EXHIBIT 10.45


                          REGISTRATION RIGHTS AGREEMENT

         REGISTRATION  RIGHTS AGREEMENT (the "Agreement")  dated  as of June 29,
2000, by and among MOTIENT CORPORATION,  a Delaware corporation (the "Company"),
each stockholder  signatory  hereto (the  "Investors") and each stockholder that
shall  become a party  hereto  pursuant to Section  12(d)  hereof after the date
hereof by executing and delivering a joinder agreement substantially in the form
of  Exhibit  A  hereto  (the  "Joinder  Agreement")  (each,  together  with  the
Investors, a "Stockholder" and, collectively, the "Stockholders").

                              Terms and Conditions

         The  Investors  have  agreed to invest in, and  provide  financing  for
Motient 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 Investment Agreement, dated as of
June  22,  2000,  by and  among  the  Company,  Newco  and  the  Investors  (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 exercise of the Parent Common Stock  Purchase
Option or the  effectuation  of the Parent  Conversions  (each as defined in the
Investment Agreement) pursuant to the Investment Agreement.

         In consideration  of the mutual  covenants and agreements  contained in
this Agreement and the Investment Agreement,  and intending to be legally bound,
the parties hereto agree as follows:

Section 1 . Definitions. As used in this Agreement, the following terms have the
meanings indicated below or in the referenced sections of this Agreement:

         "Common Stock."  The Company's Common Stock,  $.01 par value per share,
as the same may be constituted from time to time.

         "Demand Registration."  As defined in Section 4(a) hereof.

         "Exchange Act."  The Securities  Exchange Act of 1934, as amended,  and
the rules and regulations thereunder.

         "Existing   Registration   Rights   Agreements"   shall  mean  (i)  the
Registration Rights Agreement, as amended and restated on April 19, 1996, by and
among the Company,  Toronto  Dominion  Investments,  Inc.  Morgan Guaranty Trust
Company of New York and Hughes  Communications  Satellite  Services,  Inc.  (the
"Bridge   Registration  Rights  Agreement"),   (ii)  the  Amended  and  Restated
Registration  Rights Agreement,  dated as of March 31, 1998, as amended,  by and
among the Company, Hughes Electronics Corporation,  Singapore Telecommunications
Ltd., and Baron Capital Partners,  L.P. (the "Amended and Restated  Registration
Rights  Agreement"),  (iii) the Registration  Rights Agreement,  dated March 31,
1998, by and between the Company and Motorola,  Inc. (the "Motorola Registration
Rights Agreement"),  and (iv) the Exchange Agreement, dated June 7, 1999, by and
among the Company,  WorldSpace,  Inc. and XM Satellite Radio Holdings,  Inc (the
"Exchange Agreement").

         "NASD."  The National Association of Securities Dealers, Inc.

         "Person."  An  individual,  a  partnership,  a  corporation,  a limited
liability  company or  partnership,  an  association,  a joint stock company,  a
trust, a business trust, a joint venture,  an  unincorporated  organization or a
government entity or any department, agency, or political subdivision thereof.

         "Piggyback Registration."  As defined in Section 5(a) hereof.

         "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,  upon the  conversion or exercise of
the  Parent  Common  Stock  Purchase  Option  or in  connection  with  a  Parent
Conversion (or, in either case, their  transferees) in each case 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.

         "Registration Expenses."  As defined in Section 8(a) hereof.
                                                 ------------

         "SEC."  The United States Securities and Exchange Commission.

          "Securities  Act."  The  Securities  Act of 1933, as amended,  and the
rules and regulations thereunder.

         "Warrant  Registration  Rights  Agreement."   The Warrant  Registration
Rights  Agreement  dated as of March 31,  1998 by and among the  Company,  Bear,
Stearns & Co., Inc., J.P. Morgan Securities Inc., T.D. Securities (USA) Inc. and
BancAmerica Robertson Stephens.

         Capitalized  terms used in this  Agreement but not defined herein shall
have the meaning ascribed to them in the Investment Agreement.

Section 2 . Securities Subject to this Agreement.

(a)  Holders  of  Registrable  Securities.  A Person is deemed to be a holder of
Registrable  Securities whenever that Person owns, directly or beneficially,  or
has  the  right  to  acquire  Registrable  Securities,  disregarding  any  legal
restrictions upon the exercise of that right.

(b) Majority of  Registrable  Securities.  As used in this  Agreement,  the term
"majority of the  Registrable  Securities"  means 51% or more of the Registrable
Securities being registered  unless the context indicates that it is 51% or more
of the Registrable Securities then issued and outstanding.

Section 3         .  S-4 Registration.
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(a) Registration.  At the request of the Investor Group Designee for an Investor
Group exercising its right to a Parent Conversion, the Company shall register on
Form S-4 (or a similar form then in effect) the Registrable  Securities acquired
in such Parent Conversion in connection with such acquisition. In addition, such
registration  statement shall also serve to cover public  reofferings or resales
of such securities (an "S-4  Registration").  The Company shall not be obligated
to effect more than one (1) S-4 Registration per Investor Group.

(b)  Effectiveness.  The  Company  shall  use its  best  efforts  to keep an S-4
Registration  continuously  effective until the earlier of one (1) year from the
consummation of the Parent Conversion to which such S-4 Registration  relates or
at such time when all of the Registrable  Securities registered pursuant thereto
are no longer Registrable Securities.

(c) Suspension; Delay in Filing.  Notwithstanding the foregoing, the Company may
delay in filing a registration statement in connection with an S-4 Registration,
may withhold efforts to cause the registration statement to become effective and
may suspend the effectiveness of the S-4 Registration, if the Company determines
in good  faith  that  such  registration  might  interfere  with or  affect  the
negotiation  or completion of any financing,  acquisition,  disposition or other
transaction or other material event that is being  contemplated in good faith by
the Company  (whether or not a final  decision has been made to  undertake  such
transaction) at the time the right to delay or suspend is exercised (a "Blackout
Period").  The Company  may  exercise  such right to delay,  suspend or withhold
efforts  not more than  twice in any twelve  (12) month  period and for not more
than ninety (90) days at a time.

         (d) Registration  Expenses.   The Company will pay for all Registration
Expenses for one (1) S-4 Registration.

