Document:

<PAGE>   1
                                                                  EXHIBIT 10.1.8

                                                                            FORM

                           AMENDMENT NO. 8 AND WAIVER
                                       TO
                           THIRD AMENDED AND RESTATED
                       CREDIT AND REIMBURSEMENT AGREEMENT

              AMENDMENT No. 8 and WAIVER dated as of April 13, 2000 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 (the "ADMINISTRATIVE AGENT") and as Collateral Agent (the "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 from time to time, the "CREDIT AGREEMENT"); and

              WHEREAS, the Company has asked the Banks to waive compliance by
the Company with certain covenants set forth in the Credit Agreement for the
period from and including the Amendment No. 8 Effective Date (as defined in the
Credit Agreement as amended hereby) to and including January 1, 2001 (the
"WAIVER PERIOD"), and the Banks are willing to do so, subject to the terms and
conditions set forth herein;

              NOW, THEREFORE, the parties hereto agree as follows:

       SECTION 1. Definition; References. Unless otherwise specifically defined
herein, each term used herein that is defined in the Credit Agreement shall have
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 from and after the Amendment No. 8
Effective Date (as defined in Section 21 below) refer to the Credit Agreement as
amended hereby.

       SECTION 2. Amendments to the Definitions. (a) Section 1.01 of the Credit
Agreement is amended by adding therein the following definitions in alphabetical
order:

<PAGE>   2

       "AMENDMENT NO. 8 EFFECTIVE DATE" means the date of effectiveness of
Amendment No. 8 and Waiver to this Agreement.

       "**** ACQUISITION" means the ******* acquisition .

       "DEBT ISSUANCE" means any issuance of Debt by the Company or any of its
wholly-owned domestic subsidiaries after the Amendment No. 8 Effective Date
(including without limitation any Debt convertible into equity) in the capital
markets (whether in a registered offering or in a private placement).

       "MDA FINANCING" means the credit agreement to which MDA will become a
party providing for loans thereunder to be used by MDA for working capital
purposes and for [other purposes]; provided that the aggregate principal amount
of Debt that may be incurred under such credit agreement shall not exceed
$210,000,000 Canadian Dollars.

       "NML DEBT" means Debt of the Company under the Note Agreement dated as of
June 1, 1995 between the Company and The Northwestern Mutual Life Insurance
Company.

       "RESTATEMENT DATE" means the date of effectiveness of the restatement
described in Section 5.23.

       "SENIOR DEBT" means at any date (i) Debt of the Company under this
Agreement plus (ii) the NML Debt plus (iii) other Debt of the Company (other
than (x) Debt owed to any Subsidiary and (y) other Debt so long as such other
Debt is subordinated to Debt of the Company under this Agreement on terms
satisfactory to the Required Banks) plus (iv) Debt of Subsidiaries of the
Company (other than (w) any Guarantees of the NML Debt, (x) Guarantees of Debt
under this Agreement, (y) Debt of any Subsidiary owed to the Company or any
other Subsidiary and (z) other Debt of any Subsidiary Guarantor so long as such
other Debt is subordinated to such Subsidiary Guarantor's Guarantee of Debt
under this Agreement on terms satisfactory to the Required Banks).

       "SENIOR LEVERAGE RATIO" means on any date the ratio of Senior Debt on
such date to Consolidated EBITDA for the period of four consecutive fiscal
quarters most recently ended on or prior to such date.

       "SUBSIDIARY GUARANTOR" means any Subsidiary of the Company that has
Guaranteed the obligations of the Company under the Financing Documents.

       (b) The following definitions set forth in Section 1.01 of the Credit
Agreement are amended to read in their entirety as follows:

                                       2
<PAGE>   3

       "CONSOLIDATED DEBT" means at any date, without duplication, the sum of
(i) the Debt of the Company and its Consolidated Subsidiaries determined on a
consolidated basis (other than Debt consisting of performance bonds and letters
of credit issued for the account of MDA in an aggregate amount not in excess of
$48,000,000 to support certain contractual obligations, including obligations in
an aggregate amount of approximately $20,000,000 for construction of the
Radarsat 2 satellite, of approximately $8,000,000 under a contract with respect
to a Malaysian ground station, of approximately $3,000,000 under a contract with
respect to a Taiwanese ground station and of approximately $17,000,000 with
respect to miscellaneous performance bonds) plus (ii) the portion of the Debt
(other than Excluded ORBCOMM Debt) of any Person accounted for by the Company on
the equity method properly allocable to the direct or indirect interest of the
Company in such Person, all determined as of such date.

       "EQUITY ISSUANCE" means any issuance of equity securities by the Company
or any of its domestic wholly-owned Subsidiaries, other than (i) any such
issuance to the Company or any of its domestic wholly-owned Subsidiaries, (ii)
any such issuance pursuant to employee benefit arrangements in the ordinary
course of business, (iii) any such issuance pursuant to the warrants
contemplated by Section 16 of Amendment No. 6 and (iv) any such issuance
pursuant to the conversion of any Debt, so long as neither the Company nor any
of its domestic wholly-owned Subsidiaries receives any Net Cash Proceeds in
connection with any such issuance described in this clause (iv).

