Ion thruster

12 NOV 1998. Opposites attract, and likes repel. It's true in love, and even more in electricity and magnetism where the principle governs countless electrical and magnetic gizmos -- from compasses and electric motors to the cathode-ray tube that's probably allowing you to read this.
Artist's rendering of Deep Space 1.
Courtesy of NASA.

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The repulsion of opposites is supposed to drive a spacecraft called Deep Space 1 on a wide orbit of the sun. But the futuristic engine pooped out mysteriously just minutes after being turned on and the spacecraft's handlers are trying to figure out just what went wrong.

While the futuristic ship -- should it get underway -- plans to visit an asteroid and a decaying comet, its real job is to test cost-cutting technologies for upcoming space missions.

Like many a corporation, NASA is interested in downsizing, so the craft is designed to operate with little human advice -- its computers monitor its condition, set priorities, and only phone home when lonely (actually once a week). Although the plan is to test the independent operation for just one week, compare the average spacecraft, which is attended by a phalanx of warm-blooded minders hunkered in front of computer screens. Call it NASA's plot to save money -- or quell monitor tan.

Automation aside, the 25 October launching of Deep Space 1 has focused attention on ion propulsion, which we're obliged to call a "space age" engine. From the years squandered on Star Wars and Star Trek, you recognize ion propulsion as a ferociously powerful rocket alternative that accelerates charged particles (ions) to fiendish velocities. You know ion thrusters can push city-size spacecraft to distant galaxies before the first reel is half finished.

A weakling, but a persistent one
Yet even if the little thruster on Deep Space 1 finally gets going in space and works as well as it did on the ground, it will fail those expectations. True, it will accelerate ions to about 60,000 mph, but will it accelerate many of them? Even with the pedal to the metal, the thruster will push the spaceship as hard as a piece of paper pushes your hand. This ion thruster is not for the impatient since it will take one whole day to accelerate the craft by an additional 30 feet per second -- about as fast as it would accelerate after dropping in Earth's gravity for one second. Still, after a few months operation, that tiny force will speed the craft up by 8,000 mph.

Deep Space 1 is lifted from its work platform, giving a closeup view of the experimental solar-powered ion propulsion engine. The ion propulsion engine is the first non-chemical propulsion to be used as the primary means of propelling a spacecraft.
Courtesy of NASA.

It really looks real, doesn't it?
I built this in my garageSo what's the big advantage? Efficiency. The thruster pushes its exhaust about 10 times faster than chemical rocket exhaust, says Robert Nelson, mission scientist for Deep Space 1. When the large solar array is taken into account, he says, the actual advantage falls to about half that: "The bottom line is that when you factor in all the trade-offs between this engine and conventional chemical engines, you can fly a spaceship on interplanetary cruise with about one-fifth the mass for fuel." But slashing the mass by that can produce major gains in the most expensive stage -- getting off the ground in the first place. "When you reduce the mass of a spaceship by five times, that's a big deal," Nelson says.

These advantages promise to reduce weight and mission costs -- both goals of Deep Space 1.

As we await a potential fix of the thruster, we'll try to answer these nagging questions: What is ion propulsion, and how does it work?

don't try this at home
An ion thruster moves ions by electrostatic repulsion. Xenon propellant enters from the left. A cathode emits electrons which slam into the xenon atoms knocking loose an electron and creating positive xenon ions. The ions are pushed by gas pressure through holes in the positive grid. Then the electric field between the positive and negative grids accelerates the ions and sprays them out the back. The beam is neutralized by electrons. Otherwise the ions would be attracted back to the negative grid, canceling out the thrust.
Courtesy NASA.

All charged up...
An ion thruster does two simple things. It creates charged particles -- or ions -- and accelerates them opposite to the intended direction of travel. Ion thrusters expose atoms (xenon, in this case) to electrons, which knock electrons from the atoms making charged xenon ions. Ions respond to magnetic and electric fields, and these ions are attracted to a positive grid at the back of the firing chamber. The grid's electric field accelerates the ions into a ghostly blue beam traveling at about 60,000 miles per hour.

The last step is to neutralize this beam of ions. Otherwise they would be attracted back to the spacecraft's positive surfaces and cancel out the thrust. While Deep Space 1 is the first mission to use ion propulsion as its primary engine, an ion thruster, flew in 1964 and worked for 31 minutes. A second thruster operated for five months after a 1970 launch.

Despite their high-technology allure, ion thrusters depend on the same elementary Newtonian principle that conventional rocket engines do -- to every action there is an equal and opposite reaction. The exhaust goes in one direction and the rocket moves in the other.

Despite its exotic build, an ion thruster relies on the same simple equation as any other rocket engine. The force (F) it produces equals the mass (M) of propellant moved times its acceleration (A). Thus F = MA.

Since chemical rockets "only" accelerate their exhaust to about 6,000 miles an hour (only being a relative word here!), increasing the velocity of exhaust may liberate spacecraft from this dead-end physical equation: getting more thrust requires more propellant, yet accelerating that propellant takes ever-more propellant.

Instead of increasing F by adding mass , the thruster does the same thing by increasing A, which allows you to reduce M.

Aside from the extreme rapidity of the beam, the biggest novelty of an ion thruster is its source of energy. Chemical rockets store energy in the chemicals, but ion thrusters get power from solar cells, which make the electricity for the electrostatic field that moves the ions.

That limits the thruster to the 2400 watts produced by those photovoltaic cells. Thus solar-powered ion thrusters are usable in the inner solar system, where sunlight is abundant. Nelson says they might function about as far out as the asteroid belt.


-- David Tenenbaum

To learn about the amazing promise of X-ray astronomy, click here!

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