EXHIBIT 5.1





                                November 13, 1997


Board of Directors
Orion Network Systems, Inc.
2440 Research Boulevard
Suite 400
Rockville, Maryland  20850



Ladies and Gentlemen:

           We are acting as counsel to Orion Network  Systems,  Inc., a Delaware
corporation  (the "Company"),  in connection with the registration  statement on
Form S-3 (the  "Registration  Statement") filed with the Securities and Exchange
Commission  relating to the proposed public  offering of up to 5,052,202  shares
(the  "Offered  Shares")  of the  Company's  common  stock,  $.01 par value (the
"Common  Stock"),  all of  which  Offered  Shares  may be  sold  by the  Selling
Stockholders  identified in the Registration  Statement from time to time as set
forth in the prospectus  which forms a part of the  Registration  Statement (the
"Prospectus")  and  as to be  set  forth  in  one  or  more  supplements  to the
Prospectus.  The Offered Shares consist of (i) 4,030,627  shares of Common Stock
(the "Series C  Conversion  Shares")  issuable  upon  conversion  of Series C 6%
Cumulative Redeemable  Convertible Stock of the Company (the "Series C Preferred
Stock"),  (ii) 241,835  shares of Common Stock (the "Series C Dividend  Shares")
issuable in payment of  dividends  on Series C Preferred  Stock,  (iii)  779,740
shares of  Common  Stock  (the  "Debenture  Conversion  Shares")  issuable  upon
conversion of certain convertible junior subordinated  debentures of the Company
(the  "Debentures"),  and (iv)  65,455  shares of Common  Stock (the  "Debenture
Interest Shares") issuable in payment of interest on the Debentures.

           This opinion letter is furnished to you at your request to enable you
to fulfill the  requirements of Item 601(b)(5) of Regulation S-K, 17 C.F.R.  ss.
229.601(b)(5), in connection with the Registration Statement.

           For purposes of this opinion  letter,  we have examined copies of the
following documents:


Board of Directors
Orion Network Systems, Inc.
November 13, 1997
Page 2

           1.   An executed copy of the Registration Statement.

           2.   An executed copy of the Debenture Purchase  Agreement,  dated as
                of January 13, 1997,  as amended as of January 31,  1997,  among
                the Company,  British Aerospace Holdings, Inc. and Matra Marconi
                Space UK Limited (the "Debenture Purchase Agreement").

           3.   The  Certificate  of  Designations  for the  Series C  Preferred
                Stock,  as certified  by the  Secretary of State of the State of
                Delaware on November 3, 1997 and by the  Assistant  Secretary of
                the Company on the date hereof as then being complete,  accurate
                and in effect (the "Certificate of Designations").

           4.   The Restated  Certificate of  Incorporation  of the Company,  as
                certified by the  Secretary of State of the State of Delaware on
                November 3, 1997 and by the  Assistant  Secretary of the Company
                on the date  hereof  as then  being  complete,  accurate  and in
                effect (the "Certificate of Incorporation").

           5.   The Amended and Restated Bylaws of the Company,  as certified by
                the  Assistant  Secretary  of the  Company on the date hereof as
                being complete, accurate and in effect (the "Bylaws").

           6.   Resolutions of the Board of Directors of the Company  adopted on
                October 31, 1997, as certified by the Assistant Secretary of the
                Company on the date hereof as then being complete,  accurate and
                in effect,  relating to the filing of the Registration Statement
                and related matters (the "Authorizing Resolutions").

           In our  examination of the aforesaid  documents,  we have assumed the
genuineness  of all  signatures,  the legal  capacity  of natural  persons,  the
authenticity,  accuracy and  completeness of all documents  submitted to us, and
the conformity with the original  documents of all documents  submitted to us as
certified, telecopied, photostatic, or reproduced copies. This opinion letter is
given, and all statements herein are made, in the context of the foregoing.


Board of Directors
Orion Network Systems, Inc.
November 13, 1997
Page 3

           This  opinion  letter  is based as to  matters  of law  solely on the
General  Corporation  Law of the State of Delaware (the  "DGCL").  We express no
opinion herein as to any other laws, statutes, regulations or ordinances.

           Based upon,  subject to and limited by the  foregoing,  we are of the
opinion that:

           (i) when Series C  Conversion  Shares are issued and  delivered  upon
conversion of Series C Preferred  Stock in accordance  with the  Certificate  of
Designations and the Authorizing  Resolutions,  such Series C Conversion  Shares
will be validly issued, fully paid and non-assessable.

           (ii) when  Series C  Dividend  Shares are  issued  and  delivered  in
payment of validly declared  dividends on Series C Preferred Stock in accordance
with the Certificate of Designations,  with the Authorizing Resolutions and with
additional appropriate resolutions duly adopted by the Board of Directors of the
Company  authorizing and declaring such dividends and the payment thereof,  such
Series C Dividend Shares will be validly issued, fully paid and non-assessable.

           (iii) when the Debentures  Conversion Shares are issued and delivered
upon  conversion of  Debentures  in  accordance  with the terms of the Debenture
Purchase Agreement and the Debentures and with the Authorizing Resolutions, such
Debentures   Conversion   Shares  will  be  validly   issued,   fully  paid  and
non-assessable.

           (iv) when  Debenture  Interest  Shares are issued  and  delivered  in
payment  of  interest  on the  Debentures  in  accordance  with the terms of the
Debenture  Purchase  Agreement  and the  Debentures  and  with  the  Authorizing
Resolutions,  such Debenture Interest Shares will be validly issued,  fully paid
and non-assessable.

           We assume no obligation to advise you of any changes in the foregoing
subsequent to the delivery of this opinion letter.  This opinion letter has been
prepared solely for your use in connection  with the filing of the  Registration
Statement on the date of this  opinion  letter and should not be quoted in whole
or in part or  otherwise  be  referred  to, nor filed with or  furnished  to any
governmental agency or other person or entity, without the prior written consent
of this firm.



Board of Directors
Orion Network Systems, Inc.
November 13, 1997
Page 4

           We hereby consent to the filing of this opinion letter as Exhibit 5.1
to the  Registration  Statement  and to the  reference  to this  firm  under the
caption  "Validity of the  Securities" in the prospectus  constituting a part of
the Registration Statement. In giving this consent, we do not thereby admit that
we are an "expert" within the meaning of the Securities Act of 1933, as amended.


                                     Very truly yours,



                                     HOGAN & HARTSON L.L.P.