EXHIBIT 2.3


                       SUPPORT AGREEMENT


           MEMORANDUM  OF AGREEMENT made as  of  the      day  of
1995.


B E T W E E N:


                  ORBITAL SCIENCES CORPORATION,
                  a  corporation existing under
                  the  laws  of  the  State  of
                  Delaware,

                  (hereinafter referred to as "Orbital"),

                            - and -

                  MACDONALD DETTWILER HOLDINGS INC.
                  (formerly known as 3173623 Canada Inc.),
                  a  corporation existing under
                  the laws of Canada.

                  (hereinafter referred to as "Corporation").



          WHEREAS pursuant to a combination agreement dated as of
August  31,  1995 (the "Combination Agreement")  by  and  between
Orbital,  the Corporation and MacDonald, Dettwiler and Associates
Ltd.  ("MDA") the parties agreed that on the Effective  Date  (as
defined   in   the  Combination  Agreement),  Orbital   and   the
Corporation  would  execute  and  deliver  a  Support   Agreement
containing the terms and conditions set forth in Exhibit  2.3  to
the  Combination  Agreement together with such  other  terms  and
conditions  as may be agreed to by the parties to the Combination
Agreement acting reasonably;

            AND   WHEREAS   pursuant  to  an   arrangement   (the
"Arrangement") effected by articles of arrangement filed pursuant
to  the  Canada Business Corporations Act on #, 1995, each issued
and  outstanding common share of MDA (an "MDA Common Share")  was
exchanged  directly  or indirectly with the Corporation  for  0.#
issued  and  outstanding Exchangeable Shares of  the  Corporation
(the "Exchangeable Shares");

           AND WHEREAS the aforesaid articles of arrangement  set
forth   the   rights,  privileges,  restrictions  and  conditions
(collectively the "Exchangeable Share Provisions")  attaching  to
the Exchangeable Shares;

            AND   WHEREAS  the  parties  hereto  desire  to  make
appropriate  provision  and  to  establish  a  procedure  whereby
Orbital  will take certain actions and make certain payments  and
deliveries necessary to ensure that the Corporation will be  able
to  make certain payments and to deliver or cause to be delivered
shares   of  Orbital  Common  Shares  in  satisfaction   of   the
obligations  of  the  Corporation under  the  Exchangeable  Share
Provisions  with  respect  to  the payment  and  satisfaction  of
dividends,  Liquidation Amounts, Retraction Prices and Redemption
Prices all in accordance with the Exchangeable Share Provisions;

           NOW  THEREFORE  in  consideration  of  the  respective
covenants  in  this  agreement and for other  good  and  valuable
consideration  (the receipt and sufficiency of which  are  hereby
acknowledged), the parties agree as follows:

                           ARTICLE 1

                 DEFINITIONS AND INTERPRETATION

1.1        Defined  Terms.  Each term denoted herein  by  initial
capital  letters and not otherwise defined herein shall have  the
meaning  ascribed  thereto in the Exchangeable Share  Provisions,
unless the context requires otherwise.

1.2        Interpretation  not Affected by  Headings,  etc.   The
division of this agreement into articles, sections and paragraphs
and  the  insertion of headings are for convenience of  reference
only  and shall not affect the construction or interpretation  of
this agreement.

1.3        Number,  Gender,  etc.  Words importing  the  singular
number  only  shall  include the plural and  vice  versa.   Words
importing the use of any gender shall include all genders.

1.4        Date for any Action.  If any date on which any  action
is  required to be taken under this agreement is not  a  Business
Day,  such  action  shall be required to be  taken  on  the  next
succeeding Business Day.


                           ARTICLE 2

            COVENANTS OF ORBITAL AND THE Corporation

2.1       Covenants of Orbital Regarding Exchangeable Shares.  So
long as any Exchangeable Shares are outstanding, Orbital will:

                     (a)   not  declare or pay  any  dividend  on
               Orbital  Common Shares unless (i) the  Corporation
               shall  have  sufficient assets,  funds  and  other
               property  (including,  where  applicable,  Orbital
               Common  Shares  or  other securities  of  Orbital)
               available  to enable the due declaration  and  the
               due   and  punctual  payment  in  accordance  with
               applicable law, of an equivalent dividend  on  the
               Exchangeable   Shares  in  accordance   with   the
               Exchangeable   Share  Provisions  and   (ii)   the
               Corporation shall simultaneously declare  or  pay,
               as  the case may be, an equivalent dividend on the
               Exchangeable   Shares  in  accordance   with   the
               Exchangeable Share Provisions;

