While this is very encouraging, the real problem with space travel is the cost of getting materials out of the Earths gravity well. While this new technology will help with unmanned craft, I do not see a real long range manned system happening until we can launch every heavy sections of a craft to be assembled in space. I have a running bet with a Physics Professorial friend, I am betting on electromagnetic launch he thinks the space elevator is going to win the day. I have 1$ riding on it:)

The cost to orbit using chemical rockets is not as bad as you might think. The majority of cost is the cost of the rocket. If we can create a reusable rocket the fuel cost is quite reasonable. The fuel for a resupply launch to the space station only costs about $200,000. The big cost is in the rocket we have to throw away now. We are not talking about a Space Shuttle approach that was a total financial disaster costing more to refurbish after each launch than simple throw away rockets cost. We need a system that can use a simple low cost ablative heat shield for reentry and deacceleration of the booster until it reaches terminal velosity with a controlled fall through the air. With out all the fuel the crafts density is lower than that of a human being so terminal velosity is about 100 to 200 mph. When you get close to the ground restart the engine and make a controlled landing. Preferably near the next launch location. Refuel add a new heat shield and payload ready to go again. SpaceX is working on this and I believe they will succeed.

The author is very optimistic about space nuclear power's funding level, as of late the Fission Surface Power (FSP) Project has been zero funded following Obama's cancellation of the Constellation Project (from which it received its funding).
All of the nuclear systems he speaks of are PAPER reactors (meaning academic studies and computational work)! Very little has been done in terms of actually building and operating one of these little gems, which is unfortunate.
It's nice to talk about the engines that get you to the outer planets, but until the US concertedly funds a space nuclear project to power these devices, they are just an engine for making pretty lights on earth.

MontyPython... How prevalent is elemental hydrogen in deep space? I'm assuming the electro-negativity of the ions is proportional to the sputtering problems (like nickel/platinum nucleate sites for brown's gas production). With a source of fuel in space it would only be necessary to magnetically(electrostatically?) push off of it for thrust (like a jet engine intake/exhaust). (recombining to regain fuel and repeat? hydrogen 1.2eV.) Passivating the material only delays the oxidation(sputtering). Maybe something like molygraphene(?molybdenum-?sulfide??) used for the magnets in the one shown in the pic. Might be lighter weight too.

Is an Ion engine the most efficient way to turn the energy of a nuclear reactor into thrust? The reactor generates heat. Then some mechanism (therocouples or a fluid driven turbine) converts that heat into electricity with efficiency losses. The ion engine + fuel then converts the electricity to thrust. Isn't there some way to combine the heat of the reactor with the ion engine fuel to get thrust and skip the heat to electricity and electricity to thrust steps?

n2liberty, I love Space X and hope they do make their rockets reusable and self landing (they do more in a year than NASA does in a decade). I have been watching them closely and love what I see. However, rocket launching is dangerous and even if they land their rockets for reuse, you then have the issue of how many times do you reuse a part before you replace it or rebuild it. This leads to all new maintenance and safety issues. With electromagnetic launch you would still use a rockets. Very safe, low thrust rockets to keep accelerating the "truck" space vehicle into orbit. Once the vehicle drops off its load, it returns under powered flight. When I propose the idea people laugh and say "That would have to be huge!". Yes I am talking a couple miles of flat and then up the side of a mountain. We are talking billions (still cheaper than the Shuttle....) There would be only one. And the first country that made one would put everyone else out of business.....

Les, I completely understand where you are coming from. The problem is that each module Is designed separately then the modules are assembled together to form a working system. The efficiencies you are suggesting would imply that each project is designed completely customised using no 'off the shelf' modules. This would me insanely expensive. The modular approach is cheaper, but the "one module fits all" approach is never optimal.

Gwyn Rosaire, one of the nuclear reactor types that you say are are just 'Paper Reactors' is in fact not a 'Paper Reactor.' The Molten Salt Reactor Experiment at ORNL ran for several years and was poised for commercial viability until politics gutted the project. No paper designs and math here, it's a real, usable piece of technology.

"When these ion energies are large enough, they will erode material from the walls in a process called sputtering. " Is the rate of sputtering proportional to thrust output?
Just an idea. Reaction chamber damage is not easily predictable. So why waste material reinforcing the chamber when you can just repair on the fly.
Can the sputtering effect be offset by a material that repaired itself. Or more simplistically, can a directional nozzle inside the reaction chamber spray a coating of material to repair walls affected by sputtering.
In this way you can run the ion drive at maximum thrust, and every few week/months/years power down, assess the damage, repair, and power back up again. Likely at a certain thrust level, there would be a distribution of errosion in just a few spots.

@Les LaZar: Sure. You can cut out the middle man entirely and just use the reactor core as a primary heat source. Store the reactor core inside a pressure vessel, and connect to it an exhaust jet. Just flush the toilet and off you go.
The problem however, is that we're now talking about a steam powered space ship. While that's a ridiculously cool idea (and I hope someone does it some day), getting around in space is all about weight to thrust ratio, and weight to thrust comes down to exhaust velocity and mass.
When discussing efficiency in propulsion, there's more at play than simply mechanical efficiency. We have to consider that not only do we need energy, but also fuel mass. Fuel mass ultimately is the most important consideration to propulsion in a vacuum, as is by far the most scarce, and the more you bring, the more you have to burn to move it. A nuclear/steam powered rocket nozzle might (i'm pulling numbers out my ass here) provide an exhaust velocity of 1km/s. I doubt it, but let's call it 10km/s just to be safe. Conversely, the theoretical limit to the exhaust velocity of an ion thruster, is limited only first by the energy available and eventually by the speed of light. 10km/s to 300,000km/s. Assuming even 10,000km/s, you require 1,000x less fuel than our totally unrealistic steam thruster. But that's not even true because you need fuel to push that 1,000x. I believe this works out to some magical math that has to do with inverse squares and some X's and Y's or something.
So in terms of mechanical efficiency, the nuclear steam engine is ridiculously efficient when compared to a nuclear generator providing electricity. In terms though, of propulsion efficiency, with fuel mass brought into the equation, the steam engine feels like a horse drawn carriage being compared to ... well, an ion thruster I guess.