DULLES, Va.April 18, 2005--

DART Spacecraft Autonomous In-Orbit Operations Concluded Earlier than Planned; Important New Capabilities Demonstrated, But Several Mission Objectives Not Achieved

Orbital Sciences Corporation (NYSE:ORB) today issued a recap of the Demonstration of Autonomous Rendezvous Technology (DART) mission that it carried out last Friday and Saturday for the National Aeronautics and Space Administration (NASA). Orbital was the prime contractor for the mission, with responsibility for both the Pegasus launch vehicle and the DART research and development spacecraft. The company stated that the launch of the DART spacecraft aboard its Pegasus rocket was a full success; however, the in-orbit operations of the DART spacecraft achieved only a partial set of its mission objectives and concluded earlier than planned. Approximately 12 hours into a 24-hour operation, the DART vehicle's autonomous control systems correctly initiated the mission's retirement phase when low onboard fuel levels were detected. The cause for the higher than planned fuel consumption is now under investigation by a committee convened by NASA.

The DART mission began with the successful launch of Orbital air-launched Pegasus rocket at approximately 1:25 p.m. (EDT) on Friday, April 15. Following a 12-minute flight, the Pegasus rocket accurately placed the DART spacecraft into its targeted orbit, approximately 545 km (340 miles) above the Earth. It was the 22nd consecutive successful mission for the Pegasus space launch vehicle program since 1997 and its 36th flight since its debut in 1990.

Following its deployment from Pegasus, the DART spacecraft embarked on a planned 24-hour mission to locate, rendezvous with and conduct close-proximity maneuvers around another in-orbit satellite, all without the benefit of human pilots or ground controllers directing the mission operations. Orbital stated that DART successfully completed the location and rendezvous phases of its operations, closing to within approximately 92 meters (300 feet) of the mission's target satellite, the Multiple Paths Beyond Line-of-Sight Communications (MUBLCOM) spacecraft, which Orbital built and launched in 1999 and which completed its original mission several years ago. DART's Advanced Video Guidance System (AVGS) instrument also was able to acquire the MUBLCOM satellite, accomplishing one of the mission's key R&D objectives. The DART spacecraft was unable to complete all of its close proximity and circumnavigation operations near the MUBLCOM satellite due to the depleted fuel situation.

Orbital designed, developed, built and tested the DART spacecraft for NASA's Marshall Space Flight Center in Huntsville, AL. The goal of the DART mission was to test technologies that may be applied to future manned and unmanned missions of the U.S. space program that require in-space assembly, servicing, docking or other autonomous rendezvous operations.

About Orbital

Orbital is one of the world's leading developers and manufacturers of affordable space systems for commercial, civil government and military customers. The company's primary products are satellites and launch vehicles, including geostationary and low-orbit 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, the Pegasus space launch vehicle and the DART spacecraft can be found at http://www.orbital.com


    CONTACT: Orbital Sciences Corporation
             Barron Beneski, 703-406-5528
             Beneski.barron@orbital.com

    SOURCE: Orbital Sciences Corporation