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Science Fact - Ion Propulsion

Shooting a 100,000 km/h stream of ionised gas out the back of a spacecraft!

Assembled from material provided by NASA.

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An electric-powered rocket sounds like pure science fiction, but ion propulsion has been developed for space applications over four decades.

But what the hell is ion propulsion?

Ion Propulsion

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Unlike the fireworks of most chemical rockets using solid or liquid fuels, an ion drive emits only an eerie blue glow as ionised (electrically charged) atoms of xenon are pushed out of the engine. The almost imperceptible thrust from the system is equivalent to the pressure exerted by a sheet of paper held in the palm of your hand.

The ion engine is very slow to pick up speed, but over the long haul it can deliver 10 times as much thrust per kilogram of fuel as more traditional rockets.

Previous ion propulsion systems, like those found on some communications satellites, were not used as the main engines, but only to keep the satellites on track. NASA's Deep Space 1, launched in 1998, is the first spacecraft to use this important technology as its primary means of propulsion. The NASA Space Electric Rocket Test 2, launched into Earth orbit in 1970, had the previous record for ion propulsion, thrusting for about 161 days.

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"The importance of ion propulsion is its great efficiency," says Dr. Marc Rayman, Deep Space 1 project manager. "It uses very little propellant, and that means it weighs less so it can use a less expensive launch vehicle and ultimately go much faster than other spacecraft."

The fuel used in Deep Space 1's 30cm diameter ion engine is xenon, a colourless, odourless, and tasteless gas more than 4 times heavier than air. Xenon is the same gas found in photo flash tubes and many lighthouse bulbs.

When the ion engine is running, electrons are emitted from a cathode into a chamber ringed with magnets, much like the cathode in a TV picture tube or computer monitor. The electrons strike atoms of xenon, knocking away one of the electrons orbiting the atom's nucleus. This leaves each atom one electron short, giving it a net positive charge and making it into an ion.

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At the rear of the chamber is a pair of metal grids that are charged with 1,280 volts. The force of this electric field exerts a strong electrostatic pull on the xenon ions - much like the way that bits of lint are pulled to a pocket comb that has been given a static electricity charge by rubbing it on wool.

The electrostatic force in the ion engine's chamber causes the xenon ions to shoot past at a speed of more than 100,000 kilometres per hour, continuing right on out the back of the engine and into space. A second electron-emitting cathode, downstream of the grids, neutralizes the positive charge of the ion beam to keep the spacecraft neutral with respect to its environment.

"This opens the solar system to many future exciting missions which otherwise would have been unaffordable or even impossible," added Dr. Rayman.

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The technology is so efficient that it consumes only about 100g of xenon per day. At full throttle, the ion engine draws about 2.3kW of electrical power and develops 90 millinewtons of thrust.

The electrical power to run the ion propulsion engine is provided by solar cells. Cylindrical silicon lenses are used to concentrate sunlight onto cells arranged in strips. An average in-flight efficiency of about 22.5% is achieved and the cells produce 2.5kW of power.

While the ion particles shoot out at about 100,000 km/h, the vehicle doesn't move that fast in the other direction, because it's much heavier than the ion particles. But by the end of Deep Space 1's mission, the ion engine will have changed the spacecraft's speed by about 11,000 km/h.

So yes, you can increase your speed by 11,000 km/h on just 2.3kW!

And It's Kept on Going...

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The pint-sized powerplant on board Deep Space 1 NASA's Deep Space 1 probe has been doing it longer and more efficiently than anything ever launched. The spacecraft has run its unique propulsion system for more than 200 days (4800 hours). It completed its primary mission - testing ion propulsion and eleven other advanced, high-risk technologies - by September 1999.

NASA then extended the mission, taking advantage of the ion propulsion and other systems to undertake a chancy but ultimately successful encounter with the comet Borrelly.

"The ion propulsion engine on Deep Space 1 has now accumulated more operating time in space than any other propulsion system in the history of the space program," said John Brophy, manager of the NASA Solar Electric Propulsion Technology Applications Readiness project, at the agency's Jet Propulsion Laboratory in Pasadena, California.

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Exceeding the team's expectations of how this elderly spacecraft would perform, the intrepid spacefarer sent back black-and-white photos of the inner core of the comet. It also measured the types of gases and infrared waves around the comet, and how the gases interacted with the solar wind.

"Deep Space 1 plunged into the heart of comet Borrelly and has lived to tell every detail of its spine-tingling adventure!" said Dr. Rayman.

Rayman added, "After years of nursing this aged and wounded bird along - a spacecraft not structured to explore comets, a probe that exceeded its objectives more than two years ago - to see it perform its remarkably complex and risky assignment so well was nothing short of incredible."

And being pushed along with nothing more than the equivalent force of a sheet of paper resting on your hand....

Ion Propulsion Development
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Ion propulsion - also known as solar-electric propulsion because of its dependence on electricity from solar panels - has been under development since the 1950s. Dr. Harold Kaufman, a NASA engineer, built the first ion engine in 1959. In the 1960s, NASA Glenn undertook a spaceflight test program called Space Electric Rocket Test (SERT).

In 1964, a pair of NASA Glenn ion engines were launched on a Scout rocket under the name SERT 1; one of the two thrusters onboard did not work, but the other operated for 31 minutes. NASA Glenn also lead the way for a follow-up mission, SERT 2, which carried two ion thrusters, one operating for more than five months and the other for nearly three months.

Many early ion engines used mercury or caesium instead of xenon. SERT 1 carried one mercury and one caesium engine, while SERT 2 had two mercury engines. Apart from the fuel, these ion drives were similar to Deep Space 1's; the mercury or caesium would be turned into a gas, bombarded with electrons to ionise it, then electrostatically accelerated out the rear of the engine.

But mercury and caesium proved to be difficult to work with. At room temperature, mercury is a liquid and caesium is a solid; both must be heated to turn them into gases. After exiting the ion engine, many mercury or caesium atoms would cool and condense on the exterior of the spacecraft. Eventually researchers turned to xenon as a cleaner and simpler fuel for ion engines.

Beginning in the 1960s, the Hughes Research Laboratories conducted development work on ion engines. The first xenon ion drive ever flown was a Hughes engine launched in 1979 on the Air Force Geophysics Laboratory's SCATHA satellite. In August 1997, Hughes launched the first commercial use of a xenon ion engine on PanAmSat 5, a communications satellite launched on a Russian Proton rocket from the Baikonur Cosmodrome in Kazakhstan. This ion engine is used to maintain the position of the communications satellite in its proper orbit and orientation.

Ion engines for such purposes are smaller than a system like Deep Space 1's, which is designed for long-term interplanetary thrusting.

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