Document:

<PAGE>   1
                             EXECUTIVE EMPLOYMENT
                         CHANGE OF CONTROL AGREEMENT

August 9, 2000

Mr. Garrett E. Pierce
Orbital Sciences Corporation
21700 Atlantic Blvd.
Dulles, Virginia  20166

Dear Garrett:

         Orbital Sciences Corporation and its subsidiaries (together, the
"Company") consider the maintenance of a sound and vital management to be
essential to protecting and enhancing the best interests of the Company and
its stockholders. In this connection, the Company recognizes that the
possibility of a change in control may exist and that such possibility, and
the uncertainty and questions which it may raise among management, may result
in the departure or distraction of management personnel to the detriment of
the Company and its stockholders. Accordingly, the Company's Board of
Directors (the "Board") has determined that appropriate steps should be taken
to reinforce and encourage the continued attention and dedication of members
of the Company's management, including yourself, to their assigned duties
without distraction in the face of the potentially disturbing circumstances
arising from the possibility of a change in control of the Company.

         This letter agreement (the "Agreement") sets forth the severance
benefits that the Company agrees will be provided to you in the event your
employment with the Company terminates following a "Change in Control" (as
defined in Section 2 hereof) under the circumstances described below. This
Agreement is not an employment contract nor does it alter your status as an
at-will employee of the Company. No benefit shall be payable under this
Agreement except on termination of your employment with the Company as a
result of a Change in Control (as defined below).

         1. Term. This Agreement commences as of August 9, 2000, and shall
remain in effect so long as you are employed as an executive officer of the
Company, provided, however, that in the event of a Change in Control, this
Agreement shall remain in full force and effect for a 24-month period
commencing on the date of the Change in Control regardless of whether you
remain an executive officer of the Company during such 24-month period.

<PAGE>   2

Mr. Garrett E. Pierce
Orbital Sciences Corporation
August 9, 2000
Page 2

2.       Change in Control.  For purposes of this Agreement, a Change in
         Control shall mean:

         (a)      the acquisition by any individual, entity or group (within
                  the meaning of Section 13(d) or 14(d) of the Securities
                  Exchange Act of 1934 (the "Exchange Act")) (a "Person") of
                  beneficial ownership (within the meaning of Rule 13d-3 of
                  the Exchange Act) of 30% or more of either (i) the then
                  outstanding shares of common stock of the Company or (ii)
                  the combined voting power of the then outstanding voting
                  securities of the Company entitled to vote generally in the
                  election of directors;

         (b)      within any 24-month period, the persons who were directors
                  of the Company immediately prior thereto (the "Incumbent
                  Board") shall cease to constitute a majority of the Board of
                  Directors of the Company or of its successor by merger,
                  consolidation or sale of assets.  For this purpose, the
                  Incumbent Board includes any new director whose (i) election
                  to the Board resulted from a vacancy caused by the
                  retirement, death or disability of a director and was
                  approved by a vote of at least two-thirds of the directors
                  then still in office who were directors at the beginning of
                  the period, or (ii) nomination to the Board was approved by
                  a committee of the Board whose majority consisted of
                  directors who were directors in office at the beginning of
                  the period; or

         (c)      the consummation by the Company of a reorganization, merger,
                  consolidation or sale or disposition of all or substantially
                  all the assets of the Company (other than any such
                  transaction initiated by the action of the Board) (a
                  "Business Combination"), the result of which is that (i) the
                  stockholders of the Company at the time of the execution of
                  the agreement to effect the Business Combination own less
                  than 60% of the total equity of the surviving or resulting
                  entity entitled to vote generally in the election of
                  directors, (ii) a Person (excluding any corporation
                  resulting from the Business Combination) becomes the
                  beneficial owner of 20% or more of the then outstanding
                  shares of common stock of the corporation resulting from
                  such Business Combination or (iii) at least a majority of
                  the members of the board of directors of the corporation
                  resulting from such Business Combination were members of the
                  Board of Directors of the Company at the time of execution
                  of the initial agreement or other action of the Board that
                  provided for such Business Combination.