                      (b)    cause  the  Corporation  to  declare
               simultaneously   with  the  declaration   of   any
               dividend  on  Orbital Common Shares an  equivalent
               dividend on the Exchangeable Shares and, when such
               dividend  is paid on Orbital Common Shares,  cause
               the  Corporation  to pay simultaneously  therewith
               such   equivalent  dividend  on  the  Exchangeable
               Shares,  in  each  case  in  accordance  with  the
               Exchangeable Share Provisions;

                     (c)  advise the Corporation sufficiently  in
               advance  of  the  declaration by  Orbital  of  any
               dividend  on  Orbital Common Shares and  take  all
               such   other   actions  as   are   necessary,   in
               cooperation  with the Corporation, to ensure  that
               the declaration date, record date and payment date
               for  any dividend on the Exchangeable Shares shall
               be  the same as the record date, declaration  date
               and payment date for the corresponding dividend on
               Orbital Common Shares and such dates in respect of
               dividends on the Exchangeable Shares shall  be  in
               accordance   with   any   requirement    of    the
               Exchangeable  Share  Provisions  and   the   stock
               exchange  on  which  the Exchangeable  Shares  are
               listed;

                     (d)   ensure  that the record date  for  any
               dividend   declared  on  Orbital  Common   Shares,
               Orbital   Common   Share  Reorganization,   Rights
               Offering,   Special   Distribution   or    Capital
               Reorganization is not less than 10  Business  Days
               after  the  declaration date  for  such  dividend,
               Orbital   Common   Share  Reorganization,   Rights
               Offering,   Special   Distribution   or    Capital
               Reorganization;

                     (e)   take all such actions and do all  such
               things as are necessary or desirable to enable and
               permit   the   Corporation,  in  accordance   with
               applicable  law, to pay and otherwise perform  its
               obligations  with respect to the  satisfaction  of
               the  Liquidation Amount in respect of each  issued
               and   outstanding  Exchangeable  Share  upon   the
               liquidation,  dissolution  or  winding-up  of  the
               Corporation, including without limitation all such
               actions  and  all such things as are necessary  or
               desirable to enable and permit the Corporation  to
               cause  to  be  delivered shares of Orbital  Common
               Shares  to  the holders of Exchangeable Shares  in
               satisfaction  of the Liquidation Amount  for  each
               such  Exchangeable Share, in accordance  with  the
               provisions of Article 4 of the Exchangeable  Share
               Provisions;

                     (f)   take all such actions and do all  such
               things as are necessary or desirable to enable and
               permit   the   Corporation,  in  accordance   with
               applicable  law, to pay and otherwise perform  its
               obligations  with respect to the  satisfaction  of
               the  Retraction  Price and the  Redemption  Price,
               including without limitation all such actions  and
               all  such things as are necessary or desirable  to
               enable and permit the Corporation to cause  to  be
               delivered shares of Orbital Common Shares  to  the
               holders   of   Exchangeable   Shares,   upon   the
               retraction   or  redemption  of  the  Exchangeable
               Shares  in  accordance  with  the  provisions   of
               Article  5 or Article 6 of the Exchangeable  Share
               Provisions, as the case may be;

                     (g)   not exercise its vote as a shareholder
               of  the  Corporation to initiate,  consent  to  or
               approve the voluntary liquidation, dissolution  or
               winding-up of the Corporation nor take any  action
               or  omit  to  take any action that is designed  to
               result   in   the   liquidation,  dissolution   or
               winding-up of the Corporation; and

                     (h)   not exercise its vote as a shareholder
               of the Corporation to authorize the continuance or
               other  transfer of the corporate existence of  the
               Corporation to any jurisdiction outside Canada.