       "REDUCTION EVENT" means any Asset Sale, Debt Issuance or Equity Issuance.

       SECTION 3. Waiver of Compliance with Certain Covenants. (a) The Banks
waive (i) compliance by the Company with the provisions of Sections 5.01(a),
5.01(b), 5.01(c), 5.01(d), 5.08, 5.09, 5.10 and 5.17 of the Credit Agreement on
or prior to the Amendment No. 8 Effective Date and (ii) any Default arising
under Sections 6.01(c) or 6.01(d) of the Credit Agreement by reason of such
noncompliance.

       (b) Each of the waivers granted pursuant to subsection (a) above shall
expire on the earliest of (i) close of business (New York City time) on the last
day of the Waiver Period, (ii) the first date on which the Company shall, or
shall permit any other Borrower to, breach any of its obligations set forth in
Section 18 and (iii) the first date on which an Event of Default shall occur and
be continuing under the Credit Agreement.

       (c) Except as provided in subsection (a) above, this Section 3 shall not

                                       3
<PAGE>   4

operate as a waiver of any right, remedy, power or privilege of the Banks under
any Financing Document or of any other term or condition of any Financing
Document.

       SECTION 4. Increase in Commitment Reduction Amount. Sections 2.10(d) and
2.10(e) of the Credit Agreement are amended to read in their entirety as
follows:

       (d) If a Reduction Event shall occur, the Commitments shall be
automatically and ratably reduced by an amount equal to 50% of the Net Cash
Proceeds with respect to such Reduction Event; provided that (i) if such portion
of such Net Cash Proceeds, when aggregated with the corresponding portions of
the Net Cash Proceeds of all other Reduction Events not theretofore so applied,
is less than $1,000,000, such Net Cash Proceeds shall be deposited in the
Restricted Account pending application pursuant to this subsection (d) in
connection with a subsequent Reduction Event, (ii) the aggregate amount of the
reductions of the Commitments effected pursuant to this subsection (d) as a
result of Reduction Events consummated on or before August 1, 2000 shall not
exceed $40,000,000, (iii) the aggregate amount of the reductions of the
Commitments effected pursuant to this subsection (d) as a result of Reduction
Events consummated after August 1, 2000 shall not exceed $20,000,000 and (iv) in
no event shall the Commitments be reduced below $85,000,000 pursuant to this
subsection (d). Each such reduction shall be effective as of the day of receipt
by the Company or any of its domestic wholly-owned Subsidiaries, as the case may
be, of the relevant Net Cash Proceeds.

       (e) To the extent not theretofore reduced to the same or a lesser amount,
the Commitments shall be automatically and ratably reduced on each date set
forth below to the aggregate amount set forth below opposite such date:

<TABLE>
<CAPTION>
                         ---------------------------------------
                         DATE                   AGGREGATE AMOUNT
                         ---------------------------------------
                         <S>                    <C>
                         August 1, 2000         Target Amount
                         ---------------------------------------
                         July 1, 2001           $85,000,000
                         ---------------------------------------
</TABLE>

"TARGET AMOUNT" means (i) if the Restatement Date has occurred pursuant to
Section 5.23 on or prior to May 31, 2000, $125,000,000 and (ii) otherwise,
$105,000,000.

       SECTION 5. Additional Fees. A new Section 2.17 to the Credit Agreement is
added immediately after Section 2.16 thereof, to read in its entirety as
follows:

                                       4
<PAGE>   5

       SECTION 2.17. Additional Fees. On the date that is the earlier of (i) May
31, 2000 and (ii) the Restatement Date, the Company shall pay to the
Administrative Agent for the account of each Bank which was entitled to a fee
pursuant to Section 21 of Amendment No.8 and Waiver to this Agreement, a fee in
an amount equal to 0.125% of such Bank's Commitment as in effect on such date.

       SECTION 6. Amendment of the Investments Covenant. Section 5.07 of the
Credit Agreement is amended by adding the following sentence at the end thereof:

       Notwithstanding any provision of this Agreement (including the foregoing
clauses of this Section 5.07), after the Amendment No.8 Effective Date, the
Company will not and will not permit any Subsidiary to, consummate any
acquisition of any other Person or all of the components of an entire line of
business or division of any other Person (whether by purchase of stock or
assets, by merger, consolidation or otherwise) without the prior written consent
of the Required Banks, other than the .......... Acquisition.

       SECTION 7. Change in the Minimum Net Worth Covenant. Section 5.08 of the
Credit Agreement is amended to read in its entirety as follows:

       SECTION 5.08. Minimum Consolidated Net Worth. Consolidated Net Worth at
the last day of any fiscal quarter set forth below will not be less than (i) the
amount set forth in the table below opposite such fiscal quarter plus (ii) 50%
of Consolidated Net Income for each fiscal quarter of the Company ended after
March 31, 2000, which such Consolidated Net Income is positive (but with no
deduction on account of any fiscal quarter for which Consolidated Net Income is
negative) plus (iii) 100% of the aggregate amount by which Consolidated Net
Worth shall have been increased by reason of the issuance and sale after March
31, 2000 and on or prior to such date of any capital stock or the conversion or
exchange of any Debt of the Company into or with capital stock of the Company
consummated after March 31, 2000 and on or prior to such date.