         Notwithstanding the above, a Change in Control shall not be deemed to
occur as a result of a transaction where either you, individually or as an
officer, director or 5% stockholder or partner of any entity, or any employee
benefit plan (or related trust) of the Company (a) becomes the beneficial
owner of securities representing 30% or more of the combined voting power of
the

<PAGE>   3

Mr. Garrett E. Pierce
Orbital Sciences Corporation
August 9, 2000
Page 3

Company 's then outstanding securities, or (b) enters into an agreement with
the Company providing for the merger, consolidation, or sale or transfer of
all or substantially all the assets of the Company. In addition, a Change in
Control shall not be deemed to occur where you enter into an employment
agreement with the Company, any Person whose acquisition of the Company's
securities resulted in the Change in Control or any entity resulting from a
Business Combination.

         3.       Termination Following Change in Control. If a Change in
Control as described in Section 2 occurs, you shall be entitled to the
benefits provided in Section 4 of this Agreement if your employment is
terminated by the Company for Disability or Cause, as described below, or by
you for Good Reason, as described below.

                  (i)      Disability. If, as a result of your incapacity due
to physical or mental illness, you shall have been absent from your duties
with the Company on a full-time basis for nine consecutive months, and within
30 days after written notice of termination is given you shall not have
returned to the full-time performance of your duties, the Company may
terminate your employment for "Disability."

                  (ii)     Cause. Termination by the Company of your
employment for "Cause" shall mean termination on (A) the willful and continued
failure by you substantially to perform your duties with the Company in
accordance with the instructions of the Board or the executive officers to
whom you report (other than any such failure resulting from your incapacity
due to physical or mental illness), after a demand for substantial performance
is delivered to you by the Board which specifically identifies the manner in
which the Board believes that you have not substantially performed your
duties, or (B) the willful engaging by you in conduct which is demonstrably
and materially injurious to the Company, monetarily or otherwise. For purposes
of this Subsection, no act, or failure to act, on your part shall be
considered "willful" unless done, or omitted to be done, by you not in good
faith and without reasonable belief that your action or omission was in the
best interest of the Company. Notwithstanding the foregoing, you shall not be
deemed to have been terminated for Cause unless and until there shall have
been delivered to you a copy of a resolution duly adopted by the affirmative
vote of not less than two-thirds of the entire membership of the Board at a
meeting of the Board called and held for the purpose (after reasonable notice
to you and an opportunity for you, together with your counsel, to be heard
before the Board), finding that in the good faith opinion of the Board you
were guilty of conduct set forth above in clause (A) or (B) of the first
sentence of this Subsection and specifying the particulars thereof in detail.

                  (iii)    Good Reason.  You shall be entitled to terminate
your employment for Good Reason in connection with a Change in Control.  For
purposes of this Agreement, "Good Reason" shall mean:

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Mr. Garrett E. Pierce
Orbital Sciences Corporation
August 9, 2000
Page 4

                  (A)      without your written consent, the assignment to you
         of any position (including status, offices, titles and reporting
         requirements), authorities, duties and responsibilities, that are not
         at least commensurate in all material respects with the most
         significant of those held, exercised and assigned by you at any time
         during the 180-day period immediately preceding a Change in Control,
         or any other action by the Company that results in a diminution in
         such position, authority, duties or responsibilities, excluding for
         this purpose an isolated, insubstantial and inadvertent action not
         taken in bad faith and that is remedied by the Company promptly after
         receipt of notice thereof given by you;

                  (B)      a reduction by the Company in your annual base
         salary ("Annual Base Salary"), which for the purposes of this
         Agreement shall mean an amount at least equal to 12 times the highest
         monthly base salary paid or payable, including any base salary that
         has been earned but deferred, to you by the Company in respect of the
         12-month period immediately preceding the month in which the Change
         of Control occurs;

                  (C)      the Company's requiring you to be based anywhere
         other than the office of the Company in which you are based prior to
         the Change in Control or any office or location within a 50 mile
         radius of such office, except for required travel on the Company's
         business to an extent substantially consistent with your present
         business travel obligations;

