Mission Will be 33rd Flight of Company’s Air-Launched Rocket – Galaxy Observation Satellite Developed and Built by Orbital

Orbital Sciences
Corporation 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® 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.