         (e) Withdrawal.  To the extent the Company exercises its right to delay
filing of an S-4  Registration or withhold  efforts to cause an S-4 Registration
to become  effective,  the Company  shall  promptly  notify the  Investor  Group
Designee for the Investor  Group  requesting the S-4  Registration.  At any time
following  such notice,  the Investor Group Designee may withdraw is request for
an S-4  Registration  (whether or not in  connection  with a  withdrawal  of its
exercise of the Parent Conversion in accordance with the Investment  Agreement),
in which  case the S-4  Registration  shall not be deemed to  constitute  an S-4
Registration  and  such  Investor  Group  shall  retain  its  right  to  an  S-4
Registration  under  Section  3(a)  hereof.  In  the  event  an  Investor  Group
consummates  a Parent  Conversion  without the  benefit of an S-4  Registration,
Parent hereby agrees,  at the request of the Investor Group Designee relating to
such  Investor  Group,  to register,  as promptly as  practicable  following the
Blackout  Period,  on Form S-3 (or similar form then in effect) the  Registrable
Securities  held by such Investor Group covering  reofferings or resales of such
securities.  In  such  event,  for  such  Investor  Group,  (i)  the  term  "S-4
Registration" as used in this Agreement shall refer to such S-3 registration and
(ii) the Company  shall not be permitted  to exercise its right to  effectuate a
Blackout  Period during the 120 day period  following the  effectiveness  of the
Form S-3 Registration Statement.

Section 4.  Demand Registration.

(a) Request for Registration.  Subject to the provisions of Section 4(b), at any
time any holders of a majority of the Registrable Securities may demand that the
Company register all or part of its Registrable  Securities under the Securities
Act (a "Demand Registration");  provided,  however, that the Demand Registration
may not be exercised until after the consummation of all 3 Parent Conversions or
the exercise or lapse of the Investor Option. Within ten (10) days after receipt
of a demand,  the Company will notify in writing all Investor Group Designees of
the  demand.  Any  Investor  Group  Designee  who wants to include  any of their
Registrable  Securities of one or more of the Investors in its Investor Group in
the Demand Registration must notify the Company within ten (10) business days of
receiving  the notice of the Demand  Registration.  Except as  provided  in this
Section 4, the Company will include in the Demand  Registration  all Registrable
Securities  for which the  Company  receives  the  timely  written  demands  for
inclusion.  All demands for  inclusion  made  pursuant to this Section 4(a) must
specify  the  name  of  the  Investor  wishing  to be  included  in  the  Demand
Registration,  the number of  Registrable  Securities to be  registered  and the
intended method of disposing of the Registrable Securities.

(b) Number and Form of Registration.  The holders of the Registrable  Securities
shall be limited to one (1) Demand Registration,  which shall be on Form S-1 (or
a similar long-form  registration then in effect), or, if available on Form S-3;
provided,  however, that the Company include in the registration  statement such
Company-related disclosure as is customarily included in underwritten offerings,
or as reasonably  requested by the holders  requesting the Demand  Registration.
The Company will use commercially reasonable efforts to qualify for registration
on Form S-3.  Only Common  Stock may be included in a Demand  Registration.  The
Company shall not be obligated to effect more than one (1) underwritten offering
pursuant to the Demand Registration.

(c) Registration  Expenses.  The Company will pay all Registration  Expenses for
the Demand Registration;  provided,  however, that notwithstanding  Section 8(a)
hereto,  the Company will not be responsible for any printing costs and expenses
as  well  as  any  registration  filing  fees  in  connection  with  the  Demand
Registration;  and provided,  further,  that the Company will not be responsible
for  an  amount  of   Registration   Expenses  in  connection  with  the  Demand
Registration equal to any Registration  Expenses  previously paid by the Company
in connection with the S-4 Registrations pursuant to Section 3(d) hereto.

(d) Selection of Underwriters.  The holders  requesting the Demand  Registration
shall select the investment  banker(s) and manager(s)  that will  administer the
offering;  provided,  that such investment  banker(s) and manager(s) shall be of
recognized  national standing and the Company shall have given its prior written
consent to such  selection  (which  consent shall not be  unreasonably  delayed,
conditioned or withheld).  The Company and the holders of Registrable Securities
whose  shares are being  registered  shall enter into a  customary  underwriting
agreement with such investment banker(s) and manager(s).

(e)  Priority on Demand  Restrictions.  If the  managing  underwriter  gives the
Company and the holders of the Registrable Securities being registered a written
opinion that the number of  Registrable  Securities  requested to be included in
the Demand  Registration  exceeds the number of securities that can be sold, the
registration  will include only the number of  Registrable  Securities  that the
underwriters  believe can be sold. Subject to the priority rights of the holders
of  securities  requesting  registration  pursuant to the Existing  Registration
Rights Agreements, the number of securities registered shall be allocated, first
to the holders requesting the Demand  Registration,  and then pro rata among the
other  holders of  Registrable  Securities,  on the basis of the total number of
Registrable  Securities  requested  to  be  included  in  the  registration.  In
addition,  if the managing  underwriter shall advise the Company,  in writing or
otherwise, that an underwriters'  over-allotment option, not in excess of 15% of
the  total  offering  to be so  effected,  is  necessary  or  desirable  for the
marketing of such offering,  all Registrable Securities which are to be included
in such  offering  pursuant to this Section 4(e) shall be allocated on the basis
of the priority described in the preceding sentence.

(f) Delay in Filing.  Notwithstanding  the  foregoing,  the Company may delay in
filing a registration statement in connection with a Demand Registration and may
withhold efforts to cause the registration statement to become effective, if the
Company  determines in good faith that such registration might interfere with or
affect the negotiation or completion of any financing, acquisition,  disposition
or other transaction or other material event that is being  contemplated in good
faith by the Company (whether or not a final decision has been made to undertake
such  transaction) at the time the right to delay is exercised.  The Company may
exercise  such  right to delay or  withhold  efforts  not more than twice in any
twelve (12) month  period and for not more than ninety (90) days at a time.  If,
after a  registration  statement  becomes  effective,  the  Company  advises the
holders of registered  shares that the Company  considers it appropriate for the
registration  statement to be amended,  the holders of such shares shall suspend
any further sales of their registered shares until the Company advises them that
the registration statement has been amended. The 180-day time period referred to
in Section 7(a)(3) during which the registration  statement must be kept current
after its effective date shall be extended for an additional  number of business
days equal to the number of business  days during which the right to sell shares
was suspended pursuant to the preceding sentence.