                  (D)      the failure by the Company to continue in effect
         any compensation plan in which you participate, or to provide you
         with plans substantially similar, including but not limited to any
         stock purchase plan, stock option plan, incentive compensation,
         bonus, and other plan in which you were participating at the time of
         the Change in Control, or the failure by the Company to continue your
         participation therein;

                  (E)      the failure by the Company to continue to provide
         you with benefits substantially similar to those enjoyed by you under
         any of the Company's retirement, pension, 401(k), deferred
         compensation, life insurance, medical, health, accident, disability
         or other benefit plans in which you were participating at the time of
         a Change in Control, the taking of any action by the Company which
         would directly or indirectly materially reduce any of such benefits
         enjoyed by you at the time of the Change in Control, or the failure
         by the Company to provide you with the number of paid vacation days
         to which you are entitled in accordance with the Company's normal
         vacation policy in effect at the time of the Change in Control;

                  (F)      the failure of the Company to obtain a satisfactory
         agreement from any successor to assume and agree to perform this
         Agreement, as contemplated in Section 5 hereof; or

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Mr. Garrett E. Pierce
Orbital Sciences Corporation
August 9, 2000
Page 5

                  (G)      any termination of your employment which is not
         effected pursuant to a Notice of Termination satisfying the
         requirements of Section 3(iv) hereof (and, if applicable, Section
         3(ii) hereof); and for purposes of this Agreement, no such purported
         termination shall be effective.

                   (iv)    Notice of Termination. Any termination by the
Company or by you shall be communicated by written Notice of Termination to
the other party hereto in accordance with Section 6 hereof, and if by the
Company for Cause, shall not be effective unless such notice includes the
information set forth in Section 3(ii) hereof.

                  (v)      Date of Termination, etc. "Date of Termination"
shall mean (A) if your employment is terminated by reason of death or
Disability, the date of your death or 30 days after Notice of Termination is
given (provided that you shall not have returned to the performance of your
duties on a full-time basis during such 30 day period), as the case may be,
(B) if your employment is terminated by the Company for Cause or for any other
reason, the date specified in the Notice of Termination which shall not be
less than 30 days from the date such Notice of Termination is given, and (C)
if you terminate your employment for "Good Reason," the date such Notice of
Termination is given or any later date specified therein.

         4.       Benefits Upon Termination or During Disability.

                  (i)      During any period that you fail to perform your
duties hereunder as a result of incapacity due to physical or mental illness,
and in the event your employment is terminated pursuant to Section 3(i)
hereof, your benefits shall be determined in accordance with the Company's
insurance and benefit programs then in effect.

                  (ii)     If your employment shall be terminated for Cause,
the Company shall pay you your full base salary through the Date of
Termination at the rate in effect at the time Notice of Termination is given,
and the Company shall have no further obligations to you under this Agreement.

                  (iii)    If your employment shall be terminated immediately
prior to or any time after a Change in Control (a) by the Company for any
reason other than for Cause or Disability or (b) by you for Good Reason, then
you shall be entitled to all the benefits provided below:

                  (A)      The Company shall pay you on the Date of
         Termination your full base salary through the Date of Termination at
         the rate in effect at the time Notice of Termination is given.

                  (B)      In lieu of any further salary payments to you for
         periods subsequent to the Date of Termination, the Company shall pay
         to you, not later than 15 days following the Date of Termination, a
         lump sum payment equal to two times the sum of (a) your Annual

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Mr. Garrett E. Pierce
Orbital Sciences Corporation
August 9, 2000
Page 6

         Base Salary and (b) the sum of any incentive, annual and other cash
         bonuses, paid to you for the 12-month period immediately preceding
         the month in which the Change in Control occurred.

                  (C)      The Company shall also immediately fully vest you
         in all your account balances under the Company's retirement, deferred
         compensation and pension plans (the "Plans"); provided, however, that
         should the Company be unable to provide for such vesting under the
         terms of any such Plans, the Company shall pay to you in the manner
         and as directed by you, an amount that equals on an after-tax basis
         the value of any amounts that were not vested or would otherwise be
         forfeited by you under the Plans upon your termination of employment
         with the Company.

