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-- Company's Ion-Propelled Interplanetary Spacecraft Reaches One of the Solar System's Largest Asteroids Between Mars and Jupiter --

-- Dawn Provides First-Ever Close-Up Images of Arizona-Size Asteroid --

(Dulles, VA 20 July 2011) - Orbital Sciences Corporation (NYSE: ORB) announced today that the Dawn interplanetary spacecraft, which the company designed and built for NASA's Jet Propulsion Laboratory (JPL), has successfully completed rendezvous and achieved orbit around the solar system's second most massive asteroid, Vesta. These events took place after a nearly four-year, 1.7 billion-mile journey to Vesta, which resides in the main asteroid belt between Mars and Jupiter.

Dawn successfully entered orbit approximately 9,900 miles above Vesta's surface last Friday evening at approximately 10:00 p.m. PDT. Since capture, the spacecraft has continued its gradual decent toward its initial science survey orbit at an altitude of about 1,700 miles above Vesta. As the mission progresses over the next year, it will descend to additional science orbits at 425 miles and then 125 miles above the surface. Dawn will image Vesta's surface using Framing Cameras and carry out other scientific surveys using a Visible and Infrared Mapping Spectrometer and a Gamma-Ray and Neutron Detector. Dawn also will conduct a scientific survey using its radio system (along with the NASA Deep Space Network) to map Vesta's gravity field. Dawn will continue to return science data about the asteroid through July 2012, when it will depart Vesta to journey farther out in the asteroid belt.

The Dawn mission will achieve a "first" in space exploration history when it thrusts out of Vesta's orbit and continues on its journey to rendezvous in 2015 with Ceres, the solar system's largest asteroid. Ceres was recently reclassified as a dwarf planet, the same designation given to Pluto. Dawn will become the first exploration spacecraft to rendezvous with and orbit a planetary body and then transfer to and orbit a second planetary body.
This "first" is made possible by the unique onboard ion propulsion system developed by JPL, which Orbital incorporated into its spacecraft design. The ion propulsion system uses electrical power, supplied by very large solar arrays, to efficiently ionize the onboard xenon fuel and accelerate the resulting plasma from rocket thrusters at very high velocities. The resulting thrust, though of very small magnitude, is applied nearly continuously through much of the mission and accelerates the spacecraft to velocities about ten times that achievable by traditional chemical rockets. The ion propulsion system is also used as a braking system to lower the spacecraft altitude above Vesta. The spacecraft will rely on this innovative propulsion system to escape from Vesta's orbit and cruise to its second destination, a rendezvous with Ceres. It will again be used as a braking system to lower the orbit around Ceres.

To reach this important point in its mission, the Dawn spacecraft has traveled over 1.7 billion miles in its path around the Sun. Included in its journey was a close fly-by of Mars that utilized the red planet's gravitational field as a "slingshot" to accelerate the spacecraft and alter its trajectory to reach its deep space destination. When Dawn's eight-year mission is complete, it will have traveled over 3.0 billion miles.
About the Dawn Spacecraft
The Dawn spacecraft is a medium-class spacecraft that weighed 1,218 kg (2,685 lbs.) at the time of its launch in September 2007 from Cape Canaveral, FL. It carried enough xenon propellant to enable it to change its speed by more than 11 kilometers per second (almost seven miles per second) over the course of the mission, far more than any other spacecraft propulsion system has ever achieved. The solar arrays of the Dawn spacecraft are also unusually large for a spacecraft of this size due to the need to generate sufficient electrical power once the spacecraft has reached Vesta and Ceres. The solar arrays measure approximately 20 meters (65 feet) from tip to tip in their fully deployed configuration. Shortly after its launch, when Dawn was still relatively close to Earth, it was capable of generating over 10 kilowatts of power. The large solar arrays will still generate nearly 1.5 kilowatts of power while orbiting Ceres, despite being almost three times farther away from the Sun than at the beginning of the mission.
About the Dawn Mission
The primary goal of the Dawn mission is to characterize the conditions and processes of the solar system's earliest epoch by investigating two of the largest protoplanets remaining intact since their formation. Vesta and Ceres orbit the Sun in an extensive zone between Mars and Jupiter called the asteroid belt. These asteroids have followed very different evolutionary paths constrained by the diversity of processes that operated during the first epoch of solar system evolution.
The top-level question that the mission addresses is the role of size and water in determining the evolution of the planets. Ceres and Vesta are ideal bodies with which to address this question, as they are the most massive of the protoplanets, baby planets whose growth was interrupted by the formation of Jupiter. Vesta is considered to be an evolved and dry body while Ceres is thought to be primitive and contain substantial water ice. For more information about the Dawn mission, visit JPL's Dawn mission page at:
About Orbital
Orbital develops and manufactures small- and medium-class rockets and space systems for commercial, military and civil government customers. The company's primary products are satellites and launch vehicles, including low-Earth orbit, geosynchronous-Earth orbit and planetary exploration spacecraft for communications, remote sensing, scientific and defense missions; human-rated space systems for Earth-orbit, lunar and other missions; ground- and air-launched rockets that deliver satellites into orbit; and missile defense systems that are used as interceptor and target vehicles. Orbital also provides satellite subsystems and space-related technical services to U.S. Government agencies and laboratories. More information about Orbital can be found at

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