<TABLE>
<CAPTION>
       ------------------------------------------
       FISCAL QUARTER ENDED          AMOUNT
       ------------------------------------------
       <S>                           <C>
       6/30/00                       $300,000,000
       ------------------------------------------
       9/30/00                       $290,000,000
       ------------------------------------------
       12/31/00                      $270,000,000
       ------------------------------------------
       Thereafter                    $476,100,000
       ------------------------------------------
</TABLE>

                                       5
<PAGE>   6

       SECTION 8. Change in the Leverage Ratio, Addition of Senior Leverage
Ratio. Section 5.09 of the Credit Agreement is amended to read in its entirety
as follows:

       SECTION 5.09. (a) Leverage. The Consolidated Leverage Ratio will at no
date during any period set forth below exceed the ratio set forth below opposite
such period:

<TABLE>
<CAPTION>
       ---------------------------------------------------
       PERIOD                                        RATIO
       ---------------------------------------------------
       <S>                                           <C>
       Amendment No. 8 Effective Date- 12/30/00      4.50
       ---------------------------------------------------
       12/31/00 - 1/1/01                             4.80
       ---------------------------------------------------
       Thereafter                                    3.50
       ---------------------------------------------------
</TABLE>

       (b) Senior Leverage. The Senior Leverage Ratio will at no date during any
period set forth below exceed the ratio set forth below opposite such period:

<TABLE>
<CAPTION>
       ---------------------------------------------------
       PERIOD                                        RATIO
       ---------------------------------------------------
       <S>                                           <C>
       Amendment No. 8 Effective Date- 12/30/00      3.50
       ---------------------------------------------------
       Thereafter                                    3.60
       ---------------------------------------------------
</TABLE>

       SECTION 9. Change in the Consolidated Fixed Charge Ratio. Section 5.10 of
the Credit Agreement is amended to read in its entirety as follows:

       SECTION 5.10. Consolidated Fixed Charge Ratio. At the last day of any
fiscal quarter set forth below, the ratio of Earnings Available for Fixed
Charges to Consolidated Fixed Charges, in each case for the four consecutive
fiscal quarters then ended, will not be less than the ratio set forth below
opposite such fiscal quarter:

<TABLE>
<CAPTION>
       ----------------------------------------------------
       FISCAL QUARTER ENDED                          RATIO
       ----------------------------------------------------
       <S>                                           <C>
       6/30/00                                       0.75:1
       ----------------------------------------------------
       9/30/00                                       0.90:1
       ----------------------------------------------------
       12/31/00                                      0.90:1
       ----------------------------------------------------
       Thereafter                                    1.50:1
       ----------------------------------------------------
</TABLE>

                                       6
<PAGE>   7

       SECTION 10. Additional Exception For Negative Pledge. (a) Section 5.14 of
the Credit Agreement is amended by (i) deleting the "and" at the end of clause
(s) thereof, (ii) renumbering clause (t) thereof as clause (u), and (iii) adding
a new clause (t) immediately after clause (s) thereof, to read in its entirety
as follows:

              (t) Liens on assets of MDA and its subsidiaries securing Debt and
       other obligations of MDA and such subsidiaries under the MDA Financing;
       and

       SECTION 11. Change in the Subsidiary Debt Covenant, Addition of an MDA
Covenant. Section 5.17 of the Credit Agreement is hereby amended to read in its
entirety as follows:

       SECTION 5.17. (a) Subsidiary Debt. Total Debt of all of the Company's
Subsidiaries (excluding (i) Loans and Letter of Credit Liabilities hereunder and
any Guarantees thereof, (ii) Debt of a Subsidiary to the Company or to a
Wholly-Owned Subsidiary of the Company, (iii) Debt of MDA or any of its
Subsidiaries and (iv) any Guarantees of the NML Debt) will at no time exceed 5%
of Consolidated Net Worth.

       (b) MDA Equity. The Company will cause MDA to comply with the covenant
set forth in the Credit Agreement evidencing the MDA Financing requiring MDA to
maintain a minimum consolidated equity level; provided that failure by the
Company to comply with this Subsection (b) at any time shall not constitute an
Event of Default hereunder unless at such time such failure by MDA to comply
with such covenant in such Credit Agreement constitutes an event of default
thereunder which has not been waived by the lenders to the MDA Financing.

       SECTION 12. Change in Consolidated Capital Expenditures. Section 5.20 of
the Credit Agreement is hereby amended to read in its entirety as follows:

       SECTION 5.20. Consolidated Capital Expenditures. At any date the
aggregate amount of Consolidated Capital Expenditures (other than the *****
Acquisition) for the period from and including January 1, 2000 to and including
such date will not exceed $60,700,000.

