VASIMR: A Plasma Rocket

With renewed interest in voyages of exploration to other planets of the Solar System, the need to find technologies to propel space craft faster than is possible with chemical rockets has become more apparent. One of those technologies, under development at NASA’s Johnson Space Center, is called the Variable Specific Impulse Magnetoplasmic Rocket or VASIMR for short. It is the brain child of astronaut/researcher Franklin Chang-Diaz.

The way VASIMR would work is by using plasma as fuel. Plasma is an electrically charged gas that scientists consider the fourth state of matter, the others being gas, liquid and solid. Plasma is created when a gas becomes extremely hot and its atoms lose some of their electrons. Because electrons have a negative charge, atoms stripped of them become positively charged ions. An ion is an atom that is either positively or negatively charged as a result of losing or gaining electrons. The result is a kind of “soup” of charged particles, or plasma, made up of free floating negatively charged electrons and positively charged ions.
Plasma is so hot that generally no physical substance can contain it. Fortunately though, plasma is an excellent conductor of electricity, which means that it can be held, guided and accelerated by specially designed magnetic fields. It is this property of plasma that allows VASIMR to work.

A VASIMR engine would consist of three electromagnets, powered by a small nuclear generator, linked together, that would provide the magnetic fields necessary to contain and control the plasma. The first cell, or the forward cell, handles the injection of hydrogen gas that will be turned into plasma. The hydrogen is injected into the forward cell and ionized, or its atoms stripped of electrons and given an electrical charge by the magnetic field. This turns the hydrogen into plasma. Then the plasma flows into the central cell where it is heated by radio waves, just like a microwave oven heats food. After it’s heated, the plasma flows through the aft cell, which acts as a magnetic nozzle. The aft cell converts the energy in the plasma into a directed flow out the rear of the engine that provides thrust and velocity.

A VASIMR engine would have a number of advantages over a conventional chemical rocket. First, it can operate at a much higher velocity than a chemical rocket, up to hundreds of kilometers a second as opposed to about five kilometers per second. While a chemical rocket burns very quickly and at a constant rate, a VASIMR engine can be modulated and controlled. A space craft powered by a VASIMR engine could accelerate to Mars for half the journey, then decelerate for the other half.

While a space craft powered by a chemical rocket may take anywhere from six months to a year to arrive at Mars, a VASIMR powered space craft would take about three months on a similar voyage. That means astronauts who travel to Mars would spend less time exposed to the ravages of micro gravity and radiation. Also, since hydrogen is the most common substance in the universe, a space craft propelled by a VASIMR engine could be designed to refuel from sources elsewhere in the solar system, thus extending the range of the vehicle. Finally, VASIMR’s electromagnetic field and the hydrogen fuel provides a good radiation shield to protect the crew of the space craft.

When will space craft propelled by a VASIMR engine become reality? That is a question that is hard to answer. VASIMR is just one of several concepts being studied for high velocity space travel. Also, because human expeditions to Mars are at least two decades away, funding for concepts like VASIMR are considered a low priority. A space shuttle test of a VASIMR engine had been scheduled for 2003, but was cancelled in the wake of the Columbia disaster.

Still, VASIMR has a lot of potential for a solution to the problems of interplanetary travel. Only the future holds the answer to the question of whether it will or not.

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