<PAGE>   1


                                                                [EXECUTION COPY]


                                AMENDMENT NO. 15

        AMENDMENT No. 15 ("THIS AMENDMENT") dated as of May 1, 2001 relating to
the Third Amended and Restated Credit and Reimbursement Agreement dated as of
December 21, 1998 (as the same has been amended and restated by Amendment No. 14
and Waiver dated as of April 12, 2001 and as the same may hereafter be amended
from time to time, the "CREDIT AGREEMENT") among ORBITAL SCIENCES CORPORATION
(the "COMPANY"), the BANKS party thereto (the "BANKS") and MORGAN GUARANTY TRUST
COMPANY OF NEW YORK, as Administrative Agent (the "ADMINISTRATIVE AGENT") and as
Collateral Agent (the "COLLATERAL AGENT").

        The parties hereto agree as follows:

        SECTION 1. Defined Terms; References. Unless otherwise specifically
defined herein, each term used herein which is defined in the Credit Agreement
has the meaning assigned to such term in the Credit Agreement. Each reference to
"hereof", "hereunder", "herein" and "hereby" and each other similar reference
and each reference to "this Agreement" and each other similar reference
contained in the Credit Agreement shall, after this Amendment becomes effective,
refer to the Credit Agreement as amended hereby.

        SECTION 2. Amendment of Negative Pledge. (a) Section 5.12(s) of the
Credit Agreement is amended by adding "(i)" immediately after the words "of MDA"
and adding the following words at the end thereof:

        "or (ii) pursuant to Section 7(c) of the letter agreement dated April
12, 2001 between MDH and the "Purchasers" named therein."

        SECTION 3. Governing Law. This Amendment shall be governed by and
construed in accordance with the laws of the State of New York.

        SECTION 4. Counterparts, Effectiveness. This Amendment may be signed in
any number of counterparts, each of which shall be an original, with the same
effect as if the signatures thereto and hereto were upon the same instrument.
This Amendment shall become effective as of the date hereof on the date when the
following conditions are met:

                (a)     the Administrative Agent shall have received from each
        of the Company and the Required Banks a counterpart hereof signed by
        such


<PAGE>   2


        party or facsimile or other written confirmation (in form satisfactory
        to the Administrative Agent) that such party has signed a counterpart
        hereof; and

                (b)     the Administrative Agent shall have received payment in
        full of all fees and expenses payable by the Company in connection with
        this Amendment pursuant to Section 10.03 of the Credit Agreement.



                                       2
<PAGE>   3


        IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed as of the date first above written.


                                    ORBITAL SCIENCES CORPORATION


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


                                    MORGAN GUARANTY TRUST COMPANY
                                      OF NEW YORK


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


                                    THE BANK OF NOVA SCOTIA


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


                                    BANK OF AMERICA, N.A.


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:




                                       3
<PAGE>   4


                                    FIRST UNION COMMERCIAL CORPORATION


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


                                    DEUTSCHE BANK AG, NEW YORK
                                      AND/OR CAYMAN ISLAND BRANCHES


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


                                    KEYBANK NATIONAL ASSOCIATION


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


                                    BANK OF TOKYO-MITSUBISHI TRUST COMPANY


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:



                                       4
<PAGE>   5


                                    WACHOVIA BANK, N.A.


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


                                    CHEVY CHASE BANK


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


                                    MORGAN GUARANTY TRUST COMPANY
                                      OF NEW YORK, as Administrative Agent and
                                      as Collateral Agent


                                    By
                                      ----------------------------------------
                                    Name:
                                    Title:


Acknowledged by:


ENGINEERING TECHNOLOGIES, INC.


By
   ------------------------------------------
   Name:
   Title:

ORBITAL SPACE SYSTEMS, INC.


By
   ------------------------------------------
   Name:
   Title:



                                       5
<PAGE>   6


ORBITAL COMMERCIAL SYSTEMS, INC.


By
   ------------------------------------------
   Name:
   Title:


ORBITAL INTERNATIONAL, INC.


By
   ------------------------------------------
   Name:
   Title:


ORBITAL SERVICES CORPORATION


By
   ------------------------------------------
   Name:
   Title:


ORBITAL NAVIGATION CORPORATION


By
   ------------------------------------------
   Name:
   Title:


ORBLINK LLC


By
   ------------------------------------------
   Name:
   Title:



                                       6