Last week, as the world celebrated the first lunar landing, Apollo 11 astronauts Buzz Aldrin and Michael Collins both called for NASA to make Mars its next goal. But the chemical propulsion system that took them to the moon would take six months, at least, to get a man to Mars and cost hundreds of billions of dollars. However, a new ion plasma rocket being developed by another former astronaut, Franklin Chang-Diaz, could potentially reach Mars in just 39 days using a fraction of the fuel.
The problem with traditional rockets is that they’re terribly inefficient. About 90% of a mission’s initial weight is fuel, most of which is burned up escaping earth’s gravitational pull. After that, a traditional rocket could only slowly coast to Mars. Very slowly. Scientists describe rocket efficiency in terms of specific impulse, which is a rough measure of how fast fuel is ejected out of the back of the rocket. A chemical rocket has a relatively low specific impulse of 450 seconds - in other words, it gets one pound of thrust from one pound of fuel for 450 seconds.
Chang-Diaz’s prototype, however, promises specific impulses as high as 15,000 seconds. How? Well, his rocket doesn’t achieve propulsion by combusting fuel but, rather, by superheating atoms to create and expel a plasma plume.
The Variable Specific Impulse Magnetoplasma Rocket (VASIMR, for short) consists of three linked magnetic cells. The first stage works a bit like a kettle, heating the atoms of a neutral gas like argon with a radio frequency (RF) generator until electrons “boil” off, creating plasma.
The plasma is now very hot – about 50,000 degrees Celsius – but not hot enough to produce efficient thrust. The second stage of VASIMR acts as an amplifier, further energizing the plasma using electromagnetic waves. By now, the plasma reaches about a million degrees, comparable to the center of the sun.
The third and final stage is a “magnetic nozzle” that converts the energy of this superheated plasma into directed motion and, ultimately, high velocity thrust. And, in case you’re wondering how anything so hot could be possibly contained, that’s one of the reasons the cells are all magnetic. A magnetic field not only helps heat plasma but also contains it, so it won’t ever actually touch anything.
VASIMR could, theoretically, reach power levels a hundred times that of other ion engines. But there are still two big problems that need to be addressed before anyone can start packing their bags for Mars.
The first is that the 200kW VASIMR only produces a pound of thrust. That’s more than enough in the vacuum of space, where the ion engine can fire continuously for months on end and a pound of thrust can push two tonnes of cargo from the sun to Jupiter in 19 months. But it means VASIMR will never get off the earth on its own – it would need to catch a lift with one of those old gas-guzzling rockets.
The second issue is that, while the current engine can run entirely on solar power – making it perfect for moon trips and other near-earth duties – for a deep space mission, it would need more like 200MW of power. And only an on-board nuclear reactor could provide that.
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