- Mission Will be 33rd Flight of Company's Air-Launched Rocket -

       - Galaxy Observation Satellite Developed and Built by Orbital -

DULLES, Va., April 25 /PRNewswire-FirstCall/ -- Orbital Sciences Corporation (NYSE: ORB) today announced that it is in final preparations to launch the National Aeronautics and Space Administration (NASA)'s Galaxy Evolution Explorer (GALEX) satellite aboard the company's Pegasus(R) rocket. The launch is currently scheduled to take place on Monday, April 28, 2003 and will originate from Cape Canaveral, FL. The available launch window for the GALEX mission extends from 7:50 a.m. to 9:50 a.m. (Eastern Daylight Time), with a targeted launch time of 8:00 a.m. This operational schedule is subject to the completion of final pre-launch activities, as well as acceptable weather conditions at Cape Canaveral at the time of the launch.

The powered flight sequence for the GALEX mission will take approximately 11 minutes, from the time the Pegasus rocket is released from the L-1011 carrier aircraft to the time that the satellite is deployed into orbit. Orbital plans to launch the 312-kilogram (690-pound) GALEX spacecraft into a circular orbit 690 kilometers (420 miles) above the Earth, inclined at 29 degrees to the equator.

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 world's 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 Pegasus record of 18 consecutive successful missions since 1997.

The GALEX mission will be the 33rd flight of the Pegasus rocket and the second of four planned missions in 2003. In January, Orbital successfully launched another company-built satellite, SORCE, for NASA aboard Pegasus.

Following the GALEX mission, Orbital plans to launch the OrbView-3 high- resolution imaging satellite, which the company built for ORBIMAGE, in early June. Orbital is also scheduled to launch the SCISAT scientific spacecraft for NASA and the Canadian Space Agency later in 2003.

About GALEX

Orbital designed, built and integrated the GALEX satellite at its Dulles, VA manufacturing facility. It is the second satellite to be based on the company's recently introduced LeoStar-2 platform, on which NASA's SORCE satellite is also based. Since its launch in January, the SORCE satellite has exhibited outstanding in-orbit performance.

The mission of the GALEX satellite is to observe a million galaxies across 10 billion years of cosmic history to help astronomers determine when the stars and galaxies that we see today had their origins. During the course of its two-year investigation, GALEX will conduct the first ultraviolet surveys of the entire extragalactic sky, including the first wide-area spectroscopic surveys. This vast data archive will form a lasting legacy rich in objects from galaxies to quasars to white dwarf stars that will serve as a resource for the entire astronomical community.

The Jet Propulsion Laboratory (JPL) is responsible for the project management of GALEX and built the telescopic instrument. Orbital built the spacecraft bus and is responsible for instrument integration. The California Institute of Technology is responsible for the science operations and astronomical data. NASA's Kennedy Space Center is responsible for spacecraft/launch vehicle integration, countdown management, launch vehicle engineering oversight and mission assurance.

About Orbital

Orbital develops and manufactures small space systems for commercial, civil government and military customers. The company's primary products are spacecraft 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

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

    *     Live television coverage of the Pegasus/GALEX mission will be
          broadcast on NASA TV beginning at 6:30 a.m. (EDT).  NASA will also
          webcast the launch at: http://www.ksc.nasa.gov 

CONTACT: Barron Beneski, Public and Investor Relations, of Orbital Sciences Corporation, +1-703-406-5528, or beneski.barron@orbital.com.

SOURCE  Orbital Sciences Corporation
    -0-                             04/25/2003
    /CONTACT:  Barron Beneski, Public and Investor Relations, of Orbital
Sciences Corporation, +1-703-406-5528, or beneski.barron@orbital.com/
    /Web site:  http://www.orbital.com http://www.orbital.com/NewsInfo/Images http://www.ksc.nasa.gov/
    (ORB)

CO:  Orbital Sciences Corporation; National Aeronautics and Space
     Administration; NASA
ST:  Virginia
IN:  ARO
SU:  CCA


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