Jun 9, 2000

The Incredible Ions of Space Propulsion


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June 16, 2000 -- In the not so distant past when spacecraft designers had to choose a means of propulsion for their ships, ion engines were not among the viable options for long-range space travel. But today, thanks to the pioneering efforts of scientists at the NASA Glenn Research Center and the Jet Propulsion Laboratory (JPL), ion propulsion systems are a reality.

Dr. John Brophy, of JPL, discussed the past, present and future of ion propulsion systems during a session last week at the 11th annual Advanced Space Propulsion Research Workshop in Pasadena, CA.


After a development history spanning nearly 40 years and following the successful flight of Deep Space 1 in 1998-1999, ion propulsion has now entered the mainstream of propulsion options available for deep-space missions, according to an abstract written by Brophy.

Above: NASA's Deep Space 1 (DS1) spacecraft, depicted in this whimsical mission poster from JPL, was launched in 1998 and successful tested a dozen cutting-edge technologies, including its advanced ion propulsion engine. Although the DS1 primary mission ended in September 1999, ground controllers are preparing to fire up DS1's ion engines again in July to begin thrusting toward an encounter with comet Borrelly in September 2001. [more information from JPL]

"Ion propulsion has been around for a long time," Brophy said. "The first one was tested back in 1959 at NASA Glenn (then NASA Lewis) and was found to have excellent performance."

But there was a catch that scientists soon discovered.

"While it was easy to make the engines perform well, it was very hard to make them last."


Ion Propulsion -- Like Socks in the Dryer


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The principle behind an ion propulsion engine is much the same as what you experience when you pull hot socks out of the clothes dryer on a cold winter day. The socks stick together or push away from each other because they are electrostatically charged. The challenge in electric space propulsion is to charge a fluid so its atoms can be expelled in one direction, and thus propel the spacecraft in the other direction.

Right: Find out more about ion propulsion from "Ions in Action," a product of JPL's SpacePlace.

The ion propulsion fuel used by the experimental Deep Space 1 spacecraft is xenon, a gas that is more than 4 times heavier than air. When the ion engine is running, electrons are emitted from a hollow tube called a cathode. These electrons enter a magnet-ringed chamber, where they strike the xenon atoms. The impact of an electron on a xenon atom knocks away one of xenon's 54 electrons. This results in a xenon atom with a positive charge, or what is known as an ion.
Delta rocket launch of Deep Space 1
At the rear of the chamber, a pair of metal grids is charged positively and negatively, respectively, with up to 1,280 volts of electricity. The force of this electric charge exerts a strong electrostatic pull on the xenon ions. When the ion engine is running, xenon atoms with a positive charge shoot out the back of the engine at a speed of 100,000 km/h (60,000 mph). At full throttle, the ion engine consumes 2,500 watts of electrical power and puts out just 1/50th of a pound of thrust. That's far less than the thrust of even small chemical rockets. But if an ion engine can be made to run for months or even years, the tiny, constant thrust adds up to substantially reduced flight times. Plus, these engines are up to 10 times more efficient than chemical rockets.

Left: The ion-powered Deep Space 1 spacecraft blasts off aboard a Delta rocket in 1998. Once in space, DS1 used its ion engine for propulsion.

Rockets powered by electric propulsion systems cannot generate enough thrust to lift their own weight. A chemically powered launch vehicle, however, can lift an upper stage that carries a spacecraft powered by ion propulsion.

However, no one was ready to give up on these engines. NASA scientists continued to work on solutions to the longevity problem. Then, in 1992, NASA started the NSTAR (NASA Solar Electric Propulsion Technology Application Readiness) program, which had at its core the removal of the barriers to using ion propulsion on deep space missions. There were two main problems that stood in the way of successfully demonstrating the ion propulsion systems.


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"The first problem was to demonstrate that the engines could last long enough to be useful," said Brophy. "The second problem was to figure out how to guide and navigate a spacecraft with ion propulsion, which no one had done before."

After several more years of work, the NSTAR ion propulsion system was ready to be tested on Deep Space 1. However, as with many long term projects, there were issues to be resolved before the system could actually fly.

"We were told that the Deep Space 1 project could not afford to build the solar array that would be required to fly the ion propulsion system, but that NASA would fly it if we could find a free solar array to use," said Brophy. "Fortunately, we found one. The Ballistic Missile Defense Organization (BMDO) had an advanced solar array that they wanted tested, so they provided it to NASA."