2.2         Segregation  of  Funds.   Orbital  will   cause   the
Corporation to deposit a sufficient amount of funds in a separate
account  and  segregate a sufficient amount of  such  assets  and
other  property as is necessary to enable the Corporation to  pay
or   otherwise  satisfy  the  applicable  dividends,  Liquidation
Amount,  Retraction Price or Redemption Price, in each  case  for
the  benefit  of  holders from time to time of  the  Exchangeable
Shares, and will cause the Corporation to use such funds,  assets
and  other property so segregated exclusively for the payment  of
dividends   and  the  payment  or  other  satisfaction   of   the
Liquidation Amount, the Retraction Price or the Redemption Price,
as  applicable, in each case in accordance with the  Exchangeable
Share Provisions.

2.3        Reservation of Orbital Common Shares.  Orbital  hereby
represents  and  warrants  that it has irrevocably  reserved  for
issuance  out of its authorized and unissued capital  stock  such
number  of  Orbital Common Shares as is equal to  the  number  of
Exchangeable   Shares  outstanding  immediately   following   the
Effective  Date  and covenants that at all times  in  the  future
while  any  Exchangeable  Shares are  outstanding  it  will  keep
available,  free from pre-emptive and other rights,  out  of  its
authorized  and  unissued capital stock such  number  of  Orbital
Common  Shares (or other shares or securities into which  Orbital
Common Shares may be reclassified or changed) as is necessary  to
enable  Orbital  and the Corporation to perform their  respective
obligations  pursuant to this agreement, the  Exchangeable  Share
Provisions and the Voting and Exchange Trust Agreement.

2.4        Notification of Certain Events.  In  order  to  assist
Orbital to comply with its obligations hereunder, the Corporation
will give, or cause the Transfer Agent to give, Orbital notice of
each of the following events at the time set forth below:

                    (a)  in the event of any determination by the
               Board of Directors of the Corporation to institute
               voluntary  liquidation, dissolution or winding  up
               proceedings with respect to the Corporation or  to
               effect any other distribution of the assets of the
               Corporation among its shareholders for the purpose
               of  winding up its affairs, at least 60 days prior
               to   the   proposed   effective   date   of   such
               liquidation,  dissolution,  winding  up  or  other
               distribution;

                     (b)   immediately, upon the earlier  of  (i)
               receipt by the Corporation of notice of, and  (ii)
               the  Corporation otherwise becoming aware of,  any
               threatened or instituted claim, suit, petition  or
               other  proceedings with respect to the involuntary
               liquidation,  dissolution or  winding  up  of  the
               Corporation or to effect any other distribution of
               the   assets   of   the  Corporation   among   its
               shareholders  for the purpose of  winding  up  its
               affairs;

                      (c)   immediately,  upon  receipt  by   the
               Transfer Agent of a Retraction Request; and

                    (d)  as soon as practicable upon the issuance
               by  the Corporation of any Exchangeable Shares  or
               rights to acquire Exchangeable Shares.

2.5        Delivery of Orbital Common Shares.  In furtherance  of
its  obligations  under subsections 2.1(e) and (f)  hereof,  upon
notice of any event that requires the Corporation to cause to  be
delivered  Orbital  Common Shares to any holder  of  Exchangeable
Shares,  Orbital shall forthwith issue and deliver the  requisite
Orbital Common Shares to or to the order of the former holder  of
the  surrendered  Exchangeable Shares, as the  Corporation  shall
direct.   All such Orbital Common Shares shall be duly issued  as
fully paid and non-assessable and shall be free and clear of  any
Liens.   In  consideration of the issuance of each  such  Orbital
Common  Share by Orbital, the Corporation shall issue to Orbital,
or  as Orbital shall direct, such number of common shares of  the
Corporation as is equal to the fair value of such Orbital  Common
Shares.

