The Incredible Ions of Space Propulsion
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
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]
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."
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Ion Propulsion -- Like Socks in the Dryer
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
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.
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.
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.
"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
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
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.