<PAGE>   1

                                                                   EXHIBIT 10.27

                                AMENDMENT NO. 6
                                       TO
                           THIRD AMENDED AND RESTATED
                       CREDIT AND REIMBURSEMENT AGREEMENT

          AMENDMENT No. 6 dated as of December 21, 1999 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. 6 Effective Date (as defined in the Credit
Agreement as amended hereby) to and including February 22, 2000 (the "WAIVER
PERIOD"), and the Banks are willing to do so, subject to the terms and
conditions set forth herein; and

          WHEREAS, the parties hereto wish to release certain collateral and
amend certain terms of the Credit Agreement as 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. 6
Effective Date (as defined in Section 17 below) refer to the Credit Agreement as
amended hereby.


<PAGE>   2



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

          "ASSET SALE" means any sale, lease or other disposition (including any
     such transaction effected by way of merger or consolidation) by the Company
     or any of its domestic wholly-owned Subsidiaries of any asset, but
     excluding (i) dispositions in the ordinary course of business, (ii)
     dispositions to the Company or a domestic wholly-owned Subsidiary and (iii)
     dispositions of Temporary Cash Investments and cash payments otherwise
     permitted under this Agreement.

          "AMENDMENT NO. 6 EFFECTIVE DATE" means the date of effectiveness of
     Amendment No. 6 to this Agreement.

          "CONSOLIDATED CAPITAL EXPENDITURES" means, for any period, the
     aggregate amount of expenditures by the Company and its Consolidated
     Subsidiaries for plant, property and equipment during such period
     (including any such expenditure by way of acquisition of a Person or by way
     of assumption of indebtedness or other obligations of a Person, to the
     extent reflected as plant, property and equipment).

          "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 and (iii) any such issuance
     pursuant to the warrants contemplated by Section 16 of Amendment No. 6.

          "EXCLUDED CHARGES" means the non-cash charges taken by the Company
     with respect to certain matters as further described under "Pending
     Matters" in Form 10-Q filed by the Company with the Securities and Exchange
     Commission for the fiscal quarter ended September 30, 1999, a copy of which
     has been delivered by the Company to each Bank prior to the Amendment No. 6
     Effective Date.

          "MDA TRANSACTION" means, collectively, the transactions contemplated
     by the MDA Transaction Agreement.

          "MDA TRANSACTION AGREEMENT" means the Subscription Agreement dated on
     or about December 21, 1999 among CAI CAPITAL PARTNERS AND COMPANY II, a
     limited partnership formed under the laws of the Province of Ontario, CAI
     PARTNERS AND COMPANY II, a


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     limited partnership formed under the laws of the Province of Ontario, CAI
     CAPITAL PARTNERS AND COMPANY II-C, a limited partnership formed under the
     laws of the Province of Ontario (collectively, the "CAI Entities"), a
     newly-formed British Columbia corporation ("NEWCO" and, together with the
     CAI Entities, the "PURCHASERS"), MDA and the Company, containing
     substantially the terms set forth in the summary thereof heretofore
     distributed to the Banks and otherwise in form and substance satisfactory
     to the Administrative Agent.

          "NET CASH PROCEEDS" means, with respect to any Reduction Event, an
     amount equal to the cash proceeds received by the Company or any of its
     domestic wholly-owned Subsidiaries from or in respect of such Reduction
     Event (including any cash proceeds received as interest or similar income
     or other proceeds of any noncash proceeds of any Asset Sale), less (i) any
     fees, costs and expenses reasonably incurred by such Person in respect of
     such Reduction Event and (ii) if such Reduction Event is an Asset Sale, any
     taxes actually paid or to be paid by such Person (as estimated by a senior
     financial or accounting officer of the Company, after giving effect to the
     overall tax position of the Company) in respect of such Asset Sale and the
     amount of any Debt secured by a Lien on an asset which is the subject of
     such Asset Sale and required to be discharged in connection therewith.

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

          "RESTRICTED ACCOUNT" means a bank account established pursuant to
     arrangements satisfactory to the Collateral Agent which is subject to a
     Lien in favor of the Agents and the Banks and from which disbursements of
     funds may be authorized only by the Collateral Agent.

