Dawn's futuristic, hyper-efficient ion propulsion system allows Dawn to go into orbit around two different solar system bodies, a first for any spacecraft. Meeting the ambitious mission objectives would be impossible without the ion engines.
Ion propulsion was proved on NASA's Deep Space 1 mission, which tested it and11 other technologies while journeying to an asteroid and a comet.
Each of Dawn's three 30-centimeter-diameter (12- inch) ion thrust units is movable in two axes to allow for migration of the spacecraft's center of mass during the mission. This also allows the attitude control system to use the ion thrusters to help control spacecraft attitude.
Two ion propulsion engines are required to provide enough thruster lifetime to complete the mission, and the third engine serves as a spare. Since launch the spacecraft has used each of the three ion engines, operating them one at a time. Dawn will use ion propulsion with interruptions of only a few hours each week to turn to point the spacecraft's antenna to Earth. Total thrust time to reach the first science orbit will be 979 days, with more than 2,000 days of thrust through entire the mission. This surpasses Deep Space 1's 678 days of ion propulsion operation by a long shot.
The thrusters work by using an electrical charge to accelerate ions from xenon fuel to a speed 7-10 times that of chemical engines. The electrical power level and xenon fuel feed can be adjusted to throttle each engine up or down in thrust. The engines are thrifty with fuel, using only about 3.25 milligrams of xenon per second (about 10 ounces over 24 hours) at maximum thrust. The Dawn spacecraft carried 425 kilograms (937 pounds) of xenon propellant at launch. Xenon was chosen because it is chemically inert, easily stored in a compact form, and the atoms are relatively heavy so they provide a relatively large thrust compared to other candidate propellants. At launch, the gaseous xenon stored in the fuel tank was 1.5 times the density of water. At maximum thrust, each engine produces a total of 91 millinewtons—about the amount of force involved in holding a single piece of notebook paper in your hand.
You would not want to use ion propulsion to get on a freeway — at maximum throttle, it would take Dawn's system four days to accelerate from 0 to 60 MPH. As slight as that might seem, over the course of the mission the total change in velocity from ion propulsion will be comparable to the push provided by the Delta II rocket that carried it into space — all nine solid-fuel boosters, plus the Delta's first, second and third stages. This is because the ion propulsion system will operate for thousands of days, instead of the minutes during which the Delta performs.
The electrical power system provides power for all onboard systems, including the ion propulsion system when thrusting. Each of the two solar arrays is 27 feet (8.3 meters) long by 7.4 feet (2.3 meters) wide. On the front side, 18 square meters (21.5 square yards) of each array is covered with 5,740 individual photovoltaic cells. The cells can convert about 28 percent of the solar energy that hits them into electricity. On Earth, the two wings combined could generate over 10,000 watts. The arrays are mounted on opposite sides of the spacecraft, with a gimbaled connection that allows them to be turned at any angle to face the sun.
A nickel-hydrogen battery and associated charging electronics provided power during launch and continues to provide power at any time the solar arrays are directed away from the Sun.