On Monday, January 14, 2019 at 7:35:05 AM UTC-8, Chris L Peterson wrote:
On Sun, 13 Jan 2019 15:40:26 -0800 (PST), RichA
wrote:

https://phys.org/news/2019-01-steam-...y-explore.html

Of course, spacecraft that utilize water as their reaction mass have
been a staple of science fiction for many decades. And obviously, for
certain kinds of travel (as between asteroids or moons) it's a good
choice. But reaction mass is only one limiting factor for spacecraft.
The other is energy. In the inner system you have solar, which is
limitless. But further out you need some other source, presumably
nuclear. Most of our nuclear solutions at this point are either
relatively low energy (e.g. RTGs) or have relatively short lifetimes
of a few years. Still good for many applications, but not quite the
"forever" outlined in the above story!

(I recall reading that our newest nuclear submarines are powered by a
nuclear reactor with something like a 30-year lifetime, no refueling
required. I wonder how scalable that technology would be for space
operations?)

Nuclear power sources are used on the two Voyager spacecraft. Both were launched in 1977 and still producing usable power, although greatly diminished over the years.