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By Robert Lemos
The Dawn mission to the asteroid belt passed a critical test last weekend, exercising its gentle, yet reliable, ion-propulsion system to accelerate its mission to two destinations during an eight-year voyage through our solar system.
The key: Ion propulsion only needs a tenth of the fuel of a chemical rocket system to reach the same destination, and that means a smaller rocket -- and a lot less fuel -- is needed to launch missions, such as the Dawn probe. The miserly use of propellant will allow the Dawn probe to become the first to orbit two separate objects -- in this case, the asteroid Vesta in 2011 and the dwarf planet Ceres, where it's expected to arrive in 2015.
Ion propulsion uses positively charged atoms, or ions, to propel a spacecraft. An electron gun is used to knock electrons from a reservoir of xenon atoms, turning them into ions. Then, two charged plates accelerate the ions and eject them from the back of the rocket engine at speeds of 35 kilometers per second, or about 77,000 mph. To avoid accumulating a negative charge, the probe shoots electrons back into the stream of xenon ions leaving the engine.
Pushing a probe farther into space on a light stream of atoms is a trade-off. While it is 10 times more efficient than chemical propulsion, and thus only requires a tenth of the fuel needed by a chemical thruster, it also lacks power. Marc Rayman, chief engineer on the Dawn mission, has dubbed ion propulsion "acceleration with patience."
"The force of the ion thruster on the spacecraft is comparable to the weight of a single sheet of paper," he stated in an online explanation of the technology. "So here is an ion propulsion experiment you may conduct safely at home: Hold a piece of paper in your hand, and you will feel the same force that the ion thruster exerts."
If the craft's ion engine continuously thrusts for 24 hours, the probe will only expend 10 oz. of xenon. (It carries more than 900 lbs.) To go from zero to 60 mph takes the Dawn probe nearly four days.
Yet the probe's thruster is 10 times more efficient than a chemical thruster. A typical chemical spacecraft could accelerate to 1 kps in about 20 minutes, but would require 300 kg of propellant, according to Rayman. The Dawn spacecraft's ion propulsion system could attain the same speed with only 25 kg of xenon, but would take nearly 100 days of continuous thrust to reach that velocity.
Based on concepts dreamed up by rocket scientists nearly a century ago, ion propulsion has been used in just four interplanetary spacecraft, including the Dawn probe. But if the technology continues to prove itself, manned and unmanned missions will explore more of the solar system for less money, according to Chris Russell, a professor of geophysics and space physics at UCLA and principal investigator of the Dawn mission team.
"We initially modeled the mission and found that if we did it chemically, there is no rocket in our arsenal that would allow us to go to two objects," Russell said. "The cost of launch, the cost of operations and the cost of the spacecraft is just about $450 million. With a chemical system, it would have been three times more."
In 1998, Deep Space 1 became the first spacecraft to use ion propulsion to reach destinations in the solar system. Using more than 160 lbs. of xenon, the probe debuted a dozen new technologies and flew by two asteroids, operating its ion engine for 678 days -- the longest any propulsion system had been continuously run. The Dawn mission is expected to surpass that record.
"Ion propulsion has come of age now," said John Brophy, project element manager for the Dawn ion propulsion system at NASA's Jet Propulsion Laboratory. "It has been around for a long time. But it pretty much is the system of choice for (keeping satellites in orbit) and for certain types of deep space missions." While the space program has only recently adopted the technology, the theories are not new.
Noted rocket scientist Robert Goddard first suggested a propulsion system using electric fields to accelerate charged ions as early as 1906. And, like much of modern rocket science, the technology owes part of its founding to Wernher von Braun, once a Nazi scientist who created the V-2 program for Hitler. The United States snuck him out of Germany during the scramble for rocket technology at the end of World War II before the Soviets could. Von Braun led the American response to the Soviet Union's Sputnik launch in 1957.
His mentor, Hermann Oberth, had suggested as early as 1930 that electric propulsion could be used for space flight. After reaching the United States, von Braun requested that a colleague, Ernst Stuhlinger, look into possible applications of the theories. The research continued until the 1960s, when the United States pushed aside electric propulsion efforts in favor of the chemical rockets needed to reach the Moon.
The Dawn spacecraft uses giant solar panels -- the largest ever to grace an unmanned probe -- to generate the electricity needed to power the engines. As the probe moves farther from the sun, however, the arrays will not produce enough energy for the craft to operate at the highest throttle levels. For that reason, future deep-space probes may need to be powered by nuclear generators.
"As humankind engages in ever more ambitious missions in deep space, opening our frontiers, revealing otherwise inaccessible vistas, and seeking answers to new and more exciting questions about the cosmos, the tremendous capability of ion propulsion will be an essential ingredient," Rayman stated online.
Tests will continue for the next two months to evaluate the performance of the systems.
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