       SECTION 13. New Bankers' Meeting Covenant. Section 5.21 of the Credit
Agreement is amended to read in its entirety as follows:

                                       7
<PAGE>   8

       SECTION 5.21. Bankers' Meeting. The Company shall:

       (a) At the request of the Administrative Agent, after delivery of the
financial information described in Section 5.22, host a tele-conference, at
which the Company shall (i) discuss with the Banks its detailed strategic
initiatives for fiscal year 2000 and (ii) provide the Banks in reasonable detail
information regarding such strategic initiatives, as well as any other
information regarding the Company and its Subsidiaries as any Bank may
reasonably request.

       (b) At the request of the Administrative Agent, after delivery of the
financial information described in Section 5.22, (i) host a bankers meeting, at
which meeting the senior management of the Company shall present in reasonable
detail the financial condition, results of operation, current status of business
and affairs of the Company and the business plan, budget and projections of the
Company for the period from and including January 1, 2000 to and including the
Termination Date, and (ii) provide the Banks in reasonable detail information
regarding the financial condition, business plans and projections of the Company
and its Subsidiaries, as well as any other information regarding the Company and
its Subsidiaries as any Bank may reasonably request.

       SECTION 14. Delivery of Additional Financial Information. A new Section
5.22 to the Credit Agreement is added immediately after Section 5.21 thereof, to
read in its entirety as follows:

       SECTION 5.22. Additional Financial Information. The Company will deliver
to each of the Banks, on or prior to May 15, 2000 (i) a consolidated balance
sheet of the Company and its Consolidated Subsidiaries as of December 31, 1999,
December 31, 1998 and December 31, 1997 and the related consolidated statements
of operations and cash flows for each such fiscal year, together with
consolidating balance sheets, statements of operations and operating cash flows
for each such fiscal year for each of the Company's Consolidated Subsidiaries,
setting forth in each case in comparative form the figures for the previous
fiscal year, all such consolidated statements reported on in a manner consistent
with the guidelines provided to the Company by the Securities and Exchange
Commission by the Company's independent auditors with respect to the relevant
fiscal year, (ii) a projected balance sheet of the Company and its Consolidated
Subsidiaries as of December 31, 2000 and the related projected statements of
operations and cash flows for such fiscal year and (iii) summary financial
information with respect to each division of the Company and its Subsidiaries;
provided that such summary information (x) will include in any event revenues,
gross profit, operating income, selling, general and administrative expenses and
capital expenditures with respect to each such division and (y) will

                                       8
<PAGE>   9

be in such detail as shall be necessary in order to permit a reconciliation of
such information with the information set forth in the projected statements of
operations and cash flows delivered by the Company pursuant to clause (ii).

       SECTION 15. Execution of a Restatement. (a) A new Section 5.23 to the
Credit Agreement is added immediately after Section 5.22 thereof, to read in its
entirety as follows:

       SECTION 5.23. Restatement Date. (a) The Company and the Banks will use
their respective best efforts to enter into a restatement of this Agreement on
or prior to May 31, 2000 pursuant to which amendment the obligations of the
Company hereunder shall be restructured in a manner satisfactory to the Company
and the Banks.

              (b) The Company and the Banks acknowledge and agree that it is
       currently contemplated that, pursuant to the restatement of the Credit
       Agreement contemplated by Section 5.23 of the Credit Agreement as amended
       hereby, (i) the compliance levels applicable to the covenants set forth
       in Sections 5.08, 5.09 and 5.10 of the Credit Agreement will be reset for
       such fiscal quarter as the Company shall have provided financial
       projections to levels mutually agreed upon by the Company and the
       Required Banks on the basis of such updated financial projections
       consistent with the methodology and intent used in deriving the covenant
       levels applicable during the Waiver Period (as defined in Amendment No.8
       and Waiver to this Agreement), (ii) the Company shall be permitted to
       consummate the acquisition identified to the Banks as the "Cooler
       Acquisition" substantially on the terms described by the Company to the
       Banks prior to the Amendment No. 8 Effective Date (but not other
       acquisitions), (iii) the Company shall be permitted to consummate an
       issuance of convertible Debt substantially on the terms described by the
       Company to the Banks prior to the Amendment No. 8 Effective Date and (iv)
       the mandatory reductions of the Commitments effected by Sections 2.10(d)
       and (e) of the Credit Agreement as amended hereby shall not be changed.

       SECTION 16. Additional Events of Default. (a) Section 6.01(b) of the
Credit Agreement is amended to read in its entirety as follows:

       (b) (i) any fee payable pursuant to Section 2.17 shall not be paid when
due or (ii) any interest on any Loan, any fees (other than any fee described in
clause (i)) or commissions or any other amount payable under any Financing
Document, shall not be paid within one (1) Domestic Business Day after the due
date thereof;

                                       9
<PAGE>   10

       (b) Section 6.01(c) of the Credit Agreement is amended to read in its
entirety as follows:

       (c) any Borrower shall fail to observe or perform any covenant or
agreement contained in Sections 5.01(g) and 5.07 to 5.22 inclusive;

       SECTION 17. Amendment to Amendment and Waivers Section. Clause (iii) of
section 10.05 of the Credit Agreement is amended to read in its entirety as
follows:

       (iii) postpone the date fixed for any payment of principal or interest on
       any Loan, or any fees hereunder or for termination or scheduled reduction
       of any Commitment,

