DULLES, Va.Aug. 13, 2003--Orbital Sciences Corporation (NYSE:ORB) announced today that its Pegasus(R) space launch vehicle successfully launched the Scientific Satellite (SCISAT-1) Atmospheric Chemistry Experiment (ACE) spacecraft for the National Aeronautics and Space Administration (NASA) and the Canadian Space Agency. In a mission that took place on Tuesday, August 12, from Vandenberg Air Force Base, California, the 330-pound SCISAT-1 spacecraft was accurately delivered into its targeted orbit approximately 400 miles above the Earth, inclined at 73.9 degrees to the equator.

The powered flight sequence for the SCISAT-1 mission took about 11 minutes, from the time the Pegasus rocket was released from its L-1011 carrier aircraft at approximately 10:10 p.m. (EDT) to the time that the satellite was deployed into orbit. It was the 35th launch of the Pegasus rocket and its 21st consecutive successful mission.

About Pegasus

Pegasus is the world's leading launch system for the deployment of small satellites into low-Earth orbit. Its patented air-launch system, in which the rocket is launched from beneath Orbital's "Stargazer" L-1011 carrier aircraft over the ocean, reduces cost and provides customers with unparalleled flexibility to operate from virtually anywhere on Earth with minimal ground support requirements.

Pegasus is the only small launch vehicle to have earned NASA's Category 3 certification, which allows the U.S. space agency to launch its most valuable payloads aboard the rocket. A Category 3 certification is achieved through a long-term record of highly reliable launch services, such as the current record of 21 consecutive successful Pegasus missions carried out since 1997.

About Orbital

Orbital develops and manufactures small space systems for commercial, civil government and military customers. The company's primary products are satellites and launch vehicles, including low-orbit, geostationary and planetary spacecraft for communications, remote sensing and scientific missions; ground- and air-launched rockets that deliver satellites into orbit; and missile defense boosters 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

    Note to Editors:

    --  High-resolution photos of the Pegasus rocket are available on
        Orbital's web site at: http://www.orbital.com/NewsInfo/Images

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

    SOURCE: Orbital Sciences Corporation