<PAGE>   1
                                                                 EXHIBIT 10.1.13
                                                                [EXECUTION COPY]



                                AMENDMENT No. 13
                                       TO
                           THIRD AMENDED AND RESTATED
                       CREDIT AND REIMBURSEMENT AGREEMENT


         AMENDMENT No. 13 dated as of February 23, 2001 among ORBITAL SCIENCES
CORPORATION (the "COMPANY"), the BANKS listed on the signature pages hereof and
MORGAN GUARANTY TRUST COMPANY OF NEW YORK, as Administrative Agent and as
Collateral Agent.

                                   WITNESSETH:

         WHEREAS, the parties hereto have heretofore entered into a Third
Amended and Restated Credit and Reimbursement Agreement dated as of December 21,
1998 (as amended prior to the date hereof, the "EXISTING AGREEMENT"); and

         WHEREAS, the Company and the Banks wish to amend and restate the Credit
Agreement as set forth herein;

         NOW, THEREFORE, the parties hereto hereby agree as follows:

         SECTION 1. Definitions; References . Unless otherwise specifically
defined herein, each term used herein shall have the meaning assigned to such
term in Exhibit A hereto. Each reference to "hereof", "hereunder", "herein" and
"hereby" and each other similar reference contained in the Credit Agreement
shall from and after the Effective Date (as defined in Section 4 below) refer to
the Credit Agreement as amended and restated hereby.

         SECTION 2. Amendment and Restatement of the Credit Agreement . The
Existing Agreement is hereby amended and restated in its entirety to read as set
forth in Exhibit A (as so amended and restated, the "CREDIT AGREEMENT"):

         SECTION 3. Representations and Warranties . The Company hereby
represents and warrants that the representations and warranties set forth in the
Credit Agreement are true and correct.

         SECTION 4. Effectiveness . This Amendment shall become effective on the
date (the "EFFECTIVE DATE") on which the following conditions shall have been
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satisfied (except Sections 2.13 and 10.03 of the Credit Agreement, which shall
become effective when the Administrative Agent shall have received the documents
specified in paragraph (a) below):

             (a)      The Administrative Agent shall have received counterparts
hereof signed by each of the parties hereto (or, in the case of any party as to
which an executed counterpart shall not have been received, receipt by the
Administrative Agent in form satisfactory to it of telegraphic, telex, facsimile
or other written confirmation from such party of execution of a counterpart
hereof by such party);

             (b)      The Administrative Agent shall have received an opinion of
Hogan & Hartson L.L.P., special counsel for the Borrowers, substantially in the
form of Exhibit B hereto and covering such additional matters relating to the
transactions contemplated by the Financing Documents as the Required Banks may
reasonably request;

             (c)      The Collateral and Guarantee Requirement shall have been
satisfied;

             (d)      The New Agreement shall have become effective in
accordance with its terms;

             (e)      The Administrative Agent shall have received an amendment
fee for the account of each Bank from which the Administrative Agent shall have
received a signed counterpart hereof (or satisfactory confirmation of its
signing a counterpart hereof) not later than the date of satisfaction of the
condition in paragraph (a) in an amount equal to 0.60% of such Bank's
Commitment; and

             (f)      The Administrative Agent shall have received all documents
it may reasonably request relating to the existence of the Borrowers, the
corporate authority for and the validity of the Financing Documents, and any
other matters reasonably relevant hereto, all in form and substance reasonably
satisfactory to the Administrative Agent;

provided that this Agreement shall not become effective or be binding on any
party hereto unless the foregoing conditions are satisfied not later than
February 23, 2001. On the Effective Date the Credit Agreement will be
automatically amended and restated in its entirety to read as set forth in
Exhibit A. On and after the Effective Date the rights and obligations of the
parties hereto shall be governed by the Credit Agreement. Without limiting the
generality of the foregoing, on and after the Effective Date the provisions of
prior amendments of and waivers under the Existing Agreement (including
Amendment No. 4 thereto which by its terms has not become effective) shall be of
no further force and effect except insofar as the same are reflected in the
terms of the Credit Agreement. The Administrative Agent shall

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promptly notify the Company and the Banks of the effectiveness of this
Agreement, and such notice shall be conclusive and binding on all parties
hereto.

         SECTION 5. Governing Law . This Amendment shall be governed by and
construed in accordance with the laws of the State of New York.

         SECTION 6. Counterparts . 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.




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         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., f/k/a
                                       NATIONSBANK, N.A.


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


                                   FIRST UNION COMMERCIAL CORPORATION


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


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<PAGE>   5

                                   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:


                                   WACHOVIA BANK, N.A.


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




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                                   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:

ORBITAL COMMERCIAL SYSTEMS, INC.


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





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ORBITAL INTERNATIONAL, INC.


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


ORBITAL SERVICES CORPORATION


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


ORBITAL NAVIGATION CORPORATION


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


ORBLINK LLC


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



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                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                        Page
                                                                        ----

<S>             <C>                                                     <C>
      SECTION 1.  Definitions; References.................................1
      SECTION 2.  Amendment and Restatement of the Credit Agreement.......1
      SECTION 3.  Representations and Warranties..........................1
      SECTION 4.  Effectiveness...........................................1
      SECTION 5.  Governing Law...........................................3
      SECTION 6.  Counterparts............................................3
</TABLE>


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