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The Dawn spacecraft FROM NASA PRESS KIT
Most systems on the spacecraft are redundant, meaning that there is a backup available if the main system encounters a problem. Automated onboard fault protection software will sense any unusual conditions and attempt to switch to backups. With its solar array in the retracted position (for launch), the Dawn spacecraft is 2.36 meters (7 feet, 9 inches) long -- about as long as a large motorcycle. With its wide solar arrays extended, Dawn is about as long as a tractor-trailer at 19.7 meters (65 feet). Structure The core of the Dawn spacecraft's structure is a graphite composite cylinder. Tanks for the ion engines' xenon gas and the conventional thrusters' hydrazine are mounted inside the cylinder. The cylinder is surrounded by panels made of aluminum core with aluminum facesheets; most of the other hardware is mounted on these panels. Access panels and other spacecraft panels have composite or aluminum facesheets and aluminum cores. Blankets, surface radiators, finishes and heaters control the spacecraft's temperature. Telecommunication The telecommunication subsystem provides communication with Earth through any of three low-gain antennas and one 1.52-meter-diameter (5-foot) parabolic high-gain antenna. The high-gain antenna is the primary one used for most communication. The low-gain antennas are used when the spacecraft is not pointing the high-gain antenna toward Earth. Only one antenna can be used at a time. Attitude Control The attitude control system is responsible for determining the spacecraft's orientation in space, or "attitude," and providing control for maintaining and changing that attitude. Its hardware consists of two star trackers, three two-axis inertial reference units, 16 sun sensors and four reaction- wheel assemblies. The system controls gimbals to keep the solar arrays pointed towards the sun. In addition, it controls gimbaling of the ion thrusters, which can be moved in two axes. The system usually determines the spacecraft's attitude using its star trackers to sight known stars. The spacecraft's attitude is usually controlled by the reaction wheels, devices somewhat similar to traditional gyroscopes that use the momentum of spinning mass to maintain or change the spacecraft's orientation. However, the attitude can also be maintained or modified by a set of twelve 0.9-newton hydrazine thrusters that are collectively called the reaction control system. Serving primarily as a backup to the reaction wheels, the thrusters will be used shortly after launch to stop the spacecraft's spinning and point its solar panels at the sun. Ion Propulsion System The ion propulsion system will provide the Dawn spacecraft with the thrust that it will require to reach its target asteroids. The demanding mission profile would be impossible without the ion engines -- even a mission only to asteroid Vesta (and not on to Ceres) would require a much larger spacecraft and a dramatically larger launch vehicle. Ion propulsion was proved on NASA's Deep Space 1 mission, which tested it and 11 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. A total of three ion propulsion engines are required to provide enough thruster lifetime to complete the mission and still have adequate reserve. However, only one thruster will be operating at any given time. Dawn will use ion propulsion for years at a time, with interruptions of only a few hours each week to turn to point its antenna to Earth. Total thrust time through the mission will be about 2,100 days, considerably in excess of Deep Space 1's 678 days of ion propulsion operation. The thrusters work by using an electrical charge to accelerate ions from xenon fuel to a speed 10 times that of chemical engines. The electrical level and xenon fuel feed can be adjusted to throttle each engine up or down. 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 carries 425 kilograms (937 pounds) of xenon propellant. 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 miles per hour. 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. Power The electrical power system provides power for all onboard systems, including the ion propulsion system when thrusting. The ion propulsion system requires considerable electrical power, which must be available when the spacecraft is in orbit at Ceres. Since the dwarf planet is three times farther from the sun than Earth is, sunlight there is nine times fainter. Each of the two solar arrays is 8.3 meters (27 feet) long by 2.3 meters (7.4 feet) wide. On the front side, 18 square meters (square feet) 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. At 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 provide power during launch and at any time the solar arrays are directed away from the sun. Computer The Dawn spacecraft's command and data handling system provides overall control of the spacecraft and manages the flow of engineering and science data. The system consists of redundant RAD6000 processors, each with 8 gigabits of memory. Scientific Instruments To acquire science data at Vesta and Ceres, Dawn carries three instrument systems. In addition, an experiment to measure gravity will be accomplished with existing spacecraft and ground systems.
Dawn will make another set of scientific measurements at Vesta and Ceres using the spacecraft's radio transmitter and sensitive antennas on Earth. Monitoring signals from Dawn, scientists can detect subtle variations in the gravity fields of the two space objects. These variations will point to how mass is distributed in each body, in turn providing clues about the interior structure of Vesta and Ceres. |
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