2.6        Qualification  of Orbital Common Shares.   Unless  the
staff  of  the  United States Securities and Exchange  Commission
(the  "SEC") has confirmed the availability of an exemption  from
registration under the United States Securities Act of  1933,  as
amended  (the "Securities Act") as to the issuance of the Orbital
Common Shares in exchange for the Exchangeable Shares pursuant to
the Plan of Arrangement, the Exchangeable Share Provisions or the
Voting and Exchange Trust Agreement, in response to the No Action
Request (as defined in the Combination Agreement) or Orbital  has
received  an  opinion of counsel reasonably satisfactory  to  the
Corporation  to  such  effect,  then  Orbital  shall  cause  such
issuance  to  be registered under the Securities Act,  and  shall
file a registration statement covering such issuance with the SEC
and  use  all  commercially  reasonable  efforts  to  cause  such
registration statement to become effective as soon as practicable
and  remain  effective  throughout the period  during  which  the
Exchangeable Shares may be exchanged in accordance with the  Plan
of  Arrangement, the Exchangeable Share Provisions or the  Voting
and  Exchange Trust Agreement.  Orbital agrees to file  any  such
required   registration   statement   as   soon   as   reasonably
practicable.  Orbital shall use all reasonable efforts to  obtain
all  orders  required  from  the applicable  Canadian  securities
authorities  to permit the issuance of the Orbital Common  Shares
upon  any  such  exchange  of  the  Exchangeable  Shares  without
registration or qualification with or approval of or  the  filing
of  any document including any prospectus or similar document  or
the  taking of any proceeding with or the obtaining of any order,
ruling  or  consent from any governmental or regulatory authority
under  any  Canadian federal or provincial law or  regulation  or
pursuant to the rules and regulations of any regulatory authority
or  the  fulfillment of any other legal requirement  before  such
Orbital  Common  Shares  may  be  issued  and  delivered  by  the
Corporation  or Orbital to the holder thereof or  in  order  that
such Orbital Common Shares may be freely traded thereafter (other
than  any restrictions on transfer by reason of a holder being  a
"control  person" of the Corporation or Orbital for  purposes  of
Canadian  federal or provincial securities law or an  "affiliate"
for purposes of United States Federal or state securities law).

2.7        Tender Offers, etc.  In the event that a tender offer,
share  exchange  offer,  issuer bid,  take-over  bid  or  similar
transaction with respect to Orbital Common Shares (an "Offer") is
proposed by Orbital or is proposed to Orbital or its stockholders
and  is recommended by the Board of Directors of Orbital,  or  is
otherwise effected or to be effected with the consent or approval
of  the  Board  of  Directors of Orbital, Orbital  will  use  all
commercially reasonable efforts expeditiously and in  good  faith
to  take all such actions and do all such things as are necessary
or  desirable to enable and permit holders of Exchangeable Shares
to  participate  in  such Offer to the  same  extent  and  on  an
economically  equivalent basis as the holders of  Orbital  Common
Shares,  without discrimination.  Without limiting the generality
of  the  foregoing, Orbital will use all commercially  reasonable
efforts expeditiously and in good faith to ensure that holders of
Exchangeable  Shares may participate in all such  Offers  without
being  required  to retract Exchangeable Shares  as  against  the
Corporation  (or,  if  so  required,  to  ensure  that  any  such
retraction shall be effective only upon, and shall be conditional
upon,  the  closing of the Offer and only to the extent necessary
to tender or deposit to the Offer).

2.8       Ownership of Outstanding Shares.  Orbital covenants and
agrees  in  favour  of  the Corporation  that,  as  long  as  any
outstanding Exchangeable Shares are owned by any person or entity
other than Orbital or any of its Affiliates, Orbital will be  and
remain the direct or indirect beneficial owner of all issued  and
outstanding shares in the capital of the Corporation (other  than
Exchangeable  Shares  and  the Class B Preferred  Shares  of  the
Corporation issued to Canadian Imperial Bank of Commerce pursuant
to  the  Arrangement)  and  all  outstanding  securities  of  the
Corporation  carrying or otherwise entitled to voting  rights  in
any circumstances (other than Exchangeable Shares and the Class B
Preferred  Shares of the Corporation issued to Canadian  Imperial
Bank  of  Commerce pursuant to the Arrangement),  unless  Orbital
shall have obtained the prior approval of the Corporation and the
holders  of  the  Exchangeable Shares given  in  accordance  with
section 8.2 of the Exchangeable Share Provisions.