                  (D)      The Company shall also allow you the opportunity to
         surrender to the Company any then outstanding vested and unvested
         options (whether exercisable or not) to purchase Common Stock of the
         Company and any of its subsidiaries and affiliates that you own and
         that you did not previously surrender or convert in the transaction
         that resulted in the Change in Control, and the Company shall
         promptly pay to you in consideration therefor a cash payment equal to
         the difference between the respective exercise price for such options
         and the higher of the (a) highest price paid in connection with the
         transaction that resulted in the Change in Control or (b) then
         current fair-market value.

                  (E)      The Company shall also pay to you all reasonable
         legal fees and expenses incurred by you as a result of such
         termination (including all such fees and expenses, if any, incurred
         in contesting or disputing any such termination or in seeking to
         obtain or enforce any right or benefit provided by this Agreement)
         upon presentation to the Company of a reasonably detailed invoice for
         such expenses, whether or not you have already made payment for such
         expenses.

                  (F)      For a 24-month period after such termination, the
         Company shall arrange to provide you with life, disability, accident
         and health insurance benefits substantially similar to those you were
         receiving immediately prior to the Notice of Termination, provided,
         however, that should the Company be unable to provide for any such
         benefits under the terms of the benefit plans, or by law, the Company
         shall pay you an amount equal to the premiums the Company would have
         paid for such benefits under such plans.

                  (G)      You shall not be required to mitigate the amount of
         any payment provided for in this Section 4 by seeking other
         employment or otherwise, nor shall the amount of any payment or
         benefit provided for in this Section 4 be reduced by any compensation
         earned by you as the result of employment by another employer or by
         retirement benefits after the Date of Termination, or otherwise.

<PAGE>   7

Mr. Garrett E. Pierce
Orbital Sciences Corporation
August 9, 2000
Page 7

                  (H)      In addition to all other amounts payable to you
         under this Section 4, you shall be entitled to receive all benefits
         payable to you under any of the Company's plans or agreements
         relating to retirement benefits.

                  (iv)     All payments required to be made by the Company
hereunder to you shall be subject to the withholding of such amounts relating
to Federal, state, local or foreign taxes as the Company reasonably may
determine it should withhold pursuant to any applicable law or regulation.
Notwithstanding any other provision of this Agreement or of any other
agreement, contract or understanding heretofore or hereafter entered into by
you and the Company, you shall not have any right to receive any payment or
other benefit under this Agreement if such payment or benefit, taking into
account all other payments to or benefits received by you, would cause any
payment to you under this Agreement to be considered a "parachute payment"
within the meaning of Section 280G(b)(2) of the Internal Revenue Code (a
"Parachute Payment"). In the event that the receipt of any such payment or
benefit under this Agreement would cause you to be considered to have received
a Parachute Payment under this Agreement, then you shall have the right, in
your sole discretion, to designate those payments or benefits under this
Agreement which should be reduced or eliminated so as to avoid having the
payment to you under this Agreement be deemed to be a Parachute Payment.

         5.       Successors; Binding Agreement.

                  (i)      The Company will require any successor (whether
direct or indirect, by purchase, merger, consolidation or otherwise) to all or
substantially all its business and/or assets to expressly assume and agree to
perform this Agreement in the same manner and to the same extent that the
Company would be required to perform it if no such succession had taken place.
Failure of the Company to obtain such assumption and agreement no later than
ten days prior to the effectiveness of any such succession shall be a breach
of this Agreement and shall entitle you to compensation from the Company in
the same amount and on the same terms as you would be entitled under section
4(iii), except that for purposes of implementing the foregoing, a date ten
days prior to the date on which any such succession becomes effective shall be
deemed the Date of Termination. As used in this Agreement, "the Company" shall
mean the Company, as hereinbefore defined and any successor to its business
and/or assets that assumes and agrees to perform this Agreement by executing
and delivering the agreement provided for in this paragraph 5, by operation of
law, or otherwise.

                  (ii)     This Agreement shall inure to the benefit of and be
enforceable by your personal or legal representatives, executors,
administrators, successors, heirs, distributees, devisees, and legatees. If
you should die while any amount would still be payable to you hereunder, all
such amounts, unless otherwise provided herein, shall be paid in accordance
with the terms of this Agreement to your devisee, legatee or other designee or
if there is no such designee, to your estate.

