's momentum, etc. Fission is still cool for that.

> A Neptune probe?

Now we enter the nuclear fission territory. Maybe fusion, some day.

>How do you power a moon base

For those interested in near term answer:

https://www.nasa.gov/mission_pages/tdm/fission-surface-power...

Laser beams from solar concentrators are likely until aneutronic fusion pans out, if ever. Maybe after that, too.