Not exactly why this was news this week. VX-200 successfully test fired in 2009, and it looks like Ad Astra's facing a 2 year delay before they can start their space-borne VF-200-1 tests.
Economic Times
WASHINGTON: NASA is looking forward to flying a plasma-powered rocket to survey an asteroid that could take astronauts to Mars in a little over a month's time.
The rocket called Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a twin of one being developed for testing aboard the International Space Station.
VASIMR technology uses radio waves to ionise propellants like argon, xenon or hydrogen, and heat the resulting plasma to temperatures 20 times hotter than that on solar surface. It uses magnetic fields instead of metal nozzles to control the direction of the exhaust.
Equipped with an electric propulsion system, the rocket is being built to transport astronauts to Mars in 39 to 45 days someday - a fraction of the six to nine months the trip would take with conventional chemical rockets.
Why is electric propulsion important? Let's dig out our old friend the rocket equation:
The range of missions you can attend given an engine configuration and some payload mass is entirely determined by Δv. The higher this is, the more destinations you can reach. We can even ignore thrust--more accurately, acceleration--to a point, provided you're not blasting off or landing from any body with a significant amount of gravity. Once you're off Earth with a mission to encounter something, all that really matters how much can you change your velocity.
When choosing your engine, the effective exhaust velocity (v_e) is the key sorting factor. For a chemical rocket, this is going to be around 1000 to 5000 meters per second. Knowing v_e allows you to figure out the ratio of launch mass to payload (m0/m1) for a given delta-v. A lunar encounter, for example, requires a delta-v of just under 11 km/s. So, for every kilogram of payload you take with you to the moon, you'll need 8 kg of fuel. If you can improve exhaust velocity, you can reduce the amount of fuel you need to reach a given destination or increase the amount of mass you can take to the same target. Electric engines like VASIMR offer our best near term hope for increasing exhaust velocity.
There is a cost, of course. To achieve high exhaust velocities, you choose propellant with low mass and consequently low thrust. A trip to the moon that might take forty days for Apollo could take weeks using a plasma engine. On the other hand, a solar powered VASIMR spacecraft keeps accelerating towards the end of its delta-v budget, which means while a chemical rocket ship would have to cost in transfer between Earth and Mars over a period of 2 years, our electric engine ship could make the journey in a few months. With a nuclear reactor rated at 1 kg/kW, the trip could be reduced to six weeks.
But why bother with Mars? The moon has a far lower surface gravity and plenty of resources, and near Earth asteroids are even more cost-effective targets. Near Earth space is a 1300 W/m^2 gold mine of solar power. And all of it is within reach of electrically propelled spacecraft operating on nothing more than 7 kg/kW rated photovoltaics.