       SECTION 18. Limitation on New Extensions of Credit. The Company agrees
that neither Company nor any other Borrower shall deliver a Notice of Borrowing
under the Credit Agreement or a request for issuance of a Letter of Credit under
the Credit Agreement or otherwise request any Bank (including the LC Bank) to
extend any credit to the Company or any other Borrower under the Credit
Agreement, and that, notwithstanding any provision of the Credit Agreement
(including Sections 2.01, 2.03 and 3.02), on and after the date hereof, no Bank
(including the LC Bank) shall be required to make any Loan, or issue or
participate in any Letter of Credit (it being understood that nothing in this
sentence shall be construed to prohibit the Company from delivering a Notice of
Interest Rate Election with respect to any Loan outstanding prior to the
Amendment No. 6 Effective Date and continuing or converting such Loan on the
terms set forth in such Notice of Interest Rate Election).

       SECTION 19. Amendment to Pricing Schedule. The Pricing Schedule to the
Credit Agreement is amended by substituting the date "January 1, 2001" for the
date "April 30, 2000" set forth in paragraph immediately after the table set
forth therein.

       SECTION 20. New York Law. This Amendment and Waiver shall be governed by
and construed in accordance with the laws of the State of New York.

       SECTION 21. Counterparts, Effectiveness. This Amendment and Waiver 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 and Waiver shall become effective on the date (the
"AMENDMENT NO. 8 EFFECTIVE DATE") on which the Administrative Agent shall have
received:

                                       10
<PAGE>   11

       (i) duly executed counterparts hereof signed by the Company and the
Required Banks (or, in the case of any party as to which an executed counterpart
shall not have been received, the Administrative Agent shall have received
telegraphic, telex or other written confirmation from such party of execution of
a counterpart hereof by such party); and

       (ii) for the ratable account of each Bank from which approval of this
Amendment and Waiver shall have been received at or prior to the later of (i)
the time the condition in clause (i) is satisfied and (ii) 5:00 P.M., New York
City time, on April 13, 2000, an amendment fee in an amount equal to 0.125% of
the amount of such Bank's Commitment as in effect on such date; provided that
the Administrative Agent shall have received duly executed counterparts hereof
signed by the Required Banks (or, in the case of any Bank as to which an
executed counterpart shall not have been received, the Administrative Agent
shall have received telegraphic, telex or other written confirmation from such
party of execution of a counterpart hereof by such Bank). The determination of
the Administrative Agent as to timing of the receipt of a Bank's approval shall
be conclusive, absent manifest error.

                                       11
<PAGE>   12

              IN WITNESS WHEREOF, the parties hereto have caused this Amendment
and Waiver 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

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

                                       12
<PAGE>   13

                                    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:

                                       13
<PAGE>   14

                                    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:

                                       14
<PAGE>   15

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:

                                       15<PAGE>   1
                                                                  EXHIBIT 10.2.9

Execution Copy
================================================================================

                          ORBITAL SCIENCES CORPORATION

                       -----------------------------------

                                 NINTH AMENDMENT
                          Dated as of January 31, 2000

                                       to

                                 NOTE AGREEMENT
                            Dated as of June 1, 1995

                       -----------------------------------

                        Re: $13,333,333 12% Senior Notes
                                Due June 14, 2001

================================================================================

<PAGE>   2

                        NINTH AMENDMENT TO NOTE AGREEMENT

       THIS NINTH AMENDMENT to Note Agreement dated as of January 31, 2000 (the
or this "Ninth Amendment"), is entered into between ORBITAL SCIENCES
CORPORATION, a Delaware corporation (the "Company") and THE NORTHWESTERN MUTUAL
LIFE INSURANCE COMPANY (the "Noteholder").

                                    RECITALS:

       A.     The Company and the Noteholder have heretofore entered into the
Note Agreement dated as of June 1, 1995, the First Amendment to Note Agreement
dated as of June 30, 1995, the Second Amendment to Note Agreement dated as of
March 15, 1996, the Third Amendment to Note Agreement dated as of July 31, 1996,
the Fourth Amendment to Note Agreement dated as of March 31, 1997, the Fifth
Amendment to Note Agreement dated as of December 23, 1997, the Sixth Amendment
to Note Agreement dated as of August 14, 1998, the Seventh Amendment to Note
Agreement dated as of May 27, 1999 and the Eighth Amendment to Note Agreement
dated as of December 20, 1999 (as so amended, supplemented or otherwise
modified, the "Note Agreement").

       B.     The Company and the Noteholder now desire to amend the Note
Agreement in the respects, but only in the respects, hereinafter set forth.

       C.     Capitalized terms used herein shall have the respective meanings
ascribed thereto in the Note Agreement unless herein defined or the context
shall otherwise require.

       D.     All requirements of law have been fully complied with and all
other acts and things necessary to make this Ninth Amendment a valid, legal and
binding instrument according to its terms for the purposes herein expressed have
been done or performed.

       NOW, THEREFORE, in consideration of good and valuable consideration the
receipt and sufficiency of which is hereby acknowledged, the Company and the
Noteholder do hereby agree as follows:

SECTION 1.  AMENDMENTS.