     (b) The definition of "CONSOLIDATED EBITDA" set forth in Section 1.01 of
the Credit Agreement is amended to read in its entirety as follows:

          "CONSOLIDATED EBITDA" means, for any period, Consolidated Net Income
     for such period plus, to the extent deducted in determining such
     Consolidated Net Income, without duplication, the aggregate amount of (i)
     consolidated interest expense, (ii) income tax expense, (iii) depreciation,
     amortization and other similar non-cash charges, (iv) one-time accounting
     charges resulting in adjustments to earnings for each of the fiscal
     quarters of the fiscal year ended December 31, 1998, up to an aggregate
     amount equal to $35,600,000, (v) write-offs with respect to the investment
     made by the Company in CCI International N.V. for any fiscal quarter ended
     prior to December 31, 1999, up to an aggregate amount equal to

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<PAGE>   4



     $21,400,000 and (vi) solely for any period ended on or prior to February
     22, 2000, Excluded Charges, up to an aggregate amount equal to $5,000,000.

     (c) The definition of "CONSOLIDATED NET WORTH" set forth in Section 1.01 of
the Credit Agreement is amended to read in its entirety as follows:

          "CONSOLIDATED NET WORTH" means, at any date, the consolidated
     stockholders' equity of the Company and its Consolidated Subsidiaries as of
     such date; provided that Consolidated Net Worth at any date on or prior to
     February 22, 2000 shall be net of the lesser of (i) the effect of any
     Excluded Charges recorded on or prior to such date and (ii) $40,000,000 in
     the aggregate.

     (d) The definition of "EARNINGS AVAILABLE FOR FIXED CHARGES" set forth in
Section 1.01 of the Credit Agreement is amended to read in its entirety as
follows:

          "EARNINGS AVAILABLE FOR FIXED CHARGES" means, for any period,
     Consolidated Net Income for such period (excluding therefrom (i) any
     extraordinary items of gain or loss, (ii) any gain or loss of any other
     Person accounted for pursuant to the equity method, except in the case of
     gain to the extent of cash distributions received from such Person during
     the relevant period), plus the aggregate amounts deducted in determining
     Consolidated Net Income for such period in respect of (i) interest and
     rental expense, (ii) income taxes, (iii) write-offs with respect to the
     investment made by the Company in CCI International N.V. for any fiscal
     quarter ended prior to December 31, 1999, up to an aggregate amount equal
     to $21,400,000 and (iv) solely for any period ended on or prior to February
     22, 2000, Excluded Charges, up to an aggregate amount equal to $5,000,000.

     SECTION 3. Reductions in Commitments. The existing text of Section 2.10 is
redesignated subsection (a) thereof, and the following new subsections (b), (c)
and (d) are added to Section 2.10:

          (b) On the Amendment No. 6 Effective Date, the Commitments shall be
     automatically and ratably reduced to the aggregate amount of $187,000,000.

          (c) The Commitments shall be automatically and ratably reduced on the
     date of consummation of the MDA Transaction, by $22,000,000


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          (d) If a Reduction Event shall occur, the Commitments shall be
     automatically and ratably reduced by an amount equal to the lesser of (i)
     40% of the Net Cash Proceeds of such Reduction Event and (ii) the amount
     necessary to reduce the aggregate amount of the Commitments to
     $125,000,000; provided that 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. 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
     August 1, 2000 to the aggregate amount of $125,000,000.

          (f)) On the date of each reduction of the Commitments pursuant to this
     Section 2.10, the Borrowers shall prepay, together with accrued interest
     thereon, the aggregate principal amount of the Loans in excess of the
     aggregate amount of the Commitments as so reduced.

     SECTION 4. Waiver of Compliance with Certain Covenants. (a) The Banks waive
(i) compliance by the Company with the provisions of Section 5.08 of the Credit
Agreement and (ii) any Default arising under Section 6.01 (c) of the Credit
Agreement by reason of such noncompliance; provided that the waivers granted
pursuant to this Section shall be effective only so long as Consolidated Net
Worth (as defined in the Credit Agreement as amended hereby) at the last day of
any fiscal quarter ended during the Waiver Period will not be less than (i)
$375,000,000 plus (ii) 50% of Consolidated Net Income for each fiscal quarter of
the Company ended on or after September 30, 1999, for which 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 September 30, 1999 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 September 30, 1999 and on
or prior to such day.

     (b) The Banks waive (i) compliance by the Company with the provisions of
Section 5.09 of the Credit Agreement and (ii) any Default arising under Section
6.01(c) of the Credit Agreement by reason of such noncompliance; provided that


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the waivers granted pursuant to this Section 3(b) shall be effective only so
long as the Leverage Ratio will at no time during the Waiver Period exceed
5.00:1.