Deep Space 1, outfitted with BMDO's solar array and the NSTAR ion propulsion system was a success, with both working exactly as planned. New demands have followed for ion systems with increased capabilities.
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Right: This xenon ion engine prototype, photographed through a port of the vacuum chamber where it was being tested at NASA's Jet Propulsion Laboratory, shows the faint blue glow of charged atoms being emitted from the engine. A similar engine powered the Deep Space 1 spacecraft. [More images from JPL]

"The NSTAR system was conservative on purpose," said Brophy. "We wanted to see if it would work first. Now it is a legitimate option that is being considered for many new missions."

Brophy explained that many of the deep-space missions that are relatively easy to perform from a propulsion standpoint, such as planetary flybys, have already been accomplished. However, future high priority mission classes, which include sample returns and outer planet orbiters, place substantially greater demands on the onboard propulsion systems.

Ion propulsion systems make missions more affordable and scientifically more attractive by enabling the use of much smaller, lower cost launch vehicles, and by reducing flight times, according to Brophy.

Some proposed mission concepts considering ion propulsion include the Comet Nucleus Sampler Return (CNSR), the Saturn Ring Observer, the Titan Explorer, the Neptune Orbiter, and the Europa Lander.
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"The most likely candidate to use ion propulsion next will be CNSR," said Brophy. "As an example, take the European Space Agency spacecraft Rosetta, which is designed to rendezvous with comet Wirtanen in 2011. Using a conventional engine, Rosetta will take 9 years to reach the comet. The CNSR spacecraft could seize the opportunity of an ion propulsion system to get there in only 2 ½ years. Furthermore it could collect samples of the comet, and be back to Earth before the Rosetta spacecraft even arrived."

Left: This artist's rendering shows the Rosetta Lander sitting on the nucleus of comet Wirtanen after the Rosetta spacecraft arrives in 2011. Propulsion experts think that improved ion engines could eventually propel space probes to nearby comets and asteroids faster than conventional engines. Image Credit: ESA.

Despite the great possibilities presented by the successful flight of Deep Space 1, scientists are still working on improvements to the current ion propulsion technology needed for future planetary missions.

"At the moment, we are mostly working on improving how long the thrusters last," said Brophy. "We are hoping to double the engines' designed operating time, which is approximately 1 year at full power."Web Links

Advanced Propulsion Concepts -- from the Jet Propulsion Laboratory -- news and information about space transportation research from the Marshall Space Flight Center

Ion Propulsion on DS1 -- JPL

Recent Science@NASA Stories about Space Transportation:

June 9, 2000: A Little Physics and a Lot of String - Using space tethers for propulsion.

May 31, 2000: Advanced Space Propulsion Conference - Scientists meet to discuss the latest in space transportation.

May 29, 2000: What's the Matter with Antimatter?- It may be the ultimate fuel for space travel, but right now antimatter is fleeting, difficult to work with, and measured in atoms not pounds!

April 11, 2000: Where's the Edge?- NASA's Advanced Space Transportation Program looks at ways to turn science fiction into reality.

Stories from the 1999 Space Propulsion Workshop:

April 6, 1999
: Ion Propulsion -- 50 Years in the Making- The concept of ion propulsion, currently being demonstrated on the Deep Space 1 mission, goes back to the very beginning of NASA and beyond.

April 6, 1999: Far Out Space Propulsion Conference Blasts Off - Atoms locked in snow, a teaspoon from the heart of the sun, and the stuff that drives a starship will be on the agenda of an advanced space propulsion conference that opens today in Huntsville.

April 7, 1999: Darwinian Design - Survival of the Fittest Spacecraft

April 7, 1999: Coach-class tickets for space? - Scientists discuss new ideas for high-performance, low-cost space transportation

April 8, 1999: Setting Sail for the Stars - Cracking the whip and unfurling gray sails are among new techniques under discussion at the 1999 Advanced Propulsion Research Workshop

April 12, 1999: Reaching for the stars - Scientists examine using antimatter and fusion to propel future spacecraft.

April 16, 1999: Riding the Highways of Light - Science mimics science fiction as a Rensselaer Professor builds and tests a working model flying disc. The disc, or "Lightcraft," is an early prototype for Earth-friendly spacecraft of the future.