"It can thrust much, much more than any other spacecraft," said Rayman. "It's taken advantage of gravity boosts, but Dawn has the unique capability to propel itself far more than any other spacecraft ever has. It can fly to a destination, break into orbit and then break out of orbit and fly somewhere else and break into orbit there... this has never been attempted until the Dawn mission."
According to Rayman, this would be impossible with conventional propulsion.
Unlike propulsion systems that heat standard gas or put gas under pressure to produce thrust, an ion-based system uses xenon gas and gives it an electric charge. Once charged, it becomes an ion, which can be moved by a simple voltage charge. When voltage is added to ions, it shoots them out of the engine at great speeds, pushing the spacecraft in the opposite direction.
Ion propulsion has 10 times the efficiency of conventional propulsion. "That means we can take on missions that are much more ambitious," Rayman said. "When we eventually send humans to Mars, ion propulsion may figure into that mission simply by sending the cargo that the crews will need to Mars with this propulsion system."
The ion system also might be used for the spacecraft being designed to capture an asteroid and move it into orbit around the moon.
Dawn is not the first NASA spacecraft to use ion propulsion. The agency's Deep Space 1 spacecraft, launched in 1998 to test new technologies and capture data about a comet, was the first to use an alternative to traditional propulsion. The Dawn spacecraft, however, has used ion propulsion to travel much farther and to study two extraterrestrial objects.
'Acceleration with patience'
It's doubtful that an ion-based system would be used in a spacecraft that's transporting astronauts because ion propulsion doesn't move very fast.
"We only flow a very small amount of xenon through the engine at one time," explained Rayman. "It's very efficient but it's very gentle. That one piece of paper in your hand pushes on your hand as much as the xenon pushes on that spacecraft. But in the zero gravity of space flight, it can move a spacecraft."
It would take four days for ion propulsion to move a spacecraft from zero to 60 mph at full throttle, he said.
"No spacecraft has ever spent so much time in powered flight, but I like to call it acceleration with patience," Rayman said. "And if you're patient, it's a great way to explore the solar system."
To send a large payload to Mars, though, this propulsion system would do the trick. "You can get there faster with conventional propulsion, but if you want to send a massive payload to support humans, who require a lot of food, a lot of water and a lot of air and supplies, this would save you from having to use unaffordably large or unavailable rockets. It would be a slow trip, but there's no hurry."
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