(g) Effective Demand Registration.  A registration shall not constitute a Demand
Registration  until it has become effective and remains  continuously  effective
for the  lesser  of (i) the  period  during  which  all  Registrable  Securities
registered in the Demand Registration are sold and (ii) one hundred eighty (180)
days;  provided,  however,  that a  registration  shall not  constitute a Demand
Registration if (x) after such Demand  Registration has become  effective,  such
registration  or  the  related  offer,   sale  or  distribution  of  Registrable
Securities thereunder is interfered with by any stop order,  injunction or other
order or  requirement of the SEC or other  governmental  agency or court for any
reason not attributable to the holders requesting the Demand Registration or the
holders of Registrable Securities requesting to be included in such registration
and such  interference  is not  thereafter  eliminated  within five (5) business
days, or (y) the conditions  specified in the  underwriting  agreement,  if any,
entered into in connection  with such Demand  Registration  are not satisfied or
waived,  other than by reason of a failure on the part of the holders requesting
the Demand Registration, or (z) the number of Registrable Securities sold by the
holders in such  Demand  Registration  is less than fifty  percent  (50%) of the
number  of  Registrable  Securities  requested  to be  included  in such  Demand
Registration.

(h) Limitations on Demand  Registrations.  The Company shall not be obligated to
effect,  or take any action to effect,  a registration  pursuant to Section 4(a)
hereof if, upon  receipt of a  registration  request  pursuant  to Section  4(a)
hereof,  the  Company is advised in  writing by an  investment  banking  firm of
recognized  national  standing  selected  by the  Company  that,  in such firm's
opinion,  a registration  at the time and on the terms requested would adversely
affect any public  offering of securities  of the Company by the Company  (other
than in connection  with benefit and similar plans) (a "Company  Offering") with
respect to which the Company has commenced  preparations  for a registration and
filed or intends to imminently  file a registration  statement  relating to such
registration prior to the receipt of a registration  request pursuant to Section
4(a)  hereof,  until the  earlier  of (i) 90 days after the  completion  of such
Company  Offering,  or (ii)  promptly  after  any  abandonment  of such  Company
Offering.

Section 5. Piggyback Registrations.

(a) Right to Piggyback.  Whenever the Company proposes to register (including on
behalf of a selling  stockholder) any of its securities under the Securities Act
(except for the registration of securities to be offered pursuant to an employee
benefit plan on Form S-8 or Form S-3,  pursuant to a  registration  made on Form
S-4 or any successor  forms then in effect) at any time other than pursuant to a
Demand  Registration  and the  registration  form to be used may be used for the
registration of the Registrable Securities (a "Piggyback Registration"), it will
so notify in writing all Investor  Group  Designees no later than the earlier to
occur of (i) the tenth (10th) day following  the Company's  receipt of notice of
exercise of other demand registration  rights, or (ii) thirty (30) days prior to
the  anticipated  filing date.  Subject to the  provisions of Section 5(c),  the
Company will use its commercially reasonable efforts to include in the Piggyback
Registration  all  Registrable  Securities,  on a pro rata basis  based upon the
total  number of  Registrable  Securities  with respect to which the Company has
received  written  requests  for  inclusion  within  fifteen (15) days after the
Investor Group  Designees'  receipt of the Company's  notice.  A registration of
Registrable  Securities  pursuant  to this  Section 5 shall not be  counted as a
Demand Registration under Section 4.

(b) Piggyback Expenses.  The Company shall pay to the holders of the Registrable
Securities  included in a Piggyback  Registration all  Registration  Expenses of
those holders (except to the extent  prohibited by applicable  state  securities
laws).

(c) Priority on Piggyback Registrations. If the managing underwriter advises the
Company  in  writing  that the  total  number or  dollar  amount  of  securities
requested to be included in the registration exceeds the number or dollar amount
of securities  that can be sold,  the Company will include the securities in the
registration in the following order of priority,  subject to the priority rights
of the holders of securities  requesting  registration  pursuant to the Existing
Registration  Rights  Agreements:  (i) first,  all securities the Company or the
holder for whom the Company is effecting the  registration,  as the case may be,
proposes  to sell;  (ii)  second,  up to the full  number  or  dollar  amount of
Registrable  Securities requested to be included in the registration  (allocated
pro rata among the holders of Registrable Securities, on the basis of the number
of  Registrable  Securities  requested to be included,  as the case may be); and
(iii) third,  any other  securities  (provided they are of the same class as the
securities  sold by the Company)  requested to be included,  allocated among the
holders of such securities in such  proportions as the Company and those holders
may agree. In the event that the managing  underwriter  advises the Company that
an underwriters'  over-allotment option is necessary or advisable, the preceding
priority shall apply.

(d) Selection of Underwriters.  If any Piggyback Registration is an underwritten
offering,  the Company will select the investment  banker(s) and manager(s) that
will  administer  the  offering.  The  Company  and the  holders of  Registrable
Securities  whose  shares are being  registered  shall  enter  into a  customary
underwriting agreement with such investment banker(s) and manager(s).

Section 6. Holdback Agreements.

(a)  Restrictions  on Public Sale by Securities  Holders.  Each  Stockholder (i)
agrees not to make any public sale or distribution  of equity  securities of the
Company (except as part of the underwritten  registration effected pursuant to a
Demand Registration or a Piggyback  Registration),  including a sale pursuant to
Rule  144,during  the  period  beginning  seven (7) days  prior to and ending on
ninety  (90)  days  following  the  effective  date of any  underwritten  Demand
Registration  or  any  underwritten   Piggyback  Registration  as  any  managing
underwriter(s) of such underwriting may reasonably request,  and (ii) agrees not
to make any public sale or distribution of equity securities of the Company,  in
each  case  including  a sale  pursuant  to Rule  144,  during  (A) the  periods
described in Section 5.3(b)(ii) of the Bridge Registration Rights Agreement, (B)
the  periods  described  in  Section  5.3(b)(ii)  of the  Amended  and  Restated
Registration Rights Agreement,  and (C) the period described in Section 3(b)(ii)
of the Motorola Registration Rights Agreement, provided that such agreements are
in full force and effect.

