-- Mission Designed to Provide Information About the Origins of
                          the Solar System --

DULLES, Va.Sept. 27, 2007--Orbital Sciences Corporation (NYSE:ORB) announced today that Dawn, its first interplanetary spacecraft, was successfully launched this morning from Cape Canaveral, Florida. Lift-off of the Delta II launch vehicle occurred at 7:34 a.m. and approximately one hour later, Dawn separated from the final stage of the rocket. Initial data from the spacecraft indicates it is operating as planned early in the mission.

The primary objective of NASA's Dawn mission is to advance understanding of the origin and evolution of the solar system by rendezvousing with and studying Vesta and Ceres, which 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.

"After over four years of designing, building and testing the Dawn spacecraft, Orbital's entire staff is very excited that it is now on its way to completing a historic, first-of-its-kind mission to rendezvous with and study the asteroids Vesta and Ceres, which will help us understand how our solar system was formed. We are delighted to be a key member of the team that is carrying out such an important scientific investigation," said Mr. Carl Marchetto, Executive Vice President and General Manager of Orbital's Space Systems Group.

Marchetto also noted, "After a picture perfect launch into the early morning Florida sky, Orbital has entered a new market for our satellite business. With the Dawn mission, we have an excellent opportunity to display our deep space capabilities and look forward to participating in future spacecraft programs that support NASA's Vision for Space Exploration."

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

    CONTACT: Orbital Sciences Corporation
             Public and Investor Relations
             Barron Beneski, 703-406-5528

    SOURCE: Orbital Sciences Corporation