       1.1. The following shall be added as a new Section 1.3 of the Note
Agreement:

              "Section 1.3. Security for the Notes. (a) The payment by the
       Company of all amounts due with respect to the Notes is fully and
       unconditionally guaranteed by Engineering Technologies, Inc., a Virginia
       corporation ("Engineering Technologies"), Orbital Space Systems, Inc., a
       Virginia corporation ("Orbital Space"), Orbital Commercial Systems, Inc.,
       a Virginia corporation ("Orbital Commercial"), Orbital International,
       Inc., a Virginia corporation ("Orbital International"), Orbital Services
       Corporation, a Delaware corporation ("Orbital Services"), Orbital
       Navigation Corporation, a Delaware corporation ("Orbital Navigation"),
       Orblink LLC, a Delaware

<PAGE>   3

       limited liability company ("Orblink," and together with Engineering
       Technologies, Orbital Space, Orbital Commercial, Orbital International,
       Orbital Services and Orbital Navigation, collectively, the "Subsidiary
       Guarantors") pursuant to that certain Subsidiary Guaranty Agreement dated
       as of the date hereof (the "Subsidiary Guaranty Agreement") from the
       Subsidiary Guarantors to the Purchaser and each other holder of the Notes
       from time to time. Without limiting the foregoing, the enforcement of the
       rights and benefits in respect of the Subsidiary Guaranty Agreement and
       the allocation of the proceeds thereof, together with the application of
       the proceeds from the collection or payment of any Indebtedness of any
       Subsidiary of the Company due and owing under the Bank Credit Agreement
       will be subject to an Intercreditor Agreement dated as of the date hereof
       in form and substance satisfactory to the Noteholder (the "Intercreditor
       Agreement") entered into by Morgan Guaranty Trust Company of New York, as
       Collateral Agent, the Company and the holders of the Notes.

              (b)    In addition to the Subsidiary Guaranty Agreement, the Notes
       will be entitled to the benefit of (1) that certain Second Amended and
       Restated Security Agreement dated as of June 30, 1992, amended and
       restated as of August 5, 1997, and further amended and restated as of
       November 30, 1999, among the Company, each of its Subsidiaries party
       thereto, Morgan Guaranty Trust Company of New York, as Collateral Agent
       (the "Collateral Agent") and Bank of America, N.A., as Designated Lockbox
       Bank (the "Security Agreement"), (2) that certain Pledge Agreement dated
       as of November 30, 1999 among the Company, each of its Subsidiaries party
       thereto and Morgan Guaranty Trust Company of New York, as Collateral
       Agent (the "Pledge Agreement") and (3) that certain Deed of Trust,
       Assignment of Leases and Rents, Security Agreement and Financing
       Statement dated as of January 21, 2000, as further amended or
       supplemented from time to time, from the Company to the Trustee named
       therein, for the benefit of the Collateral Agent (the "Deed of Trust,"
       and together with the Security Agreement and the Pledge Agreement,
       collectively, the "Security Documents").

              1.2. Section 5.10 of the Note Agreement is hereby amended as
       follows:

                     (a) Clause (e) is amended by deleting the second proviso
              thereof in its entirety and substituting in lieu thereof the
              following proviso:

              "provided, further, notwithstanding the foregoing, nothing
              contained in this Agreement will prohibit any Lien or Liens
              consisting of (i) the Stockholders Agreement amongst the Company,
              MacDonald Dettwiler and Associates, BC Ltd. and CAI, which such
              agreement shall contain substantially the same terms and
              conditions as set forth in the CAI Letter Agreement or (ii) the
              pledge for the benefit of CAI and BC Ltd. of up to 51% of the
              outstanding common stock of MacDonald, Dettwiler and Associates."

                     (b) Clause (g) is amended by deleting the word "and" at the
              end thereof.

                                      -2-
<PAGE>   4

                     (c) Clause (h) is amended by deleting the period at the end
              thereof and substituting in lieu thereof the text ";and".

                     (d) A new Clause (i) is added following Clause (h) as
              follows:

                            "(i) liens granted for the benefit of the holders of
                     the Notes, including any liens shared by the holders of the
                     Notes and any lender under the Bank Credit Agreement."

