For some time an assumption - a rule of thumb of a sort - was that chemical rockets can get people comfortably around within Mars orbit, and to get further away

For some time an assumption - a rule of thumb of a sort - was that chemical rockets can get people comfortably around within Mars orbit, and to get further away we need something better. That "something" is likely not solid-phase nuclear engines, with Isp about twice as good as LOX/LH2, and probably not gas phase nuclear engines... but thermonuclear engines, which get closer to feasibility with advances in thermonuclear energy field. A thermonuclear engine could be more attainable than a thermonuclear electrical plant, and cheaper LEO access could be a big part of enabling technology. Having engines like that we may talk about much faster trips to Pluto and back.