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Mr. Garrett E. Pierce
Orbital Sciences Corporation
August 9, 2000
Page 8

         6.       Notice. For the purposes of this Agreement, notices and all
other communications provided for in this Agreement shall be in writing and
shall be deemed to have been duly given when delivered or mailed by registered
mail, return receipt requested, postage prepaid, addressed (i) if to the
Company, to Orbital Sciences Corporation, 21700 Atlantic Boulevard, Dulles,
Virginia 20166, Attn: Secretary of the Company, and (ii) if to you, to the
address set forth on the first page of this Agreement, or to such other
address as either party may have furnished to the other in writing in
accordance herewith, except that notice of change of address shall be
effective only upon receipt.

         7.       Miscellaneous. No provision of this Agreement may be
modified, waived or discharged unless such waiver, modification or discharge
is agreed to in writing and signed by you and such officer as may be
specifically designated by the Board. No waiver by either party hereto at any
time of any breach by the other party hereto of, or compliance with, any
condition or provision of this Agreement to be performed by such other party
shall be deemed a waiver of similar or dissimilar provisions or conditions at
the same or at any prior or subsequent time. No agreements or representations,
oral or otherwise, express or implied, with respect to the subject matter
hereof have been made by either party which are not expressly set forth in
this Agreement, and this Agreement supersedes all prior agreements between the
Company and you with respect to the subject matter herein. The validity,
interpretation construction and performance of this Agreement shall be
governed by the local laws of the Commonwealth of Virginia (regardless of the
laws that might otherwise govern under principles of conflicts of law).

         8.       Validity.  The invalidity or unenforceability of any
provision of this Agreement shall not affect the validity or enforceability of
any other provision of this Agreement, which shall remain in full force and
effect.

         9.       Counterparts.  This Agreement may be executed in several
counterparts, each of which shall be deemed to be an original but all of which
together will constitute one and the same instrument.

         10.      Arbitration. Any dispute or controversy arising under or in
connection with this Agreement shall be settled exclusively by arbitration in
Washington, D.C. in accordance with the domestic rules of the American
Arbitration Association then in effect. Pending the resolution of such dispute
or controversy, the Company will continue to pay you your full base salary in
effect when the notice giving rise to the dispute was given and you will
continue as a participant in all incentive compensation, stock option,
retirement, deferred compensation, pension, life, disability, health and
accident plans in which you were participating when the notice giving rise to
dispute was given, unless you have already received all benefits payable under
Section 4(iii) of this Agreement. Judgment may be entered on the arbitrator's
award in any court having jurisdiction; provided, however, that you shall be
entitled to seek specific performance of your right to be paid until the Date
of Termination during the pendency of any dispute or controversy arising under
or in connection with this Agreement.

<PAGE>   9

Mr. Garrett E. Pierce
Orbital Sciences Corporation
August 9, 2000
Page 9

         If this Agreement correctly sets forth our agreement on the subject
matter hereof, kindly sign both of the enclosed copies, keeping one for your
files and returning the other to the Company.

Sincerely,

ORBITAL SCIENCES CORPORATION

/s/ David W. Thompson
-------------------------------------------
By:  David W. Thompson
     Chairman and Chief Executive Officer

Agreed to:

/s/ Garrett E. Pierce
-------------------------------------------
Name:  Garrett E. Pierce
Date:  August 9, 2000<PAGE>   1
                         PERFORMANCE SHARE AGREEMENT

         This Performance Share Agreement (the "Agreement") is made as of the
9th day of August 2000 by and between Orbital Sciences Corporation, a Delaware
corporation (the "Company"), and Garrett E. Pierce (the "Executive").

         WHEREAS, the Human Resources and Nominating Committee of the Board of
Directors of the Company (the "Committee") has determined that it is desirable
and in the best interests of the Company to grant to the Executive the right
to receive performance share units (the "Performance Shares"), in order to
provide the Executive with further incentive to enhance the profitability and
financial strength of the Company by linking a component of the Executive's
compensation to Company stock value, which Performance Shares entitle the
Executive to receive an annual bonus measured by the increased value of the
Company's common stock, par value $.01 per share (the "Common Stock").