       1.3. The following shall be added as a new Section 5.18 of the Note
Agreement:

                     "Section 5.18. Additional Subsidiary Guarantors. If the
       Company or any Wholly-owned domestic Subsidiary at any time creates or
       acquires any direct Wholly-owned Subsidiary (other than any holding
       company created for the purpose of holding shares of MacDonald, Dettwiler
       and Associates Ltd.) (each, a "New Subsidiary"), the Company shall,
       within 10 days after such creation or acquisition), (a) cause such New
       Subsidiary to execute and deliver a supplement to the Subsidiary Guaranty
       Agreement in the form of Exhibit A to the Subsidiary Guaranty Agreement,
       (b) cause such New Subsidiary to become a party to the Security
       Agreement, (c) grant a perfected first priority Lien to the Collateral
       Agent for the benefit of the holders of the Notes on all of the
       outstanding capital stock or other equity interest of such New Subsidiary
       and (d) take, and cause such New Subsidiary and each other Subsidiary to
       take, all action necessary or (in the opinion of the Collateral Agent or
       the holders of the Notes) desirable to perfect and protect the Liens
       intended to be created by this Agreement and the Security Documents
       (including any documents delivered in connection with such creation or
       acquisition pursuant to clauses (a), (b) and (c) of this Section);
       provided that (1) the Company will not be required to take the actions
       described in clauses (a) and (b) of this subsection with respect to any
       New Subsidiary that is not a domestic Subsidiary or which has not become
       liable under the Bank Credit Agreement as a guarantor or otherwise, (2)
       the Company will not be required to take the actions described in clause
       (c) with respect to the capital stock or other equity interests of any
       New Subsidiary that is not a domestic Subsidiary to the extent the
       aggregate capital stock or other equity interest of such New Subsidiary
       subject to a Lien granted to the Collateral Agent for the benefit of the
       holders of the Notes would exceed 66% of the outstanding capital stock or
       other equity interests of such New Subsidiary, and (3) the Company will
       not be required to take any of the actions described in clauses (a), (b),
       or (c) of this Section 5.18 with respect to any New Subsidiary to the
       extent any such action is prohibited by the terms of any agreement or
       instrument to which (x) such New Subsidiary is a party or is bound as in
       effect on the date such New Subsidiary becomes a Subsidiary of the
       Company, so long as such agreement or such instrument was not entered
       into in contemplation of such New Subsidiary becoming a Subsidiary of the
       Company or (y) the Company or any of its Wholly-owned domestic
       Subsidiaries (other than such New Subsidiary) is a party or is bound as
       in effect on the Ninth Amendment Effective Date."

                                      -3-
<PAGE>   5

              1.4. The period at the end of SECTION 6.1 (l) is hereby deleted
and "; or" is substituted therefor and the following shall be added as a new
subsection (m) to Section 6.1 of the Note Agreement:

                            "(m) the Subsidiary Guaranty Agreement shall cease
              to be in full force and effect for any reason whatsoever,
              including, without limitation, a determination by any governmental
              authority or court that such agreement is invalid, void or
              unenforceable or the Company or any of the Subsidiaries party to
              the Subsidiary Guaranty Agreement shall contest or deny in writing
              the validity or enforceability of the Subsidiary Guaranty
              Agreement."

              1.5. The second full sentence of Section 6.3 of the Note Agreement
is hereby amended in its entirety to read as follows:

              "When any Event of Default described in paragraphs (a) through
              (i), inclusive, or paragraph (m) of said SECTION 6.1 has happened
              and is continuing, the holder or holders of 66-2/3% or more of the
              principal amount of Notes at the time outstanding may, by notice
              in writing to the Company in the manner provided in SECTION 9.6,
              declare the entire principal and all interest accrued on all Notes
              to be and all Notes shall thereupon become, forthwith due and
              payable, without any presentment, demand, protest or other notice
              of any kind, all of which are expressly waived."

              1.6. Section 6.4 of the Note Agreement is hereby amended and
restated to read as follows:

                            "Section 6.4. Rescission of Acceleration. The
              provisions of SECTION 6.3 are subject to the condition that if the
              principal of and accrued interest on all or any outstanding Notes
              have been declared immediately due and payable by reason of the
              occurrence of any Event of Default described in paragraphs (a)
              through (i), inclusive, or paragraph (m) of SECTION 6.1, the
              holders of 66-2/3% in aggregate principal amount of the Notes then
              outstanding may, by written instrument filed with the Company,
              rescind and annul such declaration and the consequences thereof,
              provided that at the time such declaration is annulled and
              rescinded:

                            (a)    no judgment or decree has been entered for
              the payment of any monies due pursuant to the Notes or this
              Agreement;

                            (b)    all arrears of interest upon all the Notes
              and all other sums payable under the Notes and under this
              Agreement (except any principal, interest or premium on the Notes
              which has become due and payable solely by reason of such
              declaration under SECTION 6.3) shall have been duly paid; and

                            (c)    each and every other Default and Event of
              Default shall have been made good, cured or waived pursuant to
              SECTION 7.1;

                                      -4-
<PAGE>   6

and provided further, that no such rescission and annulment shall extend to or
affect any subsequent Default or Event of Default or impair any right consequent
thereto."

              1.7. The following shall be added as new definitions in
alphabetical order to Section 8.1 of the Note Agreement:

              "Collateral Agent" shall have the meaning set forth in SECTION
       1.3(b).

              "Ninth Amendment" shall mean the Ninth Amendment dated as of
       January 31, 2000 to this Agreement.

              "Ninth Amendment Effective Date" shall mean the date upon which
       the Ninth Amendment becomes effective.

              "Intercreditor Agreement" shall have the meaning specified in
       SECTION 1.3(a).

              "Pledge Agreement" shall have meaning specified in SECTION 1.3(b).

              "Security Agreement" shall have the meaning specified in SECTION
       1.3(b).

              "Security Documents" shall have the meaning specified in SECTION
       1.3(b).

              "Subsidiary Guarantors" shall have the meaning specified in
       SECTION 1.3(a).

