deepspace_propulsion_000817 WASHINGTON -- A space
propulsion technology often featured in science fiction could become the method of choice for future missions exploring deep into the cosmos.The technology, called
ion propulsion, already has proved its worth aboard NASA's Deep Space 1 probe. The spacecraft this week logged a record 200 days operating its ion-drive engine. That's more accumulated engine time than any other propulsion system in the history of the space program."This is really the Energizer bunny of space missions," said Marc Rayman, project manager for Deep Space 1 at
NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. The $152 million mission, which was launched in October 1998, keeps going and going thanks to a futuristic propulsion system that runs on atomic particles rather than chemical fuel. The system, built by Hughes Space and Communications, allows a spacecraft like
Deep Space 1 to attain great speeds with a minimum amount of fuel.
Deep Space 1's ion-propulsion engine. The blue glow is caused by electrically charged particles, which are ejected from the rear of the engine at 68,000 miles (109,430 kilometers) per hour.
"The importance of ion propulsion is its great efficiency," Rayman said. "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."
Extreme space
That makes ion propulsion "an important tool in the box for mission planners," he said. "It's going to be the technology of choice for most missions where you to have to make large changes in the spacecraft's speed, or if you want to go to any extreme place."
Candidate missions include a comet-nucleus sample return, a
Venus surface-sample return, missions to explore Saturn's rings and its moon Titan, or a probe to orbit Neptune. All of those are being studied as proposed missions at the JPL."Almost all the missions that are in JPL's advanced mission set could benefit from this technology," said John Brophy, manager of the center's Solar Electric Propulsion Technology Applications Readiness project. "It's certainly a more legitimate option now. You couldn't ask for a better test."
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But don't expect to book a flight on an ion-powered rocket anytime soon.
"You're not going to be flying people around on ion propulsion because it takes too long to get where you're going," said Jerry Grey, head of aerospace and science policy at the American Institute of Aeronautics and Astronautics. "Ion rockets are much too tame. We'll never get up to light speed using ion propulsion."
Rather, Grey sees a mix of missions, some powered by chemical fuel, others by ions. Flights to
Mars, for example, could have people going on conventional rockets while cargo is relegated to the slower, ion-drive rockets."Deep Space 1's big contribution is that it has proven that long-term primary propulsion using ions makes sense," Grey said. "But you would only use it as long as you've got plenty of time to travel."
Deep Space 1 launch from Cape Canaveral on October 24, 1998 on a Delta 2 rocket
Rocket designers have been studying ion propulsion since the 1950s, and mention of the technology often turns up in works of science fiction. Ion propulsion was featured in a September 1968 episode of
Star Trek called "Spock's Brain," in which invaders steal Spock's brain and flee in an ion-powered spacecraft.The same technology is used intermittently for altitude control aboard 11 Hughes-built communications satellites in geosynchronous orbit 22,300 miles (35,885 kilometers) above Earth. Russia also has six communications satellites that use ion propulsion for station keeping.
But Deep Space 1 is the first spacecraft to use it as a primary means of propulsion.
Instead of the fiery thrust produced by typical rockets, an ion engine emits only an eerie blue glow as electrically charged atoms of xenon are pushed out of the engine. Xenon is the same gas found in photo flash bulbs and lighthouse search lamps.
Acceleration with patience
In the engine, each xenon atom is stripped of an electron, leaving an electrically charged particle called an ion. Those ions are then jolted by electricity that is produced by the probe's solar panels and accelerated at high speeds as they shoot out from the engine. That produces thrust for the probe.
The ions travel out into space at 68,000 miles (109,430 kilometers) per hour. But Deep Space 1 doesnt move that fast in the other direction because it is much heavier than the ions. Its cruising speed is closer to 33,000 miles (53,100 kilometers) per hour.
The thrust itself is amazingly light -- about the force felt by a sheet of paper on the palm of your hand.
"If you want a mission in which you want to reach your destination in a hurry or accelerate quickly, ion propulsion's not for you," Rayman said. "It takes four days to go from zero to 60 (miles per hour). I like to say it's acceleration with patience."
But once ion propulsion gets going, nothing compares to its acceleration. Over the long haul, it can deliver 10 times as much thrust-per-pound of fuel as more traditional rockets. Each day the thrust adds 15 to 20 miles (25 to 32 kilometers) per hour to the spacecraft's speed. By the end of Deep Space 1's mission, the ion engine will have changed its speed by 6,800 miles (11,000 kilometers) per hour.
Easy on xenon
Yet Deep Space 1's engine only consumes 3.5 ounces (100 grams) of xenon per day, taking about four days to use up just 1 pound (0.4 kilogram). Its solar panels generate just 2.5 kilowatts of power, the equivalent of 25 100-watt light bulbs.
Now about 200 million miles (320 million kilometers) from Earth, Deep Space 1 is headed for a September 2001 rendezvous with Comet Borrelly -- a regular visitor to the inner solar system. By then the probe will have used only about 55 pounds (25 kilograms) of its 181-pound (82-kilogram) supply of xenon. A spacecraft flying the same route using chemical propellants would need more than 900 pounds (408 kilograms) of fuel.
Deep Space 1 also will have added nearly a year to its 200-day operating-time record, for a total of 583 days.
The previous record for ion propulsion was 161 days on NASA's Space Electric Rocket Test 2, which was launched into Earth orbit in 1970. The only other system that has operated longer is a ground-based replica of Deep Space 1's engine, which has been running in a vacuum chamber at JPL for almost 500 days.
Risky software fix
Deep Space 1 came close to mission termination after its navigation system failed last November and its ion engine was shut down. That forced ground controllers to try a risky software fix in May. The fix worked and on June 21 the engine was turned back on.
Planners like Rayman can see the day when ion propulsion is no longer a novelty but a routine means of exploring the
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