VASIMR Plasma Rocket
If you’re looking for silver bullets, though, the key to getting to Mars faster may well be the Variable Specific Impulse Magnetoplasma Rocket, or VASIMR. A cousin of ion and Hall Effect thrusters, the VASIMR ionizes a working fluid and accelerates it via the use of magnetic fields. One of the virtues of this drive is that it can be “throttled" from high specific impulse (see the first article of this series) and low thrust to high thrust and low-specific impulse. Specific impulse values range from ~3000 to 30,000 seconds, far better than the Shuttle's 450s.
Obviously, such an ability has widespread implications for space travel. The most important, perhaps, is that electrical power is being used to add energy to the rocket’s exhaust; if you have a sufficiently large power source, you can dramatically reduce the amount of fuel (burn mass) required to reach your destination, which, in the world of space vehicles, is always a good thing.. Perhaps the most dramatic difference between a conventional chemical rocket and a VASIMR is in terms of transit time. A 1995 NASA technical paper discussing a nuclear-electric rocket outlines a mission profile with a 101- day outbound and 104 day inbound leg; contrast this with the Mars One Crew Manual mission time listed in the first article.
The only catch (other than improving the plasma generation process) is that issue of power. To reduce your transit time significantly, you need a reliable power source of sufficient level to substitute for all that propellant and oxidizer you didn’t want to bring with you. That becomes the real challenge… space tethers and their electric sail cousins might provide a partial solution to that problem, provided sufficient power storage capability and a little patience while the system charges, but their power generation is limited by conservation of energy to decelerating the vessel simultaneously (they could be useful for half of the trip, at least!). Fission or fusion power systems would be ideal from a power standpoint, but fission has obvious drawbacks and fusion is still years in the future. While solar power could be used, the required acreage of solar panels might well be prohibitive. For instance, the cited NASA technical paper assumed 10 MW available power. Assuming perfect efficiency, to achieve that level of power would require about 7700 square meters of solar panels at 1 AU from the sun (approximately 1.3 kW/m^3). To put this in perspective, according to NASA, the ISS has about 2500 square meters of panel area and generates 110 kW of power averaged out through the use of batteries.
In reality, the actual efficiency of our system would be closer to 20-30%, which would increase our required area by a factor of three to five. Considering our 100% efficiency solar panel area already exceeds that of the ISS, increasing it by another three times would be no trivial accomplishment. Then we have to take into account the fact that we'll have less and less available sunlight as we approach Mars and figure out a way to cope with being at an angle to the Sun... certainly possible, but a complication nonetheless.