(b)  Restrictions  on Public Sale by the Company and Others.  The Company agrees
not to make any public sale or  distribution  of its equity  securities,  or any
securities  convertible  into or  exchangeable  or  exercisable  for its  equity
securities (except as part of the underwritten registration effected pursuant to
a Demand  Registration or a Piggyback  Registration or pursuant to registrations
on Forms S-8 or S-4 or any  successor  form or on Form S-3 with  respect  to any
employee  benefit plans of the Company),  during such customary  period prior to
and following the effective date of any underwritten  Demand Registration or any
underwritten  Piggyback  Registration  as any  managing  underwriter(s)  of such
underwriting may reasonably  request.  The Company also agrees to use reasonable
efforts to cause each holder of at least 10% (on a  fully-diluted  basis) of its
equity  securities  (other  than  Registrable   Securities)  or  any  securities
convertible into or exchangeable or exercisable for its equity securities (other
than Registrable Securities), purchased from the Company at any time on or after
the date of this  Agreement  (other than in a registered  public  offering),  to
agree not to make any public sale or distribution of those securities, including
a sale pursuant to Rule 144 (except as part of the underwritten registration, if
permitted),  during the seven (7) days  prior to and the ninety  (90) days after
the effective date of the registration unless the managing underwriter(s) agrees
otherwise.

Section 7. Registration Procedures.

(a) Obligations of the Company.  Whenever the holders of Registrable  Securities
request  or are  entitled  to the  registration  of any  Registrable  Securities
pursuant to this Agreement,  the Company shall use its  commercially  reasonable
efforts to  register  and to permit the sale of the  Registrable  Securities  in
accordance  with  the  intended  method  of  disposition.   To  carry  out  this
obligation, the Company shall as expeditiously as practicable:

(1)      prepare   and  file   with  the  SEC  a   registration   statement   or
         post-effective  amendment to a registration statement, as necessary, on
         the appropriate form and use commercially  reasonable  efforts to cause
         the registration statement to become effective.  At least ten (10) days
         before filing a registration  statement or prospectus or at least three
         (3) business days before filing any amendments or supplements  thereto,
         the Company will furnish to the counsel of the holders of a majority of
         the Registrable  Securities  being  registered  copies of all documents
         proposed  to be filed for that  counsel's  review and  approval,  which
         approval shall not be unreasonably withheld or delayed;

(2)      immediately  notify  each  Investor  Group  Designee  of any stop order
         threatened  or  issued  by the  SEC and  take  all  actions  reasonably
         required  to prevent the entry of a stop order or if entered to have it
         rescinded or otherwise removed;

(3)      prepare and file with the SEC such  amendments  and  supplements to the
         registration  statement and the corresponding  prospectus  necessary to
         keep the registration  statement  effective for such period required by
         Section 3(b) or Section 4(g) or such shorter  period as may be required
         to  sell  all  Registrable   Securities  covered  by  the  registration
         statement;  and comply with the  provisions of the  Securities Act with
         respect  to  the   disposition  of  all   securities   covered  by  the
         registration  statement  during  each  period  in  accordance  with the
         sellers'   intended   methods  of  disposition  as  set  forth  in  the
         registration statement;

(4)      furnish to each Investor Group  Designee a sufficient  number of copies
         of the registration  statement,  each amendment and supplement  thereto
         (in each case including all  exhibits),  the  corresponding  prospectus
         (including each preliminary prospectus), and such other documents as an
         Investor  Group  Designee  may  reasonably  request to  facilitate  the
         disposition of the Registrable Securities held by the Investors in such
         Investor Group;

(5)      use its  commercially  reasonable  efforts to  register  or qualify the
         Registrable   Securities   under   securities   or  blue  sky  laws  of
         jurisdictions  in the United  States of America as any  Investor  Group
         Designee  requests and do any and all other  reasonable acts and things
         that  may  be  necessary  or  advisable  to  enable  the  Investors  to
         consummate the  disposition of the  Registrable  Securities held by the
         Investors  in  such  Investor  Group  in such  jurisdiction;  provided,
         however,  that the  Company  shall not be  obligated  to  qualify  as a
         foreign  corporation to do business under the laws of any  jurisdiction
         in which it is not then  qualified  or to file any  general  consent to
         service of process;

(6)      notify each Investor Group  Designee,  at any time when a prospectus is
         required to be delivered  under the  Securities  Act, of any event as a
         result of which the prospectus or any document  incorporated therein by
         reference  contains an untrue  statement of a material fact or omits to
         state any material fact  necessary to make the  statements  therein not
         misleading in light of the  circumstances  under which such  statements
         were made,  and  promptly  prepare a  supplement  or  amendment  to the
         prospectus or any such document incorporated therein so that thereafter
         the prospectus will not contain an untrue  statement of a material fact
         or omit to state any material  fact  necessary  to make the  statements
         therein not misleading in light of the  circumstances  under which such
         statements were made;

(7)      cause  all  registered  Registrable  Securities  to be  listed  on each
         securities exchange,  if any, on which similar securities issued by the
         Company are then listed;

(8)      provide  an  institutional  transfer  agent and  registrar  and a CUSIP
         number for all  Registrable  Securities on or before the effective date
         of the registration statement;

(9)      enter  into  such  customary  agreements   (including  an  underwriting
         agreement in customary  form) and take all other  actions in connection
         with  those   agreements  as  the  Investor   Group   Designee  or  the
         underwriters,  if any, reasonably request to expedite or facilitate the
         disposition   of  the   Registrable   Securities   including,   without
         limitation,  the participation of senior management in "road shows" and
         similar activities, provided that such activities do not interfere with
         the  duties of  senior  management  in a manner  that  would  likely be
         detrimental to the best interests of the Company;

(10)     make  available for  inspection  by any Investor  Group  Designee,  any
         underwriter   participating   in  any   disposition   pursuant  to  the
         registration statement, and any attorney, accountant, or other agent of
         any Investor Group Designee or  underwriter,  all reasonable  financial
         and other records, pertinent corporate documents, and properties of the
         Company,  and cause the Company's officers,  directors and employees to
         supply all  information  reasonably  requested  by any  Investor  Group
         Designee,   underwriter,   attorney,  accountant,  or  other  agent  in
         connection  with  the  registration   statement;   provided,   that  an
         appropriate and customary  confidentiality agreement is executed by any
         such Investor  Group  Designee,  underwriter,  attorney,  accountant or
         other agent;

