EXHIBIT 5.1






                                 March 31, 1997



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

Gentlemen:

                  This firm has acted as counsel to Orion Network Systems,  Inc.
(the "Company"),  a Delaware  corporation,  in connection with its registration,
pursuant  to a  registration  statement  on Form S-8  filed on or about the date
hereof (the  "Registration  Statement"),  of 380,000  shares (the  "Shares")  of
common stock, par value $.01 per share, of the Company, issuable under the Orion
Network Systems, Inc. Non-Employee Director Stock Option Plan (the "Plan"). This
letter is  furnished to you pursuant to the  requirements  of Item  601(b)(5) of
Regulation  S-K,  17  C.F.R.   ss.   229.601(b)(5),   in  connection  with  such
registration.

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

                  1.      An executed copy of the Registration Statement.

                  2.      A copy  of the  Plan, as certified by the Secretary of
                          the Company on the date hereof as then being complete,
                          accurate and in effect.

                  3.      The  Restated  Certificate  of  Incorporation  of  the
                          Company, as amended  (the "Charter"),  as certified by
                          the  Secretary  of State  of the State of  Delaware on
                          January 29, 1997 and by  the  Secretary of the Company
                          on the date hereof as  then being  complete,  accurate
                          and in effect.

                  4.      The By-laws of the Company,  as amended,  as certified
                          by the  Secretary of the Company on the date hereof as
                          then being complete, accurate and in effect.






Board of Directors
Orion Network Systems, Inc.
March 31, 1997
Page 2


                  5.      Resolutions and consents of the  Board of Directors of
                          the Company  adopted on  March 20, 1996  and March 12,
                          1997 as  certified by the  Secretary of the Company on
                          the date hereof as  then being complete,  accurate and
                          in effect  relating to,  among other things,  approval
                          of the Plan.

                  We have not, except as specifically identified above, made any
independent  review or investigation of factual or other matters,  including the
organization,  existence,  good  standing,  assets,  business  or affairs of the
Company or its subsidiaries.  In our examination of the aforesaid  certificates,
records, and 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 as originals,  and the authenticity,  accuracy
and  completeness  and conformity  with the original  documents of all documents
submitted to us as certified, telecopied,  photostatic, or reproduced copies. We
have assumed the  authenticity and accuracy of the foregoing  certifications  of
corporate  officers,  on which we are  relying,  and  have  made no  independent
investigations thereof. This opinion is given in the context of the foregoing.

                  This  opinion  letter is based as to  matters of law solely on
the  General  Corporation  Law of the State of  Delaware.  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 the Shares,  when issued and  delivered in the manner and on
the terms  contemplated  in the  Registration  Statement  and the Plan (with the
Company  having  received the  consideration  therefor,  the form of which is in
accordance  with  applicable  law),  will be  validly  issued,  fully  paid  and
non-assessable by the Company.

                  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 letter,  and should not be quoted in
whole or in part or  otherwise be referred to, nor be 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.
March 31, 1997
Page 3


                  We hereby  consent  to the  filing of this  opinion  letter as
Exhibit 5.1 to 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.