To achieve his much-repeated claim that VASIMR could enable a 39-day one-way transit to Mars, Chang Diaz posits a nuclear reactor system with a power of 200,000 kilowatts and a power-to-mass ratio of 1,000 watts per kilogram.BW:
There is a very near term solar power system which can achieve 1 kg/kw energy density at megawatt levels.
It is interesting that Zubrin minimizes the hazards of cosmic rays. Perhaps he has not considered the fact that the LEO ISS missions have the Earth's magnetic field to shield from cosmic rays. Once outside that protective barrier, there would be no protection.
I am not always in agreement with Zubrin.
There is a possibility of the James Webb Telescope's being canceled due to budget cuts. Here is an argument against that. You have to admire the guy's enthusiasm.