(11)     in connection with any underwritten offering, obtain a "comfort" letter
         from the Company's independent public accountants in customary form and
         covering those matters  customarily covered by "comfort" letters as the
         Investor Group Designee or the managing underwriter reasonably requests
         (and,  if the  Company  is able  after  using  commercially  reasonable
         efforts,  the letter shall be  addressed to holders of the  Registrable
         Securities, the Company and the underwriters);

(12)     in connection with any underwritten  offering,  furnish, at the request
         of any Investor Group Designee or  underwriter(s)  of the offering,  an
         opinion of counsel  representing  the Company  for the  purposes of the
         registration,   in  the  form  and  substance   customarily   given  to
         underwriters  in  an   underwritten   public  offering  and  reasonably
         satisfactory  to counsel  representing  the Investor Group Designee and
         the  underwriter(s) of the offering,  addressed to the underwriters and
         to the holders of the Registrable Securities being registered;

(13)     use  its  best   efforts  to  comply  with  all  applicable  rules  and
         regulations of the SEC;

(14)     cooperate  with  each  Investor  Group  Designee  and each  underwriter
         participating  in the  disposition of such  Registrable  Securities and
         their respective  counsel in connection with any filings required to be
         made with the NASD; and

(15)     take all other steps reasonably necessary to effect the registration of
         the Registrable Securities contemplated hereby.

(b) Seller  Information.  In the event of any registration by the Company,  from
time to time, the Company may require each Investor Group Designee to furnish to
the Company information regarding each seller of Registrable  Securities subject
to the  registration  and the  distribution  of the  securities  subject  to the
registration,   and  such  Investor   Group  Designee  shall  furnish  all  such
information requested by the Company.

(c) Notice to  Discontinue.  Each  holder of  Registrable  Securities  agrees by
acquisition of such securities that, upon receipt of any notice from the Company
to the Investor  Group  Designees of any event of the kind  described in Section
7(a)(6), the holder will discontinue disposition of Registrable Securities until
the Investor  Group  Designee  receives  copies of the  supplemented  or amended
prospectus  contemplated  by  Section  7(a)(6).  In  addition,  if  the  Company
requests,  each Investor Group Designee will cause each holder to deliver to the
Company (at the Company's expense) all copies,  other than permanent file copies
then in the holder's  possession,  of the  prospectus  covering the  Registrable
Securities  current at the time of receipt of the notice.  If the Company  gives
any such notice,  the time period mentioned in Section 7(a)(3) shall be extended
by the number of days elapsing between the date of notice and the date that each
seller  receives  the  copies  of  the   supplemented   or  amended   prospectus
contemplated in Section 7(a)(6).

(d) Notice by Holders.  Whenever  the  holders of  Registrable  Securities  have
requested  that  any  Registrable  Securities  be  registered  pursuant  to this
Agreement, those holders shall notify the Company, at any time when a prospectus
relating  thereto is required to be delivered  under the Securities  Act, of the
happening of any event, which as to any holder of Registrable  Securities is (i)
to his or its  respective  knowledge,  (ii) solely within his or its  respective
knowledge  and  (iii)  solely  as to  matters  concerning  that  holder  of  the
Registrable  Securities,  as a result of which the  prospectus  included  in the
registration  statement contains an untrue statement of a material fact or omits
to state any material fact necessary to make the statements therein, in light of
the circumstances under which they were made, not misleading.

Section 8. Registration Expenses.

(a) Generally.  All Registration  Expenses incident to the Company's performance
of or  compliance  with  this  Agreement  shall  be  paid  as  provided  in this
Agreement.  The term "Registration Expenses" includes,  without limitation,  all
registration  filing fees,  reasonable  professional  fees and other  reasonable
expenses of the Company's  compliance with federal,  state and other  securities
laws  (including  fees and  disbursements  of counsel  for the  underwriters  in
connection with state or other securities law qualifications and registrations),
printing expenses,  messenger,  telephone and delivery expenses; reasonable fees
and  disbursements of counsel for the Company;  reasonable fees and disbursement
of all independent  certified public accountants  (including the expenses of any
audit or  "comfort"  letters  required  by or  incident  to  performance  of the
obligations   contemplated  by  this  Agreement);   fees  and  expenses  of  the
underwriters  (excluding  discounts and  commissions);  fees and expenses of any
special  experts  retained  by the  Company  at  the  request  of  the  managing
underwriters in connection with the registration;  and applicable stock exchange
and NASD registration and filing fees. The term "Registration Expenses" does not
include the Company's  internal expenses  (including,  without  limitation,  all
salaries  and  expenses  of its  officers  and  employees  performing  legal  or
accounting  duties),  the expense of any annual  audit and the fees and expenses
incurred in  connection  with the listing of the  securities to be registered on
each securities  exchange on which similar  securities issued by the Company are
then  listed,  all of which  shall be paid by the  Company,  nor does it include
underwriting fees, discounts or selling commissions,  brokerage fees or transfer
taxes, or the fees and  disbursements  or counsel for the sellers,  all of which
shall be paid by the sellers of Registrable Securities.

(b)  Other  Expenses.  To  the  extent  the  Company  is  not  required  to  pay
Registration  Expenses,  each holder of securities  included in any registration
will pay those  Registration  Expenses  allocable to the holder's  securities so
included,  and any  Registration  Expenses  not  allocable  will be borne by all
sellers in proportion to the number of securities each  registers.  Furthermore,
the Stockholders hereby agree amongst themselves that any Registration  Expenses
or other cost and expenses in connection with an S-4  Registration or the Demand
Registration  not  required  to be paid by the  Company  that  are  borne by the
holders of the Registrable  Securities in accordance with the previous sentence,
shall be shared pro rata among the Investor  Groups that are  exercising or have
previously  exercised  their  right to a Parent  Conversion,  including,  to the
extent  necessary,  subsequent  reimbursement  by Investors who have  previously
exercised a Parent  Conversion  and requested an S-4  Registration  to Investors
then exercising a Parent Conversion and requesting an S-4 Registration.

Section 9. Indemnification.