         NOW THEREFORE, in consideration of the mutual promises and covenants
contained herein, the parties hereby agree as follows:

         1.       GRANT OF PERFORMANCE SHARES.  The Company hereby grants to
the Executive a total of 50,000 Performance Shares, which shall have the
features set forth below.  The date of the grant of the Performance Shares is
August 9, 2000.

                  a.       Vesting.  The 50,000 Performance Shares shall vest
                           immediately.

                  b.       Performance Bonus Calculation. Until expiration or
                           termination, each vested Performance Share shall
                           entitle the Executive to receive a bonus (the
                           "Performance Bonus"). The Performance Bonus shall
                           be calculated on each of August 9, 2001 and 2002
                           with respect to the aggregate number of Performance
                           Shares that have vested as of August 1 of such
                           year. The Performance Bonus shall be equal to the
                           increase, if any, from the Base Price to the
                           Anniversary Valuation Price, as illustrated below
                           for each applicable valuation period.

<TABLE>
<CAPTION>
         NUMBER OF                                                                      ANNIVERSARY
         ---------                                                                      -----------
         PERFORMANCE SHARES         BASE DATE                 BASE PRICE                VALUATION PRICE
         ------------------         ---------                 ----------                ---------------
<S>                                 <C>                       <C>                       <C>
         50,000                     August 9, 2000            Fair Market Value         Fair Market Value
                                                              on August 9, 2000         on August 9, 2001

         50,000                     August 9, 2001            Fair Market Value         Fair Market Value
                                                              on August 9, 2001         on August 9, 2002
</TABLE>

<PAGE>   2

                 c.        Payment of Performance Bonus. The Performance Bonus,
                           if any, shall be paid in cash in the form of a credit
                           to the Executive's account under the Company's 1995
                           Deferred Compensation Plan. Such payment shall be
                           made as soon as practicable after calculation of the
                           Performance Bonus. Fifty percent (50%) of such credit
                           shall vest immediately, with the other Fifty percent
                           (50%) vesting on August 9 of the subsequent year.

                  d.       Expiration.  The Performance Shares shall expire on
                           August 10, 2002 unless this Agreement is terminated
                           according to its terms at an earlier time.

                  e.       Fair Market Value. For purposes of this Agreement,
                           the Fair Market Value shall be equal to the average
                           closing sales price of the Common Stock on the
                           national securities exchange on which the Common
                           stock is then principally traded, calculated for
                           the 20 trading days immediately preceding August 9
                           of the applicable valuation year.

         2.       LIMITATION ON TRANSFER. The Performance Shares are not
transferable by the Executive.

         3.       PERFORMANCE SHARE ADJUSTMENTS.

                  a.       Changes in Stock.  If the outstanding shares of
                           Common Stock of the Company are increased,
                           decreased, changed into or exchanged for a
                           different number or kind of shares of the Company
                           through reorganization, recapitalization,
                           reclassification, stock dividend, stock split or
                           reverse stock split, upon authorization of the
                           Board, a proportionate adjustment shall be made in
                           the number or kind of Common Stock subject to the
                           Performance Shares, so that the proportionate
                           interest of the Executive immediately following
                           such event shall, to the extent practicable, be the
                           same as immediately prior to such event.  Any such
                           adjustment to Performance Shares shall include a
                           corresponding proportionate adjustment in the Base
                           Price per share of Common Stock.

                  b.       Reorganization in Which the Company is the
                           Surviving Corporation.  Subject to subparagraph (c)
                           below, if the Company shall be the surviving
                           corporation in any reorganization, merger or
                           consolidation of the Company with one or more other
                           corporations, the Performance Shares shall pertain
                           to and apply to the securities to which a holder of
                           the number of shares of Common Stock subject to the
                           Performance Shares would have been entitled
                           immediately following such reorganization, merger
                           or consolidation, with a corresponding
                           proportionate adjustment in the Base Price per
                           share of Common Stock so that the aggregate Base
                           Price thereafter shall be the same as the aggregate
                           Base Price of the Common Stock remaining subject to
                           the Performance Shares immediately prior to such
                           reorganization, merger or consolidation.

