DULLES, VAAug. 16, 2003--Orbital Sciences Corporation (NYSE:ORB)

    --  Company's Interceptor Booster Demonstrates Silo-Launch
        Capabilities and Validates Vehicle Design and Flight
        Performance --

    --  Launch Represents Orbital's 25th Consecutive Successful Space
        Mission in the Past 21 Months --

Orbital Sciences Corporation (NYSE: ORB) announced today that the second launch of the missile defense interceptor boost vehicle the company is developing and manufacturing for the U.S. Missile Defense Agency's (MDA) Ground-based Midcourse Defense (GMD) system was fully successful. In a mission that originated from Vandenberg Air Force Base (VAFB), California, Orbital's GMD boost vehicle was launched from a silo at approximately 2:00 p.m. (EDT). The boost vehicle flew a ballistic trajectory over the Pacific Ocean, reaching an altitude of approximately 1,165 miles and traveling about 3,300 miles down range from the launch site.

Following preliminary post-flight analysis of the data collected from the mission, Orbital confirmed that all the primary objectives for the second launch of its GMD boost vehicle were achieved. The objectives for the mission included demonstrating the vehicle's silo-launch capabilities, verifying the vehicle design and flight characteristics, and confirming the planned performance of its guidance, control and propulsion systems. Orbital's GMD boost vehicle is a three-stage system based on flight-proven hardware that has flown 45 times on missions carried out by the company's Pegasus(R), Taurus(R) and Minotaur space launch vehicles.

"With the second successful launch of our GMD boost vehicle, the Boeing and Orbital team has clearly demonstrated that it is on track to provide a reliable, high-performance interceptor vehicle system to defend the U.S. from the threat of ballistic missile attack," said Mr. Ron Grabe, Executive Vice President and General Manager of Orbital's Launch Systems Group.

Under a contract from The Boeing Company, Orbital is developing, manufacturing and testing interceptor boost vehicles for MDA's GMD system. The firm portion of the company's contract, awarded in early 2002, is valued at approximately $450 million and extends through 2007.

In addition to providing boost vehicles for flight test, the contract includes manufacture and delivery of five boost vehicles for emplacement at Ft. Greely, Alaska by September 2004. Current government plans for increasing the number of deployed boost vehicles in 2005 could potentially increase the value of Orbital's contract later this year.

Mr. James R. Thompson, Orbital's Vice Chairman and Chief Operating Officer, said, "Our working relationship with Boeing on the GMD booster program has been outstanding. That cooperation is reflected in the operational success of our new booster, which has been seamlessly designed, manufactured, tested, delivered and launched twice in just over a year and a half. This aggressive schedule is helping MDA meet its schedule, cost and performance goals for the GMD program."

The successful GMD boost vehicle launch was the 12th space mission Orbital has carried out in 2003 and the 25th consecutive success dating back to December 2001. During that 21-month period, seven Orbital-built satellites and related systems and 18 company-supplied rockets have carried out a wide range of military, scientific and commercial missions for domestic and international customers.

Orbital's space launch vehicles, missile defense interceptors and related suborbital rockets are primarily produced at the company's engineering and manufacturing facility in Chandler, Arizona and its vehicle assembly and integration facilities at VAFB. The launch vehicles are used by commercial and government customers to deliver small satellites into low altitude orbits above the Earth and in missile defense systems, both as threat-simulating target vehicles and potentially as interceptor boosters for U.S. national defense systems.

In addition to its launch vehicle systems, Orbital's other primary products are satellites and related space systems, which are also used by commercial, civil government and military customers. These products include low-orbit, geostationary and planetary spacecraft for communications, remote sensing and scientific missions. In addition, Orbital 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