(a)  Indemnification by Company. In the event of any registration of Registrable
Securities  under the  Securities  Act pursuant to this  Agreement,  to the full
extent  permitted  by law,  the  Company  agrees  to  indemnify  each  holder of
Registrable Securities, its officers, directors,  trustees, partners, employees,
advisors and agents, and each Person who controls the holder (within the meaning
of the Securities Act and the Exchange Act) against all losses, claims, damages,
liabilities and expenses caused by any untrue or allegedly  untrue  statement of
material  fact  contained  in  any  registration   statement  under  which  such
Registrable  Securities were registered under the Securities Act, any prospectus
or preliminary  prospectus contained therein or any omission or alleged omission
to state a material fact required to be stated  therein or necessary to make the
statements therein not misleading in light of the circumstances under which such
statements  were  made,  except to the extent  the  untrue or  allegedly  untrue
statement or omission or alleged  omission  resulted from  information  that the
holder  furnished  in writing  to the  Company  expressly  for use  therein.  In
connection  with a firm or best  efforts  underwritten  offering,  to the extent
customarily required by the managing underwriter, the Company will indemnify the
underwriters,  their  officers  and  directors  and each Person who controls the
underwriters (within the meaning of the Securities Act and the Exchange Act), to
the extent customary in such agreements.

(b)   Indemnification   by  Holders  of  Securities.   In  connection  with  any
registration statement, each participating holder of Registrable Securities will
furnish to the Company in writing such information and affidavits as the Company
reasonably  requests for use in connection  with any  registration  statement or
prospectus and each holder agrees to indemnify,  to the extent permitted by law,
the Company, its directors,  officers, trustees, partners,  employees,  advisors
and agents,  and each Person who controls the Company (within the meaning of the
Securities  Act and the  Exchange  Act)  against  any losses,  claims,  damages,
liabilities and expenses resulting from any untrue or allegedly untrue statement
of a material fact or any omission or alleged  omission to state a material fact
required  to be  stated  in the  registration  statement  or  prospectus  or any
amendment thereof or supplement thereto necessary to make the statements therein
not misleading in light of the  circumstances  under which such  statements were
made,  but only to the extent that the untrue or allegedly  untrue  statement or
omission or alleged  omission is contained in or omitted from any information or
affidavit  the holder  furnished  in writing to the  Company  expressly  for use
therein and only in an amount not  exceeding  the net  proceeds  received by the
holder with respect to securities sold pursuant to such registration  statement.
In connection with a firm or best efforts underwritten  offering,  to the extent
customarily required by the managing  underwriter,  each participating holder of
Registrable  Securities  will  indemnify the  underwriters,  their  officers and
directors and each Person who controls the  underwriters  (within the meaning of
the  Securities  Act and the  Exchange  Act),  to the extent  customary  in such
agreements.

(c) Indemnification  Proceedings.  Any Person entitled to indemnification  under
this  Agreement  will (i) give prompt  notice to the  indemnifying  party of any
claim with  respect  to which it seeks  indemnification  and (ii)  unless in the
indemnified party's reasonable judgment a conflict of interest may exist between
the indemnified and indemnifying  parties with respect to the claim,  permit the
indemnifying  party to assume the defense of the claim with  counsel  reasonably
satisfactory to the indemnified party. If the indemnifying party does not assume
the defense,  the indemnifying  party will not be liable for any settlement made
without its consent  (but that  consent may not be  unreasonably  withheld).  No
indemnifying  party will consent to entry of any judgment or will enter into any
settlement that does not include as an unconditional term thereof the claimant's
or plaintiff's  release of the indemnified  party from all liability  concerning
the claim or litigation.  An indemnifying party who is not entitled to or elects
not to assume the defense of a claim will not be under an  obligation to pay the
fees and  expenses of more than one counsel for all parties  indemnified  by the
indemnifying party with respect to the claim,  unless in the reasonable judgment
of  any  indemnified  party  a  conflict  of  interest  may  exist  between  the
indemnified  party and any other indemnified party with respect to the claim, in
which  event  the  indemnifying  party  shall be  obligated  to pay the fees and
expenses of no more than one additional counsel for the indemnified parties.

(d) Contribution.  If the indemnification provided for in Section 9(a) or (b) is
unavailable to an indemnified party in respect of any losses,  claims,  damages,
liabilities  or  expenses  referred  to therein,  then each  indemnifying  party
thereunder  shall  contribute to the amount paid or payable by such  indemnified
party as a result of such losses,  claims,  damages,  liabilities or expenses in
such  proportion as is  appropriate to reflect the relative fault of the Company
and the participating  holders of Registrable  Securities in connection with the
statements  or  omissions  that  resulted  in  such  losses,  claims,   damages,
liabilities or expenses, as well as any other relevant equitable considerations.
The relative fault of the Company and the  participating  holders of Registrable
Securities shall be determined by reference to, among other things,  whether the
untrue or alleged untrue statement of a material fact or the omission or alleged
omission to state a material fact relates to information supplied by the Company
or by the  participating  holders of  Registrable  Securities  and the  parties'
relative intent and knowledge.

         The parties  hereto  agree that it would not be just and  equitable  if
contribution  pursuant this Section 9(d) were  determined by pro rata allocation
or by any other method of allocation that does not take account of the equitable
considerations   referred   to   in   the   immediately   preceding   paragraph.
Notwithstanding  anything  herein to the contrary,  no  participating  holder of
Registrable  Securities  shall be required to contribute any amount in excess of
the amount by which the net proceeds of the offering (before deducting expenses,
if any) received by such participating  holder exceeds the amount of any damages
that such  participating  holder has otherwise been required to pay by reason of
such untrue or alleged  untrue  statement  or omission or alleged  omission.  No
Person  guilty of  fraudulent  misrepresentation  (within the meaning of Section
11(f) of the Securities Act) shall be entitled to  contribution  from any Person
who was not guilty of such fraudulent misrepresentation.

Section  10 . Rule 144.  The  Company  covenants  that it will file the  reports
required to be filed by it under the Securities Act and the Exchange Act and the
rules  and  regulations  adopted  by the SEC  thereunder,  and it will take such
further action as any Investor Group Designee reasonably may request, all to the
extent  required  from  time to time,  to enable  such  holders  of  Registrable
Securities to sell such Registrable  Securities  without  registration under the
Securities Act within the limitation of the exemptions  provided by (i) Rule 144
under the  Securities  Act, or (ii) any  similar  rule or  regulation  hereafter
adopted by the SEC.  Upon the request of any holder of  Registrable  Securities,
the Company will deliver to such holder a written statement as to whether it has
complied with Rule 144's or any successor rule's requirements.  The Company also
covenants  that in such event it will provide all such  information  and it will
take such further action as any holder of Registrable  Securities reasonably may
request  to  enable  such  holder  to  sell   Registrable   Securities   without
registration  under the  Securities  Act within the limitation of Rule 144 under
the Securities Act or any successor rule requirements.

