-- Spacecraft is Performing as Expected Early in Its Mission --

     -- AMC-21 to Begin Several Weeks of In-Orbit Testing Prior to
                 Commencement of Commercial Service --

DULLES, Va.Aug. 18, 2008--Orbital Sciences Corporation (NYSE:ORB), the world's leading manufacturer of smaller-sized geosynchronous (GEO) communications satellites, announced today that the AMC-21 satellite, which is based on the company's STAR-2 platform, was successfully launched into orbit during a mission that took place on Thursday, August 14, 2008. In conjunction with the program's prime contractor, Thales Alenia Space, Orbital built, integrated and tested the AMC-21 spacecraft for SES Americom. The satellite was delivered into its initial orbit by an Ariane 5 rocket launched from the European space launch complex located near the Equator in French Guiana. Results from early tests of the satellite conducted over the weekend indicate that it is operating as planned for this phase of its mission.

Once it reaches it final operational location, the AMC-21 satellite will occupy an orbital slot at 125 degrees West longitude, providing Ku-band broadcast television and mobile broadband services to the Continental United States (CONUS), Southern Canada, Mexico, the Caribbean and Central America. The AMC-21 is fitted with 24 Ku-band transponders, each 36 MHz in bandwidth, and is designed for an operational life of 15 years. At launch, the spacecraft weighed approximately 2,500 kilograms.

"The early mission results point to an excellent start for the AMC-21 spacecraft. The satellite is performing just as we would expect prior to embarking on the extensive set of in-orbit tests and orbit raising maneuvers to be performed over the next several weeks," stated Mr. Christopher Richmond, Senior Vice President and head of Orbital's GEO satellite programs division. "We also congratulate Arianespace for another excellent launch campaign, resulting in the accurate deployment of the AMC-21 satellite."

During the next several days, the AMC-21 satellite will perform a series of orbit-raising burns using its onboard rocket engines to achieve a circular orbit approximately 22,300 miles above the Earth. For several weeks afterwards, the mission engineering teams from SES Americom, Thales Alenia Space and Orbital will conduct a comprehensive series of tests to ensure the spacecraft is ready for final hand-over to the customer.

Over the past several years, Orbital has established itself as the world leader in the market for smaller-sized GEO commercial communications satellites. Orbital currently has 10 additional GEO spacecraft in various stages of design, production and testing at its Dulles, VA satellite manufacturing facility. The commercial communications satellite business is in addition to Orbital's long-established presence in the market for civil government space- and Earth-science satellites and its rapidly growing position in the national security space systems market. To accommodate the rapid growth rate of its total satellite business, Orbital has completed an expansion of its manufacturing plant and has added approximately 500 people to its satellite design, manufacturing and testing operations at its Dulles, VA campus in the past two years.

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-Earth orbit, geosynchronous-Earth orbit and planetary 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 government agencies and laboratories.

Note to Editor: High-resolution images of the AMC-21 are available on Orbital's web site at: http://www.orbital.com/NewsInfo/Images/GEO/index.html

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

    SOURCE: Orbital Sciences Corporation