PROSPECTUS SUPPLEMENT DATED SEPTEMBER 23, 1998
TO PROSPECTUS DATED MARCH 12, 1998

ORBITAL SCIENCES CORPORATION
5% CONVERTIBLE SUBORDINATED NOTES DUE 2002
SELLING SECURITYHOLDERS

This Prospectus Supplement should be read in conjunction with the
Prospectus dated March 12, 1998 (the "Prospectus"). The information on pages 30,
31 and 32 of the Prospectus setting forth information concerning the Selling
Securityholders is superceded by the following table, which revised table is
based upon information provided by or on behalf of such Selling Securityholders
as of September 18, 1998, unless otherwise indicated.

COMMON
PRINCIPAL PRINCIPAL STOCK
AMOUNT OF AMOUNT OF NOTES OWNED PRIOR COMMON STOCK
NAME OF SELLING SECURITYHOLDER NOTES OWNED OFFERED HEREBY TO OFFERING(1) OFFERED HEREBY(2)
------------------------------ ----------- --------------- -------------- -----------------

Fidelity Financial Trust:
Fidelity Convertible Securities Fund... $ 12,200,000 $ 12,200,000 438,814 435,714
General Motors Employees Domestic Group
Trust.................................. 3,150,000 3,150,000 112,500 112,500
Deutsche Bank Securities Inc.(3)......... 3,000,000 3,000,000 107,142 107,142
SMALLCAP World Fund, Inc. ............... 2,500,000 2,500,000 89,285 89,285
Fuji US Income Open...................... 2,000,000 2,000,000 71,428 71,428
Lipper Offshore Convertibles, L.P. ...... 2,000,000 2,000,000 71,428 71,428
Lipper Convertibles, L.P. ............... 1,500,000 1,500,000 53,571 53,571
Froley, Revy Investment Co., Inc.
Account: Arkansas PERS................. 1,350,000 1,350,000 48,214 48,214
Equitable Life Assurance Separate Account
Convertibles........................... 1,305,000 1,305,000 46,607 46,607
Alexandra Global Investment Fund 1
Ltd. .................................. 1,300,000 1,300,000 46,428 46,428
Froley, Revy Investment Co., Inc.
Account: Delaware PERS................. 1,060,000 1,060,000 37,857 37,857
Allstate Insurance Company............... 1,000,000 1,000,000 64,414(4) 35,714
Hudson River Trust Growth & Income
Account................................ 945,000 945,000 33,750 33,750
Delaware State Employees' Retirement
Fund................................... 730,000 730,000 26,071 26,071
Hudson River Trust Balanced Account...... 690,000 690,000 24,642 24,642
Memphis Light, Water and Gas Retirement
Fund................................... 615,000 615,000 21,964 21,964
Hudson River Trust Growth Investors...... 545,000 545,000 19,464 19,464
COVA Bond Debenture Fund................. 500,000 500,000 17,857 17,857
Oxford Strategic Income Fund............. 500,000 500,000 17,857 17,857
Forest Alternative Strategies Fund A-5... 450,000 450,000 16,071 16,071
Forest Global Convertible Fund A-5....... 450,000 450,000 16,071 16,071
Froley, Revy Investment Co., Inc.
Account: ICI American Holdings Trust... 440,000 440,000 15,714 15,714
Froley, Revy Investment Co., Inc.
Account: Zeneca Holdings Trust......... 440,000 440,000 15,714 15,714
BancBoston Robertson Stephens............ 350,000 350,000 12,500 12,500
Bancroft Convertible Fund, Inc. ......... 475,000(5) 350,000 16,964(5) 12,500
Ellsworth Convertible Growth & Income
Fund, Inc. ............................ 475,000(5) 350,000 16,964(5) 12,500

1
2

Thermo Electron Balanced Investment
Fund................................... 280,000 280,000 10,000 10,000
Black Diamond Partners, L.P. ............ 665,000(6) 237,000 23,750(6) 8,464
Black Diamond Ltd. ...................... 631,000(7) 210,000 22,535(7) 7,500
Froley, Revy Investment Co., Inc.
Account: Nalco Chemical Retirement
Trust.................................. 210,000 210,000 7,500 7,500
Declaration of Trust for the Defined
Benefit Plans of ICI American Holdings
Inc. .................................. 160,000 160,000 5,714 5,714
The Frist Foundation..................... 160,000 160,000 5,714 5,714
Declaration of Trust for the Defined
Benefit Plans of ZENECA Holdings
Inc. .................................. 130,000 130,000 4,642 4,642
Hillside Capital Incorporated Corporate
Account................................ 90,000 90,000 3,214 3,214
First Church of Christ, Scientist --
Endowment.............................. 85,000 85,000 3,035 3,035
Christian Science Trustees for Gifts and
Endowments............................. 75,000 75,000 2,678 2,678
Equitable Life Assurance Separate Account
Balanced............................... 70,000 70,000 2,500 2,500
World Wide Transactions Ltd. ............ 100,000(8) 61,000 3,571(8) 2,178
David Lipscomb University General
Endowment.............................. 40,000 40,000 1,428 1,428
Highbridge Capital Corporation........... 64,000(9) 31,000 2,285(9) 1,107
Forest Alternative Strategies Fund
A-5I................................... 30,000 30,000 1,071 1,071
LLT Limited.............................. 30,000 30,000 1,071 1,071
Forest Alternative Strategies Fund
A-5M................................... 20,000 20,000 714 714
Summer Hill Global Partners L.P. ........ 15,000 15,000 535 535