2.9        Orbital  Not  To  Vote Exchangeable  Shares.   Orbital
covenants  and  agrees  that it will  appoint  and  cause  to  be
appointed  proxyholders with respect to all  Exchangeable  Shares
held  by  Orbital  and  its Affiliates for the  sole  purpose  of
attending each meeting of holders of Exchangeable Shares in order
to  be  counted  as  part of the quorum for  each  such  meeting.
Orbital  further covenants and agrees that it will not, and  will
cause its Affiliates not to, exercise any voting rights that  may
be  exercisable by holders of Exchangeable Shares  from  time  to
time pursuant to the Exchangeable Share Provisions or pursuant to
the  provisions  of  the CBCA with respect  to  any  Exchangeable
Shares  held by it or by its Affiliates in respect of any  matter
considered  at  any  meeting of holders of  Exchangeable  Shares,
including without limitation any approval to be given by  holders
of   Exchangeable  Shares  pursuant  to  section   8.2   of   the
Exchangeable Share Provision.

2.10       Due  Performance.  On and after  the  Effective  Date,
Orbital  shall  duly  and timely perform all of  its  obligations
provided   for   in  the  Plan  of  Arrangement,  including   any
obligations that may arise upon the exercise of Orbital's  rights
under the Exchangeable Share Provisions.

2.11      Economic Equivalence.  Orbital hereby acknowledges that
it  will  be  bound by any determination of economic  equivalence
made  by  the Board of Directors pursuant to section 5.6  of  the
Plan  of  Arrangement  or section 9.1 of the  Exchangeable  Share
Provisions, where applicable.


                           ARTICLE 3

                            GENERAL

3.1        Term.   This agreement shall come into  force  and  be
effective as of the date hereof and shall terminate and be of  no
further  force  and  effect  at  such  time  as  there   are   no
Exchangeable Shares (or securities or rights convertible into  or
exchangeable  for  or  carrying rights  to  acquire  Exchangeable
Shares) held by any party other than Orbital and its Affiliates.

3.2        Changes  in  Capital of Orbital and  the  Corporation.
Notwithstanding  the provisions of section  3.4  hereof,  at  all
times  after  the  occurrence of any event effected  pursuant  to
section  2.7  hereof as a result of which either  Orbital  Common
Shares or the Exchangeable Shares or both are in any way changed,
this  agreement  shall  forthwith  be  amended  and  modified  as
necessary  in  order  that it shall apply  with  full  force  and
effect,  mutatis  mutandis,  to all  new  securities  into  which
Orbital Common Shares or the Exchangeable Shares or both  are  so
changed  and  the  parties hereto shall execute  and  deliver  an
agreement  in  writing  giving  effect  to  and  evidencing  such
necessary amendments and modifications.

3.3        Severability.  If any provision of this  agreement  is
held  to  be  invalid,  illegal or unenforceable,  the  validity,
legality  or  enforceability of the remainder of  this  agreement
shall  not  in any way be affected or impaired thereby  and  this
agreement  shall  be  carried  out  as  nearly  as  possible   in
accordance with its original terms and conditions.

3.4        Amendments, Modifications, etc.   This  agreement  may
not  be  amended  or modified except by an agreement  in  writing
executed  by  the  Corporation and Orbital and  approved  by  the
holders  of  the Exchangeable Shares in accordance  with  section
10.2 of the Exchangeable Share Provisions.

3.5       Ministerial Amendments.  Notwithstanding the provisions
of  section  3.4, the parties to this agreement may  without  the
approval  of the holders of the Exchangeable Shares, at any  time
and  from time to time, amend or modify this agreement in writing
for the purposes of:

                     (a)   adding to the covenants of  either  or
               both parties for the protection of the holders  of
               the Exchangeable Shares;

                     (b)  making such amendments or modifications
               not  inconsistent with this agreement  as  may  be
               necessary or desirable with respect to matters  or
               questions  which, in the opinion of the  Board  of
               Directors of each of the Corporation and  Orbital,
               it  may  be expedient to make, provided that  each
               such  board  of directors shall be of the  opinion
               that such amendments or modifications will not  be
               prejudicial to the interests of the holders of the
               Exchangeable Shares; or

                     (c)   making  such  changes  or  corrections
               which, on the advice of counsel to the Corporation
               and  Orbital,  are  required for  the  purpose  of
               curing  or  correcting any ambiguity or defect  or
               inconsistent  provision or  clerical  omission  or
               mistake  or  manifest error herein, provided  that
               the boards of directors of each of the Corporation
               and  Orbital  shall  be of the opinion  that  such
               changes or corrections will not be prejudicial  to
               the  interests of the holders of the  Exchangeable
               Shares.