     (c) The Banks waive (i) compliance by the Company with the provisions of
Section 5.10 of the Credit Agreement and (ii) any Default arising under Section
6.01(c) of the Credit Agreement by reason of such noncompliance; provided that
the waivers granted pursuant to this Section 3(c) shall be effective only so
long as the ratio of Earnings Available for Fixed Charges to Consolidated Fixed
Charges, on the last day of any fiscal quarter in each case for the four
consecutive fiscal quarters then ended, will at no time during the Waiver Period
be less than 1.15:1.

     (d) Each of the waivers granted pursuant to this Section shall expire on
the earlier of (i) close of business (New York City time) on the last day of the
Waiver Period and (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
11.

     (e) Except as provided in subsections (a), (b) and (c) above, this Section
3 shall not 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 5. Consent to MDA Transaction. The Banks hereby consent to the MDA
Transaction and agree to release the Lien on the capital stock of MacDonald
Dettwiler Holdings Inc. ("MDH") pledged to them under the Pledge Agreement, upon
and subject to the following terms and conditions:

     (a) The MDA Transaction shall be consummated in accordance with the MDA
Transaction Agreement for gross cash proceeds not less than $75,000,000.

     (b) Immediately prior to the merger between MDA and MDH contemplated by the
MDA Transaction Agreement, the Collateral Agent shall release the Lien of the
Banks on the capital stock of MDH so as to permit consummation of such merger
(which shall be consummated on the same day as such release), and immediately
following consummation of such merger and of the other principal components of
the MDA Transaction (all of which shall in any event be consummated on the same
day or the following day), the Company shall deliver to the Collateral Agent in
pledge under the Pledge Agreement all capital stock of MDA owned by the Company
following consummation of the MDA Transaction.

     (c) Upon formation of the new holding company ("NEW MDH") contemplated by
the MDA Transaction Agreement, the Collateral Agent shall release the Lien of
the Pledge Agreement on the capital stock of MDA pledged pursuant to clause (b)
above, provided that (i) New MDH is a wholly owned


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Domestic Subsidiary of the Company and (ii) prior to or substantially
simultaneously with the release of such lien, the Company shall have delivered
all capital stock of New MDH to the Collateral Agent in pledge under the Pledge
Agreement.

     (d) The net cash proceeds to the Company of the MDA Transaction shall be
applied on the date of consummation of the MDA Transaction as follows: (i) an
amount equal to $22,000,000 shall be applied to prepayment of Loans outstanding
under the Credit Agreement (ratably amoung the Banks, to be applied to such
Group or Groups of Loans as the Administrative Agent may determine); (ii) an
amount equal to $10,000,000 shall be deposited in the Restricted Account and
(iii) the balance shall be released to the Company to be used for working
capital purposes. Amounts so deposited in the Restricted Account shall be
released therefrom from time to time upon request of the Company, so long as at
the time no Default shall have occurred and be continuing, as follows: (A) upon
consummation of the security arrangements contemplated by Section 5.19(d) of the
Credit Agreement, $5,000,000 shall be released; (B) at such time as the
aggregate amount of the Commitments shall have been reduced to $150,000,000 or
less, $10,000,000 shall be released; (C) amounts not theretofore released
pursuant to clause (A) or (B) shall be released from time to time, so long as
after giving effect to such release the Company's cash balances do not exceed
$10,000,000 and (D) any such request for release shall be accompanied by a
certificate signed by a duly authorized officer of the Company (on which the
Collateral Agent may conclusively rely) to the effect that the applicable
conditions set forth above have been satisfied in connection with such release.

     (e) The Company agrees to cause the foregoing conditions to be satisfied in
the event that the MDA Transaction is consummated, and agrees that any failure
to do so shall constitute an Event of Default.

     (f) The Banks hereby authorize the Collateral Agent to give the
acknowledgments contemplated by Sections 4.1(2) and (3) of the Unanimous
Shareholders' Agreement contemplated by the MDA Transaction Agreement.

     (g) The Banks hereby waive (i) the requirement under Section 5.19(b) of the
Credit Agreement that New MDH become a Guarantor and (ii) the limitations of
Section 5.14 of the Credit Agreement to the extent, and only to the extent,
necessary to permit New MDH to grant the Lien on the shares of MDA pursuant to
Section 15 of the Secondary Option Agreement contemplated by the MDA Transaction
Agreement, provided that the documentation establishing such Lien is
satisfactory to the Collateral Agent.


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     (h) The figure "60 days" appearing in Section 6.01(i) of the Credit
Agreement is changed to "30 days".

