— Companys First Planetary Mission to Rendezvous with Protoplanets Vesta and Ceres Between Mars and Jupiter —

— Ion-Propelled Dawn Spacecraft to Investigate the Origins of the Solar System —

Orbital Sciences Corporation (NYSE: ORB) announced today that it is in final preparations for the launch of its Dawn spacecraft on a historic NASA/Jet Propulsion Laboratory (JPL) mission to rendezvous with and study Vesta and Ceres, the two largest asteroids in the main belt between Mars and Jupiter. The Dawn spacecraft is currently scheduled for launch by NASA from Cape Canaveral Air Force Base aboard a three-stage Delta II rocket on September 27 during an available time window of between 7:20 a.m. and 7:49 a.m. (EDT). This operational schedule is subject to the completion of final pre-launch activities for the Dawn spacecraft and the Delta II launch vehicle, as well as acceptable weather conditions at Cape Canaveral at the time of the launch.

The Dawn mission’s primary objective is to advance understanding of the origin and evolution of the solar system. Vesta and Ceres are located in the main asteroid belt, a large region located between the orbits of Mars and Jupiter. Both large asteroids, also known as “protoplanets,” are believed to have formed at the same time and in similar environments as the solar system’s rocky inner planets Mercury, Venus, Earth and Mars. Vesta and Ceres were selected for study because available evidence shows that each has distinct characteristics that may reveal clues to the conditions and processes early in the formation of planets in our solar system. Vesta appears to be dry and perhaps volcanic, whereas Ceres has a primitive surface with possible concentrations of ice or liquid water in its polar regions or beneath its surface. By studying these contrasts and comparing the two asteroids, scientists hope to develop an understanding of the transition from the rocky inner regions to the icy outer expanses of the solar system.

“After over four years of working with NASA, the University of California at Los Angeles (UCLA) and JPL to make this mission a reality, Orbital’s Dawn spacecraft team is ready to get started with actual flight operations,” said Mr. Carl Marchetto, Executive Vice President and General Manager of Orbital’s Space Systems Group. “The Dawn program is especially important to Orbital because it represents the company’s first opportunity to demonstrate our ability to execute and support a planetary mission for NASA and JPL. With a successful Dawn launch and in-space operations, we are hopeful that we will be able to support more of these vitally important scientific missions in the future.”

About the Dawn Spacecraft

Orbital is partnered with the Dawn program’s Principal Investigator, Dr. Chris Russell of UCLA, and JPL, led by Project Manager Keyur Patel. The company’s role on the program has been to design, develop, manufacture, integrate and test the Dawn spacecraft that will carry out the asteroid mission and to support mission operations.

The 1,218 kg (2,685 lb) Dawn spacecraft will establish several “firsts” as it carries out its mission. It will be the first dedicated scientific mission to utilize an ion propulsion system developed by JPL, which was integrated with the spacecraft platform by Orbital. The Dawn spacecraft will carry enough Xenon propellant to enable it to change its speed by more than 10 kilometers per second (about six miles per second) over the course of six years of thrusting, far more than any other spacecraft’s propulsion system has ever achieved. The Dawn spacecraft will also be the first 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 ion propulsion system.

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 launch, while Dawn is still relatively close to Earth, it will generate over 10 kilowatts of power. However, 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 Orbital

Orbital develops and manufactures small rockets and space systems for commercial, military and civil government customers. The company’s primary products are satellites and launch vehicles, including low-orbit, geosynchronous-orbit and planetary spacecraft for communications, remote sensing, scientific and defense 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 offers space-related technical services to government agencies and develops and builds satellite-based transportation management systems for public transit agencies and private vehicle fleet operators. More information about Orbital can be found at http://www.orbital.com