Larry's comment was correct. However to further elaborate, VASIMR stands for "VAriable Specific IMpulse Rocket". VASIMR's specific impulse can be dialed up at the loss of thrust or visa-versa. Being able to throttle the thrust is very import because higher thrust is more desireable for getting out of the Earth's gravity well. At maximum Isp, VASIMR is capable of ridiculous specific impulse (around 10,000 seconds). The ion propulsion system on DS-1 was in the ballpark of 5000 seconds which is pretty darn good. Nuclear Thermal Rockets (NTRs) have a specific impulse in the ballpark of 1000 seconds. NTR's are ideally suited for getting people to Mars. However if you want to get large amounts of hardware to Mars then VASIMR or ion propulsion systems are much better. If you want to get hardware to Jupiter-through-Pluto quickly then nuclear electric propulsion systems based upon VASIMR or ion rockets are the only way to go.

The key word here is "nuclear". If you want to get people to Mars or hardware quickly to the outer planets then you MUST use nuclear power. The Space Age is OVER, finished, kaputt if the greenies succeeded in banning nuclear energy from space (something they would dearly love to do). Make no mistake, the radical environmentalists are the mortal enemies of Space Exploration (this is the opinion of an ex-Sierra Club member). The President's announcement that the US is still pursuing this technology means we're still in the game.

Gary