SUBTOTAL........................ $ 42,825,000 $ 41,654,000 1,561,264 1,487,642
------------ ------------ --------- ---------

Unnamed holders of previously registered
Notes(10).............................. $ 44,405,000 $ 45,576,000 1,585,892(12) 1,627,714

Unnamed holders of Notes or any future
transferees, pledgees, donees or
successors of or from any such unnamed
holders(11)............................ $ 12,770,000 $ 12,770,000 456,072(12) 456,072
TOTAL........................... $100,000,000 $100,000,000 3,603,228 3,571,428
============ ============ ========= =========

- ---------------
(1) Comprises the shares of Common Stock owned by each Selling Securityholder
prior to the offering, including the shares of Common Stock into which the
Notes held by such Selling Securityholder are convertible at the initial
conversion rate, excluding fractional shares. Fractional shares will not be
issued upon conversion of the Notes; rather, cash will be paid in lieu of
fractional shares, if any. The Conversion Rate and the number of shares of
Common Stock issuable upon conversion of the Notes are subject to
adjustment under certain circumstances. See "Description of
Notes -- Conversion Rights." Accordingly, the number of shares of Common
Stock issuable upon conversion of the Notes may increase or decrease from
time to time.

(2) Assumes conversion into Common Stock of the full amount of Notes held by
the Selling Securityholder at the initial conversion rate and the offering
of such shares by such Selling Securityholder pursuant to this Prospectus.
The Conversion Rate and the number of shares of Common Stock issuable upon
conversion of the Notes is subject to adjustment under certain
circumstances. See "Description of Notes -- Conversion Rights."
Accordingly, the number of shares of Common Stock issuable upon conversion
of

2
3

the Notes may increase or decrease from time to time. Fractional shares will not
be issued upon conversion of the Notes; rather, cash will be paid in lieu of
fractional shares, if any. The Selling Securityholders may offer and sell
pursuant to the Prospectus, their Notes, their Conversion Shares or both.

(3) Deutsche Bank Securities Inc. ("DBSI"), formerly known as Deutsche Morgan
Grenfell Inc., acted as an Initial Purchaser in the original placement of
the Notes. In addition, Deutsche Bank AG, New York Branch, an affiliate of
DBSI, is the trustee under the Indenture for the Notes. In addition,
Deutsche Bank, another affiliate of DBSI, provides an uncommitted $25
million demand line of credit to the Company.

(4) Includes (i) 18,300 shares of Common Stock owned by Allstate Insurance
Company, (ii) 1,100 shares of Common Stock owned by Allstate Life Insurance
Company, (iii) 5,500 shares of Common Stock owned by CTC Illinois Trust
Company, as trustee for the Allstate Retirement Plan, and (iv) 3,800 shares
of Common Stock owned by CTC Illinois Trust Company, as trustee for Agents
Pension Plan.

(5) Includes $125,000 principal amount of previously registered Notes (and the
Common Stock into which such Notes are convertible).

(6) Includes $428,000 principal amount of previously registered Notes (and the
Common Stock into which such Notes are convertible).

(7) Includes $421,000 principal amount of previously registered Notes (and the
Common Stock into which such Notes are convertible).

(8) Includes $39,000 principal amount of previously registered Notes (and the
Common Stock into which such Notes are convertible).

(9) Includes $33,000 principal amount of previously registered Notes (and the
Common Stock into which such Notes are convertible).

(10) Based on information provided to the Company by the Trustee as of September
21, 1998. Principal amount of Notes Owned excludes $1,171,000 principal
amount of Notes that are owned by the Selling Securityholders identified
herein.

(11) No such holder may offer Notes pursuant to this Prospectus until such
holder is included as a Selling Securityholder in a supplement to this
Prospectus in accordance with the Registration Rights Agreement.

(12) Assumes that the unnamed holders of Notes or any future transferees,
pledgees, donees or successors of or from any such unnamed holder do not
beneficially own any Common Stock other than the Common Stock issuable upon
conversion of the Notes at the initial conversion rate.