<PAGE>   3

                  c.       Reorganization in Which the Company is Not the
                           Surviving Corporation or Sale of Assets or Stock.
                           Upon the dissolution or liquidation of the Company,
                           or upon a merger, consolidation or reorganization
                           of the Company with one or more other corporations
                           in which the Company is not the surviving
                           corporation, or upon a sale of substantially all
                           the assets of the Company to another corporation,
                           or upon any transaction (including, without
                           limitation, a merger or reorganization in which the
                           Company is the surviving corporation) approved by
                           the Board which results in any person or entity
                           owning Eighty percent (80%) or more of the combined
                           voting power of all classes of the stock of the
                           Company, unless provision is made in writing in
                           connection with such transaction for the assumption
                           of this Agreement with such adjustments as the
                           Board deems appropriate with respect to the
                           features, terms and conditions of the Performance
                           Shares, the Performance Shares hereunder shall
                           immediately entitle the Executive to a Performance
                           Bonus in an amount equal to the difference between
                           the applicable Base Price and the Fair Market Value
                           of the per share Common Stock on the trading date
                           immediately preceding the closing date of such
                           Transaction, and any unpaid portion of a previously
                           vested Performance Bonus shall be immediately paid.

                  d.       Adjustments.  In all cases, the nature and extent
                           of adjustments under this Section 3 shall be
                           determined by the Committee in its sole discretion,
                           and any such determination as to what adjustments
                           shall be made, and the extent thereof, shall be
                           final and binding.

         4.       WITHHOLDING OF TAXES. The parties hereto recognize that the
Company may be obligated to withhold federal, state and local income taxes and
Social Security taxes to the extent that the Executive realizes ordinary
income in connection with receipt of the Performance Bonus pursuant to this
Agreement. The Executive agrees that the Company may withhold amounts needed
to cover such taxes from payments otherwise due and owing to the Executive.

         5.       DISCLAIMER OF RIGHTS. No provision in this Agreement shall
be construed to confer upon the Executive the right to be employed by the
Company, or to interfere in any way with the right and authority of the
Company either to increase or decrease the compensation of the Executive at
any time, or to terminate any employment or other relationship between the
Executive and the Company.

         6.       INTERPRETATION OF PERFORMANCE SHARE AGREEMENT.  All
decisions and interpretations made by the Committee or the Board of Directors
of the Company with regard to any questions arising under this Agreement shall
be binding and conclusive on the Company and the Executive.

         7.       TERMINATION. Except as otherwise provided herein, this
Agreement shall terminate and all rights and obligations of the parties
hereunder shall be void and of no effect immediately upon the date the
Executive ceases to hold the same, equivalent or higher grade

<PAGE>   4

office as that office set forth in the first paragraph of this Agreement or
August 9, 2003, whichever occurs first. For purposes of this Agreement, an
approved leave of absence shall not be deemed an event resulting in the
Executive ceasing to hold such office under this Agreement. An approved leave
of absence shall mean an absence approved by the Committee for military leave,
sick leave, or other bona fide leave, as long as the Executive's right to
re-employment is guaranteed by contract, statute or the policy of the Company.

8.       MISCELLANEOUS

         a.       Title and Headings.  Titles and headings of sections of the
                  Agreement are for convenience of reference only and shall
                  not affect the construction of any provision of this
                  Agreement.

         b.       Governing Law.  This Agreement shall be governed by,
                  interpreted under and construed and enforced in accordance
                  with the internal laws, and not the laws pertaining to
                  conflicts or choice of laws, of the State of Delaware.

                  IN WITNESS WHEREOF, the parties have executed this agreement
as of August 9, 2000.

ORBITAL SCIENCES CORPORATION

By:   /s/ David W. Thompson                       /s/ Garrett E. Pierce
      --------------------------------            ---------------------------
      David W. Thomspon                           Garrett E. Pierce
      Chairman and Chief Executive Officer

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