3.6        Meeting  to Consider Amendments.  The Corporation,  at
the  request of Orbital, shall call a meeting or meetings of  the
holders of the Exchangeable Shares for the purpose of considering
any   proposed  amendment  or  modification  requiring   approval
pursuant  to  section 3.4 hereof.  Any such meeting  or  meetings
shall  be called and held in accordance with the by-laws  of  the
Corporation and the Exchangeable Share Provisions.

3.7        Waivers  Only in Writing.  No waiver  of  any  of  the
provisions of this agreement otherwise permitted hereunder  shall
be  effective unless made in writing and signed by  both  of  the
parties hereto.

3.8        Inurement.  This agreement shall be binding  upon  and
inure  to  the benefit of the parties hereto and their respective
successors and permitted assigns.

3.9        Orbital Successors.  Orbital shall not enter into  any
transaction  (whether  by way of reconstruction,  reorganization,
consolidation,  merger,  transfer,  sale,  lease  or   otherwise)
whereby  all  or substantially all its undertaking, property  and
assets  would become the property of any other person or, in  the
case  of  a  merger,  of  the  continuing  corporation  resulting
therefrom, unless:

                      (a)    such   other  person  or  continuing
               corporation   is  a  corporation   (the   "Orbital
               Successor")  incorporated under the  laws  of  any
               state  of the United States or the laws of  Canada
               or any province thereof; and

                     (b)  the Orbital Successor, by operation  of
               law, becomes, without more, bound by the terms and
               provisions of this agreement or, if not so  bound,
               executes, prior to or contemporaneously  with  the
               consummation of such transaction, an agreement  to
               be bound by the provisions hereof as if it were an
               original  party hereto and to observe and  perform
               all  of  the covenants and obligations of  Orbital
               pursuant  to  this agreement, in form satisfactory
               to the Corporation, acting reasonably.

Nothing  herein shall be construed as preventing the amalgamation
or  merger of any wholly-owned subsidiary of Orbital with or into
Orbital.

3.10        Notices   to   Parties.   All   notices   and   other
communications between the parties shall be in writing and  shall
be  deemed  to  have  been given if delivered  personally  or  by
confirmed telecopy to the parties at the following addresses  (or
at such other address for either such party as shall be specified
in like notice):

                    (a)  if to Orbital at:


                         Orbital Sciences Corporation
                         21700 Atlantic Boulevard
                         Dulles, VA 20166

                         Attention: General Counsel

                                   Telecopy: (703) 406-5572

                    (b)  if to the Corporation at:

                         MacDonald Dettwiler Holdings Inc.
                         13800 Commerce Parkway
                         Richmond, British Columbia
                         V6V 2J3

                         Attention: #

                         Telecopy: (604) #

Any  notice  or  other  communication given personally  shall  be
deemed to have been given and received upon delivery thereof  and
if  given  by  telecopy shall be deemed to have  been  given  and
received on the date of confirmed receipt thereof unless such day
is  not  a Business Day in which case it shall be deemed to  have
been  given and received upon the immediately following  Business
Day.

3.11       Counterparts.   This  agreement  may  be  executed  in
counterparts, each of which shall be deemed an original, and  all
of  which  taken  together  shall constitute  one  and  the  same
instrument.

3.12       Jurisdiction.  This agreement shall be  construed  and
enforced  in accordance with the laws of the Province of  British
Columbia and the laws of Canada applicable therein.

3.13        Attornment.   Orbital  agrees  that  any  action   or
proceeding  arising out of or relating to this agreement  may  be
instituted  in  the  courts  of  British  Columbia,  waives   any
objection which it may have now or hereafter to the venue of  any
such   action   or   proceeding,  irrevocably  submits   to   the
jurisdiction of the said courts in any such action or proceeding,
agrees to be bound by any judgment of the said courts and not  to
seek,  and  hereby waives, any review of the merits of  any  such
judgment  by  the  courts  of any other jurisdiction  and  hereby
appoints  the  Corporation at its registered office as  Orbital's
attorney for service of process.

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


                                   ORBITAL SCIENCES CORPORATION


                                   by ___________________________


                                      ___________________________


                                   MACDONALD DETTWILER HOLDINGS
                                   INC.


                                   by ___________________________


                                      ___________________________