Russell got interested in ion engines in 1992, when he met Scott Benson, an engineer at NASA’s Lewis Research Center in Cleveland (now the Glenn Research Center), who had recently begun experimenting with ion propulsion. In fact, NASA had explored the technology as far back as the 1960s but lost interest as the agency’s focus shifted to the space shuttle; ion engines had been developed only to make minor adjustments in the paths of Earth-orbiting satellites. When NASA started its New Millennium program in the 1990s to develop innovative spaceflight technologies, research on ion thrusters began again, this time in earnest. “One of the features of ion propulsion is that it essentially allows you to fly on a smaller launch vehicle, at lower cost, to destinations that would require a larger vehicle with chemical propulsion,” says Benson. At first Russell’s instinct was to use ion propulsion to go back to the moon. As a postgraduate researcher on the Apollo program, he had developed instrumentation for the command module that measured the lunar magnetic field. With Benson he spent two years on a sequel of sorts, a lunar orbiter that used an ion engine, but the idea was passed over. He next worked up a proposal to go to Vesta but again failed to win backing from NASA. Russell suspects that ion propulsion was deemed too risky—it had never been used on a space probe. He tried to be philosophical: “Each time you lose,” he says, “you learn something.”
With its twin solar panel arrays fully extended after launch, Dawn is 65 feet long.
Image courtesy of NASA
The challenge was to turn an engine intended for occasional use on a satellite into a trustworthy interplanetary thruster. Deep Space 1, an engineering test mission launched by NASA in 1998, demonstrated that an ion engine could be used to move around the solar system. “That excited people,” Russell says. “That was a winner.” In December 2001, NASA gave Dawn a green light.
“Dawn really reflects a big departure from what we used to do in planetary exploration,” Russell adds. “The way we’re probing these bodies is very cost-effective.” NASA considered the cost of exploring both Vesta and Ceres with chemical rockets and concluded that it would have required two missions at $750 million each, as opposed to Dawn’s sub-$500 million price tag. “We’re saving a billion dollars compared with what it would have cost us to do it any other way,” Russell says.
In Russell’s proposal, Dawn used the same basic engine design as Deep Space 1 but needed a larger xenon fuel tank and other changes to ensure the system would survive its eight-year mission. Making these alterations nearly doomed the project, forcing it way over its $373 million budget. “The design parameters of Dawn were ambitious,” says Tom Jones, a former shuttle astronaut and now a consultant to NASA. “No probe had ever gone to one body, slowed down and achieved orbit, and then turned around and gone to a second body. That puts a lot of stress on an engine, and you have to make it reliable.”
By October 2005 Dawn was $73 million over budget. That, combined with concerns over the fuel tank’s design and the mission’s management, prompted NASA to pull the plug, canceling the project altogether in March 2006. NASA was also scrambling for funds to cover President George W. Bush’s moon program. Despite having already spent hundreds of millions of dollars, administrators may have been willing to scrap Dawn to avoid spending any more. Russell insists the project’s technical troubles were nothing out of the ordinary for such a complicated mission, and that NASA’s decision to cancel the project was foolhardy. “I don’t have any logical reason for why they did that,” he says. “To throw away the roughly $300 million that had been invested was crazy. Why not just finish off the project and get a return on this investment?” Fortunately, NASA’s chief administrator, Michael Griffin, allowed an appeal, and the mission was reinstated.
Now journeying outward, Dawn is following a flight plan unlike that of a conventional spacecraft. To set course for Vesta, a chemical rocket would burn for a few minutes near Earth, putting it on a path that intersected Vesta’s orbit, and then burn again to enter that orbit. Dawn’s ion engine, by contrast, has to accelerate the spacecraft continuously for months on end, spiraling outward until its trajectory matches Vesta’s orbit. The thrust from each of Dawn’s three ion engines is minuscule, a force equivalent to that of the weight of a piece of paper resting on the palm of your hand. But an engine will be firing during 90 percent of the trip, building up a speed as high as that attained by any chemical rocket.
A Mars flyby in February 2009 will help things along, giving Dawn a gravitational kick. In August 2011 it will begin slowing down as it approaches and then settles into orbit about Vesta. The craft will fire up its engines again in May 2012 to set course for Ceres. It will arrive in February 2015, once again slowing down to enter orbit and snap photos.
Taking snapshots will be a major part of its mission, because Dawn is not exactly a flying lab bristling with instruments. It has only three—part of the trade-off necessary to keep its weight and cost under control. A camera will create detailed maps of the two asteroids, with a resolution of about 225 feet for Vesta and 400 feet for Ceres. A spectrometer will measure the light absorbed by the asteroids’ surfaces, which will tell much about their composition. And a gamma ray and neutron detector will measure cosmic radiation bouncing off the surface of the asteroids. (It will be able to scan several yards below Ceres’ surface, searching for ice or liquid water.) In addition, variations in Dawn’s radio signal will be monitored to provide information about the gravitational pull—hence the internal structure—of the asteroids.
During the years of proposals and rejections, Russell had plenty of time to think about what Dawn might find when it finally reached its mystery worlds. His interests naturally led him to McCord, another asteroid hunter, who had gotten into the business indirectly. At Caltech in the 1960s, McCord helped develop instruments for remote spectrometry—analyzing the light coming off planets and stars. The first thing McCord and his colleagues trained their new instruments on was the moon, but soon they began measuring everything in sight. They worked their way through the planets and down to the asteroids, and eventually Vesta found itself in their crosshairs.
“It doesn’t sound like exploration, but that’s the way it really works,” McCord says. “You’ve got an instrument and you just go out and do everything you can with it. New data are power in science, and if you can measure something 10 times better than somebody else can, you’re going to learn a lot of exciting things.”
