AMENDMENT NO. 1 TO
                          REGISTRATION RIGHTS AGREEMENT

                  THIS AMENDMENT NO. 1 TO  REGISTRATION  RIGHTS  AGREEMENT (this
"Amendment")  is  entered  into as of  January  8, 2001 by and  between  Motient
Corporation,  a  Delaware  corporation  (the  "Company")  and  each  stockholder
signatory hereto (the "Stockholders").

                  WHEREAS,  the  parties  entered  into  a  Registration  Rights
Agreement on June 29, 2000 (the "Registration  Rights  Agreement"),  pursuant to
which Company  granted  certain  registration  rights to the  Stockholders  with
respect to the shares of Common  Stock that may be acquired by the  Stockholders
pursuant to the  Investment  Agreement  dated June 29,  2000 among the  Company,
Mobile Satellite Ventures LLC (formerly Motient Satellite Ventures LLC)("Newco")
and the Stockholders.

                  WHEREAS,  the parties desire to amend the Registration  Rights
Agreement as more fully described  herein to reflect the January 2001 Investment
Agreement dated as January 8, 2001 among the Company,  Newco,  the  Stockholders
and the other parties named therein (the "January Investment Agreement").

                  NOW  THEREFORE in  consideration  of the mutual  covenants and
agreements set forth below, the parties,  intending to be legally bound,  hereby
amend the Registration Rights Agreement as follows:

                    1.  Amendments  to  Registration   Rights   Agreement.   The
                        -------------------------------------------------
Registration Rights Agreement is hereby amended as follows:

                  (a) The first  two  paragraphs  of the  Terms  and  Conditions
section of the  Registration  Rights Agreement is hereby deleted in its entirety
and replaced with the following:

                           The  Investors  have agreed to invest in, and provide
                  financing  for  Mobile  Satellite  Ventures  LLC,  a  Delaware
                  limited liability company ("Newco"), and the Company and Newco
                  have agreed to issue, sell and grant to the Investors, certain
                  securities and options pursuant to the January 2001 Investment
                  Agreement,  dated as of  January  8,  2001,  by and  among the
                  Company,  Newco,  the  Investors  and the other  parties named
                  therein (the "Investment Agreement");

                           The  Parties   desire   herein  to  provide   certain
                  registration  rights to the  Stockholders  with respect to the
                  shares  of  Common   Stock  that  may  be   acquired   by  the
                  Stockholders  pursuant  to  the  effectuation  of  the  Parent
                  Conversions (as defined in the Investment Agreement).

                  (b) The definition of "Registrable Securities" in Section 1 of
the Registration Rights Agreement is hereby deleted in its entirety and replaced
with the following:

                           "Registrable  Securities." Any shares of Common Stock
                  of the  Company  held by the  Stockholders,  and any shares of
                  Common Stock that such  Stockholder  has the right to acquire,
                  or does acquire,  in connection  with a Parent  Conversion (or
                  their  transferees)  pursuant  to  the  Investment  Agreement,
                  including  any shares of Common Stock issued or issuable  with
                  respect to such shares by reason of a stock  dividend or stock
                  split  or  in  connection   with  a  combination   of  shares,
                  recapitalization,     merger,     consolidation    or    other
                  reorganization;  provided,  that a Registrable Security ceases
                  to be a Registrable  Security when (i) it has been  registered
                  and  sold  under  the  Securities  Act,  (ii)  it is  sold  or
                  transferred in accordance  with the  requirements  of Rule 144
                  (or similar provisions then in effect)  promulgated by the SEC
                  under the Securities Act ("Rule 144"), or (iii) it is eligible
                  to be sold or  transferred  under  Rule  144  without  holding
                  period or volume limitations.

                  2.  Miscellaneous.  Capitalized  terms  used  herein  and  not
defined  herein  shall have the  meaning  ascribed  to them in the  Registration
Rights  Agreement.  All other terms and  provisions of the  Registration  Rights
Agreement  shall  continue in full force and effect and unchanged and are hereby
confirmed in all respects.  This Amendment  shall terminate upon the termination
of the January  Investment  Agreement  and all of the terms of the  Registration
Rights  Agreement shall continue in full force and effect (without giving effect
to this Amendment). This Amendment may be executed in any number of counterparts
which,  taken together,  shall  constitute a single,  binding  instrument.  This
Amendment  shall be governed by and construed in accordance with the laws of the
Commonwealth  of Virginia,  without regard to its principles of conflicts of law
that would give effect to the application of the law of another jurisdiction.






                  IN WITNESS WHEREOF, the parties hereto have duly executed this
Amendment, or have caused this Amendment to be duly executed on their behalf, as
of the day and year first above written.


                                   COMPANY:

                                   MOTIENT CORPORATION


                                   By:/s/Randy S. Segal
                                      -------------------------------------
                                      Randy S. Segal, Senior Vice President


                                   STOCKHOLDERS:
                                   ------------


                                   TELCOM SATELLITE VENTURES INC.


                                   By:/s/ Hal B. Perkins
                                      -------------------------------------
                                      Name: Hal B. Perkins
                                      Title: Vice President







                                   COLUMBIA SPACE (QP), INC.


                                   By: /s/ Donald A. Doering
                                      -------------------------------------
                                      Name: Donald A. Doering
                                      Title: CFO


                                    COLUMBIA SPACE (AI), INC.


                                    By: /s/ Donald A. Doering
                                       ------------------------------------
                                       Name: Donald A. Doering
                                       Title: CFO


                                    COLUMBIA SPACE PARTNERS, INC.


                                    By: /s/ Donald A. Doering
                                       ------------------------------------
                                       Name: Donald A. Doering
                                       Title: CFO


                                    SPECTRUM SPACE EQUITY
                                    INVESTORS IV, INC.


                                    By: /s/ Kevin J. Maroni
                                       ------------------------------------
                                       Name: Kevin J. Maroni
                                       Title: Chairman and CEO


                                    SPECTRUM SPACE IV PARALLEL, INC.


                                    By: /s/ Kevin J. Maroni
                                       ------------------------------------
                                       Name: Kevin J. Maroni
                                       Title: Chairman and CEO


                                    SPECTRUM SPACE IV MANAGERS, INC.


                                    By: /s/ Kevin J. Maroni
                                       ------------------------------------
                                       Name: Kevin J. Maroni
                                       Title: Chairman and CEO