     SECTION 6. Additional Reporting Requirements. Subsections (i) and (j) of
Section 5.01 are amended to read in their entirety as follows:

          (i) (x) no later than 10 Business Days after the Amendment No. 5
     Effective Date, a statement of projected cash flows of the Company and its
     Consolidated Subsidiaries for each month in the six month period from and
     including October 1, 1999 to and including March 31, 2000 and (y) no later
     than the 22nd day after the end of each calendar month (starting with
     October 1999), a statement of projected cash flows of the Company and its
     Consolidated Subsidiaries for each month in the immediately succeeding six
     month period and for the period beginning on the first day after such
     calender month and ending December 31, 2000, taken as a whole, setting
     forth (a) in each case, at least the financial information set forth in the
     projected statement of cash flows of the Company and its Consolidated
     Subsidiaries for the third and fourth fiscal quarters of the fiscal year
     1999 and each fiscal quarter of the fiscal year 2000, a copy of which has
     been delivered to the Banks prior to the Amendment No. 5 Effective Date,
     (b) accounts receivables at the last day of the most recently ended
     calendar month, (c) available cash on hand and amounts available for
     borrowing under this Agreement at the last day of the most recently ended
     calendar month, and (d) in the case of any such statement delivered
     pursuant to clause (y), the actual cash flows of the Company and its
     Consolidated Subsidiaries for the most recently ended calendar month
     setting forth the operating cash flows as a single line item and the
     financing and investing activities as separate line items;

          (j) no later than (x) 10 Business Days after the Amendment No. 5
     Effective Date and (y) the 22nd day after the end of each calendar month
     (starting with October 1999), a summary report setting forth (1)(i) total
     number of units sold by ORBCOMM Global, (ii) total number of units
     installed by ORBCOMM Global and (iii) revenues of ORBCOMM Global derived
     from subscriber airtime usage (i.e. net of product sale revenues), in each
     case (a) for such calendar month (or, in the case of the summary report
     delivered within 10 Business Days after the Amendment No. 5 Effective Date,
     for September 1999 (the "FIRST SUMMARY REPORT")) and (b) for the period
     from creation of ORBCOMM Global to the last day of such calendar month (or
     September 1999, in the case of the First Summary Report), and (2) the
     numbers and revenues described in subclauses (i), (ii) and (iii) of clause
     (1) projected for the immediately succeeding calendar month (or, in the
     case of the First Summary Report, for October 1999) and


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     for the period beginning on the first day after such calender month and
     ending December 31, 2000, taken as a whole; and

     SECTION 7. Additional Permitted Investments. Section 5.07 of the Credit
Agreement is amended by:

          (i) substituting the dollar amount "$157,500,000" for the dollar
     amount "$154,000,000" set forth in clause (d) thereof; and

          (ii) substituting the dollar amount "$45,500,000" for the dollar
     amount "$35,000,000" set forth in subclause (y) under clause (j) thereof.

     SECTION 8. Exception to the Subsidiary Debt Covenant. Section 5.17 of the
Credit Agreement is amended to read in its entirety as follows:

                    SECTION 5.17. Subsidiary Debt. Total Debt of all of the
     Company's Subsidiaries (excluding (i) Loans and Letter of Credit
     Liabilities hereunder, (ii) Debt of a Subsidiary to the Company or to a
     Wholly-Owned Subsidiary of the Company and (iii) Debt consisting of
     performance bonds and letters of credit issued for the account of MDA in an
     aggregate amount not in excess of $30,000,000 to support certain
     contractual obligations, including obligations in an aggregate amount of
     approximately $10,000,000 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 $9,000,000 with respect
     to miscellaneous performance bonds) will at no time exceed 55% of
     Consolidated Tangible Net Worth."

     SECTION 9. Additional Covenants Regarding Collateral. New subsections (d)
and (e) are added immediately after subsection (c) of Section 5.19 of the Credit
Agreement, to read in their entirety as follows:

          (d) The Company shall use its commercially reasonable efforts to
     obtain all necessary consents for the pledge to the Collateral Agent for
     the benefit of the Banks of all of the capital stock and other equity
     interests of Magellan held directly or indirectly by the Company and,
     within 5 days of obtaining such consents, shall (i) enter into, or such
     cause its Subsidiaries to enter into, a pledge agreement in form and
     substance reasonably satisfactory to the Collateral Agent pursuant to which
     the Company or such Subsidiaries, as the case may be, shall grant a
     perfected first priority Lien to the Collateral Agent for the benefit of
     the Banks on all of such capital stock or other equity interests, (ii) take
     all action necessary or (in


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     the opinion of the Collateral Agent or the Required Lenders) desirable to
     perfect and protect the Liens intended to be created by the pledge
     agreement described in clause (i) and (iii) deliver to the Collateral Agent
     and the Lenders such opinions of counsel, in form and substance reasonably
     satisfactory to the Collateral Agent, as the Collateral Agent shall
     reasonably request.