              "Subsidiary Guaranty Agreement" shall have the meaning specified
       in SECTION 1.3(a).

              1.8.   The definition of "Bank Credit Agreement" set forth in
Section 8.1 of the Note Agreement shall be amended and restated to read as
follows:

                    "Bank Credit Agreement" shall mean the Third Amended and
       Restated Credit and Reimbursement Agreement dated as of December 21, 1998
       among the Company, the Banks listed therein, and Morgan Guaranty Trust
       Company of New York, as Administrative Agent and as Collateral Agent,
       including any extensions, renewals or replacements thereof.

SECTION 2.    REPRESENTATIONS AND WARRANTIES OF THE COMPANY.

              2.1.   To induce the Noteholder to execute and deliver this Ninth
Amendment (which representations shall survive the execution and delivery of
this Ninth Amendment), the Company represents and warrants to the Noteholder
that:

              (a)    this Ninth Amendment has been duly authorized, executed and
       delivered by it and this Ninth Amendment constitutes the legal, valid and
       binding obligation, contract and agreement of the Company enforceable
       against it in accordance with its terms, except as enforcement may be
       limited by bankruptcy, insolvency, reorganization,

                                      -5-
<PAGE>   7

       moratorium or similar laws or equitable principles relating to or
       limiting creditors' rights generally;

              (b)    the Note Agreement, as amended by this Ninth Amendment,
       constitutes the legal, valid and binding obligation, contract and
       agreement of the Company enforceable against it in accordance with its
       terms, except as enforcement may be limited by bankruptcy, insolvency,
       reorganization, moratorium or similar laws or equitable principles
       relating to or limiting creditors' rights generally;

              (c)    the execution, delivery and performance by the Company of
       this Ninth Amendment (i) has been duly authorized by all requisite
       corporate action and, if required, shareholder action, (ii) does not
       require the consent or approval of any governmental or regulatory body or
       agency, and (iii) will not (A) violate (1) any provision of law, statute,
       rule or regulation or its certificate of incorporation or bylaws, (2) any
       order of any court or any rule, regulation or order of any other agency
       or government binding upon it, or (3) any provision of any material
       indenture, agreement or other instrument to which it is a party or by
       which its properties or assets are or may be bound, including, without
       limitation, the Third Amended and Restated Credit and Reimbursement
       Agreement dated as of December 21, 1998 among the Company, the Banks
       listed therein, and Morgan Guaranty Trust Company of New York, as
       Administrative Agent and as Collateral Agent, or (B) result in a breach
       or constitute (alone or with due notice or lapse of time or both) a
       default under any indenture, agreement or other instrument referred to in
       clause (iii)(A)(3) of this SECTION 2.1(c); and

              (d)    as of the Effective Date and after giving effect to this
       Ninth Amendment, no Default or Event of Default has occurred which is
       continuing, excepting (i) only as expressly set forth in (A) that certain
       Waiver dated October 31, 1999 from the Noteholder to the Company, (B)
       that certain Waiver dated November 30, 1999 from the Noteholder to the
       Company and (C) that certain Waiver dated December 23, 1999 from the
       Noteholder to the Company and (ii) any Default or Event or Default, if
       any, relating to Section 5.21 of the Bank Credit Agreement.

SECTION 3.    PAYMENT OF NOTEHOLDER'S COUNSEL FEES AND EXPENSES.

       The Company agrees to pay the reasonable fees and expenses of Chapman and
Cutler, counsel to the Noteholder, in connection with the negotiation,
preparation, approval, execution and delivery of this Ninth Amendment.

SECTION 4.    MISCELLANEOUS.

          4.1.   This Ninth Amendment shall be construed in connection with
and as part of the Note Agreement, and except as modified and expressly amended
by this Ninth Amendment, and by the waivers granted by the Noteholder prior to
the date, hereof all terms, conditions and covenants contained in the Note
Agreement and the Notes are hereby ratified and shall be and remain in full
force and effect.

                                      -6-
<PAGE>   8

              4.2.   Any and all notices, requests, certificates and other
instruments executed and delivered after the execution and delivery of this
Ninth Amendment may refer to the Note Agreement without making specific
reference to this Ninth Amendment but nevertheless all such references shall
include this Ninth Amendment unless the context otherwise requires.

              4.3.   The descriptive headings of the various Sections or parts
of this Ninth Amendment are for convenience only and shall not affect the
meaning or construction of any of the provisions hereof.

              4.4.   This Ninth Amendment shall be governed by and construed in
accordance with Illinois law.

              4.5.   This Ninth Amendment shall become effective on January 31,
2000 (the "Effective Date").

                                      -7-
<PAGE>   9

              4.6.   The execution hereof by you shall constitute a contract
between us for the uses and purposes hereinabove set forth, and this Ninth
Amendment may be executed in any number of counterparts, each executed
counterpart constituting an original, but all together only one agreement.

                               ORBITAL SCIENCES CORPORATION

                               By
                                 Its
                                     --------------------------------

Accepted and Agreed to:

                               THE NORTHWESTERN MUTUAL LIFE INSURANCE COMPANY

                               By
                                  Its
                                     --------------------------------

                                      -8-

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