Section  11  .  Participation  in  Underwritten  Registration.   No  Person  may
participate  in any  underwritten  registration  without  (a)  agreeing  to sell
securities on the basis provided in  underwriting  arrangements  approved by the
Persons  entitled  hereunder  to approve such  arrangements  (the holders of the
Registrable Securities in a Demand Registration pursuant to Section 4(d) and the
Company  in  a  Piggyback  Registration  pursuant  to  Section  5(d)),  and  (b)
completing and executing all  questionnaires,  powers of attorney,  indemnities,
underwriting  agreements  and  other  documents  required  by  the  underwriting
arrangements.

Section 12. Miscellaneous.

(a)  Recapitalizations,  Exchanges,  etc. The provisions of this Agreement shall
apply to the full  extent  set forth  herein  with  respect to (i) the shares of
Common Stock held by the Stockholders and acquired  pursuant to the transactions
contemplated by the Investment  Agreement (or their  transferees),  (ii) any and
all shares of voting  common  stock of the Company into which the shares of such
Common Stock are converted,  exchanged or substituted in any recapitalization or
other  capital  reorganization  by the  Company  and  (iii)  any and all  equity
securities of the Company or any successor or assign of the Company  (whether by
merger,  consolidation,  sale of  assets  or  otherwise)  which may be issued in
respect of, in conversion of, in exchange for or in substitution of, such shares
of Common Stock and shall be  appropriately  adjusted  for any stock  dividends,
splits, reverse splits,  combinations,  recapitalizations and the like occurring
after the date hereof. The Company shall use its commercially reasonable efforts
to cause any successor or assign (whether by sale, merger or otherwise) to enter
into a new  registration  rights  agreement  with  the  holders  of  Registrable
Securities on terms  substantially  the same as this Agreement as a condition of
any such transaction.

(b)  Amendment.  This  Agreement  may be amended or  modified  only by a written
agreement executed by the Company and the Stockholders.

(c)  Attorneys'  Fees. In any legal action or proceeding  brought to enforce any
provision of this Agreement,  the prevailing  party shall be entitled to recover
all reasonable expenses, charges, court costs and attorneys' fees in addition to
any other available remedy at law or in equity.

(d)  Benefit of Parties;  Assignment.  All of the terms and  provisions  of this
Agreement  shall be binding on and inure to the benefit of the parties and their
respective successors and assigns, including,  without limitation, any Person to
whom the Investors may transfer any of its Registrable Securities (or any rights
to acquire  Registrable  Securities);  provided  any such  successor or assignee
shall  agree in  writing  to  become  bound by the  terms of this  Agreement  by
executing and delivering a Joinder Agreement.

(e) Captions. The captions of the sections and subsections of this Agreement are
solely for convenient reference and shall not be deemed to affect the meaning or
interpretation of any provision of this Agreement.

(f)  Cooperation.  The parties agree that after execution of this Agreement they
will from time to time,  upon the request of any other party and without further
consideration,  execute,  acknowledge  and  deliver in proper  form any  further
instruments and take such other action as any other party may reasonably require
to carry out effectively the intent of this Agreement.

(g)  Counterparts;   Facsimile   Execution.   This  Agreement  may  be  executed
simultaneously  in two or more  counterparts,  each of which  shall be deemed an
original, but all of which together shall constitute one and the same agreement.
Facsimile  execution and delivery of this  Agreement  shall be legal,  valid and
binding execution and delivery for all purposes.

(h) Entire Agreement.  This Agreement  contains the entire  understanding of the
parties with respect to the subject  matter of this Agreement and supersedes all
prior  agreements and  understandings  between the parties with respect thereto.
There are no promises,  covenants or undertakings other than those expressly set
forth or provided for in this Agreement.

(i) Governing Law. The internal law of the  Commonwealth of Virginia will govern
the  interpretation,  construction,  and  enforcement  of this Agreement and all
transactions and agreements  contemplated  hereby,  notwithstanding  any state's
choice of law rules to the contrary.

(j) No Inconsistent Agreements.  Except with respect to the rights granted under
the Existing  Registration Rights Agreements and the Warrant Registration Rights
Agreement,  the Company  represents  and warrants that it has not granted to any
Person the right to request or require the Company to  register  any  securities
issued by the Company other than the rights  contained  herein.  Except with the
prior written consent of the holders of Registrable Securities, the Company will
not enter into any agreement with respect to its securities  that shall grant to
any Person  registration  rights that in any way  conflict  with or are prior in
right to the rights provided under this Agreement. The parties to this Agreement
acknowledge  and agree that it is their intent that this Agreement be consistent
with the  terms of the  Existing  Registration  Rights  Agreements  and that the
Company  have no  obligation  under  this  Agreement  which  conflicts  with its
obligations  under the Existing  Registration  Rights  Agreements.  To that end,
notwithstanding  anything to the contrary  contained in this  Agreement,  to the
extent that any of the  provisions  of this  Agreement  conflict with any of the
provisions of the Existing Registration Rights Agreements, the Company shall not
be obligated to comply with its obligations under the conflicting  provisions of
this  Agreement  until it can do so  without  violating  the  provisions  of the
Existing Registration Rights Agreements.

(k) Notices.  All notices,  requests,  demands, or other communications that are
required  or may be given  pursuant to the terms of this  Agreement  shall be in
writing and delivery shall be deemed sufficient in all respects and to have been
duly given on the date of service if delivered  personally  to the party to whom
notice is to be given,  or upon  receipt if mailed by first class  mail,  return
receipt requested,  postage prepaid,  and properly addressed to the addresses of
the parties set forth on the signature pages hereto or in the Joinder  Agreement
or to such other address(es) as the respective parties hereto shall from time to
time designate to the other(s) in writing.

(l) Specific  Performance.  Each of the parties agrees that damages for a breach
of or default under this  Agreement  would be inadequate and that in addition to
all other  remedies  available  at law or in equity  that the  parties and their
successors  and assigns shall be entitled to specific  performance or injunctive
relief,  or both,  in the  event  of a breach  or a  threatened  breach  of this
Agreement.