          (e) On or prior to 30 days after the Amendment No. 6 Effective Date,
     the Company shall (i) execute and deliver to the Collateral Agent a
     mortgage in form and substance reasonably satisfactory to the Collateral
     Agent pursuant to which the Company shall grant to the Collateral Agent for
     the benefit of the Banks a perfected first priority Lien on (x)
     approximately 11.5671 acres of real property, comprising lots 1 and 2 as
     recorded in Deed Book 948, page 1041, among the land records of Loudoun
     County, Virginia, and (y) approximately 9.2922 acres of real property,
     comprising lots 3 and 4, as recorded in Deed Book 948, page 1041, among the
     land records of Loudoun County, Virginia, (ii) deliver to the Collateral
     Agent such surveys, title reports and title insurance as the Collateral
     Agent shall reasonably request, (iii) take all action necessary or (in the
     opinion of the Collateral Agent or the Required Lenders) desirable to
     perfect and protect the Liens intended to be created by the mortgage
     described in clause (i) and (iv) deliver to the Collateral Agent and the
     Lenders such opinions of counsel, in form and substance reasonably
     satisfactory to the Collateral Agent, as the Collateral Agent shall
     reasonably request.

     SECTION 10. Additional Covenant to Limit Consolidated Capital Expenditures.
A new Section 5.20 is added immediately after Section 5.19 of the Credit
Agreement, to read in its entirety as follows:

                    SECTION 5.20. Consolidated Capital Expenditure. At any date
     the aggregate amount of Consolidated Capital Expenditures for the period
     from and including December 1, 1999 to and including such date, will not
     exceed $9,000,000.

     SECTION 11. Bankers Meeting; Strategic Plan. A new section 5.21 to the
Credit Agreement is added immediately after Section 5.20 thereof, to read in its
entirety as follows:

                    SECTION 5.21 January 2000 Bankers Meeting. The Company
     shall:


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          (a) On or prior to January 27, 2000 (i) host a bankers meeting, at
     which meeting the Company shall discuss with the Banks its detailed
     strategic plan to address liquidity for the fiscal year 2000, and (ii)
     provide the Banks in reasonable detail information regarding the strategic
     plans of the Company and its Subsidiaries, as well as any other information
     regarding the Company and its Subsidiaries as any Bank may reasonably
     request.

          (b) At least three Domestic Business Days prior to the date of the
     bankers' meeting referred to in subsection (a) (but in any event no later
     than January 24, 2000), deliver to each of the Lenders a strategic plan
     setting forth in reasonable detail the strategic plans of the Company which
     plan shall include the information set forth in clause (i) of subsection
     (a) of this Section.

     SECTION 12. Additional Events of Default. 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 5.21 inclusive;

     SECTION 13. 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 date hereof and continuing or
converting such Loan on the terms set forth in such Notice of Interest Rate
Election).

     SECTION 14. Amendment to Pricing Schedule. The Pricing Schedule to the
Credit Agreement is amended by substituting the date "February 22, 2000" for the
date "December 30, 1999" set forth in paragraph immediately after the table set
forth therein.

     SECTION 15. New York Law. This Amendment shall be governed by and construed
in accordance with the laws of the State of New York.


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     SECTION 16. Warrants. The Company will, not later than January 15, 2000,
deliver to the Administrative Agent for the account of the Approving Banks (as
defined below) ratably in proportion to their Commitments, warrants in form
satisfactory to the Administrative Agent entitling the holder to purchase up to
100,000 shares of Common Stock, par value $.01 per share, of the Company for a
period expiring on the fifth anniversary of the date of issuance thereof at an
exercise price of $0.01 per share, subject to customary antidilution protection
and registration rights.

     SECTION 17. 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 on the date (the "AMENDMENT NO. 6
EFFECTIVE DATE") on which the Administrative Agent shall have received:

          (a) 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

          (b) an amendment fee in the amount of $500,000, for the ratable
     account of those Banks (the "APPROVING BANKS") from which approval of this
     Amendment shall have been received at or prior to the later of (i) the time
     the condition in clause (a) is satisfied and (ii) 12:00 noon, New York City
     time, on December 22, 1999. The determination of the Administrative Agent
     as to timing of the receipt of a Bank's approval shall be conclusive.


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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:


                                       13
<PAGE>   14



                         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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<PAGE>   15



                         CHEVY CHASE BANK

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


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

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



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