There are many technological goals required to go BEO: high Isp propulsion with some Newton’s of thrust is one of those. Ion drives are a nice development, but they fail in many other ways: including that they wear out. VASIMR provides a “first generation” propulsion for real deep space capability.
Yes, it requires Nuclear power to be viable for “real” work BEO; this is one of the technological goals needed, but it has many other roles besides propulsion.
• Lunar base: any habitat on the moon is going to require power, and due to the lunar day, photovoltaic is out.
• Mining and materials processing: if you want to do any mining, or materials processing while you’re hanging around out there, it has to be powered by something. Again nuclear power is the answer: it’s not like we’re going to burn coal or oil there.
• Radiation shielding: if we send people into space for duration, they need to be protected from the radiation. There are 2 ways to shield personnel in space; some volume of material (water is the most likely) or something electrical (electrostatic, or electromagnetic). Which do you think would already be on a manned BEO craft being pushed around with first generation deep space propulsion?
• Earth based military: back here on Earth, besides the obvious need for power to replace the use of oil; there are the new military research weapons that are about to come on line: the Navy has railgun research underway with working prototypes, as well as ship based laser weapons. Lets also add the idea of missile defense, and its use of lasers that have been continually being worked on.
It seems to me, small, safe, easily operated nuclear power plants are a major technological goal, with payoff in many areas. There is potential in the Bussard polywell fusion reactor, but it’s still being researched, and it may not work. Understand, I want it to work, but I’m being practical here and acknowledging that a working fusion power plant has not yet been built.
My money to fill all these needs would be LFTR (Liquid Fluoride Thorium Reactor). Thorium is abundant here on Earth, and on the moon. It does not require water to run, has inherent safety features designed in to reactor design, waste produced is much easier to deal with (less of it, much shorter half life).
It also sits nicely into the ISRU desires of many BEO efforts: we can mine it from the moon, use it to power the moon facilities, and supply BEO craft with the nuclear fuel used to go into deep space.
Those should be the 2 biggest goals NASA should focused on: high Thrust, high Isp propulsion, and the energy technology to power it, along with everything else.