(m) Validity of Provisions.  Should any part of this Agreement for any reason be
declared by any court of competent  jurisdiction  to be invalid,  that  decision
shall not affect the validity of the remaining portion,  which shall continue in
full force and effect as if this  Agreement  had been  executed with the invalid
portion  eliminated,  it being the  intent of the  parties  that they would have
executed the remaining  portion of the Agreement  without  including any part or
portion that may for any reason be declared invalid.

                  [remainder of page intentionally left blank]




                            SEPARATE SIGNATURE PAGE
                         REGISTRATION RIGHTS AGREEMENT


         IN WITNESS WHEREOF,  the parties hereto have executed this Agreement as
of the date first above written.

                                     MOTIENT CORPORATION

                                     By:  /s/Gary M. Parsons
                                          Name:  Gary M. Parsons
                                          Title: Chairman

                                          Address for Notices:

                                          Motient Corporation
                                          10802 Parkridge Boulevard
                                          Reston, Virginia  20191-5416
                                          Fax:  (703) 758-6134
                                          Attention:   Randy S. Segal, Esq.
                                                       Senior Vice President and
                                                       General Counsel




                            SEPARATE SIGNATURE PAGE
                         REGISTRATION RIGHTS AGREEMENT


     IN WITNESS WHEREOF, the parties hereto have duly executed this Agreement as
of the date first above written.

                                      INVESTOR GROUP 1:

                                      TELCOM SATELLITE VENTURES INC.

                                      By:  /s/Rahul Prakash
                                           Name:  Rahul Prakash
                                           Title: President

                                      Address for Notices to the Investors in
                                      Investor Group 1:

                                      Telecom Satellite Ventures Inc.
                                      c/o Telcom Ventures, L.L.C.
                                      211 North Union Street
                                      Suite 300
                                      Alexandria, Virginia 22314
                                      Fax:  (703)706-3801
                                      Attention:  Hal Perkins, Esq.
                                                  General Counsel



                            SEPARATE SIGNATURE PAGE
                         REGISTRATION RIGHTS AGREEMENT


     IN WITNESS WHEREOF, the parties hereto have duly executed this Agreement as
of the date first above written.

                                      INVESTOR GROUP 2:

                                      COLUMBIA SPACE (QP), INC.

                                      By:  /s/James B. Fleming
                                           Name:  James B. Fleming
                                           Title: President


                                      COLUMBIA SPACE (A1), INC.

                                      By:  /s/James B. Fleming
                                           Name:  James B. Fleming
                                           Title: President


                                      COLUMBIA SPACE PARTNERS, INC.

                                      By:  /s/James B. Fleming
                                           Name:  James B. Fleming
                                           Title: President




                            SEPARATE SIGNATURE PAGE
                         REGISTRATION RIGHTS AGREEMENT

                                      Address for Notices to the
                                      Investors in Investor Group 2:

                                      Columbia Space (QP), Inc.
                                      211 North Union Street
                                      Suite 300
                                      Alexandria, Virginia 22314
                                      Attention:  James Fleming
                                      Tel:  (703)519-3000
                                      Fax:  (703)519-3028

                                      with a copy to:
                                      Edwards & Angell, LLP
                                      101 Federal Street
                                      Boston, MA 02100
                                      Attention:  Stephen Meredith, Esq.
                                      Tel. (617)951-2233
                                      Fax: (888)325-9120



                            SEPARATE SIGNATURE PAGE
                         REGISTRATION RIGHTS AGREEMENT


     IN WITNESS WHEREOF, the parties hereto have duly executed this Agreement as
of the date first above written.

                                      INVESTOR GROUP 3:

                                      SPECTRUM SPACE EQUITY
                                      INVESTORS IV, INC.

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

                                      SPECTRUM SPACE IV PARALLELL, 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


                            SEPARATE SIGNATURE PAGE
                         REGISTRATION RIGHTS AGREEMENT


                                      Address for Notices to the Investors in
                                      Investor Group 3:

                                      Spectrum Space Equity Investors IV, Inc.
                                      One International Place
                                      29th Floor
                                      Boston, MA 82110
                                      Attention:  Kevin Maroni
                                      Tel:  (617)464-4600
                                      Fax:  (617)464-4601

                                      with a copy to:
                                      Edwards & Angell, LLP
                                      101 Federal Street
                                      Boston, MA 02100
                                      Attention:  Stephen Meredith, Esq.
                                      Tel:  (617)951-2233
                                      Fax:  (888)325-9120



                                                                      EXHIBIT A

                            FORM OF JOINDER AGREEMENT

                                JOINDER AGREEMENT

         This Joinder  Agreement dated as of  ___________,  200_ is executed and
delivered  pursuant to Section 11(d) of the Registration  Rights Agreement dated
as of June ___, 2000 (the "Registration  Rights Agreement") by and among Motient
Corporation   (the   "Company")   and  the   stockholders   party  thereto  (the
"Stockholders").  Capitalized  terms used by not defined  herein  shall have the
respective meanings assigned to such terms in the Registration Rights Agreement.

         The undersigned,  _______________  (the "New Stockholder") has acquired
shares of Common Stock of the Company and therefore desires to become a party to
the  Registration  Rights  Agreement on the terms set forth in the  Registration
Rights Agreement. Accordingly, the New Stockholder hereby agrees as follows:

         1.       By its signature below,  the New Stockholder  shall be a party
to the  Registration  Rights  Agreement  fully and completely the same as if its
signature were affixed to the Registration Rights Agreement.

         2.       The  New Stockholder is a Stockholder  under the  Registration
Rights Agreement with all the rights of obligations of a Stockholder thereunder.

         3.       This  Joinder  Agreement shall be governed by and construed in
accordance with the laws of the Commonwealth of Virginia.





         IN WITNESS WHEREOF,  the undersigned has caused this Joinder  Agreement
to be executed and delivered as of the date first above written.

                                     [NEW STOCKHOLDER]



                                     By:_______________________________
                                     Name:
                                     Title:

                                                     Address for Notices:
                                                     =================
                                     Fax:

                                     Attention:

ACCEPTED:

MOTIENT CORPORATION


By: _______________________________
    Name:
    Title: