A radical, highly fuel-efficient plasma rocket whose interior operates at temperatures close to those found inside the sun has passed a key test milestone. Trials of the space drive aboard the International Space Station are expected within the next few years.

The superpowered rocket design is known as the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), and it is under development by former NASA astronaut and physicist Franklin Chang Díaz at his spinout company Ad Astra Rocket. Last week the experimental VX-200 test rig - which is partly British made - reached its full rated 200 kilowatt power, an achievement described by the firm as a "highly coveted milestone".

VASIMR works by turning Argon reaction mass into terrifically hot plasma - nearly as hot as "the interior of the sun" according to Ad Astra - and blasting it out of the engine's rocket nozzle. Because the argon is ejected with such terrific speed, a VASIMR-propelled ship would get a lot more poke out of a given amount of propellant than one driven by everyday chemical rockets.

One downside is that the amount of thrust produced is small in comparison to the plasma drive's mass, meaning that VASIMRs couldn't lift themselves out of Earth's atmosphere. The plasma drive would outspeed chemical rockets not by pushing harder, but by pushing for much longer, so achieving a much higher velocity over time. Another snag is that significant amounts of electrical power are required, the generation of which is an issue in space.

For relatively easy missions close in to the Sun, VASIMRs could run usefully on solar panels, the mainstream means of generating electricity in spacecraft. Solar-powered plasma drives, Chang Díaz says, could save fuel over chemical ones doing tasks such as maintaining the ISS in orbit, or driving ships between the Earth and Moon.

Solar VASIMR would also, according to the veteran astronaut's calculations, be of use on a Mars mission. However he has suggested in the past that a solar-powered Mars mission would be "technically feasible but operationally fragile", adding that "as their robotic precursors have done, future human interplanetary spacecraft will rely on nuclear power to explore the far reaches of the solar system and beyond".

The greater power offered by nuclear reactors would make for much more puissant interplanetary ships. If small dustbin-sized reactors (of the same class nowadays used in submarines) could be employed, Chang Díaz believes that VASIMR ships could reach Mars in just 39 days, a huge improvement over chemically-propelled craft which would have to coast almost all the way and take six months. (The concept vid above shows a longer trip using slightly different assumptions, but still very fast compared to a conventional ship). Manned missions to the moons of Jupiter or other destinations in the outer Solar System would also become realistic with nuclear VASIMR.

The use of nuclear power in space is actually fairly routine, but it has occurred mostly in top-secret spy satellites and so hasn't caused much debate. Anti-nuclear protests have occurred when such tech has been used in civil craft.

For now, though, Chang Díaz and his team at Ad Astra are focused on building a flight-rated version of their technology, the VF-200-1. This is already underway, and the successful VX-200 fullbore trial has made engineers confident that it will work. Plans call for the VF-200-1 to be tested in space aboard the ISS in 2013.

“This latest achievement is a fitting tribute to the outstanding Ad Astra team. I am proud and honored to have the opportunity to work alongside this superb group of individuals who turn dreams into reality," says Chang Díaz. ®