Exhibit 5.1

                            July 30, 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 1,300,000  shares (the "Shares") of
common stock, par value $.01 per share, of the Company, issuable under the Orion
Network  Systems,  Inc.  1997 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.
July 30, 1997
Page 2


     5. Resolutions  and  consents  of the  Board of  Directors  of the  Company
        adopted  on  March  12,  1997  and July 16,  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.
July 30, 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.